Compression test expected figures

[blaze_away]

I have been given a compression testing kit so thougth I might as well give it a twirl.

What figures should I be seeing ?

4.6 top hat liners, pocketed pistons, stage 3 heads, gas flowed.

[sparkythecat]

I don't know what the figures are likely to be, but the important thing is that all 8 should be the same.

[Boosted LS1]

180 psi would be ball park at a guess. Look for similar readings across all cylinders give or take a few psi.

[blaze_away]

Thanks folks will give it a go when its a bit warmer I also bought a boroscope/endoscope thingy for my phone so will try that to and have a look indide each cylinder and I an record the footage too.
Quite looking forward to doing it always good to have a reference point for any future use.

[Steve_D]

If it is not in any instructions you have then remember to hold the throttle open during the compression tests. Prop it open with w bit of wood in the throttle stop.

Prepare to be disappointed with you scope endeavors.

Steve

[blaze_away]

Thanks for the tip about using WOT (wide open throttle)
What is to be expected to be disappointing about the scope ?

[QBee]

Boosted LS1 said:

180 psi would be ball park at a guess. Look for similar readings across all cylinders give or take a few psi.

I didn't think the 450 was that high a compression engine?

[blaze_away]

Ots not a 450 its a 400 fitted with a Dulford built 4.6

[QBee]

Thanks for clarifying

[phazed]

It is very difficult to make out any detail with one of these scopes.

It will infect tell you if there is a piston in the bore.

[Hedgehopper]

Ideally, if everything was in perfect working order, shouldn't the cylinder pressure be 14.7 lbs/sq.in. x the compression ratio? [14.7 lbs/sq.in. being atmospheric pressure].
Therefore, a 9.35 cr. engine should produce 137.5 lbs/sq.ft.?

Edited by Hedgehopper on Sunday 10th March 20:16

Edited by Hedgehopper on Sunday 10th March 23:34

[Chimp871]

Besides Wot make sure the engine is warmed up too.

[Boosted LS1]

Hedgehopper said:

Ideally, if everything was in perfect working order, shouldn't the cylinder pressure be 14.7 lbs/sq.in. x the compression ratio? [14.7 lbs/sq.in. being atmospheric pressure].

Something seems way wrong with that. Perhaps it's how hard you do the squashing?

[Boosted LS1]

QBee said:

Boosted LS1 said:

180 psi would be ball park at a guess. Look for similar readings across all cylinders give or take a few psi.

I didn't think the 450 was that high a compression engine?

Crikey how low can it be? I was being generous .

[ianwayne]

Hedgehopper said:

Ideally, if everything was in perfect working order, shouldn't the cylinder pressure be 14.7 lbs/sq.in. x the compression ratio? [14.7 lbs/sq.in. being atmospheric pressure].
Therefore, a 9.35 cr. engine should produce 137.5 lbs/sq.ft.?

It's been many years since I did a compression test but don't you crank the engine until the max compression reading is met?

Your figure would be for one cycle I think, and the idea is to keep compressing until it reaches a point where air begins to escape past the rings / valve seats. i.e. The max retained compression for when fuel ignites to produce the power.

[Classic Chim]

Keep turning it over until no higher reading can be obtained. Clearly you need well oiled bores so warm engine should suffice and a strong battery.
Can’t remember now but is it worth removing spark plugs to increase engine cranking speed?
I’d disconnect fuel pump too personally.

I think the idea is to find the one with most compression and hope the others are close.

[Boosted LS1]

I do the cranking with the throttle wide open and count 5 or 6 seconds as I do each cylinder. Obviously you need a fully charged battery to have a healthy cranking speed.

[Hedgehopper]

ianwayne said:

Your figure would be for one cycle I think, and the idea is to keep compressing until it reaches a point where air begins to escape past the rings / valve seats. i.e. The max retained compression for when fuel ignites to produce the power.

You can't keep compressing it because after every compression stroke the exhaust valve opens and lets the pressure out.

[rev-erend]

Don't know about you but I disconnect the coil and fuel at the injectors too..

[ianwayne]

Hedgehopper said:

You can't keep compressing it because after every compression stroke the exhaust valve opens and lets the pressure out.

Yes of course. That sounds right; but the needle on compression gauges tends to flick up at first crank, and then creep up a bit higher after a few cycles, so why is this?