Re: Playing with cam timing
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.
Re: Playing with cam timing
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.
Re: Playing with cam timing
Quote:
Originally Posted by
glhs875
No problem! All is forgiving on my end! I agree cam timing is interesting!
Right on, I appreciate your understanding reply!!
Thanks
Randy
Re: Playing with cam timing
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.
Re: Playing with cam timing
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.
Re: Playing with cam timing
Quote:
Originally Posted by
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.
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
Re: Playing with cam timing
Quote:
Originally Posted by
GLHS60
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.
Re: Playing with cam timing
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
Re: Playing with cam timing
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.
Re: Playing with cam timing
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."
Re: Playing with cam timing
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!
Re: Playing with cam timing
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.
Re: Playing with cam timing
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
Re: Playing with cam timing
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.
Re: Playing with cam timing
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
Re: Playing with cam timing
Quote:
Originally Posted by
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.
Gonna take it out for a test hit?
http://www.turbo-mopar.com/forums/at...id=58550&stc=1