Playing with cam timing

[Joe_Van_Duno]

So looking for some opinions here. I had a huge story written out, but I'm just going to cut to the chase....

When advancing cam timing you gain bottom end power and loose top end power, but how does this affect A/F ratio? I advanced cam timing 3 degrees and all of a sudden I was really rich I'm hoping this is related....

If thats correct, then could I retard cam timing, get more top end power (which the wonderful 2.5 lacks) and lean out the A/F ratio? I know this would slow down how fast I build boost, which is all ready a major concern of mine, but if I can make my max power closer to the 5000 mark I think it would help me out at the track. I'll spray enough juice to make up for any bottom end power loss.

[cordes]

There was just a great discussion about this, but I forget which thread it was. Some of the dyno graphs were amazing and highlighted the advantages of retarding the cam on our motors. It doesn't look like there's much to lose IMO.

[tryingbe]

Only way you can really play with the cam timing is on the dyno.

[cordes]

Only way you can really play with the cam timing is on the dyno.

You can't do that at the track?

[glhs875]

Only way you can really play with the cam timing is on the dyno.

Disagree!

[tryingbe]

You can't do that at the track?

What track gives you a print out of the engine torque curve, HP curve, boost curve, and A/F curve?

[glhs875]

There was just a great discussion about this, but I forget which thread it was. Some of the dyno graphs were amazing and highlighted the advantages of retarding the cam on our motors. It doesn't look like there's much to lose IMO.

How much hp is a slightly ported 1pc intake good for is the thread

[cordes]

What track gives you a print out of the engine torque curve, HP curve, boost curve, and A/F curve?

You need all of those to play with cam timing?

[Vigo]

Trap speed is king. If you change cam timing and gain trap speed, it worked. You may still have to 'tune it in' but that's a 100% valid way to verify gains.

I talked about changing cam timing in my TBI van project thread. You may not be able to get away with as much as i did because you're auto and large turbo. Unless you have enough converter stall to make up for it or are willing to do anti-lag or nitrous to spool etc. Sounds like you might be.....

In any event you want to be making power high enough that when you upshift you never fall below the 'full spool' rpm of the turbo. You can experiment with cam retard to see if you gain more top end than you lose in spool, you may or may not end up with net gains that way.

But, it should make peak power at 5000rpm anyway with stock cam timing.

[Joe_Van_Duno]

Thanks for the input guys. Il check out that "1pc hp" thread later tonight.
I'll also see if I can find the thread Cordes was taking about, that sound like an interesting read also.

But overall it sounds like I have nothing to lose with retarding my cam timing a little bit. Guess that's something you'll be playing with at SDAC 26

[glhs875]

Thanks for the input guys. Il check out that "1pc hp" thread later tonight.
I'll also see if I can find the thread Cordes was taking about, that sound like an interesting read also.

But overall it sounds like I have nothing to lose with retarding my cam timing a little bit. Guess that's something you'll be playing with at SDAC 26

The "1pc hp" thread is the one Cordes was talking about!

[wheming]

I need to re-read that thread also. I was thinking similarly Joe, specifically how does changing the cam timing affect AFR. What can i expect for an AFR change as i retard cam timing (if/when i choose to test).

[contraption22]

If I am not mistaken, changing the cam timing will shift where peak power is made a bit, but it of course will not change the physical limitations of the engine. The valve events are not the limiting factor in making the 2.5 happy at high RPM.

[Joe_Van_Duno]

The "1pc hp" thread is the one Cordes was talking about!

I misunderstood I thought we were saying there was two different threads I need to read

[glhs875]

If I am not mistaken, changing the cam timing will shift where peak power is made a bit, but it of course will not change the physical limitations of the engine. The valve events are not the limiting factor in making the 2.5 happy at high RPM.

Not disagreeing with you but changing cam timing can help a 2.5!

[Vigo]

I misunderstood I thought we were saying there was two different threads I need to read

The gist of what i posted in my van thread was that i was limited to ~230hp by the tiny inducer on the stock mitsu turbo, and i retarded the cam 6 degrees and still was able to make 230hp. The only reason i did it was to limit midrange torque (typically the highest cylinder pressures it would see) because i was trying to tiptoe around my tbi pistons at first. In the end the cam retard slowed my spool up, but my turbo is so small it still spools early, full boost by ~2400. On the top end i was able to move my powerband enough that i go to the 6200 limiter on a completely stock top end whereas before i NEVER EVER hit the rev limiter in a 2.5 because it didn't feel like it made any power up there. It was a pretty dramatic change according to the seat of the pants. I didn't make any more power (limited by the turbo) but it changed the way it drove and where i shifted it.

[OmniLuvr]

On the top end i was able to move my powerband enough that i go to the 6200 limiter on a completely stock top end whereas before i NEVER EVER hit the rev limiter in a 2.5 because it didn't feel like it made any power up there. It was a pretty dramatic change according to the seat of the pants. I didn't make any more power (limited by the turbo) but it changed the way it drove and where i shifted it.

this almost makes me want to put a 2.5 in my omni... hmmmm

[tryingbe]

You can't do that at the track?

Way to many variables on the track, plus, I am lucky to run two runs on the entire night where as on Dyno, I can change settings, run it, get a graph every time to see if it improved or not.

Silly not to do it on the dyno.

http://www.thelostartof.net/tryingbe/dodge/omniproject/2015build/dyno1.jpg

http://www.thelostartof.net/tryingbe/dodge/omniproject/2015build/dyno2.jpg

[GLHS60]

You need all of those to play with cam timing?

I was going to suggest "work on Engine, play with self...
Better not :)

Thanks
Randy

PS: This from Gary, gold as usual:

"I discovered the wonders of cam centerlining and the effects of rolling the timing from as low as 106 to 114. One would give me buckets of torque that would blow the tires away.. and the other end of the range would make the car soggy down low but have a hell of a top end charge. The trick is to find that sweet spot with just enough torque to keep the tires from getting blown away on the bottom end of the track while having enough juice for the top end charge."

[Joe_Van_Duno]

I will definitely be playing with my cam timing this weekend unfortunately weren't here to get on the dyno for tuning is way to expensive. Sometimes you can get $75 for three pulls but there's no tuning in between it's just three pulls and you're done. I'll just take the van up to the country and do some testing.

[glhs875]

I was going to suggest "work on Engine, play with self...
Better not :)

Thanks
Randy

PS: This from Gary, gold as usual:

"I discovered the wonders of cam centerlining and the effects of rolling the timing from as low as 106 to 114. One would give me buckets of torque that would blow the tires away.. and the other end of the range would make the car soggy down low but have a hell of a top end charge. The trick is to find that sweet spot with just enough torque to keep the tires from getting blown away on the bottom end of the track while having enough juice for the top end charge."

This holds true if a person is not willing to make changes to the combo like final drive ratio and or converter stall speed if an auto. Or play around with the compression ratio, etc.

[glhs875]

The gist of what i posted in my van thread was that i was limited to ~230hp by the tiny inducer on the stock mitsu turbo, and i retarded the cam 6 degrees and still was able to make 230hp. The only reason i did it was to limit midrange torque (typically the highest cylinder pressures it would see) because i was trying to tiptoe around my tbi pistons at first. In the end the cam retard slowed my spool up, but my turbo is so small it still spools early, full boost by ~2400. On the top end i was able to move my powerband enough that i go to the 6200 limiter on a completely stock top end whereas before i NEVER EVER hit the rev limiter in a 2.5 because it didn't feel like it made any power up there. It was a pretty dramatic change according to the seat of the pants. I didn't make any more power (limited by the turbo) but it changed the way it drove and where i shifted it.

I always wanted to experiment with increasing the compression ratio enough after retarding a cam so that the cranking psi would at least equal the factory cranking psi. I have a thinner .027 headgasket just for this purpose. Raising the compression ratio may bring back some or all of the lost low end power and also help with the turbo spool up and also increase the upper RPM HP even more!

[Vigo]

I always wanted to experiment with increasing the compression ratio enough after retarding a cam so that the cranking psi would at least equal the factory cranking psi. I have a thinner .027 headgasket just for this purpose. Raising the compression ratio may bring back some or all of the lost low end power and also help with the turbo spool up and also increase the upper RPM HP even more!

I'm going to experiment with this myself. I've been planning to build a 10:1 G-head engine with carb pistons, a big cam, and an upgraded vnt turbo to see if i can build something that makes at LEAST stock power down low but keeps working up to ~7500+. I have everything but the cam and valvetrain (decisions..) but haven't done any of the work!

[glhs875]

I'm going to experiment with this myself. I've been planning to build a 10:1 G-head engine with carb pistons, a big cam, and an upgraded vnt turbo to see if i can build something that makes at LEAST stock power down low but keeps working up to ~7500+. I have everything but the cam and valvetrain (decisions..) but haven't done any of the work!

I like where you are headed! I was having problems with rocker arms being spit out above 7500 rpm (w/ Taft S3 cam)! The cause could of been the valve springs. Anyhow I was looking into using the mousetrap spring setup from a 2.3/2.0 Ford engine to help with the problem. Warren Stramer got with me asking about any problems I might be having with increased RPM's. And he went with a setup that he designed to help keep the rockers planted on the lifter. Maybe check his idea out!

[OmniLuvr]

I'm going to experiment with this myself. I've been planning to build a 10:1 G-head engine with carb pistons

I ended up using t3 pistons, they worked awesome with stock g-head and my block decked 15 thou. Not sure how close the valve clearance might get if there was excessive machining on the head gasket surface or large cam... but I'm bout to try both shortly...

[GLHS60]

This holds true if a person is not willing to make changes to the combo like final drive ratio and or converter stall speed if an auto. Or play around with the compression ratio, etc.

I don't understand your point. Gary explained how he fine tuned his cam timing at the track with the final drive, converter and compression he had at that point in time. He always gives excellent, precise, direct advice.

Thanks
Randy

[mopar-tech]

I don't understand your point. Gary explained how he fine tuned his cam timing at the track with the final drive, converter and compression he had at that point in time. He always gives excellent, precise, direct advice.

Thanks
Randy

Thank you Randy- I have to point out that I did play with some factors like compression (minor) and final drive ratios. Low ET was always the goal and lowering the short time drove most of the changes along with making more power in general.

Balancing the chassis = equalizing the traction to the front wheels
Changing the gear ratio in the final drive = reduce wheel spin/torque multiplication
Removing mass = faster short time and less parts breakage

etc, etc.

[glhs875]

I don't understand your point. Gary explained how he fine tuned his cam timing at the track with the final drive, converter and compression he had at that point in time. He always gives excellent, precise, direct advice.

Thanks
Randy

If a change is made to an engine combo and lets say the change makes more power up top but loses some down low. A person can then play with other things (stall speed,final drive,etc.) to compensate for the loss in lower RPM power but will take advantage of the top end power increase. Not doubting Gary's abilities, but what works for him may not work for someone else. If molds are not broken how can progress be made. I know I most likely get frowned upon by many for not always agreeing with what Gary says or do, but that is okay with me!

[MILKCARTON]

∆∆∆ what this guy said

[Vigo]

I ended up using t3 pistons, they worked awesome with stock g-head and my block decked 15 thou. Not sure how close the valve clearance might get if there was excessive machining on the head gasket surface or large cam... but I'm bout to try both shortly...

I just mounted up some t3 pistons on LW rods ;) for my spirit but it is 782 head. They are still out so i can compare them to the carb pistons i have as far as compression height. Anyone know the total 'dish/valve relief' volume of a t3 piston? As far as valve clearance i don't care about freewheeling if the belt skips. In all this time i can only think of one time i had a belt skip or break out of all the shitty cars and builds ive done, and i cant remember the last time i bought a new 8v timing belt! lol

[GLHS60]

If a change is made to an engine combo and lets say the change makes more power up top but loses some down low. A person can then play with other things (stall speed,final drive,etc.) to compensate for the loss in lower RPM power but will take advantage of the top end power increase. Not doubting Gary's abilities, but what works for him may not work for someone else. If molds are not broken how can progress be made. I know I most likely get frowned upon by many for not always agreeing with what Gary says or do, but that is okay with me!

I don't want to sound disrespectful but your not making any sense in criticizing Gary for fine tuning his combination, whatever it might be, at the track.

Anyone who is consistent enough and smart enough to fine tune cam timing at the track is very talented.

I'm sure, without even asking Gary, that if he changes a torque converter, or gearing or anything else, he considers where his cam is at.

I cant understand the criticism towards someone who shares his success.

I don't mean to speak for Gary but I appreciate and trust what he says.

Actually, I think this trend of retarding the cam is breaking the mold, and here is Gary and Vigo confirming my theory to some extent with their testing.

I'm not frowning on any of your accomplishments, I just don't understand the criticizing.

However, if you have a good reason to not fine tune cam timing then I will have to eat my words!!:)

Thanks
Randy

[mopar-tech]

If a change is made to an engine combo and lets say the change makes more power up top but loses some down low. A person can then play with other things (stall speed,final drive,etc.) to compensate for the loss in lower RPM power but will take advantage of the top end power increase. Not doubting Gary's abilities, but what works for him may not work for someone else. If molds are not broken how can progress be made. I know I most likely get frowned upon by many for not always agreeing with what Gary says or do, but that is okay with me!

What I get a chuckle about is people assume why I did something and have lengthy discussions without asking me why or for more data. I take no offense, questions are always good, more data is always good assuming the input is sound.

Feel free to play with cam timing; as I stated with the GTX it was a valuable tuning aid for street racing and with the Reliant a valuable for for maximizing elapsed time. Different venues that have their own unique requirements IMHO.

I was also clear that there are 2 schools of thought regarding engine tuning- Tune for peak power and seal the bay and tune the chassis or if you can't tune the chassis very much then tune the engine to the track. I'm for which ever gets you the lowest E.T.

[glhs875]

I don't want to sound disrespectful but your not making any sense in criticizing Gary for fine tuning his combination, whatever it might be, at the track.

Anyone who is consistent enough and smart enough to fine tune cam timing at the track is very talented.

I'm sure, without even asking Gary, that if he changes a torque converter, or gearing or anything else, he considers where his cam is at.

I cant understand the criticism towards someone who shares his success.

I don't mean to speak for Gary but I appreciate and trust what he says.

Actually, I think this trend of retarding the cam is breaking the mold, and here is Gary and Vigo confirming my theory to some extent with their testing.

I'm not frowning on any of your accomplishments, I just don't understand the criticizing.

However, if you have a good reason to not fine tune cam timing then I will have to eat my words!!:)

Thanks
Randy

Never did I say I was against fine tuning cam timing! I have been changing/playing with cam timing since at least the early 1980's! I am not new at this! BTW I was the one who brought up cam timing in another recent thread. And I don't feel that I am criticizing Gary! Things that I have posted goes for myself as well! And I agree Gary has done a lot for our hobby and never said that he didn't! But that does not make me a blind follower of Gary! I like to question things that others may have done with MY OWN experiences! Personally I was wondering why you went off the deep end! I feel I made a mistake on commenting on your post in the first place! Normally If I disagree with a post I tend to just let it go! I need to keep doing that! And the comment that I made about your post about Gary's thoughts on cam timing was not directed at just Gary! It was for anyone! I am getting real close to kissing Turbo-Mopar goodbye!

[glhs875]

I don't want to sound disrespectful but your not making any sense in criticizing Gary for fine tuning his combination, whatever it might be, at the track.

Anyone who is consistent enough and smart enough to fine tune cam timing at the track is very talented.

I'm sure, without even asking Gary, that if he changes a torque converter, or gearing or anything else, he considers where his cam is at.

I cant understand the criticism towards someone who shares his success.

I don't mean to speak for Gary but I appreciate and trust what he says.

Actually, I think this trend of retarding the cam is breaking the mold, and here is Gary and Vigo confirming my theory to some extent with their testing.

I'm not frowning on any of your accomplishments, I just don't understand the criticizing.

However, if you have a good reason to not fine tune cam timing then I will have to eat my words!!:)

Thanks
Randy

I wanted to add that the whole purpose of my post was not to be disrespectful to anyone or even disagree with anyone but only to maybe make others aware that there are things a person can do to their combo to work with or to compensate for whatever effect the cam timing changes may have to engine power output. I feel as though you have added MANY WORDS to my post that I didn't even say or even mean!

[Gaboon]

Fine tuning for max power on a dyno is fine but keep in mind nobody races a dyno. All a dyno is, is a tool.
Fine tuning at the at the track for quickest et is what I look for and what all racers should be shooting for.



We are taking about cam timing and it's affects on power output. Yes there are loads of other things we can change that will have an affect on ET but this conversation is about cam timing.

[Shadow]

Fine tuning for max power on a dyno is fine but keep in mind nobody races a dyno. All a dyno is, is a tool.
Fine tuning at the at the track for quickest et is what I look for and what all racers should be shooting for.



We are taking about cam timing and it's affects on power output. Yes there are loads of other things we can change that will have an affect on ET but this conversation is about cam timing.

Like it! :thumb:

[Vigo]

Well i think we should decide whether we're really talking about tuning for POWER (trap speed) or tuning for ET since referring to both in the same post can be confusing. I think cam timing can affect either one but you may not necessarily gain on both unless you can hook up all the power you're making, so at some point you may have to decide which one is more important to you, as Gary was saying about different venues.

[GLHS60]

I wanted to add that the whole purpose of my post was not to be disrespectful to anyone or even disagree with anyone but only to maybe make others aware that there are things a person can do to their combo to work with or to compensate for whatever effect the cam timing changes may have to engine power output. I feel as though you have added MANY WORDS to my post that I didn't even say or even mean!


When you said this about my post that quoted Garys input on cam timing:

"This holds true if a person is not willing to make changes to the combo like final drive ratio and or converter stall speed if an auto. Or play around with the compression ratio, etc.?

It sounded like you were disagreeing with Garys input on cam timing. It doesn't add anything to the cam timing discussion is why I said I don't understand your point.

If for example you would have stated you found advancing the cam was beneficial at 7500 RPM that would have been on point.

Anything you learned since the 1980's about cam timing would have been of interest !!

I usually don't post much but the cam timing topic has been of great interest.

I wasn't trying to add any words in your post only trying to understand your point and I still don't.

Sorry if I offended you or anyone else, that wasn't my intention.

Thanks
Randy

[Gaboon]

My camshaft is bigger than your camshaft...;)


That's all I got and of no value to this thread.

[glhs875]

When you said this about my post that quoted Garys input on cam timing:

"This holds true if a person is not willing to make changes to the combo like final drive ratio and or converter stall speed if an auto. Or play around with the compression ratio, etc.?

It sounded like you were disagreeing with Garys input on cam timing. It doesn't add anything to the cam timing discussion is why I said I don't understand your point.

If for example you would have stated you found advancing the cam was beneficial at 7500 RPM that would have been on point.

Anything you learned since the 1980's about cam timing would have been of interest !!

I usually don't post much but the cam timing topic has been of great interest.

I wasn't trying to add any words in your post only trying to understand your point and I still don't.

Sorry if I offended you or anyone else, that wasn't my intention.

Thanks
Randy

No problem! All is forgiving on my end! I agree cam timing is interesting!

[glhs875]

I will try and explain my comment a little! Say on a combo a person finds that retarding a cam (which has a similar effect of adding a larger camshaft) now has more top end HP which resulted in a higher MPH than before but the ET dropped do to a loss in lower end power which netted a slower 60' time. A person can then do things to his set up like lower the final drive ratio (which increases torque to the wheels and allows the engine to stay at a higher rpm during the run), increase the stall speed of the converter (if an auto) which allows a higher launch rpm and also will help the engine stay in the powerband during gear changes, raise the compression ratio a little to make up for a loss of cylinder pressure (loss of low end power) in the lower R's due to the later closing event of the intake valve from the cam timing change. These are some of the things I was referring to in my post.

[wheming]

I'll just contribute this, which will be of no real technical value whatsoever.
I appreciate reading all experienced comments, suggestions and real world results. Anyone who is willing to share results from thorough testing is giving those of us who haven't done so, or those without the ability a real gift. -Knowledge without firsthand experience and the trial and error.
For me the discussion and digression to other factors like TC stall speed and final drive is still interedting. But, the topic was about the cam timing and for me at this point in my build those other factors are no longer variables i am changing. Those of us who are not about to go into transmissions at this point have what we have. But as we can see from those who have performed thorough testing, you can still optimize the combo you have.

I remember suggestions that i was making a bad move by using .91 transfer gears in Mini along with the higher stall converter and big turbo. On the street tires which are taller it already leaves a tad soft - initially. The slicks are shorter and i only have bested a 1.88 for the 60'. Maybe messing with some cam timing would help? Maybe retarding it won't hurt the short time so much as help some top end power?
But, it is easy enough to change and go back. I'm going to get a good pic of just where the cam is referenced on the Fidanza scale/indicator. Then if i want to try a change i can set it back. Right now it is just properly degreed at the spec centerline.

Anyone that provides enough details to be useful to all users is ok in my book. When you only get part of the story that's not really productive sharing because it'll just set someone up for failure who doesn't have all the relavent/prerequisite data or configuration prior to trying to apply those principles.

[GLHS60]

No problem! All is forgiving on my end! I agree cam timing is interesting!

Right on, I appreciate your understanding reply!!

Thanks
Randy

[contraption22]

The ET and trap speed tells everything, and will ultimately dictate what's best for your combo. Actually more so the ET, because trap speed doesn't turn on the win light.
That said, having to set your cam timing far out of the intended settings to get the car to run better is probably a good indicator that one piece, or several pieces of your hardware combination is pretty mismatched. I know this because I've been there. I ran my cam way advanced to compensate for a poorly matched turbocharger and a tight converter. To get the car to 60ft, I knew I ended giving up a lot of top end power. Going with a better matched turbo, I chopped short time AND gained trap with the cam back in normal range.

[Ondonti]

I don't think the facebook style defensiveness belongs or the Millennial inability to disagree. We can't have discussion when we all fear to share.

Cam timing knowledge has been around as long as the camshaft has. The people who dont know are we the people when we first get into the scene. Been in all those books forever.

I tried advancing the timing on a low compression 3.0 built motor with a big cam to avoid the super low oem rev limiter. It did keep AFR in check as it didnt lean out so bad up top (stock n/a ecu). It didn't move the powerband enough to keep me In power when on slicks. If I babied it, bog on shift. Car performed poorly with a careful driver. That was 8 degrees advanced (a lot). With cam straight up and a higher limiter on tunable fuel megasquirt, it woke up on half the boost.

So my experience retarding the Cam's was due to a rev limiter restraint not for optimizing a car with many other adjustments possible to make up for either a soggy bottom and or waiting power up top. We all have reasons and desires to experiment. My suggestion is to not run different timing each pass combined with boost changes till It blows up because you unplugged that annoying knock sensor last pass.

[GLHS60]

The ET and trap speed tells everything, and will ultimately dictate what's best for your combo. Actually more so the ET, because trap speed doesn't turn on the win light.
That said, having to set your cam timing far out of the intended settings to get the car to run better is probably a good indicator that one piece, or several pieces of your hardware combination is pretty mismatched. I know this because I've been there. I ran my cam way advanced to compensate for a poorly matched turbocharger and a tight converter. To get the car to 60ft, I knew I ended giving up a lot of top end power. Going with a better matched turbo, I chopped short time AND gained trap with the cam back in normal range.

Good point Mike and one might conclude a larger turbo, ported head, etc. might be a slightly mis matched combination with stock cam timing. Some are finding that 2.5's are responding very well with late cam timing at high RPM. Luckily, while our cams are hard to centerline, they are real easy to advance or retard!!

Thanks
Randy

[glhs875]

Good point Mike and one might conclude a larger turbo, ported head, etc. might be a slightly mis matched combination with stock cam timing. Some are finding that 2.5's are responding very well with late cam timing at high RPM. Luckily, while our cams are hard to centerline, they are real easy to advance or retard!!

Thanks
Randy

The 2.5 has a around a 1.45 rod to stroke ratio stock. A 1.45 is considered a small/short number for rod to stroke ratios. Engines set up in this manner tend to respond well to later intake valve closing events (camshaft retard) A Reason why a short rod to stroke ratio responds well to a later intake valve closing events is due to on shorter rod to short ratios the piston travel on Bottom dead center area is slowed down more than that of a long rod to stroke ratio setup.

[88C/S]

Great info here! I have a 2.5 with a mildly ported swirl head with +2 valves, a .499 lift/260 duration cam, BMF intake, log header and GT35 clone turbo. I retarded the cam about 2 degrees, in hopes to give the engine more top end past 5500 rpms. Now I have yet to drive it on the street, due to items that have to be corrected. Reading this thread has given me an idea what I could do with my other engine, should this one not work out as hoped. My 2.5 will see more autocross than drag racing, unless I have my friend run it. He's better at it.

Please keep the info going and make this a sticky

[glhs875]

I thought I would talk some about what happens on the exhaust side of things when changing cam timing. When the cam is retarded the opening of the exhaust valve is delayed until later in the power stroke and then the exhaust valve will close later in the TDC area at the end of the exhaust stroke. Of course the opposite will happen when the cam is at an advanced position. When I say advanced or retarded for the cam that actually means the centerline of the cam will most likely be in the area of around 104 to around 116 degrees. Some may like to venture even further than that.

[GLHS60]

I copied this paragraph from a David Vizard cam advance/retard discussion. I tried to copy/paste more but timed out on the site.

One of his points is a perfectly matched cam does not benefit from advance or retard.

He spends lots of time designing and testing cams so he has been able to spec cams very accurately, not for our engines of course, mostly N/A V8's. Very hard to spec a cam perfectly but I believe that's one of his life long ambitions.

128 is his secret decoder ring formula.

All very interesting then you bring up R/S ratio's!! Up next: squish : slider vs roller : eyebrows:

Thanks
Randy



"Notice the use of the term ‘overall’ to define engine output. That’s because a timing change does not necessarily increase or decrease output in a uniform manner throughout the rpm range. Usually , but certainly not always, a cam retarded from optimal for the best average curve will drop low speed output due to later intake valve closure but picks up at high speed for the very same reason. Advancing tends to do the reverse, that is pick up low speed and drop high speed. Just how much the power and torque curves are skewed by this really comes back to how optimal the cam spec was in the first place. For the most part you can assume that the more optimal the cam spec the less there is to be gained by attempting to tune outside the timing called for. I have seen well specced cams where either more or less advance dropped output everywhere."

[glhs875]

I can agree with David Vizard's paragraph. I have read some on him myself. In my opinion R/S ratio's has a huge influence on cam choice and cam timing for an engine. If a person finds that changing cam timing can change engine output a great deal. then that info can also lead into maybe choosing a different cam or having a custom ground cam made that will increase engine output even more!

[glhs875]

Having mentioned that R/S ratio's having an effect on cam/cam timing requirements, piston pin offset has a similar effect on piston speed at the TDC and BDC areas as connecting rod length.

[GLHS60]

Again not to be argumentative, while such things have an effect they are minuscule and not always positive.

Dave Zelkowski summed up his research nicely on The Dodge Garage !!!

Thanks
Randy

[Johnny]

I found out by mistake it makes a difference. In my GLHS autocross car, I got the cam one tooth off and presto...had much better low end. Great for corner to corner. I loved it and left it there.

[GLHS60]

I found the same thing running a slider cam on roller rockers. I had a winter driver 1988 TBI Omni and wanted to keep the cam.

With the stock slider cam and rollers it had exceptional bottom, great for short track autocross.!!

However it ran out of breath early, so I retarded the cam one tooth and it was back to normal.

I wanted to see how long the cam would last and it gave no problem, although I was willing to accept the consequences if it chewed up the cam. Might have been a rare case as its accepted policy to not run this combination.

With my stock GLHS, I added a roller cam and rockers that apparently retards cam timing 4 deg.

With a 525 I never pushed it off the line, but it was way more comfortable at high RPM.

Nothing was degreed, just dot to dot stuff, + or -1 tooth.

Thanks
Randy

[contraption22]

I will definitely be playing with my cam timing this weekend unfortunately weren't here to get on the dyno for tuning is way to expensive. Sometimes you can get $75 for three pulls but there's no tuning in between it's just three pulls and you're done. I'll just take the van up to the country and do some testing.


Gonna take it out for a test hit?

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