Max cranking psig, DCR for pump gas?

[Nut124]

Asking for a friend: He built a 1980 iron block, alu head 4 cyl engine for street. Unknown SCR, possibly in the 10.5 range. Lame stock cam, possibly off timing, could be advanced.

Cranking compression is 210psig, cold. Worried about detonation w pump gas.

What should i tell him?

[Tom68]

Cold air intake, sub 180 thermostat, premium fuel, listen for knock.

[ClassAct]

Cranking compression means nothing.

[rebelrouser]

Asking for a friend: He built a 1980 iron block, alu head 4 cyl engine for street. Unknown SCR, possibly in the 10.5 range. Lame stock cam, possibly off timing, could be advanced.

Cranking compression is 210psig, cold. Worried about detonation w pump gas.

What should i tell him?

I would be worried on pump gas. Not to start a flame war, but I do pay attention to cranking compression. If it has a lot of cranking compression, it is an indicator that it may detonate. And while it may not detonate on a cool fall day, what about a hot humid summer day?
I use performance trends engine program; it will generate a timing curve and tell you if detonation can be a problem. Usually what it tells me is that in the middle rpm range is where it may be an issue. So, by delaying the advance you can sneak under the wire. I use the generated timing curve as a starting point, it is usually pretty close. Since distributor machines are not common anymore, you can have someone hold the throttle at 500 rpm increments and then graph out the curve with a timing light. Change springs and weights until it matches what you want. And if it has vacuum advance hook to ported vacuum not manifold vacuum.

[PRH]

If you run the numbers thru a cranking compression calculator, does it show a similar result?

Has it been verified with another gauge?

FWIW, I used to have a 1995 dohc neon. The cranking pressure spec was like 180-200....... and that’s what it had.

[Stan Weiss]

Asking for a friend: He built a 1980 iron block, alu head 4 cyl engine for street. Unknown SCR, possibly in the 10.5 range. Lame stock cam, possibly off timing, could be advanced.

Cranking compression is 210psig, cold. Worried about detonation w pump gas.

What should i tell him?

What is the bore, stroke and rod length of his engine? What the specs on this lame stock cam? Also at what elevation is he?

Stan

[PackardV8]

The aluminum head may allow him to sneak by. With an iron head on a 100-degree day on an uphill pull at cruise RPM, probably not.

jack vines

[mag2555]

I can't see how a any street motor made in 1980 could have been intended to pump even 190 with how automotive computer control was still taking baby steps back then.

I would wager a bet that the Cam is out of proper time.

[Nut124]

Thanks for the replies.

Bore: 84mm, stroke: 90mm, rod c/c: 145mm.

He put in domed pistons and an early head with a smaller chambers.

[juuhanaa]

You could tell your friend not to mix blindly 1600/2000 TC ignition parts.



-juhana

[panic]

Engines with identical CCP @ 165 psi and DCR @ 8.04:1 can have completely different tunes, especially as to when the torque curve begins.
1. high static CR + late intake valve closure: 12.00:1 CR, IVC 82 degrees ABDC; torque will be low for quite a while
2. mild static CR + early intake valve closure: 9.00:1 CR, IVC 47 degrees ABDC, torque will rise immediately above idle

[Stan Weiss]

Thanks for the replies.

Bore: 84mm, stroke: 90mm, rod c/c: 145mm.

He put in domed pistons and an early head with a smaller chambers.

What is a rough idea of the stock cam specs?

Stan

Nut124_CGP_IVC.gif

[Nut124]

Stan, thanks for that interesting graph.

That 66 ABDC IVC seems just about right for the stock cam.

He could retard the intake cam one tooth, which is 17 crank degrees for 83 IVC, which is what some hotter street cams run about.

Any chance I could ge the same chart for my engine?

Same bore, 84mm. Stroke: 79.2mm, Roc C/C: 136mm, cranking compression 185psig.

[panic]

That's 17 degrees out of the overlap area...

[Nut124]

That's 17 degrees out of the overlap area...

Understood. What do you think that would do other than lower dynamic CR? The stock cam has a pretty short duration, in the 250 deg seat to seat range.