Thread: Cranking compression & pump gas

Tweet

I need to get something nailed down here, if anybody can help?

I have a calculated compression ratio of 12.3-1. that is with calculator.
http://www.rbracing-rsr.com/compstaticcalc.html

The heads are 122 cc the head gaskets are .039 the deck is .005 and the dome is 48. 4.31 bore x 4.00 stroke.

I did a compression test today, and 6 cylinders are at 195, one is at 190 and one at 200. I read somewhere that the max for premium 92 pump gas was 180 then add 1 point in octane for every extra 10 lbs of cranking compression. is this true or is there a real formula to base this on? I am running pump gas, and VP-C5 octane booster with 38 degrees total timing. I cant hear any detonation, and the plugs are not peppering.

Sheldon

Originally Posted by Grumpy 427

I need to get something nailed down here, if anybody can help?

I have a calculated compression ratio of 12.3-1. that is with calculator.
http://www.rbracing-rsr.com/compstaticcalc.html

The heads are 122 cc the head gaskets are .039 the deck is .005 and the dome is 48. 4.31 bore x 4.00 stroke.

I did a compression test today, and 6 cylinders are at 195, one is at 190 and one at 200. I read somewhere that the max for premium 92 pump gas was 180 then add 1 point in octane for every extra 10 lbs of cranking compression. is this true or is there a real formula to base this on? I am running pump gas, and VP-C5 octane booster with 38 degrees total timing. I cant hear any detonation, and the plugs are not peppering.

Sheldon

That seems like a lot of timing to me.
There are a lot of variable's.
Cam profile,are the combustion chambers polished,piston domes ploished,spark plug heat range etc,.

The heads have been CNC machined, they are AFR 305 heads. the cam is a cam Motion. solid roller. .691/.684 .262/.272 @.050 On a 112 lSA. My engine builder pollished the pistons removed all the sharp edges, and made room for the spark plugs. The plugs are a race plug from NGK they are a 7 heat range.

the rest of the cam specs are- intake valve open 23.8close at 58.1

exhaust valve open at 69.9 and close at 17.5

overlap is 41.3

That cranking compression is about perfect for a street motor,inbetween 180 and 200 psi.Street/strip about 220 and a race motor about 240 psi.
If you are getting below those numbers,which you are not,then your dynamic compression stroke is going to be very short ,giving up hp and torque all thru the rpm band.Effectivly reducing the stroke of the engine and worse yet ,all that stroke you dont use just drags the engine down.
Thos numbers look good to me,inspect the plugs closely for metal particles and back the timing off a few degrees if its worries you.

as for timing, the motor wanted 38 degrees. We started at 34 then went up in 2 dgree incriments. we would gain 20-24 Hp every 2 degrees

Originally Posted by Grumpy 427

The heads have been CNC machined, they are AFR 305 heads. the cam is a cam Motion. solid roller. .691/.684 .262/.272 @.050 On a 112 lSA. My engine builder pollished the pistons removed all the sharp edges, and made room for the spark plugs. The plugs are a race plug from NGK they are a 7 heat range.

the rest of the cam specs are- intake valve open 23.8close at 58.1

exhaust valve open at 69.9 and close at 17.5

overlap is 41.3

Why are you running a wide L/S.
Does this car run on nitrous??
Is this a street car or a drag car??

I am going to run a 300 shot of nitrous. The motor made 708 Hp and 622 Tq on the motor, and 1006 Hp and 1000 TQ on Nitrous. the car is a street car that i take to the track 2 times a year.

I ran it on race fuel on the dyno, and will only spray it with race gas. i just thought as far as running around to the local A&W it would be safe on pump and octane booster

Originally Posted by Grumpy 427

I am going to run a 300 shot of nitrous. The motor made 708 Hp and 622 Tq on the motor, and 1006 Hp and 1000 TQ on Nitrous. the car is a street car that i take to the track 2 times a year.

OK.,now I understand.
First I would not even try and run it on pump gas with that comp. ratio and that L/S with 38 total timing.
If you run that much nitrous on pump gas I will bet you will blow a hole in the piston very quickly.
With that much nitrous I would try and run 112 octane and I would have the timing down around 30 degree's.

No i know Eric i would never run it on pump gas and nitrous. C-12 only. I have been running it as is on pump and booster just tooling around town, it appears fine.

The Canadian pump gas is about 100 octane ?

No our premium here is 92 octane

I spoke to a guy from JE pistons and a guy from VP fuels. They Both said i am on the edge and should take some timing out just to be safe. The guy from VP said that the octane booster would increase me from 91 to 95 and thats it. So i will pull 6 degrees out for street driving just to be safe.