What would the ideal CR be for a built motor to withstand a high amount of boost (say 15-22#'s), for maximum power with race gas?

 

Sorry don't count my vote for 11:1. I am still hung up on the discussion of alchohol motors and I didn't see race gas. Make mine another vote for 8.5:1

 

I voted 8.5 but I think it depends on the exact setup as there are a few variables. I wouldn't go over 9:1 the max but allot safer around 8.5. JFYI I am building a 370 right now with 8.7 and plan to boost it b/w 15-18 or so with meth injection. Good luck, will be a blast I know I can't wait for mine!

 

What are you doing Ben? Already contemplating a head change? 8.8:1 here my friend.

 

Lol, no Silas, I started this thread as evidence for some guys in another thread about CR and boost as they seem to think running with a high CR and boost is a good idea and don't believe me when I say its not.

I'll be leaving my CR just were it is (8.8:1).

 

Very subjective poll, it depends on the car and what type of race fuel.
I am 9.0 because I want to be able to do 15+psi on gas and e85.
If it was a track only car I would have gone to 11.0 for more power on e85 only.
More static compression will always make much more power at the same boost if detonation is controlled.
I guarantee all the big boys are running more compression than you would think.

 

The point I am making is more that to achieve a detonation free engine and for max boost, even on race gas it will not allow for such a high CR. You can have a high CR but even with race gas not be able to safely achieve the max boost levels as with a lower CR.

 

I agree with you completely that a street car on pump 93 lower CR is safer,
but the rules are a little different on high octane race fuel or alcohol based fuels.
If you are able to increase the compression, the power will come up quite a bit.
Remember, boost is just the measure of restriction of airflow through the engine.
As an example, you can have a 10:1 engine at 15psi outpower a 8.5:1 at 20psi,
there are just too many variables to say one option is always better than the other.
It just depends on the goals and intended usage for the car.

 

It would be hard to have a definative answer for sure. Keep in mind that a lower CR car can make more boost with the race fuel or alcohol based fuel as well if reaching the peak performance without getting into detonation. It would only be fair to give that when compared to a higher CR with race gas and a slightly lower amount of boost.

Personally if there were a difinative answer, my guess is the power levels would be similar. I guess the whole point of this is to show that just because you are running race gas with high CR with it tuned for max power without detonation, doesn't mean it will out power a lower CR motor on race gas with higher boost.

 

Opinions are like A$$holes. When you see how much faster a higher CR motor spools and revs, even if you have to run 1-2 psi less boost (not really likely though), you'll still make alot more HP.

I think this thread is redundant.

 

I am running 9.5:1 and 13psi of boost I went 8.84@154 1/4 5.69@124 1/8 on a 1.32 60' on M/T radials with a 3700lbs race weight with a pump gas tune

 

Wouldn't intercooling and methonal injecting at the same time make it possable to raise the CR? Just asking, I am thinking of doing this to my 408, which looks like it will be 10.5:1 with the heads and piston choice I have made. I also do not plan to go over 15psi. I do think it will take close to that though.

From what I understand, detination does not acure from the boost pressure, but instead from the heated intake air charge. That is why you run the intercooler. If you have a method to lower the intake air charge back down to ambient temp, then you should be able to run the same CR as a NA engine on the same fuel. Farther more, if you implement a system that will farther lower the intake air temp below ambient, You should be able to run even higher CR. Isn't this the idea behind Air to Water intercoolers that use ice buckets? So wouldn't the correct question be, what CR at X intake air temp be safe, due to detination?

 

The problem with your logic is the more air you've pushed into the cylinder the hotter it gets as you compress it. But you've improved the situation by lowering the initial air intake temp, which will allow more detonation free compression given everything else remains the same.

 

You're pretty much dancing around the whole concept. Yes, detonation occurs from higher intake temps, however the higher intake temps can be caused by greater boost and higher CR, so saying adding meth injection and/or an intercooler would allow you to run a higer CR with 15#'s of boost may not be true. It will depend on how far you can get safely on 15#'s of boost with the given octane fuel you are using with intercoolers and methanol to show you if you could raise the CR.

I am using both meth injection and a large FMIC with 15#'s of boost in my car, and its maxed out at 8.8:1 CR. For me, to raise the CR safely, I would have to lower my boost levels (or run a much higher octane fuel like C16), which would result in less power from the lower boost. Although the higher CR would give more power. How much power? Too many specifics to tell if the gain in power from raising the CR would be the same as a lower CR with more boost. I am guessing they are not far off, but in my case, it sure would be a lot more inexpensive to run pump gas and high boost than C16 and low boost.

Which when it all comes down to it is probably why you see so many votes from people in here for a lower CR for high boost.

 

just adding some .02$ but this is proven at for every 1pt of compression its a 4% increase in power. it was either car craft of super chevy... so if at sea level air pressure of 14.7:1 is 4% so if you add another 15psi of boost you should in theroy get 8% per CR point................. so im gonna leave it at that 1000hp * 8% = 80hp per CR pt.

P.S. whats with all the PUMP GAS talk. this guy is askin for MAX affort RACE GAS.

 

Huh?

 

I have been reading your posts on this subject in both this thread and the other thread you mentioned. Yes, you are right, lowering the compression ratio as you incress the boost pressure is a "safer" angle. But why do you do that? By lowering the compresion ratio, you are lowering the peeck pressure in the cylinder before the engine fires that cylinder. The higher the compression ratio, the higher the peeck presure is before the cylinder fires. The higher the pressure the higher the temp in the cylinder will be. Heat is what causes the fuel to detonate. There are alot of factors, or ways to manipulate the peak firing pressure and temp in the cylinder. CR is only one way. You can control this also by cam shaft specification, ignition timing, a/f ratio, material of the head, temperature of the engine jacket water, temperature of the engine oil, rod to stroke ratio, combustion chamber shape, stroke, and bore also play into it. The exhaust system can play into it as well. Preignition can lead to detonation. Preignition can be caused from hot spots in the combustion chamber. You can also control it from the fuel octane rating, as well as the intake air temp charge. To say that compression ratio is the only factor involved is not true. You can build a very safe engine with low compression and alot of boost. One that will run on pump fuel. But if you control the other engineering aspects that lead to detination, you can also build a high compression engine with lots of boost that can and is just as safe on pump fuel.

You are right, going with lower compression and adding more boost is a safe way, but it is also the easier way to go. To go high compression with lots of boost is more dificult, but can also be done, even on pump gas. There are people all over doing both ways and they all make their points. If there was only one majic way to do it, we would all be building the same thing. Who is to say who is right? Thats were the fun at the track comes in.

 

Why are we talking about pump gas? You are right, he did say race gas not pump gas, my bad.

It was edited, I think it started out as pump gas. Maybe I am wrong.

 

I want my vote back, Ive been robbed.

 

I agree, I said 8.5, race gas is a WAY different story IMO though!

edit - The more I think about it I stand with the lower cr and higher boost, this is what I'm doing anyhow!