LocostUSA.com

Hey everyone I have been planning a single seater formula car for a while. The engine I want to use is the k8de from mazda and turbo it. I want to copy the new f1 engine rules. I was looking at pistons that were already on the market(cheap) and found one that will give me a compression ratio ~7:1. My question is will that be to low for off boost performance. Interested to hear your feedback.

Vince

The F1 rules? If so there's a bit more to it than that...

_________________
Midlana book: Build this mid-engine Locost!, http://midlana.com/stuff/book/
Kimini book: Designing mid-engine cars using FWD drivetrains
Both available from https://www.lulu.com/

And. .. define "performance". Fuel efficiency? Output? If output, in torque or hp? etc, etc.
Some early 20th century engines were as low as 6.5-1 but they wouldn't meet a lot of today's definitions of "performance" despite being outstanding for their time.

_________________
"There are times when a broken tool is better than a sound one, or a twisted personality more useful than a whole one.
For instance, a whole beer bottle isn't half the weapon that half a beer bottle is ..." Randall Garrett

There is a lot of way to build turbo motor, low compression with high pressure or high compression with low boost or somewhat in the middle. The question is what do you want to do? What rev you want? What rev will this motor sustain? What pressure will this block sustain before the block let go? How much power do you want? How much torque do you want? What type of gas you want to run? Lots of question to answer before picking a set of piston.

On pump gas, I've built Subaru ej25 motor at 9.5:1 compression at 21psi @ 420cfn (small turbo). The motor did deliver at the wheel ~320lbs torque at 2000 rpm at full boost, peak at ~250whp, flat torque curve. The turbo did feel like a super charger. The car was built for autocross. Quick response, full power band, usable power everywhere but no top end.

On a different setup(still pump gas), we did 10:5.1 compression @ 21 psi @ ~500 cfn (big turbo). The car push about 380whp once the NA block gave up, block had to flip to a turbo block. Less bottom end, way more top end. Subaru motor don't like to rev, you have to use torque.

On a different setup 7:1 compression @ ~20psi on the similar motor as the first one. Well, the block go pulled out and change it for a higher compression........ Gut less... The engine took a long time to built boost. As I said Suby motor don't like to rev. You're cap in the 7k range (specially the 2.5). If you could have rev this engine to 12-14K rpm, we could have built 40psi, get a 2x4 behind the head on the power finally kick in, we could have produce twice the HP but a low torque number.

I've driven and race car many combo of compression, mostly all in the same HP range. My favorite setup was #2 of the example above. The 10.5:1 compression gave the base motor (with head work and cam) gave about 200hp vs 165 stock. The extra gas produce by the base motor, the speed the base motor can start spinning on it's own, make the turbo spin really quickly, therefor reducing the lag.

Finally, high compression does not equal low boost. You need to calculate the ECR (effective compression ratio) base on the gas you want to use. On 94 octane, you can do 10.5:1 @ ~23PSI before you start detonate the motor.

Ok let me clarify a bit. By f1 rules I mean small displacement v6 and turbo, that's it. Alf pretty much told me what I needed to hear. My question was more geared toward having a little weight car, how much will the compression ratio actually effect the performance (acceleration, lap times, etc). That's where I was headed. Thanks guys for the help. Anyone else care to chime in?

Vince

the critical point here as alf says in the last paragraph, effective compression ratio, not the static compression ratio.

the point in the cycle when the inlet valve closes is when compression starts, on boost, the speed of cylinder fill is faster than off boost, what you have trapped in the cylinder at tdc is what goes bang, how it got there and at what revs is a combination of boost, when the inlet closed and was the exhaust valve still open when the inlet opened, because you can use boost to evacuate the cylinder of spent gas.

see, there's a bit of science to it, the least of which is spark timing, you may have a problem stopping detonation with 10 to 1 without boost if your crevice volume is too large and unburnt fuel is still present after combustion is over, turbulance is the secret, either by boost or by quench area of the piston/chamber design or both, a fast burn flame front with small crevice volume is the way and the limiting factor, the more that is in the chamber at spark time the longer it takes to burn so the more turbulance you need.
Visard, incidentally another Englishman, has writen the definative on crevice volume and worth a read.

bring the fuel to the flame front, don't just let the flame wander across the chamber because it is inherantly lazy and won't burn what is not put in it's way.

you can only hang soo much fuel in a vapor state in soo much air, a bit like desolving sugar in tea, if the boost goes up, the air is less capable of carrying the fuel, the later in the intake tract that you inject the fuel the more arrives in the chamber, all be it not in a vapor state but in droplets now you have to remix it in the chamber and feed it to the flame front.
there's a whole lot more to this than meets the eye.

any combustion that commences after tdc is wasting stroke time.

be aware that "ported pistons" will increase the crevice volume.

even more speed secrets going free here, soon i won't have any left.

_________________
this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

Interesting how small changes in compression can change the power curve

Oh great..... at some point lost in the mists of time I was told that electricity was lazy, now you're telling me that fire is too?
Oh the horror!
I need another beer

_________________
"There are times when a broken tool is better than a sound one, or a twisted personality more useful than a whole one.
For instance, a whole beer bottle isn't half the weapon that half a beer bottle is ..." Randall Garrett

Hey Jack! Think of it this way... If electricity is lazy and fire is lazy, then me and you are at least in good company!!!

JD "Born lazy and gotten worse since" Kemp

_________________
JD, father of Quinn, Son of a... Build Log
Quinn the Slotus:Ford 302 Powered, Mallock-Inspired, Tube Frame, Hillclimb Special
"Gonzo and friends: Last night must have been quite a night. Camelot moments, mechanical marvels, Rustoleum launches, flying squirrels, fru-fru tea cuppers, V8 envy, Ensure catch cans -- and it wasn't even a full moon." -- SeattleTom