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best compression for 91 gas

2582 Views 46 Replies 14 Participants Last post by  Donnie J
I live in SoCAL so 91 gas is expensive and i don't know ANYWHERE nor do i want to spend the extra money to get 93. As i plan to get stage 2 cams, not sure what yet since there are so many stage 2 cams out there, by raising compression a little, i'll get better gains with the cams right? I have a k20z1 so would raising it to 11.5 with the type R pistons do any good? And whats the highest compression i can run 91 octane gas with, being that my car will still be reliable.
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People have run 12.1:1 but I chose 11.8:1 with alot of research on it finding it to be the most reliable (within reason, obviously) on 91 and still providing power.
sweet...thx kevin. how did you achieve 11.8? did you just get new pistons or did you do other stuff.
sweet...thx kevin. how did you achieve 11.8? did you just get new pistons or did you do other stuff.
Just new pistons. I didn't want to mess with thinner head gasket or flat valves to raise compression. Just personal choice, no real reason I guess. Just seemed to me like it would potentially cause more issues than just swapping pistons.

If you do get pistons though, make sure you do NOT get buttons for the lock type. Wrist Pins are the best to get, so make sure you get pins. Not rails, not buttons. Just trust me and save the hassle. ;)
yes, trust him lol.
sorry if i am a little bit off topic,
kevin, what compression would be good for 94 gas? 12.5:1?
btw...what kinda pistons did you get. Did you just use the ones that come with the BC strokert kit? CRSX only sells wiesco pistons with 11.7, or is that basically the same thing. And if anybody knows, on stage 2 cams, how much more whp is gained by raising the compression? Skunk 2 claims like 10-15 more.
You can always get Import Builders prep work done on pistons. He takes for example an 12.1 CR and does prep work and it ends up an 11.7 but makes more power.
wut the....i'm very interested, care to elaborate?
wut the....i'm very interested, care to elaborate?
Here's the word from the man himself.

I charge $150 a set, please compare to other companies offering prep. The service uses a 3 step process from NHRA people at the highest level.

Have a look.


Thank you.

Jeff

Above is 89 MM Stock shelf Wiseco 13.7:1 pistons, prepped down to just about 13:1.

I charge $150 for the service, but what customers are finding is the prepped pistons in lower compression actually make more HP than the 13.7 pistons if you put them in un-prepped. And higher RPM you take it, the more HP disperity.

Jeff
Edit: Can't load image whoops.
btw...what kinda pistons did you get. Did you just use the ones that come with the BC strokert kit? CRSX only sells wiesco pistons with 11.7, or is that basically the same thing. And if anybody knows, on stage 2 cams, how much more whp is gained by raising the compression? Skunk 2 claims like 10-15 more.
a general rule of thumb is 1.5-2hp per .1 compression... of course it varies depending on setup, so there's not really an exact number... but by skunk2 giving a range of 10-15 with 11.7 pistons would fit with 1.5-2 per .1 cause that would be between 10.5 and 14hp.
Wiseco and CP pistons are the best pistons, IMO
Brian Crowers stroker comes with CP pistons which are what I ended up using. 89mm, 11.8:1 CR CP Pistons.

Best compression for 94 octane? I don't think I'd risk higher than 12.5:1. Could probably go a little higher, but to what end? Probably need to start pulling alot of timing. Plus, as CR gets higher, power gain gets less. Similar to a diminishing returns system.
Another option is to run water/alcohol injection:
  • K-Pro can control this and advance ignition and adjust fuel when squirting. Look at the Nitrous tabs (you use these same controls for water injection).
  • Your K-Pro maps would be set for normal non-squirting driving, with ignition set so as not to knock.

This lets you run high compression, 91, and still get max power and advance at WOT. I still wouldn't go nuts and install 14:1, but it would be a safe way to get the most out of something like 12.5:1.

Settings for use with water would be something like in the following image (mostly the reverse of how the settings would be for nitrous).

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To start, there's no 91 pump gas around that can guarantee and maintain its consistency. So I do not believe there's an answer for the "best" compression for it.
what do you mean by 91 gas can't guarantee consistency?
what do you mean by 91 gas can't guarantee consistency?
on pumps that share all the octane feeds through the same pump, there's a possibility that you could get a bit of gas of the octane the person before you got. So you pay for 91 but you probably get a little 87 in there in the beginning.

Also, people talk alot about "summer gas". *Shrug* I've never experienced this I guess. Other than that, I've had no problems with it, but that's not to say others won't as well.
what do you mean by 91 gas can't guarantee consistency?
As gasoline gets old, so as its qualities. My point being that is it really worth it to use a regular 91 unleaded fuel from your local gas station after all the time, money, and labor that you have spent by putting a well balanced motor with high compression pistons? Remember, gas is cheaper than a motor so please I strongly suggest to put 2-3 gallons of a higher octane to have a piece of mind.
As gasoline gets old, so as its qualities. My point being that is it really worth it to use a regular 91 unleaded fuel from your local gas station after all the time, money, and labor that you have spent by putting a well balanced motor with high compression pistons? Remember, gas is cheaper than a motor so please I strongly suggest to put 2-3 gallons of a higher octane to have a piece of mind.
Unfortunately it's not quite as simple for some of us to get a higher octane fuel anywhere. The only places I know of to get higher octane fuel is 100 octane chevron "Cool Blue". I can either get it at the pump for (last time I went) 7+ a gallon or buy a 5 gallon container of it for however the store would charge. I'd LOVE to stockpile 100 octane, for sure, but I have no place to put an ample supply. Personally, that is.
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