High Compression Pistons

James

Staff member
Would there be a noticeable difference between 13.5:1(stock) and 13.8:1? What about 14:1?

Also any idea at what point 100 octane (race) gas is required?
 

James

Staff member
Pro Circuit claims that their high compression piston is designed to run on pump gas. However I'm not sure if they're referring to only the 13.8:1 or both. Might have to email them on that.


Still curious on the low end power gains, is going from 13.5:1 to 13.8:1 really even that much?
 
Why not get a 450? I have never been a fan of putting in high performance anything as reliability and longevity suffer in my opinion. I have a buddy put in high compression pistons in his Harley and then starters began breaking. Stroking and over size are one thing but changing compression is another. I suppose it depends on the amount of altaration but this is just my opinion though. :noidea:
 

James

Staff member
Why not get a 450?
:shocked: :lol::cry::lol: We've met before right?..... :lol:
I have never been a fan of putting in high performance anything as reliability and longevity suffer in my opinion. I have a buddy put in high compression pistons in his Harley and then starters began breaking. Stroking and over size are one thing but changing compression is another. I suppose it depends on the amount of altaration but this is just my opinion though. :noidea:
That is a valid concern. :thumb:
 

SRAD97750

Moderator
Staff member
The term "Pump Gas" is misleading, as my premium is 91 octane here in CA where in other states I've seen 94 octane as premium.

As a rule of thumb I say 12:1 and up requires 91 octane as a safe fuel. It's a high enough ratio that a higher octane fuel won't rob you of power, and when your engine gets really hot and hot spots appear on the head/piston, you won't ping/detonate.

Running a high octane fuel in a lower compression engine will rob you of power because the fuel is more resistant to ignition.
 
Pro Circuit claims that their high compression piston is designed to run on pump gas. However I'm not sure if they're referring to only the 13.8:1 or both. Might have to email them on that.


Still curious on the low end power gains, is going from 13.5:1 to 13.8:1 really even that much?
It makes a larger difference than you'd think. A stock 150F trail bike runs 9.5 to 1
 

James

Staff member
The term "Pump Gas" is misleading, as my premium is 91 octane here in CA where in other states I've seen 94 octane as premium.

As a rule of thumb I say 12:1 and up requires 91 octane as a safe fuel. It's a high enough ratio that a higher octane fuel won't rob you of power, and when your engine gets really hot and hot spots appear on the head/piston, you won't ping/detonate.

Running a high octane fuel in a lower compression engine will rob you of power because the fuel is more resistant to ignition.
Our premium is 93-94, think that that'll be enough for at least the 13.8:1?
 
There are a ton of variables involved. All other things being constant I doubt that you would notice much difference from 13.5 to 13.8. Maximum compression that an engine can take before pre-ignition sets in can be wildly different from one design to another. The exact conditions like air pressure (altitude), temperature and even humidity can all have a big effect. The different piston can change the airflow characteristics inside the cylinder and this is a very important part of engine design.

Another factor is what the manufacturer claims vs actual reality. These can differ. Is it really 13.8?
 
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