J Chen

Hi,
Has anyone ever tried a 944S heads
on the 951 engine ?

Danno

It would work, but you'd need custom intake-manifold. Also the 944S doesn't have the double-sided support for the chain-tensioner like the later cars. You'll want to weld one on. There's an article in Excellence where David Chen did this mod.

Epic2112

What were the performance results like?

Mike1982

I have thought of doing this for sometime now. Putting a 16v head on my turbo. What all do you have to do to make it work then? Just the head and you mentioned the chain-tensioner then? A custom head I won't think would be that hard to get now with all the different performance places out there.

Songzzz

I think he meant the 8V NA head. Larger valves, better flow, able to port exhaust side as there are no ceramic liners.... IIRC.

From what I gather, the NA head should be good for high end HP, might not be as good for low-end though. Point to note: 968 TurboS/RS head is much closer to the 951 in terms of valve size.

LukeSportsman

Just as Songzzz says, anytime you go to a bigger ported head to get flow your going to move the rpm band up. There is a reason outside of cost that factories put restrictive heads on an engine for OEM use. That being said, a larger cc engine is going to negate this effect to some degree and could balance the equation for a similar curve simply moved UP.

For the same reason you must flow test a head during porting and that bigger is not always better. You must maintain port velocity. I think its GURU's website that talks of this in relation to ceramic exh ports. Port velocity is critical for turbo spool up and there is always a trade off for the street.

seb928s

Don't know if it's worth the 16V head on the turbo since if you remember the 968 Turbo S had an 8V head. Had to do something with how the air flows in and out.

J Chen

Hi Guys,
Songzzz, it's 16v I'm talking about.
There is real potential in going this
route especially when you're looking
for big power like 400 rwhp.
As for the 968RS being 8v, I think it
was more about development cost more
than anything else. Porsche could have
used the 944S/S2 heads but that would
be like slapping themselves on the face
in lue of the existance of the variocam
heads. Do remember that the 924 GTR
had a 16V head. If it were'nt due to cost
cutting reasons, the our cars would have
been powered by V6 engines instead.

Russ Murphy

I'd like to pursue the 16v route as well. That's why I resurrected Konstantin's old thread. There's certainly a reason why current turbo race cars don't use 2v heads.

Danno

Yeah, as I recall, the 968 TurboRS got 8V heads because the rule restricted the cars to 350bhp or so. Porsche figured they can get better low-end torque with a 2V head and stay below that power-limit. There's no reason a car developed over 10-years AFTER the 924-GTP should be getting less power from a larger engine, eh? If anything, the 968 TurboRS should be laying down 500bhp just from shear displacement alone, all else being equal.

Ok, here's back-to-back comparison of 8V vs. 16V head on the same car with the same configuration, turbo, boost, etc. Just swapping heads:



Since boost is the same, roughly the same air-flow is moving into the engine, so peak-torque remains the same at 450 lb•ft or so. But it's been moved up the powerband by about 800rpm. That means you can be in one lower gear in the 16V engine at the same speed. With the same torque from the engine, end up with 20% more torque at the wheels. Thus faster acceleration at any given speed compared to the 8V car. The wider powerband builds up more gradually, so easier to control without spinning the tires instantly.

Power is up dramatically from 390HP to 450HP. But it's still not made at redline, so there's some tuning and adjustments that can be done here.

seb928s

Ok I thought it was something to do with the air flow. Learned something new got it 16V Best 8V good

J Chen

Danno,will the standard cam timing
work with turbocharging ? I wonder
if theres a way to fit two adjustable
cam sprockets so that we can dial in
the cam timing.

TonyG

>>>Since boost is the same, roughly the same air-flow is moving into the engine,<<<

Huh? One is a 2.8L and one is a 3.0 liter, thus at the same pressure ratio, same VE (which they are not), they cannot by definition be moving the same quantity of air.

Huge difference in cfm potential just by the displacement alone.

The comparision is invalid based on the displacement differences...

>>>so peak-torque remains the same at 450 lb•ft or so. But it's been moved up the powerband by about 800rpm.<<<

Actually, both of these engines are not breathing well since both roll over well before a measly 6000 rpms. Hardly an example of a well designed application.

>>>Power is up dramatically from 390HP to 450HP. But it's still not made at redline, so there's some tuning and adjustments that can be done here.<<<


That's because of the displacement difference, and the fact that the 2.8L 8V engine was running a non-exhaust ported 2.5L 951 head which is by default handicapped due to the huge exhaust port restrictions built in....

Put my Milledge top end on that 2.8L engine and the dyno would look much different. Then punch it out to 3.0L and then see what you have to compare..

My little 2.5L 8V makes more HP at a lower boost than that 2.8L 8V, and I'm still running through a stock intercooler (which both of those cars were not). Given this, and with a larger intercooler, at the same 1.3 bar boost, you could easily see a 2.5L 8V that makes the same HP as the 3.0L 16V application.

You have to have an apples to apples comparision....

The point is well taken, but the example is not accurate.

The differences are much smaller than what are represented on that dyno chart..

TonyG

951and944S

What could be more "apples to apples" than two 3.0 liters.....?

The 944S/S2/968 head turns on at 4000/4100 rpms noticeably like a mini turbo boost. An early onset of boost with a properly matched turbo would be brutal when the head begins to really breathe 2/3 through the rev band, just exactly what I'm planning.

From looking at 968 exhaust manifolds (it's said you need "custom" exhaust) and mocking up the parts, it looks like you could cut off the 3 bolt flanges and add the type from the 951 or scratch build some, no problem. You could actually turn the 968 ones to line up with the crossover but the 3 bolt pattern on each header is a shade off, too much to simply enlarge the bolt holes.

From what I've read the 16v head is 20% more efficient than the 8v heads.
There's definitely no need for porting except for cleaning up the casting since you could easily lose something the size of a cell phone in there... You'd be building a scratch intake manifold (easily done) so one of the restrictive problems with the 951 is already addressed.

Now, to variocam or to not variocam, that is the question.....

TS

TonyG

951and944S & Danno

>>>What could be more "apples to apples" than two 3.0 liters.....?<<<

Is that Graham Gillies dyno? If so, his engine, at least before it blew up was 2.8L I thought.... This is why I made that statement....

Looks like I was incorrect about the assumption.


How ever, it's still not a real good comparison. There can be too many things that are different on each car (like the turbo for one...) to say that this chart represents the difference between an 8V engine and a 16V engine, apples-to-apples.

My 2.5L 8V makes more HP at less boost than that 3.0L 8V. How would that factor in to that comparision? What happens when I get the 2.5L 8V up to 450RWHP at a similar boost level?


TonyG



But th