I'm starting to look at perhaps some motorwork beyond Stage 1. One of the self imposed constraints I have identified is that it has to be able to run on 91 Octane pump gas.

What is the highest compression I could build into the engine to safely operate on 91? 10:1?

Thanks.

 

All depends on the how much cylinder pressure the cam builds and the tune you get. I'm running 10.2:1, some friends are running 10.5:1, and I think PhilM's new motor is 11:1.

 

10.5:1 on any street motor.

 

So any of the SE 103 kits would run on 91 then?

 

I have NO problem running 10.5:1 on my build.

 

http://www.daytona-sensors.com/tech_tuning.html

Raising compression by 1 unit (ie from 10:1 to 11:1) increases power by about 3%, assuming you dont have to retard timing to stop knocking.

In reality 10:1 is about right for street, you can go higher (mine is 11:1) but the gain isnt huge and you risk knocking.

 

10.25 on my last build with no issues using 91 octane.

 

Cam overlap will bleed of “static” compression and bring compression ratios as high as 11:1 to a point where you can run pump gas, but at the same time it moves the “power band” up the rpm scale and you lose some bottom end power.

 

Troy is right. The static compression isn't the be all and end all to determining required octane. Cam timing plays a huge role. Also the efficiency of the combustion chamber comes into play as does the rest of the intake system (and the exhaust too if you get picky.)

Doug Coffee at HQ can get 11.5:1 to run on pump gas. As with anything near the edge, there are lots of IFs, ANDs & BUTs attached.

 

Thanks for all the input so far. Please keep the info coming.