I am about to order some pistons for my stage project, what compression is the best to choose, I don;t really think I;ll be going over 30 psi boost though on c-16 maybe 23 psi on alky and 93. looking at a 230 cam about or so in size.
Ordering pistons what compression?
- Thread starter norbs
- Start date
maybe 32 psi MAX.
- Joined
- May 28, 2001
You can go 9 to 1. You will find the motor wanting more fuel at lower rpms but that is nothing you can't handle.
I went from 11.25 to 9.25 with methanol and only felt a slight difference. On top of that, I have a very safe arrangement for in case a tuneup goes awry.
I think some get too stuck on pinching every bit of compression they can out of their combination.
I think some get too stuck on pinching every bit of compression they can out of their combination.
turbobitt
Well-Known Member
- Joined
- Feb 14, 2002
I think its plenty safe and could go higher compression at your self imposed 30 psi limit. This is why I say it is still conservative.
Allan G.
turbobitt
Well-Known Member
- Joined
- Feb 14, 2002
Not sure what answer your looking for but when I went from 8:1 to 9:1 I went considerably faster with lower boost.
Allan G.
That could of been due to the camshaft you were using, too.
A couple interesting stories. When I was running 11.25 with the second motor, I had set a boost limit of 22 psi. The engine did good with the high static CR and the 22 psi boost. First time out on a 1/4 mile track, low nines. On the last launch of the night the boost spiked to over 22 psi and a rod folded due to a lean condition which, with the rest of the combination of high static CR and over boost, set off preignition. The front of the car rose, and then pop!
The next motor had 9.25. The boost limit for this engine was set at 40-45. On a 1/4 mile run, the car performed an 8.70s pass with about 30 psi boost. The most interesting thing about this run was that a dangerously lean condition had developed for most of the pass. So lean that the engine was lean surging across the finish line. But, the engine suffered zero damage. The engine survived to make another pass, after fixing the lean condition.
How lean was it? The a/f reading was bouncing off 14.64 to one, averaging in the 13s!!!
The next motor had 9.25. The boost limit for this engine was set at 40-45. On a 1/4 mile run, the car performed an 8.70s pass with about 30 psi boost. The most interesting thing about this run was that a dangerously lean condition had developed for most of the pass. So lean that the engine was lean surging across the finish line. But, the engine suffered zero damage. The engine survived to make another pass, after fixing the lean condition.
How lean was it? The a/f reading was bouncing off 14.64 to one, averaging in the 13s!!!
Great info. Thank you for sharing.
Just drawing some lines on this graph it looks like about 3% more power going from 8:1 to 9:1. You can see that the increase in thermal effeciency (power) gets lower and lower the higher you go. But, the risk of detonation gets higher and higher. I'm sure compression ratio has some effect on spool up and converter also, but I don't know to what extent.
Thermal effeciency is direcly related to hp: HP = TE x FUEL FLOW (PPH) x 7.466
For a 1000hp engine 3% is 30 hp. What do most v6's in the 1000hp range pick up with 1 lb. of boost?
I scanned this from the book "Engineering Fundamentals of Internal Combustion Engines" by Pulkrabek.
C
coach
Guest
On a standard V8 engine, 1 compression point was an average of 6-8hp. That was years ago when I lowered my Max Wedge car from 13.5 to 11. On a naturally aspirated motor, more comp allows you a little more cam.
We don't have that issue. It's another 1/2lb of boost for us. My thought would be, the better baseline V6 you have, the healthier it will be when your feeding it!!! 9:1 is what I think my stroker will be.
We don't have that issue. It's another 1/2lb of boost for us. My thought would be, the better baseline V6 you have, the healthier it will be when your feeding it!!! 9:1 is what I think my stroker will be.
Dusty Bradford
Well-Known Member
- Joined
- May 24, 2001
I like 9.1 to 9.5-1. I have run as much as 10.5-1 but as Don hit on, the higher compression will make your tuning window smaller. I tune a local turbo car with 12.2-1 and 22psi of boost and it's very touchy. We are trying this because of the small turbo limitation in the class keeps the boost under 25psi. I feel I have to pull the timing back so far that I could make just as much power with 10.1 and more timing. I'm not sure going higher than 9.5-1 on gas has any gains because you have to run less timing to keep cylinder pressure in check.
My answer. 9.1-9.5-1
My answer. 9.1-9.5-1
This is a street application, correct? Is it really worth it to push things to the edge for a street car where maybe an extra psi of boost would make up the difference anyway?
I vote for keeping it on the safe side, and not trying to push it out to the edge. Who knows what street gas is going to be like when Obama gets done with us.
I vote for keeping it on the safe side, and not trying to push it out to the edge. Who knows what street gas is going to be like when Obama gets done with us.
Norbs, you have the engine sim. What is it telling you?
Here's another story. Someone came to me for advice on what CR to use for his unique combination. I determined he needed to be in the 6s. He wouldn't hear of it. He had other experts telling him he needed to be in the 10-12s. He is burning methanol. After blowing up I don't know exactly how many motors, he finally settled on a CR that has been durable for him, and he's been getting good performance out of. Last I'd seen his CR was in the 6s.
I know high numbers for static CR sound cool, and that's how I thought when I started out, but believe me, the difference in power is certainly not worth it on a street car, and from the pocketbook stand point, again, it's not worth it.
Here's another story. Someone came to me for advice on what CR to use for his unique combination. I determined he needed to be in the 6s. He wouldn't hear of it. He had other experts telling him he needed to be in the 10-12s. He is burning methanol. After blowing up I don't know exactly how many motors, he finally settled on a CR that has been durable for him, and he's been getting good performance out of. Last I'd seen his CR was in the 6s.
I know high numbers for static CR sound cool, and that's how I thought when I started out, but believe me, the difference in power is certainly not worth it on a street car, and from the pocketbook stand point, again, it's not worth it.
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