I did I search but got mutiple answers. Some say 12:1 is too high for 91 octane, and some say 12:1 and 91 octane would be fine with the right tunning. Can you guys tell me if I can run 12:1 CR in a SOHCvtec with 91 octane? What's the downfall? I know 11:1 CR is ideal with 91, but is 12:1 pushing it too far for 91? Thanks.

 

it may be a little too much, but i think it will be fine. are you suppose to advance or retard the timing if you have too much compression with too luttle octane?

 

p29 pistons and z6 head i am assuming? Depends on cam and tuning. obd1, with a healthy overlap cam and 8-10 degrees base timing, you should be fine. Or, 14 degrees base timing and a CHEETAH programmed obd1 ecu with said NA cam.


[Modified by GTI 1.8t, 10:47 PM 12/28/2002]

 

Ask anyone from IB about it and this is what they'll tell you:

"It all depends on the tuning."

Not only it's just do-able, but it has been done. They say 13.0:1 is very realistic too on 91 octane pump gas.

Jay Kim

 

TUNING is everything... but yes it is possible...

 

Okay. Thanks guys.

 

Actually it's gonna be an OBD2 D16Y8 with D16A1 pistons.

 

Why not use a turbo type head gasket (thicker, drops compression some). Even with the overlap of the VTEC cam, the low rpm cam will be less tolerent of the 91 octane fuel especially at full throttle with that compression ratio. Im assuming the piston valve clearances will be adequate.


[Modified by dennisman17, 1:01 PM 12/29/2002]

 

What's the point of putting 12:1 pistons if you're going to cancel it with a thicker turbo gasket?

 

my brother had a 12:1 compression ratio on his ls-vtec and he used 91 octane, his car ran OK, but im not sure if it was as good as it could've been with more octane rich gas...

 

YOU have GOT to be kidding me, .. and BTW, i dont think 91 is healthy for the motor.. If your planning on spending thousands of dollars for building a motor, why not spend the extra 10 CENTS per gallon on 93????

 

Because I dont think you can get 93 octane in Cali??

 

Correct, and don't be surprised as other states adapt the same rating...

 

12.5:1 is ok with pump fuel @ 2700ft so I would guess you could run it where ever you are. Thanks to Kataku2K3 I'm planning on running 13:1 on pump fuel... it CAN be done.

 

Damn I run Sunoco 94 Or no less than a name brand 93.I put some no name 93 gas in my car and it ran like garbage.
Dan

 

I have built dozens of 12.5 plus motors running 91 no prob.
tuning is key

 

Let me add to this question. I am planning on running a 75-150 shot of NOS via DP on my H22. I have acess to 93 and 94 octane gas and thats all I currently put in my engine now.

Knowing that I have a shot of NOS on tap(Along with an AEM EMS and a wideband O2 sensor for tuning)what would be the highest CR I can run safely?? I am also using a Stage 2 Skunk2 camshaft which has an intake lift of .505 so I know I may have to mill the indents on the pistons a bit. BUt thats beside the point. Anyway, whats the highest I can go with a large shot of NOS??

 

What exactly do you guys mean by tunning?? Do you mean cams, cam gears, port and polish head, valves, springs, injectors, high flow header and exhaust.. What would you need to be able to run the engine correctly on pump gas??

 

Yup. Don't skimp on this.

 

ECU... optimizing timing and fuel maps, tuning on the dyno and on the street.

 

You will be closer to 13:1 with that combination, 12.8:1 to be exact.

It is much cheaper to buy a thicker head gasket than to obtain a different head/piston combination...duh.

Depends on fuel quality and what your pistons/exhaust valves/ head bolts/rods/main bearings can stand. 15:1 is great with juice. W/o knowing your intent and hardware, it's impossible to supply an accurate answer.


[Modified by GTI 1.8t, 2:42 PM 12/30/2002]

 

Actually it will be around 11.9:1 CR. I will be using the 86-87 LS pistons which is a bit lower and a CRX head gasket which will be thicker.

 

i used to have a si with b20 with endyne 12:1 and there was no way in hell my car ran on less than 100 octane what a headache