I always see different results seeing how everyone has their own opinion but what would y'all say is the highest CR to run a car on with 93 octane pump gas? Also, still being streetable etc...

 

I was just thinking we needed a 99th thread on this subject . . .

 

Maybe sticky worthy then?

Simple question, for some reason when ever I would search it wouldn't bring up any results. It's working now, don't trip chocolate chip...

 

depends when the intake valve closes. (IE your camshaft timing)

what displacement and what cam you plan on running

 

Currently at around 12.9:1 ish with Jun 3's, B20V with stock bore. Check sig, it's been researched and I know everyone will get different results but most thread I found were pretty old and there's been a lot developmentally wise with parts now-a-days. I'm just curious...

 

Instead of static compression ratio, think about dynamic compression and how high that can be.

 

This is correct. Static compression ratio has nothing to do with what octane fuel you can run. It's the cam timing that's important. The higher your static compression ratio the later your intake valve needs to close. What this means is that a high compression engine needs a cam with more duration. The compression ratio measured from the point the intake valve closes is called the effective compression ratio and on a 4-valve aluminum head engine should be about 8.5:1, regardless of make. It's the same whether its a Ferrari or a Honda.

 

I wanted to bump this because I'm doing some figuring....

According to the stock specs I put in this calculator - http://www.rbracing-rsr.com/comprAdvHD.htm

A stock engine and cam @ 9.8:1 statc CR and runs on 87 octane with cranking pressure of 180.05 and Effective Dynamic CR of 8.81:1.

Is that within your window of 8.5:1 varying by make? If so, then the same specs with the static CR of 11.4:1 and much hotter cam (ADBC 41.7 degrees stock vs 66 degrees for the Crane Cam) results in cranking pressure of 180.31 and Effective Dynamic CR of 8.82:1

This might sound ridiculous with specs like this, but wouldn't it mean I could run the car on 87 even with the higher CR and this cam (with the same 87 octane tune/program/pcm/ecu)? As you said octane and static ratio have no relation to each other and I find this stuff extremely interesting since static CR numbers are often thrown around in relation to what runs on pump gas.

Given we know the previous parameters what numbers should be taken into account for 91 octane?

Higher cranking pressures (assuming also higher cylinder temperatures), say a 5% over stock?

In other words what determines what octane is needed? For example I can't give you any specific data but let's assume that I can run 28 degrees advanced spark and experience no knock suppression (retard) with a car running at normal operating temperature at the stock compression ratio and camshaft. This tune is spec'ed for 92 octane and have no way to change it, it was spec'ed when 92 was available across the country, think of it as a "chip". In fact this "tune" likes 92-93 but drops a couple of degrees because of 91 octane, so I would assume it shoots for max timing at all times vs quality of fuel.

Thanks.