428 compression

[Jakobs67gt500]

Hi - good morning

I have pull my 67gt500 engine out to change crankshaft bearings.

We did a compression test to see if any work was required further up.
The result attached, between 175 and 210.
Does this look ok or is work required?

Any input is much appreciated.

Best regards
Jakob

[shelbymann1970]

Hi - good morning

I have pull my 67gt500 engine out to change crankshaft bearings.

We did a compression test to see if any work was required further up.
The result attached, between 175 and 210.
Does this look ok or is work required?

Any input is much appreciated.

Best regards
Jakob

I have been told a healthy engine should have no more than a 10 percent variance. With that said my 351W is a little over that variance on one cylinder only  and that was over 3 decades ago and still runs fine by my standards.

[Cobrask8]

Agreed on the evenness. #4 seems a little down, but still healthy. If you pull the heads, look at the bore, or maybe a valve  or valve seat?

I would be leaning towards leaving it be.

[shelbymann1970]

Agreed on the evenness. #4 seems a little down, but still healthy. If you pull the heads, look at the bore, or maybe a valve  or valve seat?

I would be leaning towards leaving it be.

+1 and my engine is now 55 years old and never been rebuilt. Factory 10.75:1 compression(well not in all cylinders now  )

[JD]

rotated...

[Royce Peterson]

Hi - good morning

I have pull my 67gt500 engine out to change crankshaft bearings.

We did a compression test to see if any work was required further up.
The result attached, between 175 and 210.
Does this look ok or is work required?

Any input is much appreciated.

Best regards
Jakob

Looks like the RH cylinder head could benefit from a valve job. I would pull both heads and do them. Cold cranking pressure test is not the same as calculating compression ratio. Neither one has anything to do with bearings.

[Bob Gaines]

Hi - good morning

I have pull my 67gt500 engine out to change crankshaft bearings.

We did a compression test to see if any work was required further up.
The result attached, between 175 and 210.
Does this look ok or is work required?

Any input is much appreciated.

Best regards
Jakob

Looks like the RH cylinder head could benefit from a valve job. I would pull both heads and do them. Cold cranking pressure test is not the same as calculating compression ratio. Neither one has anything to do with bearings.

+1. On a big block with the engine out is the best time to pull the heads. It is better to be preemptive and do it now rather then wait and determine later they need done. Head removal is so much easier to do everything needed with the engine out of the car compared to in the car.

[FL SAAC]

Hi - good morning

I have pull my 67gt500 engine out to change crankshaft bearings.

We did a compression test to see if any work was required further up.
The result attached, between 175 and 210.
Does this look ok or is work required?

Any input is much appreciated.

Best regards
Jakob

If you have the engine out, now is the time to go through the motor completely.

The last thing you want to do is fix one thing or just put a band aid on it for now and have to pull out that behemoth of a motor out again.

Ask me how I know, pay me know or pay me later....

picture of motor (tight fit) installed for illustration purposes only

[shelbydoug]

It is entirely possible that I am wrong on this BUT I am not a novice at all.

The DESIRED compression readings should be in the 125 to 135 psi range.

The readings you are posting are WAY TOO HIGH. They are APPROACHING (but not quite there) what you would expect to see in a diesel engine.

Diesel engines work without spark. They just compress the fuel with high pressure until it explodes.


The largest component in high readings like yours is the camshaft timing. I would say that it does not have enough overlap, i.e., the intake and exhaust open at the same time to permit the compression to bleed out.

All that you are going to get with those numbers that you have is an engine that can not run on pump gas as we know it and an engine that is difficult to turn over hot on a restart.


As I said, I may be completely wrong here on this but I don't think so. I've gone through this so much more then just a few times myself.

In any case, that engine is showing good compression and more then likely could use a nice "valve job" and new valve train components and those procedures will balance out the compression to a great deal.

[shelbymann1970]

It is entirely possible that I am wrong on this BUT I am not a novice at all.

The DESIRED compression readings should be in the 125 to 135 psi range.

The readings you are posting are WAY TOO HIGH. They are APPROACHING (but not quite there) what you would expect to see in a diesel engine.

Diesel engines work without spark. They just compress the fuel with high pressure until it explodes.


The largest component in high readings like yours is the camshaft timing. I would say that it does not have enough overlap, i.e., the intake and exhaust open at the same time to permit the compression to bleed out.

All that you are going to get with those numbers that you have is an engine that can not run on pump gas as we know it and an engine that is difficult to turn over hot on a restart.


As I said, I may be completely wrong here on this but I don't think so. I've gone through this so much more then just a few times myself.

In any case, that engine is showing good compression and more then likely could use a nice "valve job" and new valve train components and those procedures will balance out the compression to a great deal.

Now in the pic below a guy quotes a 1968 Pontiac service manual and how to check for proper compression.

[shelbydoug]

All I can do is speak of my experience.

As I said, I may be wrong. You can verify that with my wife. She will tell you that I am always wrong.

My experience is that those readings are too high. Proceed as you will.


...oh...it isn't the static compression ratio, it is the cam timing. You blow the useless compression out through the cam timing.

[Royce Peterson]

All I can do is speak of my experience.

As I said, I may be wrong. You can verify that with my wife. She will tell you that I am always wrong.

My experience is that those readings are too high. Proceed as you will.


...oh...it isn't the static compression ratio, it is the cam timing. You blow the useless compression out through the cam timing.

I think you are clueless. FE cold cranking numbers I am familiar with - they vary greatly depending on camshaft and engine condition - but those numbers are typical. If you are saying otherwise you simply have no experience.

[shelbydoug]

All I can do is speak of my experience.

As I said, I may be wrong. You can verify that with my wife. She will tell you that I am always wrong.

My experience is that those readings are too high. Proceed as you will.


...oh...it isn't the static compression ratio, it is the cam timing. You blow the useless compression out through the cam timing.

I think you are clueless. FE cold cranking numbers I am familiar with - they vary greatly depending on camshaft and engine condition - but those numbers are typical. If you are saying otherwise you simply have no experience.

Thanks Royce. Love to you too Babe. 

[Lincoln tech]

" Valve overlap is the period during engine operation when both intake and exhaust valves are open at the same time " ==== should be fairly easy to see why it would affect compression at cranking speed.You should also do a wet test that will tell a lot also .

[Road Reptile]

Hi all involved,
Just a few points that may help. According to the shop manual "engine" section a variation of 20 psi from specified pressure is satisfactory. It also says an accurate test is done on a fully warmed up engine, and goes on to say primary  throttle plates open-5 pumping strokes-then record the highest reading. Repeat on all cylinders. Unless you have magical talent this is impossible with an engine installed in a car. The most accurate way to determine an engines condition is a leakdown tester. Knowing most people only dream of owning one they are rarely used. Knowing this engine after years of use it is normal to find worn valve guides which leads to poor valve sealing. The rocker arm geometry on/with oem parts causes the valve to wear out the guide. Worn guides lead to poor valve sealing and  if only 1 cylinder is low on compression it is the most likely cause. Best advice is open it up while it is out and inspect everything. If you are deep enough to be doing bearings you should not have any trouble finding out why compression is lower in one cylinder. Spend whatever time it takes to clean all parts and be certain to prime the oil system prior to running the engine.
Hope this is useful information. Keep us in the loop as you progress.
Regards R.R.