52 replies to this topic

[Johno]

I’ve not done a compression test on a car before and just want to know what I need to do with fuel system while doing the test.

I know you have to disconnect coil but do I also need to put my foot to the floor on the accelerator while cranking?

Do I need to also remove any fuel system fuses?

Motor is 355 with edelbrock 600.

[S pack]

Compression test done with engine at normal operating temp. Remove all spark plugs, remove dist cap and leads, I disconnect power to the coil and ensure the connector is insulated from grounding on anything.

Install compression tester starting with cylinder # 1 and crank for approx 5 - 10 seconds with wide open throttle, record the reading on the gauge and move onto the next cylinder and so on until finished.

 

From memory the cylinder pressure readings should all be within 20 or 25psi of each other.

If one or more cylinders are way down on compression you can squirt a few drops of oil into the cylinder, crank it over a few times to distribute the oil around the cylinder and repeat the compression test on that cylinder.

If the pressure has increased then the oil is showing the compression rings are worn. If the reading stays the same then that cylinder may have a leaky valve or head gasket.

Edited by S pack, 03 April 2023 - 08:55 PM.

[RallyRed]

Pull elect fuel pump fuse..if it is not old school mech. pump?

[warrenm]

Yes, you do need to hold the throttle wide open.

[Johno]

It’s a mechanical fuel pump.

What’s the reason for putting the throttle to floor when doing this test?

[RallyRed]

To let in the air that you are trying to compress.

[Shiney005]

, I disconnect power to the coil and ensure the connector is insulated from grounding on anything.

This is very important. I have a mate who got burned by having an errant spark ignite the fuel mixture that was still being drawn into another cylinder and blown out the plug hole.

[N/A-PWR]

Standard Compression is 149PSI

 and Performance Compression is upwards to 250PSI.

 

Edit:

At the higher end Compression,

 you will need High Octane Fuel, like the Old days 100 Octane Super.

Edited by N/A-PWR, 04 April 2023 - 04:20 PM.

[Dr Terry]

Standard Compression is 149PSI

 and Performance Compression is upwards to 250PSI.

In all my years working on cars of all types I've never seen as high as 250 psi compression out of a petrol engine.

 

"Performance" engines have can a high 'static' compression, but because they usually have a long duration cam, their cranking compression is quite often not much higher than a stock engine.

 

We had a 13:1 SBC drag engine with a 300+ roller camshaft & it's cranking comp was around 180 psi. Racecar engine builders usually use a leak down tester to test how good the valve & piston sealing is.'

 

Dr Terry

[N/A-PWR]

In all my years working on cars of all types I've never seen as high as 250 psi compression out of a petrol engine.

 

"Performance" engines have can a high 'static' compression, but because they usually have a long duration cam, their cranking compression is quite often not much higher than a stock engine.

 

We had a 13:1 SBC drag engine with a 300+ roller camshaft & it's cranking comp was around 180 psi. Racecar engine builders usually use a leak down tester to test how good the valve & piston sealing is.'

 

Dr Terry

Hi Dr Terry,

 I have achieved this 250PSI myself,

 as is why I stated it. 

using a 208CI with 100 thou shaved head, possibly flat top pistons and a 30/70 cam. Regards

[ozyozyozy]

Compression test is ok but a leak down test is far better.
Can work out were or if it’s losing seal of individual components. If i really want to see what’s going on with an engine, I do a leak down.
Google a video and check them out.

[Dr Terry]

Hi Dr Terry,

 I have achieved this 250PSI myself,

 as is why I stated it. 

using a 208CI with 100 thou shaved head, possibly flat top pistons and a 30/70 cam. Regards

I'm not saying that it's not achievable, I've just never seen it personally. You would need a very high static comp ratio with a relatively small cam.

 

With a 208ci with 100 thou off & flat tops, your compression ratio must have been 13:1 plus, so a 30/70 cam is quite small for that combo.

 

Dr Terry

[Uncle Chop Chop]

I'm with the doctor here. 250 psi is way out from what I would expect from a compression check. Has the guage been dropped?

[Johno]

Thanks all.

So I’ll disconnect the distributor, put the throttle wide open and crank a few times.

Simple as that?

[N/A-PWR]

Yes John,

and watch the gauge increase in pressure,

 then stop cranking over engine,

when the gauge stops gaining pressure.

 

You will see the pressure pump-up quickly and slow down.

That's it. 

 

Do after engine has been running. 

[Johno]

Did the test and all cylinders were from 110 - 130 PSI

[Johno]

I got the following compression test results will all plugs out and throttle wide open. 355 stroker with fairly lumpy cam. What do you make of this? I’m happy there’s no one cylinder that’s really low and all are within a reasonable range.

Drivers side:
120
110
120
120

Passenger side:
120
110
110
130

I noticed the compression tester looked like it couldn’t screw in all the way to the rubber o ring to seal it. Not sure how to get around that as I’m only meant to screw it in finger tight.

[Heath]

Normally, a motor that has consistent compression between the cylinders is in good condition, and I've never paid all that much attention to the peak pressures - just the relative readings. BUT those figures are lower than I would expect.

 

If it's a 355 it obviously isn't some ridgey didge 50 year old Holden motor... do you know what static compression ratio of the motor?

If it has a giant camshaft, that could bleed off a fair bit of compression at idle, but these readings do seem concerningly low.

 

What was the reason for doing the compression test in the first place?

 

(Also, I don't believe you need to have the throttle wide open when doing this test as some others have said. As long as you do the test with the throttle in the same positon for each tested cylinder, that is the main thing. Throttle position and cranking speed can both influence the peak readings, and I would imagine engine temperature etc. may play a part as well, but none of those things matter when looking for consistency)

[Johno]

Thanks Heath, I did the test initially because the car seems to idle a bit rough. Previous owner said it lacked power. I don’t know the static compression ratio of the motor. I estimate it may be a 20 year old engine build.

It seems to have a vac leak that I can’t find yet.

The cam is pretty lumpy but probably nowhere as big as what it could be.

The 130 reading was taken while motor was warm, the rest the motor cooled a lot. Cylinders 4 - 8 were done cold.

I’m happy to run around on this motor if it’s just tired and keeps going for a few years while continue the rebuild.

Edited by Johno, 11 April 2023 - 11:15 AM.

[rexy]

Maybe it was a low comp build for forced induction down the track?

 

If it goes ok without any red flags then as you said, just keep driving it. As soon as you pull the engine and take it to someone its probably gonna cost you more than 10k to get it back.

[yel327]

If it's a proper Holden built 350 those are getting close to 30 years old now since they were first made, and they were only about 8.5:1 anyway. People have been building Holden 308 to 350 strokers a lot longer than Holden did though, back in the late 70's and early 80's they were doing it using modified 3.48" stroke (350) SBC crankshafts with a modified thrust arrangement and a 3/4" thick spacer at the back of the block, long before locals started to cast cranks with 3.48" stroke to suit the 308 blocks. SBC 377 pistons for the 308-350 stroker have been around since Chevrolet built their first 377 strokers in 1962-3 for the 1963 Corvette Grand Sports using 327 SJ blocks. So even though it might just be a recent build with some miles on it, it may also be near 50 years old. It may also be low compression if the builder just used stock 377 dished pistons and didn't deck it, it may be as low as 8:1 or lower.

Edited by yel327, 11 April 2023 - 05:18 PM.

[Johno]

I actually don’t know how it drives because it’s not registered. I’m currently just trying to get it to idle and rev without any hesitation. It seems to idle ok but has a weird exhaust leak type sound (sounds like vw beetle a bit when you rev it)

The next thing to do is check the timing and try and figure out where the apparent vac leak is coming from (vacuum is 11 in gf and it should be closer to 20). And check the exhaust from extractors all the way to the muffler.

[yel327]

What carby is on it? Or is it EFI?

Edited by yel327, 11 April 2023 - 08:00 PM.

[Johno]

Edelbrock 600.

It’s apparently pretty new but I’ll definitely rebuild it I suspect it’s part of the problem.

[yel327]

About $350 or thereabouts for a full rebuild. Worth every cent.