stangiac03

has anyone ever put NPI or PI heads on a 99 cobra block? or just used the 4.54 dished pistons with the NPI or PI heads? would the compression ratio be to high for pump gas and cause detonation?

oxfordgt

The compression ratio would be 9.8:1 with NON-PI heads and 10.7:1 with PI heads. With Non-PI heads you will be fine on pump gas but with PI you will have to run 93 octane and pull timing out.

stangiac03

thanks. are you sure on this cause i have asked numerous people and i have gotten so many diffrent answers one guy told me 11.3:1 on NPI heads. lol.

contentsunderpressur

Yes, oxford knows his ****.

Sxynerd

Im not smart so I could be wrong. But wouldn't the CR drop?

Nitti00GT

+1 Ox def knows what he is talking about.

11.3:1 sounds high and from what I have been told you will have to run race fuel.

oxfordgt

Yes I'm sure. If you were to run flattop pistons with PI heads you would get 11.2:1 CR which means if you wanted 11.3:1 with non pi heads you would have to use domed pistons with about a 2cc dome. LOL have fun running that on the street.

Mark99GT

What is the piston dish on the 99 cobra?

oxfordgt

4.54

Mark99GT

So what numbers are you using to calculate the CR then?

Ive got:

Bore: 3.552"
Stroke: 3.543"
Piston Dish: 4.54cc (I read somewhere else they were 3cc)
Head G Bore: 3.625"
HG comp thickness: .03"
Deck height: .012"
CC volume: 51cc (I also read these were 49.77cc)

I'm getting a CR of 10.2:1 with the NPI heads and 11.5:1 with the PI heads. What numbers do you use?