wowchad

I did a compression check on my 454 in a Baja Outlaw last night.
The four corners all read 110-120 but the inner siamese cylinders were only 75-80
Is this normal, due to the heat, that the inner cylinders are that far off?!?

It still runs good, I just did a check for the hell of it.

JaayTeee

That past the old 10% rule
between cylinders.

Do you have the throttle open
when checking ?

75-80 psi is not real good

wowchad

No jt I didn't have the throttle open for any cylinder. What difference will that make, more or less pressure?

So that's the rule hey, 10%?
Guess I'd better keep an eye out for a long block on eBay again.

Dennis Moore

Being a Mercruiser mechanic for many years I know that all 454 engines have 155 pounds of compression if they are healthy (throttle closed, Snap-On compression tester). You probably need a valve job.
Good Luck
Dennis Moore

wowchad

I'm pricing new Edelbrock marinized square port loaded heads. For about the price of a quality valve job I can go to aluminum!

Yeah I ran her dry on oil, new valve covers without vents, forgot to remove oil fill cap and I spewed all my oil out of the dipstick tube

I felt like a dipstick what a stupid mistake, any how I'm glad i run full synthetic oil, it probably saved my lower end.

I've never done a leak down test, hows it done?

JaayTeee

I've alway's opened the throttle
when testing compression, per what
various manuals say, ( not just marine)
the theory is that it lets a non restricted intake
charge into the cylinder.

In your case, I doubt it's going
to help.

Like Dennis has said, should be in
the 150 Neighborhood.

To do a leak down test, remove valve
covers, loosen up the rocker arms ,
remove spark plug, instert leak down tester
into spark plug opening, connect up air,
guage will show % of leakage.

In your case, (low compression) you then
listen for where the leakage is going,
out the exhaust ports (ex valve), back up through the
intake ( intake valve), into the crankcase,out the breathers, ( rings)

jt

mcollinstn

Dennis

why throttle closed?

I've always opened them up and on some engines (bikes with vacuum slide carbs) I've taken carbs OFF to get unrestricted airflow for consistent readings.

robyw1

I just got 162 to 164 in all 8 of my 7.4 MPI. Seems kind of high to me, Better go buy a new gauge and try again. However it made no difference with throttle closed or open in my case

Roby

formula31

And valve overlap will change those numbers considerably. Like was said , its not the number, its the difference that counts. Ive seen many serious horsepower engines that read 90 cranking pressure. But , what do I know.

clearcut3

I am pretty sure by Dennis's quote above he was just stating this is what the reading should be with throttle closed and a snap on compression tester. I in no way got the impression that he was saying the throttle had to be closed. Just my .02, probably only worth .01