TraviZ

iTrader: (2)

what compression would u run for a 15psi charged system? would it differ from turbo and super? assuming both intercooled?

F-BIRD'88

A very generallized question that leaves a lot of info not stated. So I'll give you a very generallized answer. 8.5:1 or less. Less is better.

junkcltr

iTrader: (1)

15PSI charged without a turbocharger or supercharger? As in a leaf blower?

15PSI is 15PSI no matter how you do it. The air temp. matters. 15PSI at what air temperature? What size cam?

Anywhere between 7:1 to 10:1 compression. I thought you already asked this same question in another thread.

TraviZ

iTrader: (2)

Kinda the same, kinda the not, I want to run on california pump gas (91). Without having to pull lots of timing. Think 8.2:1 would do it? I am not sure on cam, something that compliments a miniram. If I do a supercharger, it will be with longtubes, if a turbo, I will re-use my shorties.

F-BIRD'88

Not a chance.

TraviZ

iTrader: (2)

CR too high? I hear the power is in the timing more so then it is compression.

CrazyHawaiian

How are you planning on tuning the setup? Will you be using any type of injection or will it just be 91 pump all the time? If you don't want to have to pull alot of timing then you want low compression and as much detonation prevention as possible. I personally think its impossible to avoid pulling some ammount of timing at 15psi. Here's a formula that might help:

Final Compression Ratio or FCR = (Boost/14.7)/1 x (Compression Ratio)

From what I've read its accepted that 92 octane pump gas will support up to a 12.4:1 FCR. Any higher than that on the same octane gas means you'll probably need to do something to supress detonation (pulling timing is one of em). If your local premium pump is 91 octane then then max FCR will be a little lower, probably around 12.2:1 or 12.3:1. I think the 2 bar MAP sensor will max out somewhere around 14psi or 14.5psi. An engine with a 8:1 static compression ratio boosting 14psi has a FCR of 15.6:1. So that gives you an idea of where boosting 15psi will lead you. Some form of detonation prevention will be needed with only 91 pump. Even if the engine was 6.5:1 compression ratio and you boosted 14psi you'd have a 12.7:1 FCR, little bit higher than what 92 octane could support with no detonation prevention. Now I don't know if these formulas can be considered an exact science, there are so many other variables that can affect your outcome. Like junkcltr said the cam also matters, and so does the air temp. But these formulas do give you a good general idea of what you're going to see. Another "rule of thumb" I've read is that the octane rating will need to go up one point for every pound of boost you run on any given compression ratio. So if the car is tuned good N/A on 87 octane, you could run up to 5psi boost on 92 octane and not need alot of detonation prevention. Don't know how accurate that "rule of thumb" is but I found that one in one of the books I have. When my project is complete I plan to run an 8:1 static compression ratio and my first goal is a good tune for 8 psi on 92 octane pump gas. Once I can reach that then I'll work on tuning for higher boost (10-14psi) with Meth Injection.

Tony89GTA

iTrader: (7)

I would run 8.3-8.5.

TraviZ

iTrader: (2)

I can get pistons that will net me 8.2 with 75cc chambers.

I am gonna be tuning with the accel gen 6 dfi. I want to remain on pumpgas. I think I will go for the low 8ish compression to stay safe. Thanks for that spit of information CrazyHawaiian. I cant/dont want to run anything besides gasoline for this is a street car

Damon

When you start boosting some timing reduction is necessary just becuase you're shoving a much denser charge of air&fuel into the cylinder which burns faster than a less dense mix. You can drop compression in the toilet but still doesn't mean you would necessarily want to run a full 32-38* of spark advance. Now every combo is different, but what you're really shooting for is a combo that will alow optimum spark advance- i.e. you can advance it until more advance doesn't help make any more power, but still be shy of detonation.

If you're pulling spark advance out just to stay out of detonation when the motor would really like more advance to make maximum power then you've got too much compression. This is typical of a N/A motor that someone slaps a blower onto. They often need to pull out more advance than the motor would really like just to stay out of detonation. Now it still makes more power than it did N/A obviously, but it doesn't mean it's optimum.

Take a Buick GN motor as an example. It has a very low sub-8 compression ratio and guys often shove 15+ PSI into them with an intercooled turbo. Even on race gas that doesn't mean they run 36* of timing. Often it's down in the 20s. That's becuase it doesn't want or need any more timing than that to make maximum power, even if it isn't on the verge of detonation.

I know with my (inefficient) Roots blower 8:1 is about the lid at only 6PSI of boost on pump premium and I'm still only running 26-28* of advance. I am definitely detonation limited. With a more efficient centrifugal or turbo, especially with an intercooler, and the accuracy of a well-tuned EFI system I'm sure you could push the limits considerably further. I have learned, however, that it is MUCH easier to pulley-up for higher boost if you find you have gone too low with the compression than it is to lower the engine's mechanical compression ratio if you find it's too high.

I wouldn't be "a-skeered" of an 8:1 or lower compression ratio for a motor that's going to see serious boost on pump gas.

1 DwnCam

I'm running a -36cc dish in my JE's with a final compression 8:6:1 using a 64cc chambered AFR 210. My D1-SC is pullied for 15psi, though it only produced 13.

84camaroturbo

i run 8.3to1 on 13psi on a eldelbrock proflo system and isnt very turbo friendly. the accel fuel injection or the computer system you can order it with a 2bar map and is alot nicer with boost. next year i am going to bump it to 19psi on pump gas going for 1000rwhp.

TraviZ

iTrader: (2)

Thanks, I ordered a accel gen 6 unit off a fellow member here. So that should help definitely.

1 DwnCam, can you show or tell me what kind of timing map you are running. Much appreciated!

1 DwnCam

I would love to but, the ECM is not connected to a power source. So I have no way of pulling maps out of the ECM.

RacerRick

36 degrees timing is too much for a turbo setup on pump gas.

How efficent your combustion chamber is plays a part as well as the burn characteristics of the denser cylinder charge. A lot of the big boost buicks run low 20's for total timing to keep out of detonation.

My mild SBC turbo setup at 7 psi on top of 7.83:1 compression likes 30 degrees of timing on 91 octane pump gas. Mostly because the stock open chamber heads have lousy chambers. It likes the same timing on 9psi and 94 octane gas.

I would run 7:1 compression and throw lots of boost at it. You loose around 4% power for every 1 compression point you drop but you will gain more than that back through boost. And you will be able to run more boost.