sportscarclassics

Would big valves, polished ports make that much difference in HP if you combine it with a RRFPR, 2.5" dual exhaust, headers?

ErnestSw

Your GT's HP is limited by the amount of air/fuel you can get through the engine, THEN by the design of the engine itself such as compression ratio etc. The cheapest HP increase will come from exhaust and fueling modifications. Word is that the stock intake system is good for over 500HP.

epbrown01

Say what? Then why all the supercharging? I'm a novice to tuning, but it sounds like you're saying the engine could make 500 hp if someone could increase the fuel pressure, remove the exhaust, and get the computer to go along with both. Why go to forced induction if the intake is so good?

Emanuel

ErnestSw

What's the purpose of supercharging? To ram more air into the engine so you can mix more fuel with it to produce more power. It's been shown that the stock intake can produce 500HP+ if the air pressure is increased. That's called supercharging.

epbrown01

Ah - I get it now. You're saying that the heads and intake can flow enough air, but supercharging increases that airflow. Hmm, shouldn't moderate gains be possible with a high-flow filter or sort of ram air set-up too? Not as many gains, but more.

ErnestSw

It's been tried by lots of people and the bottom line seems to be that Porsche didn't leave much on the table in this area. Louis Ott has seen a 6HP gain modifying his filter/MAF setup, but that's about it. REMOVING the air filter doesn't increase HP.
The biggest bang for your buck will be in the area of exhaust modifications with a rising rate fuel regulator. Louis Ott's X pipe gave about 30HP on the dyno and work is being done on an X pipe which may give even better numbers. 928 Specialists will be dynoing their X pipe soon as well.

Jim_H

Here is what a little extra air can do for you. This was with very little tuning. Ignore the 5th gear before #, it's the 3rd gear run at 4 in the morning that was a hoot!

ErnestSw

Oh, yeah, supercharging doesn't hurt either.

sportscarclassics

Assuming you have a great exhaust (2.5 dual, X pipe, high flow mufflers & headers) with a rrfpr, good intake WOULD BIG VALVES HELP. No turbo or supercharger.

We all know that if you force cold air down the intake it greatly increases the hp.
Thanks
Carl

ErnestSw

Increasing the size of the intake valves alone would change the torque curve, but would not increase the HP.

IcemanG17

Big valves alone would not make much of a difference in a near stock engine...however in a modified engine (especially with a bigger cam) would make a difference! In a stock engine the exhaust mods + a chip are your best bets!
Brian

sportscarclassics

I am in the process of changing the exhaust, RRFPR, headers. If I take the heads off to raise the compression ratio I could put larger valves and polish the ports while I am there. I know little things add up, 5 here 10 there.

ErnestSw

You're a Chevvy guy. Every Chevvy configuration known to man has been tweaked AND dynoed and there is all sorts of information on what changes give what results. It's obvious that GM engineered their motors with lots of design room left in almost every area.
Porsche does things differently. ESPECIALLY where the 928 is concerned there is very little information on the results of tuning efforts and in many areas it's been found that Porsche didn't leave much on the table in terms of design.
I STRONGLY suggest you talk to Mark Thomas at Devek and Mark Anderson at 928 International as well as some of the other people who've BTDT BEFORE you make any assumptions about the results of "polishing and porting" or any of the other tricks that work on a Chevvy.

Fastest928

39 mm valves with mild porting on a 89 Gt made an approximate additional 30 rwhp.

James-man

What is the additional cost to change valves and have porting done - assuming one is in the middle of an engine rebuild anyhow???

Thanks.