FredR

Is there anything in the GTS exhaust cam profile design to suggest that it is "less than optimal"?

ptuomov

What's your objective function?

My opinion:

If you want to pass emissions with certainty even if the car is somewhat neglected and produce a flat torque curve in the factory decided power band, the factory GTS cams are probably pretty good for the GTS motor.

I think most of the views about GTS exhaust cam being "bad" come from people taking the factory manual misinformation at face value. This includes me a decade ago.

If you want to make more mid-range torque, I think you need more overlap than the factory GTS cams. So earlier IVO and later EVC. That will make the emissions tail pipe test a little more sensitive, but if everything else is set up well it's going to pass with much more overlap than what's ground into the stock GTS cams. If you want to make more power at high rpms, you probably want earlier EVO and later IVC. Those changes together should allow you to increase compression a little bit by shaving the heads and using the thin stock gasket. If you make all those changes, headers will start really helping. I think Roger Tyson's GTS has all those changes (including Elgin 65-6 cams) and he appears to be very happy with his motor. GT cams would also help, but less.

That's just my opinion. Don't confuse that with a fact.

FredR

About 15 years ago there was a widely held opinion around that the GTS exhaust cam was constrained by design to stop the GTS from outclassing the 911 turbo's of the day on top end performance. When I was putting my current motor together [a 90 S4] I received a recommendation to use the GTS inlet cams with the S4 exhaust- no real clue whether it is an improvement or not but it seems to work ok. Given design tools available today it seems some of you chaps are better armed to evaluate prospective performance mods than folks were 15 years plus ago. What with the likes of Colin cams and other grinds around the possibilities presumably are more evident than at any other time previously.

ptuomov

We learn new stuff every day. Often, the learning comes in the form of a conspiracy theory being discredited.

Will the S4 exhaust cam run better or worse in a GTS or S4 engine than the GTS exhaust cam? I don't have the info in my fingertips about the LCAs available for GTS and S4 intake and exhaust cams. One theory is that the S4 exhaust cam, although shorter in duration, increases the overlap. The increased overlap will boost the mid range torque, and the exhaust port of the 928 heads flows so stupidly well to start with that the shorter duration doesn't hurt too much. It's just a theory. If Rob posts the measured LCAs for these cams, then we can go on and theorize further on a firmer ground.

Strosek Ultra

Some basics about cams. The exhaust cam usually has less influence on engine power than the intake cam. Very mild cams like the 928 stock cams, usually perform best having a retarded setting (intake valve closes late). Medium performance cams or some may call them sports cams usually perform best in a split overlap setting. Very hot racing cams usually peform best in an advance setting.
Åke

Mark Anderson

Reminds me of something I tried years ago

Bigfoot928

Looks very similar to my original variocam experiment. I have several broken camshafts from experimenting.

Strosek Ultra

Did it work well? Tell us more.
Åke

Strosek Ultra

Tell us more about your broken cams. What happened? Any solutions/fix?
Åke

SwayBar

There's a thread on that somewhere and it's a sad story.

One thing I remember was that the newly ground cams were broken during shipping.

Bigfoot928

One cam was broken in shipment. I broke another showing someone how brittle the factory cams are. Stand on one end of the cam with the toothed gear high center. Stomp on the other end of the cam & viola 2 pieces. The cam gear failures were catastrophic. just when you think something was strong enough, It decides to visit you by a sudden exit through the cam cover during idle & whiz by your head. I don't recommend that one. I don't know if I kept any of the broken bits. We are packing up for a move so it will be months before I find them again... if I even have them.

Mark Anderson

The first set broke after just a few hours. This is the replacement part but we never tried them. If I remember correctly these had more teeth which required a longer chain

Bigfoot928

Strosek Ultra

Sorry to hear about your camshaft problems. One guy over here in Sweden had one of his Lizard cams to break off during shipping. The 928 chilled cast iron camshafts are not more brittle than any other cams. One has to take care when torquing down the camshafts. I agree the chain gear being an integrated part of the camshaft is not suitable for the kind of drilling you guys have made in order to make it adjustable. Chilled cast iron is the far best camshaft material in respect of wear characteristics.
Åke

There are various materials crankshafts are manufactured from-

Generally, the camshaft is made from Chilled iron castings, because this material has far superior wear characteristics than any other material.

Majorly two materials are used - IRON and STEEL

I) IRON

1. Hardenable iron (developed in 1930)- low cost replacement for steel.
2. Spheroidal graphite cast iron (used by FIAT in 1980s) - failed due to its low hardness value in cast form.
3. Chilled chrome cast iron- Extremely good scuff resistance as mentioned above and most used material for camshafts.

II) STEEL

1. Carbon Steel- EN8/EN9 (used during 1930–1945)- used nowadays for induction hardened camshafts due to its through-hardening characteristics.
2. Alloyed steels-EN351 AISI 8620 and EN34 etc. - Best when run against a chilled cam follower.
3. NITRIDING STEEL-EN40B- The best steel for camshafts. After nitriding it gives a surface hardness and finish similar to chilled iron.

Now when to use STEEL or IRON

• IRON - High volume production

• STEEL - Low volume production or high quality camshafts

V2Rocket

If you use 2x exhaust cams per head you can fit adjustable camshaft gears instead.
By enlarging the center bore you can fit Honda or the like gears that are sourced inexpensively from eBay. Just match the "donor cam gear" tooth count against your crank drive gear (y = 2x). Lots of options.