T-Smec 2.5 G-head Thoughts

[turbovanman²]

Cool, really when you compare total timing there isn't that much difference between the two from the factory, except the timing comes in faster and is different in high vacuum. That said, the general concensus seems to be that bathtub heads like lots of timing. The timing curve for them is very much like the old V8's.

Correct, swirl piston engines that use G-heads have a lower compression and need a ton of timing due to the dead chamber.

[Force Fed Mopar]

Looking at the cals in MP Tune, basically the higher the boost goes, the more the total timing becomes the same. Conversely, the at the lower boost levels and in vacuum, the bathtub uses much more timing. At 10 psi, Rob's custom 2.5 G-head ATX cal has 8-10* more than the swirl cal does.

[1BADVAN]

I am starting to think all i did was a waste of time. IDK what happened but today it just wants to detonate no matter what boost from 4-5k i even put in my old cal that used to run great and that did the same thing. Another weird thing i have had happen before when things go all weird is that the engine just started idling 200 lower than it usually does????
For the detonation, my guess is that the past few days have been about 20* cooler, but Why doesn't the old cal work, and why does it idle lower

[smdandb2]

I am starting to think all i did was a waste of time. IDK what happened but today it just wants to detonate no matter what boost from 4-5k i even put in my old cal that used to run great and that did the same thing. Another weird thing i have had happen before when things go all weird is that the engine just started idling 200 lower than it usually does????
For the detonation, my guess is that the past few days have been about 20* cooler, but Why doesn't the old cal work, and why does it idle lower

Those are the exact demons I have been fighting. My idle stays the same, but one day I will have it running great and knock free and the next day it just wants to light up the CEL everywhere.

I think I am going to start checking my plugs more frequently. If there is really as much knock as the sensor sees than the plugs should show it too I would imagine.

I suspect you have a different issue though. What kind of vacuum are you seeing? What are your air/fuels under boost now?

[1BADVAN]

Those are the exact demons I have been fighting. My idle stays the same, but one day I will have it running great and knock free and the next day it just wants to light up the CEL everywhere.

I think I am going to start checking my plugs more frequently. If there is really as much knock as the sensor sees than the plugs should show it too I would imagine.

I suspect you have a different issue though. What kind of vacuum are you seeing? What are your air/fuels under boost now?

It seems that the knock picks up more if it is hot. I let it sit for a few hours and it is cooler out, again it runs fine and the idle is back up to what it was???
I only have 8 inches of vacuum, and 11.5-11.7 A/Fs I know that is on the leaner side but i am working on that.

[smdandb2]

8" seems super low.

The Lancer pulls about .4-.5bar (11-14"ish).

[88C/S]

Have you checked the cam/ distributor timing. I had 11" at idle. Checked the cam and found it was retarded and advanced it to where it should be and now I have 20" at idle. I have a ported 287 G-head that was CC'd, with +1 valves stock 2-piece, TU cast header and 3" exhaust. The block is a stock '89 TII w/.020 mahles.

[turbovanman²]

8" is really low, even at your altitude, I would suspect maybe the timing belt jumped.

And you found the other issue with a G-head, your on the ragged edge so a temp change and it detonates like a mofo,

[Force Fed Mopar]

My 2.5 idles at 20" also. Another thing, sometimes knock sensors are overly sensitive, and the stock knock threshold is pretty conservative anyway. See my thread about it here http://www.turbo-mopar.com/forums/sh...lds&highlight=

[1BADVAN]

My 2.5 idles at 20" also. Another thing, sometimes knock sensors are overly sensitive, and the stock knock threshold is pretty conservative anyway. See my thread about it here http://www.turbo-mopar.com/forums/sh...lds&highlight=

I have roughly scanned that but i will have to look into it more. -Thanks

Thanks all of you on the cam. I will have to check the Cam, Maybe It jumped a tooth, but I probably just have the adjustable gear messed up. I thought i had it set at zero. It has had low vacuum for years i noticed it when i switched to a big FMIC and thought it was that.

Any tips on setting the cam? I know how to do it with a stock pully but how a bout for making sure the adjustable one is right?

I have heard things like adjust it until you get Max vacuum. Is that just a myth or are there tricks like that?

[smdandb2]

I've never seen a vehicle idle at 20in/hg up here, LOL. That would be neat!

On decel I see about 23in/hg. Looking at my math from last night I think I was off a bit. I usually see about 14in/hg at idle in the Lancer. I retarded the cam about 5 degrees before I broke it, so I am not sure what its actually idling at now. I never really looked.

[speedfreek500]

Is the adj cam gear a new one that fits on so the bolts are on the outside? i had an old one that was backwards and made a collar so i could flip the gear around so i could adjust it.

If you pull the cam gear off you will see the slot for the key way, if you look just above it there will be a small hole in line with it. I used that hole to line my cam up inbetween the 2 bolts at the front cam tower with the valve cover off. You could count up 2 ridges from the front of the valve cover if you didn't want to remove it.

[turbovanman²]

I have roughly scanned that but i will have to look into it more. -Thanks

Thanks all of you on the cam. I will have to check the Cam, Maybe It jumped a tooth, but I probably just have the adjustable gear messed up. I thought i had it set at zero. It has had low vacuum for years i noticed it when i switched to a big FMIC and thought it was that.

Any tips on setting the cam? I know how to do it with a stock pully but how a bout for making sure the adjustable one is right?

I have heard things like adjust it until you get Max vacuum. Is that just a myth or are there tricks like that?

When I had my 8 valve, I played alot with cam timing, I found that adjusting it for max compression made it awesome around town and the midrange rocked but it stopped revving around 4500 rpm. So you can do that then back off to suit your driving habits but of course, the best thing is to actually degree it so you know where 0 is.

[1BADVAN]

so I retarded the cam all that the adjuster would let me and that netted a 7" of vacuum.
I advanced the cam all the way and that netted only 9-10" of Vacuum.

Both seem way low still. I find it hard to believe it is that far off to be more than one tooth off on the belt, it just runs too good in my opinion.

I ran a compression test and it reads 90 across the board. With the cam advanced it went up to 120ish.
I am guessing it is so low do to the low Compression ratio.
Any thoughts guys??

[turbovanman²]

150 is the normal I've seen, 120 is about normal for a higher mileage engine.

[1BADVAN]

150 is the normal I've seen, 120 is about normal for a higher mileage engine.

I guess i should add that my engine has less than 4,000 miles would those numbers change with the lower compression?

[turbovanman²]

I guess i should add that my engine has less than 4,000 miles would those numbers change with the lower compression?

Forgot you have a G-head, so 110-120 should be about right.

[smdandb2]

Silly question, what kind of boost gauge do you have? Have you tried another?

[1BADVAN]

I advanced the cam, and the engine couldn't flow air over 4200 the A/Fs went straight to 9.0 lol, Then i retarded the cam and it was way laggy and no low end, so I guessed by deduction the cam was on the right tooth.

so i then changed my throttle body and gasket, and i know have 14"! I think my Idle speed motor was bad, maybe causing a constant leak. The new throttle body is a 52mm, I guess i will see how that affects the tune, it is now all messed up from chasing all these demons, hopefully i can find an original cal somewhere. I want to get it running good for the track on Friday.

Makes me wonder if the low compression test numbers is due to the cheap Harbor freight gauge. I will have to try a different gauge sometime.

[Force Fed Mopar]

Just go back to the 2.5 G-head cal and start from there again.