Compression test

[S2Mick]

Morning all, quick question re a compression test.
The test was conducted on a cold 450 engine with 75k on the clock and unknown service history.
To be fair, there is evidence of a good service regime in that the oil was relatively new and the coolant was certainly fresh.
I'm getting 155 on cylinder 1 with a gradual reduction in the reading as i go toward the back of the block.
readings were as follows

1 - 155
2 - 145
3 - 150
4 - 132
5 - 145
6 - 138
7 - 135
8 - 130

Before I start automatically going down the road of cam replacement etc, does anyone know what this could be indicative of ?
My guess is that the front and back of the engine have run at slightly different temperatures meaning the rearmost valves could be sealing less effectively ?
The plugs were very sooty when I removed them although the car has been pretty much stationary for the last 6 months with only brief start ups to move short distances so no decent 'thrashes' for a wee while
Engine coming out anyway to check clutch status and do a tidy up but before I rag the thing apart, does anyone have any idea what could be causing these numbers ?
Oh, I should add that the manifold has been removed from the port side but not the starboard side (don't ask) so I'm not sure how this may affect the readings but the starboard side (2,4,6 & 8) does seem generally weaker.
Thanks

[S2Mick]

Sorry, quick update, I should add that after testing 1 - 8 I then re tested 1 and got 155 again so the reduction in test values couldn't be attributed to the oil starting to move better.
Thanks, Mick

[pmc_3]

Did you do the test with the throttle wide open?

[Belle427]

Id expect to see around 160 psi across all cylinders, i think the general rule of thumb is that they are within 10% of each other.
Maybe worth taking the car for a good run and perform the test again.
Does the car run and perform ok? Thats what id be more concerned about.
A leak down test may tell you more but it could be a worn cam, poorly seated valves etc, id doubt its the rings.
If your pulling the engine it makes sense to refresh it in my opinion but that will depend on costs.

[Englishman]

As others have said, I'd give it a run and measure again as doesn't look too bad to me.

FYI, these were the measurements on my SEAC engine before rebuild needing liners and new piston rings etc.:
- Cylinders 1 & 2 ~ 30psi
- Cylinders 3 & 4 ~ 85 psi
- Cylinders 5 & 6 - 130 to 150psi
- Cylinders 7 & 8 - 115 psi

[S2Mick]

pmc_3 said:

Did you do the test with the throttle wide open?

Unfortunately the engine is already half torn down so running not an option.
Fuel pump disconnected, exhaust, Y piece and one manifold off.
Just thought I'd do a compression test before I took the starter motor off to get access to the starboard side lower manifold bolts.

[S2Mick]

Belle427 said:

Id expect to see around 160 psi across all cylinders, i think the general rule of thumb is that they are within 10% of each other.
Maybe worth taking the car for a good run and perform the test again.
Does the car run and perform ok? Thats what id be more concerned about.
A leak down test may tell you more but it could be a worn cam, poorly seated valves etc, id doubt its the rings.
If your pulling the engine it makes sense to refresh it in my opinion but that will depend on costs.

Thanks for the reply and unfortunately I'm no longer in a position to start the car.
I drove it briefly and also started her in the garage a couple of times before starting the tear down and she runs pretty much rattle free so I'm probably going to look at the valves first.
I'll be keeping her for the duration so it'll cost what it costs, need to do the job right but valves are my first port of call.
Thanks again for interest, Mick

[S2Mick]

Englishman said:

As others have said, I'd give it a run and measure again as doesn't look too bad to me.

FYI, these were the measurements on my SEAC engine before rebuild needing liners and new piston rings etc.:
- Cylinders 1 & 2 ~ 30psi
- Cylinders 3 & 4 ~ 85 psi
- Cylinders 5 & 6 - 130 to 150psi
- Cylinders 7 & 8 - 115 psi

Ugh....some grim looking numbers there, thanks for the interest and how's the patient now ?

[Englishman]

S2Mick said:

Ugh....some grim looking numbers there, thanks for the interest and how's the patient now ?

Had the engine rebuilt by Dom during the Covid lockdown as he built it originally! He reported 'wash past the piston rings' was the main issue and that the pistons, cam etc. were all reusable. He also mentioned the early 420 SEAC engines (proper ones in his words!) were all individually made, and this one (a) had never been apart before and (b) had all the performance parts fitted including Cosworth pistons, solid lifters, big valves and a 214 cam. He said the only difficultly was getting the bespoke piston rings. However he contacted the person that made the originals over 30 years ago who made some more for him!

The patient was measured as producing 316bhp by Surrey Rolling Road in 2021 which makes it quite rapid

[Belle427]

Regarding throttle wide open for compression testing i was under the impression id didnt matter. This seems to confirm that and hes a fairly well respected person.
https://youtube.com/shorts/zLCgTbSx60I?si=XNu3lAl-...

[lwt]

A leak down test is really the only way to find out if its valves or rings. Mine had a couple of broken rings, the main symptom was burning oil which misted over the rear end and lights. As it got worse a blue haze appeared under acceleration. One day the oil caused one catalytic converter to disintegrate (a custom system built by Janspeed with twin racing cats) with a spectacular bang and cloud of smoke, followed by a limp home as one exhaust was now blocked by an unravelled converter (think giant pan scourer). Engine rebuilt by Rob at V8D (though it did take an age…) and used as an opportunity for a Steath cam and Stage 4 heads, finished with an Emerald ECU by Jools at Kits’n’Classics. Result 323.6 bhp and 345 ftlbs and a big grin…