CR for SC

[Chilliman]

You guys with experience,

Is 10.5:1 too high for SC'ing? I'm expecting to have to change the cam (or possibly not) Is there an ideal/optimum CR when SC'ing? Appreciate there will be alot of variables, but is there an optimum combination of charge/CR etc?

I'll speak to the SC suppliers when I can but thought some of you guys might have the answers...

Thanks in advance,

Chilli

[carsy]

Daxtojeiro with his Cobra is running about that, maybe worth having a chat with him and see how he is going.

http://www.pistonheads.com/gassing/topic.asp?h=0&a...

[Chilliman]

carsy said:

Daxtojeiro with his Cobra is running about that, maybe worth having a chat with him and see how he is going.

http://www.pistonheads.com/gassing/topic.asp?h=0&a...

Good shout Carsy, Phil is local to me and I'm running MS2

[Pupp]

Are you aiming for that CR or is that what you believe it to be? If the latter, why do you think its anything like that high? Have you buretted the heads? I'd be astonished (and in awe) if you have got a 2 valve motor running reliably on pump petrol with that CR....

[900T-R]

Depends on what your aims are. If you want high numbers, lowering the static CR is where its at but for a road car, I'd rather leave the CR where it is and run a low boost configuration to preserve throttle response. FWIW the Lotus Evora S(which I regard to be a very good S/C road sports car implementation, runs the same 10.5:1 CR as the N/A version. In the Evora, the point where peak power is made also is raised from 6,400 to 7,000 rpm.

You only have to pull 1 tonne of car along, so loads of lazy torque is not necessarily an advantage.

[Chilliman]

Pupp said:

Are you aiming for that CR or is that what you believe it to be? If the latter, why do you think its anything like that high? Have you buretted the heads? I'd be astonished (and in awe) if you have got a 2 valve motor running reliably on pump petrol with that CR....

Engine was rebuilt last year by V8Dev (see my profile for more details). Build spec was 10.5:1

[Chilliman]

900T-R said:

Depends on what your aims are. If you want high numbers, lowering the static CR is where its at but for a road car, I'd rather leave the CR where it is and run a low boost configuration to preserve throttle response. FWIW the Lotus Evora S(which I regard to be a very good S/C road sports car implementation, runs the same 10.5:1 CR as the N/A version. In the Evora, the point where peak power is made also is raised from 6,400 to 7,000 rpm.

You only have to pull 1 tonne of car along, so loads of lazy torque is not necessarily an advantage.

I'm not after loads of lazy torque Eric. I just need a teensy weensy bit more power

[SILICONEKID340HP]

Chilliman said:

900T-R said:

Depends on what your aims are. If you want high numbers, lowering the static CR is where its at but for a road car, I'd rather leave the CR where it is and run a low boost configuration to preserve throttle response. FWIW the Lotus Evora S(which I regard to be a very good S/C road sports car implementation, runs the same 10.5:1 CR as the N/A version. In the Evora, the point where peak power is made also is raised from 6,400 to 7,000 rpm.

You only have to pull 1 tonne of car along, so loads of lazy torque is not necessarily an advantage.

I'm not after loads of lazy torque Eric. I just need a teensy weensy bit more power

What do you gain by having higher CR ? You must have forged pistons so thats a plus if your going down the forced induction..

Edited by SILICONEKID340HP on Monday 18th June 21:31

[MPoxon]

Leave it as it is Chilli, you have a cracking car. Put fuel in it and enjoy it.

[Pupp]

Chilliman said:

Engine was rebuilt last year by V8Dev (see my profile for more details). Build spec was 10.5:1

So *you* don't actually know then

Ok, well V8D certainly build a good motor (I've got one too) so the cam must be a monster (which would enable a high static CR of that order), Problem is, when you add boost, you're then going to be trying to open the exhaust valve against a high (very high and possibly still rising) cylinder pressure which could cause valve train problems (bent pushrods for example). Assuming you get the valve open, if the exhaust restricts gas escape such that pressure exceeds that of the incoming charge, then you're into reversion city

By changing to a less wild cam as you mention (which is often done for exactly these reasons), the engine will be effectively 'seeing' much more of the static CR so the inherent risk of detonation and thermal problems will be consequently higher.

I'm not sure the comparison with the Evora is that helpful really, I have no idea what the engine architecture for that is but I'll wager it's not a big pushrod 2 valver. Its combustion chamber design will be way way more efficient than even the very best of ours, as will its ability to cope with high combustion pressures.

There's no definitive rights or wrongs and, certainly with turbos, thinking seems to be heading toward using longer duration cams on the basis that modern turbos are way less 'restrictive' to exhaust flow that they used to be so maybe with a very free flowing exhaust you'll be ok with moderate boost. Then again....

[hiltonig]

Why don't u ask rob ? Seems obvious to me ! Talk to the organ grinder

[900T-R]

Pupp said:

I'm not sure the comparison with the Evora is that helpful really, I have no idea what the engine architecture for that is but I'll wager it's not a big pushrod 2 valver. Its combustion chamber design will be way way more efficient than even the very best of ours, as will its ability to cope with high combustion pressures.

It's not about the exact value the Evora S runs in se - more about the static c.r. (and probably the cams) being identical for N/A and S/C versions. Your observations would hold true for both (Rover and modern Toyota V6) engines, just at different cylinder pressure levels.

The choice in both cases are to run lower static cr and shorter duration cams to allow high boost and big numbers, to leave them alone and restrict yourself to whatever boost you can still run in the given configuration (taking ignition mapping, intercooling etc into account), or something inbetween. For a road sports engine, personally I'd always go for the second option, if it means an extra 40-50 bhp rather than 150-200 so be it.

Then again, if I already had a performance built N/A engine on the far side of 300 bhp in there, I'd leave well enough alone for a road car.

Edited by 900T-R on Tuesday 19th June 05:22

[Chilliman]

hiltonig said:

Why don't u ask rob ? Seems obvious to me ! Talk to the organ grinder

Obvious as it might be, if the organ grinder doesn't answer his phone then it's a bit difficult to ask the question

[Chilliman]

Pupp said:

So *you* don't actually know then

You're quite right, I don't know, I have only taken Rob's word on it.

[Bluebottle]

If you are looking to just fit the SC Power kit, then you should know that Dave and Colin designed their setup to work with the stock 10.5:1 cr. However what i would say to you is make sure your heads have been ringed!.
If you're looking for a bit more then the general consensus from the gurus seems to be 9.2:1 to 9.35:1 cr, with ringed heads and cross bolted block, forged omega pistons and H beam rods.

[v8 racing]

Chilliman said:

hiltonig said:

Why don't u ask rob ? Seems obvious to me ! Talk to the organ grinder

Obvious as it might be, if the organ grinder doesn't answer his phone then it's a bit difficult to ask the question

i did try ringing you back chilli

[Boosted LS1]

"Problem is, when you add boost, you're then going to be trying to open the exhaust valve against a high (very high and possibly still rising) cylinder pressure which could cause valve train problems (bent pushrods for example). Assuming you get the valve open, if the exhaust restricts gas escape such that pressure exceeds that of the incoming charge, then you're into reversion city"

The exhaust opens when cylinder pressure is quite low though and it's got manifold as well as mechanical pressure behind it. Once it's off the seat there will be a huge pressure drop.

ETA that if you meant the inlet valve then it'll have boost pressure behind it until it lifts off the seat.

Edited by Boosted LS1 on Tuesday 19th June 11:38

[Chilliman]

v8 racing said:

Chilliman said:

hiltonig said:

Why don't u ask rob ? Seems obvious to me ! Talk to the organ grinder

Obvious as it might be, if the organ grinder doesn't answer his phone then it's a bit difficult to ask the question

i did try ringing you back chilli

Hi Rob

I'm offshore at the moment so no mobile signals here.... I'll try again this afternoon.....

[SILICONEKID340HP]

Chilliman said:

v8 racing said:

Chilliman said:

hiltonig said:

Why don't u ask rob ? Seems obvious to me ! Talk to the organ grinder

Obvious as it might be, if the organ grinder doesn't answer his phone then it's a bit difficult to ask the question

i did try ringing you back chilli

Hi Rob

I'm offshore at the moment so no mobile signals here.... I'll try again this afternoon.....

Will you still be useing the Carbon Plenum ? Is it forsale !!!

[Chilliman]

SILICONEKID340HP said:

Will you still be useing the Carbon Plenum ? Is it forsale !!!

Possibly Daz, possibly