I've been looking at the factory specs for the S3, S4, GT and GTS cams. I was curious if anyone had run the GTS (or even the S3) intake cam advanced one tooth on the cam chain.

Here's my thinking: There are 19 teeth (or 18?) on the cam chain sprocket. If you advance or retard the cam by one tooth, that's a difference of approximately 19 degrees. Stock GTS intake cam peak lift is at 127.0º ATDC and lobe separation angle is 118.5º. One tooth advance results in peak lift at 108.1º ATDC and 109.0º LSA. Stock GT intake cams have peak lift at 112.5º ATDC and 110.0º LSA.

This LSA of 109º or 110º seems more in line with typical high performance cams I've read about. The stock GTS LSA seems too large. The overlap at 1mm lift on GT cams is about -8.0º (no overlap) and -6.1º on GTS-1tooth.

I was thinking it might bump the dynamic compression ratio too high but I noticed Louis Ott has successfully run GT cams in a GTS. The intake valve closes at 42.0º ABDC on a GT and 42.1º ABDC on a GTS-1t. Plus we're able to alter the overall cam timing a few degrees in either direction so the DCR should be ok.

I know the lower lift and shorter exhaust duration of the GTS cams limit their power potential. That said, I was still wondering if anyone had tried the GTS-1t mod and if so, what were the results.

 

I'm running my S4 cams 4 degrees advanced (how do you get the degree symbol).

Using Porkens tool and his advice that advanced cam timing would improve low end power I gave it a shot. Seat of the pants dyno did indicate sucess but a subsequent compresion test showed all cylinders 5-8 to be consistant but a good 15% diff (lower) than the 1-4 cylinders.

I'm going back to stock timing as one of many winter projects and shall post comp results shortly thereafter.

 

Right, I've seen Louis' dyno graphs comparing advanced vs. retarded cam timing overall. It seems like he found some worthwhile gains.

Just to be clear though, I'm talking about adjusting only the intake cam timing by relocating it by one tooth on the chain linking it to the exhaust cam. Like a very poor man's Variocam.

Oh and the degree symbol º is ALT-167 (hold down ALT key and type 1-6-7 on your keypad).

 

I just reread your post; got your idea but no I've not been there.

Just overall cam timing is my experience.

 

I knew a Fiat mechanic who would alter the intake cam timing. Made for a great engine, but he wouldn't give me the specs. I think you are onto something, but is one tooth a little radical ? I am guessing it would be a lot.

 

http://www.tedmontgomery.com/tutorial/ALTchrc.html

 

Malcolm,

What did you set the ⅝ side to? To get 4° advance you would need to set it at ~2° advance with the engine cold, to compensate for expansion. The ¼ side would be 4° advance.

Did you do the compression check with the engine hot? Checked cold, the ⅝ side will be lower because of the cold retard.

 

AFter doing more searching it looks like Greg Gray (and probably many others) thought of this years ago:
https://rennlist.com/forums/928-forum/41374-had-a-crazy-idea-about-camshafts.html

The question remains though - has anyone tried it?

 

This sounds promising.... "if" it works....... I'm sure some of the other experienced engine builders will comment?

 

Dennis,
If you already have the GTS cams, you can use the GTS intake cams and an S4 exhaust cam. That will get you better performance than either a GTS or an S4 cam set. Not as good as regular GT or S3 cams. It's the GTS exhaust cams that seem messed up. I'm not sure I believe the WSM GTS cam specs since the numbers don't look right. However, I never spent time actually degreeing the GTS cams. I guess reground they'd be good for something. Phil Threshie reground a set of GTS cams for Dennis Wilson and they worked well in his stroker motor. I set the intake lobe center for 110 deg ATC hot (113 deg cold). The GT cams work extremely well in a GTS engine. Such an engine needs some touchup ECU tuning around the idle area as all seem to not return to idle very well. It's been my completely unsupported opinion that Porsche made the GTS engine first with GT cams. When testing it they realized it would blow the doors off of whatever VW derivitive they were pushing at the time. The illuminati would then revolt. They'd never sell another 911 and have to keep making the 928.

You do raise an interesting point though. If the timing was changed on the GTS intake cam by a tooth, then time the intake cam for about 110 deg ATDC hot (113 cold) that would change the exhaust cam timing from the stock reference. That could be why the exhaust cam seems to be the one that causes the problem. Maybe not because I do know the GTS intake cam works good with the S4 exhaust cam with the GTS intake cam set normally. Dunno. It's late and I'm tired.

 

So this would make S3 intake basically like GT or do I remember wrong what is different between S3 and GT cams?

 

The difference in compression between sides on a hot engine shows the cam timing is not in sync. Advance the side with the low compression until they are even. Or, retard the side with high compression. The compression check has to be done ASAP after shutting down the engine. Cold engine compression should always show a difference between sides. This doesn't apply to the leak down method of checking compresion.

 

MAN you guys know your stuff ! I`m amazed :-)

Tell me; what is an S3 ?

 

Unofficial name of '85-86.0 US 32V model.

 

Hi Louie -

I agree w/ you that there was a probably a hierarchy to the hp numbers for each model. I think the 911 turbos at the time were at 355 hp, which is why they chose 345 hp for the GTS.

This GTS cam idea is not for my car - it's just something I'm throwing out there . . . It's extremely interesting that you've found the S4 exhaust cam works better than a GTS exhaust cam when they're combined with a GTS intake cam.

The S4 ex cam appears to be the worst of all the factory 4-valve exhaust cams. It has only 195º duration @ 1mm with 8.0mm lift. A GTS ex has 196º duration with 8.5mm lift. By the numbers, the GTS exhaust cam should be better.

However the GTS intake/S4 exhaust combo has a LSA of 113.3º, which is theoretically better than the stock GTS intake/GTS exhaust LSA of 118.0º. A GTS-1tooth intake/GTS exhaust would have an LSA of 109.0º, similar to a GT set.

Maybe there's power to be unlocked from a GTS using the stock parts and a relatively easy adjustment?

-Dennis