Compression Test

wowchad · 08-15-2003, 10:30 AM

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 · 08-15-2003, 10:53 AM

That past the old 10% rule
between cylinders.

Do you have the throttle open
when checking ?

75-80 psi is not real good

wowchad · 08-15-2003, 11:15 AM

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.

08-15-2003, 11:24 AM

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 · 08-15-2003, 12:05 PM

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 · 08-15-2003, 12:30 PM

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 · 08-16-2003, 09:21 AM

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 · 08-16-2003, 02:04 PM

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 · 08-16-2003, 11:14 PM

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 · 08-16-2003, 11:23 PM

Quote:

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.

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