Chris White

968 exhaust valve is 1 mm smaller, I have ordered up enough custom ones to know this. You can fit the S2 valve in the 968 head - but it not the correct size.
The ports are different - but not by much - both are big enough!
I have used the vario cam and it is a good idea for NA, but not anywhere near optimum for a turbo. Too much lobe overlap and too big a timing change to be really useful - better off going to a adjustable gearing system. Ever notice the 16v heads have crappy low end performance...that can be fixed.

Chris White

sorry, my bad. I was thinking about some modded heads I did, both the S2 and 968 heads have 33mm exhaust valves stock. 37mm intake for S2 and 39mm for 968.

Bass GT3

Chris,

You're right, i never did detail the requirements. There is a decision to make in the future about the engine in my race chassis. See my thread & pics in the 924/031 section.
now i have to stick with a porsche engine, of the type fitted, so naturally, the 3 ltr engines seem to be the best route. As the car is a Hillclimber & Sprinter, she does a mix of circuit work and tight sinuous hills. So i want an engine with good top end, circa 400 crank is sufficient, but with ample low end torque/grunt. I really can't afford to sit around waiting for the turbo to wake up and deliver. S from my estimations, the 400 mark seems a good mid point, in terms of the overall package. Don't forget, my ar will weight circa 950kgs, so very light.
So i was asking which would be the better base to start with. Given that i would be dry sumping it, fitting custom rods/pistons etc, there will be a fair amount of work done, but nothing compared to some here. obviously, 968 engines comand a premium, so given my objective, would the S2 be the better starting point?
ECU is a Pectel SQ6, which is a phenomenal ECU.
And as this is a race car, i would look to mount the turbo in front of the motor, ala the WRC cars, and as i have done on my 924 engine. This engine has produced almost 400bhp at the crank on fancy Elf race fuel, but naturally, it drives like a hand grenade!! all or nothing. So getting a meaningful off boost performance allied to a healthy top end is the goal.
And considering these goals, would a turbo even be the best solution. Maybe an S/C such as Rotrex etc.
Or alternatively, is their a better "hybrid" route, by combining different heads/blocks from particular models etc ?
And as per your comment above, could you elaborate further?


Cheers,

Steve

333pg333

Steve, with your requirements given, I think you could save yourself a LOT of money and achieve what you want with an 8 valve. You could get the 400bhp comfortably and also have some decent low down torque. Although I believe that the idea that 16v motors immediately mean you have a high revving, slower off the mark motor are not necessarily true, you can get what you want with the 8v and you would save a heap of cash to put into other components. By all means go for a 16v motor as we all want one of these, but just my 0.02c worth.
Patrick

Bass GT3

Hi Patrick,

i'm not hung up on a 16V per se, but the thought of 3 litres as opposed to 2.5 does appeal. Or will an 8v head fit on a 3 litre block? Forgive my ignorance here!!
I know my planned HP is not spectacular, but really want to look at the best compromise possible WRT TRQ and BHP.
I know there's been a lot of interchange on these engines, but i'm a bit confused as to what goes with what.
and if i do persue this, it is a longer term project, but i may start aquiring the parts gradually over time.
My idea option would be the Audi 5 cylinder from an S2, wound up nicely, as these bolt straight in with only the clutch to resolve. But that would preclude me from the Porsche Championship. So 944 derivative it has to be
Steve

vette951s

Here is my experience with factory rods: Every 951 engine I've seen apart had the forged rods up to cars built around December 1988. Of the two later production 1989 951S engines I've disassembled, both had the cast rods similar to the S2. All S2 motors I've seen apart had the cast rods. The turbo forged and cast rods weigh exactly the same! Early production 968 engines built prior to Oct. 1, 1993 have a lightweight narrow rod beam forged rod. The factory then introduced a transition beam forged rod and recommended that any early 968 engine have their rods replaced with the later version! I'd have to do some digging around to get some pictures of all the rods.

John

gt37vgt

when i said standard 968 motor bolt on gt30 or 35 I also said e85 and meant to say conservative boost ..
In wich case it would be fine to make 400/450 crank hp ... with no fear of rod failure and the high compression would make it responsive off boost .
I think Chris seems to want to future proof every engine and build it for 500+ hp

Chris White

Yeah, I probably need some sort of rehab...
BTW – from the description of use I would go with the 8v head unless budget is not an issue! The 2.7 8v head will bolt on to a 3.0 block or a 2.5 8v head can be modified to fit. A well flowed 8v head will run with a 16 v head until you get the revs way up there.

edh

Steve

Maybe you cold have a chat with some of the guys running 3.0 8V motors in the UK? Paul Smith (diver944 on this forum and PCGB) has been running his for quite a while now. It sounds like you would get all the power you'll ever need from this setup.

Bass GT3

Hiya,

Good thinking!! I'll drop them a line. Cheers.

edh

no problem - can I have a drive when it's finished ?

....I read this morning that Mark Dalton's car (3.2l 8V) is making 400+ hp & 500lb ft. That should be enough

333pg333

What he said....what I said.

Paul's 3.2L would be a good starting point, but perhaps the method on which that motor was built won't be on the budget side either. If you've already got a good 3L block, then pick up a 2.7L head as Chris suggests and you'll have all the power in the world that you need for this application and it won't cost the earth. Then with all that extra cash you save, you can put it into suspension and brakes or play with the head a little more and get a good cam. That will make a nice bit of difference.

Bass GT3

Patrick,

If/when i come to build this motor, it will be the final expense. Chassis is fitted with all the bells & whistles, such as JRZ suspension, inc coilovers in the rear.
I didn't realise that the 8v head could be grafted onto the 3ltr block. Makes it interesting. But why do it? Not trying to be obtuse. If i sourced an S2 engine for example, what advantage does the 8v head offer over the 16v head fitted?
My initial thoughts were that the 3ltr would always offer more inherent torque than the 2.5, purely due to displacement. So working on this assumption, i wondered which was the better option, S2 or 968. Both offer 16v heads(which is not critical in the overall plan), with slight differences in the mechanicals, such as harmonic balancers or oil squirters etc
And now there's the curve ball of the 2.7 head. Was this the short lived 944S 16valve model?? Whats the advantatages here, as opposed to the S2 or 968 head?
I'm really struggling to understand all the various permeatations, with regard to the head/block combinations and what their respective benefits/downsides are
This is all too confusing!!!!!

edh

The 2.7l motor had an 8v head & was fitted to later Lux models. The 944S was a 2.5

I'm assuming that most of the advantages in the 8V head are about cost & not having to fabricate custom intakes? That may not be too much of an issue for you I guess?

Bass GT3

Re the intake, not an issue. What would be idea is a chart type affair, that lists each configuration, it's swept capacity and it's strengths & weaknesses.
ie, X block + Y head= 3 ltr/ high rev motor/ poor low end etc
There seems to be too many variables here!! I always thought the 2.7 was the short lived 16valve model??
Waaaay to hard!