Here's a comparison that is very close comparing a stock shortblock with GN1 ported heads with .040 gasket for a net of 8.3:1 CR to a good strong rotating assembly with a girdle netting 9.3:1 CR. With just about everything the same except the cam @28-29psi the stocker with 8.3:1 trapped 135-136mph. At 25-26psi the 9.3:1 engine trapped 137-138mph. Both were numbers were acquired with a tnetics 6680 that was run for all it was worth in lousy 80*+ air at 1100'. There's a lot more than 3% there for 1 point increase in CR. Those that say oh it's only one or two psi more boost to offset it clearly don't understand why that's not really possible in the examples I just gave. There a window on the CR just like the turbo. If I had a turbo on there that would flow over 80lbs/min that one or two psi statement would be more accurate. If the turbo flows 75lbs/min then it doesn't matter what boost or rpm. 75 is 75 and it's the limitation. if the turbo is being used close to where it's flowing it's max CR increases will help a lot if they are below the window. Especially with a back pressure combo like just about everyone else on here is using on their street cars.Alky V6 said:
Ordering pistons what compression?
- Thread starter norbs
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Come on. You're attributing the difference completely to the CR? We're supposed to simply ignore that they were using different cams? Cam differences are pretty important in my book. What sort of heads were on the 9.3? What was the difference in cubes? There's a lot of variables you're not bringing up here.
Same heads. The cam in the 9.3:1 engine isn't giving it all that gain either. That cam peaks right where the turbo is out of breath. Less intake duration might have helped the 9.3:1 engine because it would have complemented the turbo better. Same converter and tire too.
Gearing? Fuel? Tuning?
Cubes are the same?
The 3% was an estimate pulled off a very limited graph in my book. It could be higher. It is a theoretical graph after all. My hope was that the graph would illustrate that there is a point of diminishing return on CR. 8:1 to 9: is a larger increase in power than it is from 9:1 to 10:1.
I'm with the rest of the group that says 9:1 or a little more is fine as long as you run good fuel. In fact that's where mine will be.
For guys running primarily pump gas, run lower CR so you can have a safe tune. But when you go to the track, fill it full of good fuel and turn up the boost. I would never build a 9.5:1 pump gas motor because the relatively small increase in power from compression is wasted because you can't max out a turbo on the street anyways. But you will have the option of turning it up at the track to make up for it.
This sort of discussion, comparison, whatever you want to call it, reminds me of when I was out on the island tuning that alcohol Buick V6. The engine was built by TA Performance, knowing that the engine would be burning methanol and would be eventually running with 40+ psi boost. They are the people that picked the 8.9 CR for the methanol. I had no input on that. I thought it was a wise choice on their part. This would be like choosing 7-8:1 for a blown gasoline application. Very conservative.
When we were at the track and we were all sitting around after the testing, an acquaintance of the owner of the car was sitting with us and gave us his professional opinion on the performance of the car. Bear in mind, this guy was what I call a professional spectator. He told us that he wasn't really impressed with the performance and guessed the car was capable of doing mid to high nines (quarter mile times). The clocks weren't up during our testing. He was actually pretty close. Later on, using the datalog, we were able to determine that the car was doing 5.90s in the 1/8. Low nine second quarter mile capability.
With his broken english that they use down there, I was pretty lost trying to understand what they were talking about. I was able to catch the times he was guessing. I interrupted him and asked, "Let me get this straight. So you're saying that the car is probably only doing mid to high nines for a quarter mile track?"
He confirmed my understanding of what he was saying. I reacted a little different than what he was expecting. I answered with, "Well then, I hope you're right about that."
You could see that big question mark appearing above his head. I went on to explain to him the exact parameters of the testing he was watching. 2 psi launch, max 18 psi during the run, not even kicking in the secondary injectors yet, very conservative rev limits. His face sort of melted away. Ooops.
You see, a lot of meaning can be misunderstood without knowing the full facts of the tests.
When we were at the track and we were all sitting around after the testing, an acquaintance of the owner of the car was sitting with us and gave us his professional opinion on the performance of the car. Bear in mind, this guy was what I call a professional spectator. He told us that he wasn't really impressed with the performance and guessed the car was capable of doing mid to high nines (quarter mile times). The clocks weren't up during our testing. He was actually pretty close. Later on, using the datalog, we were able to determine that the car was doing 5.90s in the 1/8. Low nine second quarter mile capability.
With his broken english that they use down there, I was pretty lost trying to understand what they were talking about. I was able to catch the times he was guessing. I interrupted him and asked, "Let me get this straight. So you're saying that the car is probably only doing mid to high nines for a quarter mile track?"
He confirmed my understanding of what he was saying. I reacted a little different than what he was expecting. I answered with, "Well then, I hope you're right about that."
You could see that big question mark appearing above his head. I went on to explain to him the exact parameters of the testing he was watching. 2 psi launch, max 18 psi during the run, not even kicking in the secondary injectors yet, very conservative rev limits. His face sort of melted away. Ooops.
You see, a lot of meaning can be misunderstood without knowing the full facts of the tests.
I really appreciate everyone's input here, but I don't wan't to start a war on this subject, Donnie can you run some graphs on the Sim for compression vs hp vs boost at 30 psi? I am using a front mount IC standard bar and plate 5" thick, 20" tall 31" long approx dimensions...........at 20 psi on my last defunked combo inlet temps were no more than 20 degrees above ambient on the dyno. Bison has tested a lot of combo's and I think it would make sense what he is saying that gains can be well over 3% in a turbo application because of the increased efficiency. I also would like to add that I want the car to have lots of low end power off boost for the street.
I did that a long time ago. Why do you think I'm pushing the safer CR. But, with enough pulling back of the timing, 9.5:1 should be just fine at 32 psi boost.
Here is a comparison of horsepower numbers with compression ratios of 7.5 through 10.0:1.
This is using a lot of your engine specs. Not too sure if the turbo matches what you're going to use. I used an intercooler efficiency of 75%, typically a minimum efficiency reserved to liquid intercoolers.
34 psi boost. blower rich 116 octane gasoline.
This is using a lot of your engine specs. Not too sure if the turbo matches what you're going to use. I used an intercooler efficiency of 75%, typically a minimum efficiency reserved to liquid intercoolers.
34 psi boost. blower rich 116 octane gasoline.
Here's a knock index comparison using most of the same specs above, comparing only 8:1 and 9.5:1.
The boost was lowered to 32 psi for these calcs.
Any knock index number between 1.0 and 2.0 is considered to be safe. Any knock index number above 2.0 is considered a knock likely area.
These calcs were done using blower rich 116 octane gasoline.
The intercooler efficiency was 75% (liquid intercooler). Using an intercooler efficiency percentage lower than 75% makes knock more likely.
The boost was lowered to 32 psi for these calcs.
Any knock index number between 1.0 and 2.0 is considered to be safe. Any knock index number above 2.0 is considered a knock likely area.
These calcs were done using blower rich 116 octane gasoline.
The intercooler efficiency was 75% (liquid intercooler). Using an intercooler efficiency percentage lower than 75% makes knock more likely.
You can see that 9.5:1 is doable, but is riding on the edge of safe, with a very good intercooler. Be very careful with your tuneup if you use 9.5.
These calcs used timing settings that gave best power, not necessarily the safest timing numbers. Backing off the timing in the lower rpm areas would lower the knock index in that range. Power would also drop where the timing was backed off.
These calcs used timing settings that gave best power, not necessarily the safest timing numbers. Backing off the timing in the lower rpm areas would lower the knock index in that range. Power would also drop where the timing was backed off.
Thanks so the consensus is to stay with 9.5 to 1? Looks like 100 hp @7000 rpm from 8.5 to 9.5 that would equate to a 10% power change correct? But the knock index is pretty high.
Do you think your intercooler is 75% efficient? If so, go for it. Start out with the timing down a little and work up with it.
Keep in mind that if you have to back off timing to stay away from detonation, you will start to erode that hp advantage. How much timing might you have to back off? Would you simply end up making just a tiny bit more hp than if you ran the lower CR and didn't have to back off timing?
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