ptuomov

It is completely obvious that you are better off starting with a 4-valve pent roof head for any forced induction project.

That's not the right way about think about it. Boost doesn't make your car go faster, average hp makes your car go faster. You can produce the same average hp with a turbo at lower boost than with any mechanically driven supercharger. This is because turbo takes only about 5% of the crank hp to drive in pumping losses, whereas a mechanically driven supercharger will take 25% or so. In addition, you can easily tailor the boost curve per rpm to stay the desired distance from the knock limit. Therefore, with a correctly sized turbo, the engine will last longer producing the same power. Not dissing superchargers here, they are very good bang for the buck -- turbo is just more bang in absolute sense.

The 4-valve pent roof head can safely take almost a point more compression than 2-valve wedge (or whatnot) head. It makes no sense whatsoever to use the 2-valve heads to lower compression to reduce knock.

If you want a lower compression (which is only necessary at a really high boost levels if using S4 heads and pistons), I think you can machine a dish to the S4 pistons. The thick top '87 S4 pistons are especially good for this purpose. Here are some photos of work in progress related to this. These are pistons dished with the primary concern of keeping the pistons strong. The dish design is by Jim Morton. The actual dish was machined by Eric Henke of REBCO. REBCO is a firm that lightens drag racing pistons as one of their main businesses.

9x8

Thanks!
This is exactly what I wanted to hear.

Scrapping the whole hybrid idea then!
Will start with stock S4 engine.

Regards,

Fabio421

If you already have the 16V heads I see no reason not to use them. The 951 came out of the box w/ 11psi and alot of those guys are running 15psi on stock head gaskets and internals. The 951 16V head and the Euro S 16V head are virtually the same except for sodium filled exhaust valves and exhaust port liners which can be added to the 928 heads. You aren't planning on boosting that high so I don't think it would be an issue. Do the 32V heads flow more? Sure they do, but that doesn't mean that the 16V heads are trash. Theres plenty of ways to skin a cat. The 951 guys have been skinning cats for many years with what basicly amounts to half of a 928 16V engine. I'd like to think they have alot of experience we can use and learn from.

BTW, a mid mount turbo is not a bad way to go. Mark Robinson never really workerd out all of the details regarding the cold side plumbing. It fit, but it wasn't pretty. I saw a home brewed mid mount S4 turbo while in Bogota Colombia that was very nicely done. I believe I posted a thread with pictures a few years back. Maybe search for that. I'll see if I can dig it up when I get time later.

Here are some videos of that car. Sorry about using links but embedding has been disabled.

http://www.youtube.com/user/monoito#p/a/u/2/XdaqmSKqHhg

http://www.youtube.com/user/monoito#p/a/u/0/NqHTuBB6ndo

ptuomov

And it's true that one can possibly piggyback off some 951 R&D with 2-valve heads. This is not to say that one can't make turbo power with 2-valve heads. As you say, there are a bunch of 951 2-valve half motors making close to 400 hp.
However, since he has _both_ the S4 heads and 2-valve heads in his hands and is plannign to use the S4 pistons and short block, I think it's better to use the S4 heads. The combustion chamber burns the charge so much faster on the 4-valve head.

Right after cat might work really well, as the cat burns the residual fuel and increases pressure and heat.

mark kibort

actually, it was legal in NASA racing, so there was no issues with being "legal".

I did this with the 4.7 stock motor and the 5 liter part euro motor for a short whle.

hacker-pschorr

Try 500rwhp:
https://rennlist.com/forums/944-turb...493-498-a.html

He is far from the only one too.

I agree 100% with your reasoning for using a 32V head. There is still a lot of untapped potential with the 16V.
IMO the 928 boosted world is playing a big game of "catch-up" with the 951 guys.
There is no reason why boosted 928's shouldn't be making 500-600rwhp with 16V's much less a 32V engine.

We are behind the eightball with our engines thanks to the cam profile and intake design which are superior in the 951 (valve size is the same as EuroS heads). Not to mention a few other bits from the factory with boost in mind like sodium filled valves, ceramic coated exhaust valves, lower compression and fordged pistons do not hurt anything either.

It's nice to see from all different avenues the 928 progressing like we've seen this past decade. It wasn't all that long ago that every knew for at fact that 400rwhp was the max a stock based 928 engine could produce. Then it was 450, 500, 550...... IMO there is no reason why 1,000rwhp should be possible with a boosted 928 and mostly stock bits.

Iwanna928

I thought he said the 4v heads were bad and needed work. If watching his outlay of cash and keeping it buget minded I would use the 16v heads like Fabio stated.

If money isn't a factor then rebuild the 4v heads and boost away!

Lizard928

He has said the S4 heads will most likely need a few valves to be replaced.
But even so the S4 heads will be better. ESP with a nice set of my cams in there

a mid-rear mount turbo should be fun, but I prefer two close to the engine.

And I agree with Erik that the stock manifold is a huge bottleneck.

Fabio421

When I found that my 32v heads had all of the valves either bent or broken, I started pricing valves. It became quickly apparent that sourcing another set of good used heads was a hell of alot cheaper than buying all new valves. I bought some S4 heads for a good price ( thanks Tuomo ) but then I realized that I have two sets of Euro S heads lying around. Hmmmm? Decisions decisions. :-) It's nice to have choices.

9X8, before this goes unmentioned, I see no need to lower compression at low to medium boost pressures. Tim Murphy and John Kuhn are running fairly high boost #'s with stock compression ratios. I believe they are both running about 11 - 12psi ( Tim's wifes car ). Getting a really good tune is much more important than lowering the comp. ratio since you said you were only planning on running 1/2 bar.

hacker-pschorr

This cannot be repeated enough.

Far too many people have no appreciation for how much time and $$$ is spent tuning. Getting everything bolted up is the "easy" part. Getting a good tune that not only keeps the engine from grenading but also keeps the characteristics of a stock car for daily driving is where the fun begins.

We cannot thank John Speake enough for the Shark Tuner.

dprantl

My opinion is that if you are thinking about forced-induction, you should first get the customizable engine management system before you even begin adding the fast parts. That would mean either convert to standalone EFI or get a SharkTuner. Once you have full control over engine parameters, then you can add the turbo/supercharger and already be up to speed on changing the parameters as needed.

Dan
'91 928GT S/C 475hp/460lb.ft

Jim Morton

Under the fair practices and disclaimers act:

Tuomo's piston dish was derived from info posted here on the 928 forum. There have been several posts showing the piston cross-section(s). I do not recall who authored these thread / posts, but the info was very useful to ensure we kept the piston deck thick enough, yet got the total chamber volume to a point where cylinder pressures would be manageable.

ptuomov

One of the sources was Tony's cut '87 piston photos:

https://rennlist.com/forums/928-foru...machining.html

Lizard928

I will concure on having a proper tune/management!

Running a motor even a stroker without it is just plain stupid IMHO.

And with a good set of cams one should be able to get away with 14PSI on a stock S4 engine. Provided that they had the ability to properly tune it.

I went with S3 heads on an S4 bottom to give me roughly 8:1 CR (plus I prefer the S3 combustion chamber and ports). But with that I will have the ability to run a ton of boost should I so desire. I will probably tune it on a dyno and crank the boost up just to see what I can actually get out of the motor, then limit it to 650 RWHP. I can always increase it later.

Iwanna928

Colin You are one twisted individual!