rebuilt engine low compression

[77carrerabt]

I am looking at buying a 75 911 with records indicating a rebuild less than 3K ago. The compression numbers on the engine are all between 135 - 145. If I am not mistaken, should they not be significantly higher? I thought they should be around 160 - 170 range. The pistons, cylinders and everything else where changed and the owner has documents showing all work yet the numbers seem low to me. Should I pass on this or take a chance? Thanks for any help.

[Joe Bob]

How was the test done? Warm, cold, WOT?

[jyporscheboy]

I saw where you spoke of a CIS test kit (where are these accessed from ?) I have been thinking of changing from cis to carbs on my 1973.5 911T ,, i have had cold and hot start problems as long as i have owned the car (10 yrs) thanks for your help john young jyporscheboy@aol.com

[T77911S]

is it a MM rebuild? wait, it has 3k miles on it, cant be.

how much?

[dad911]

They are fairly even, which is a good thing. If it was me, I'd do a leakdown test, a fresh engine should be under 2% across the board.

[cgarr]

Those tests don't mean a lot except that they are around the same. You can change that number by changing the cam overlap. Do you know where it's set?


Sent from my iPhone using Tapatalk

[Flat6pac]

75S is only 8,5 compression, you would be in good shape to hit 150#
Bruce

[77carrerabt]

Originally Posted by Joe Bob

How was the test done? Warm, cold, WOT?

not sure how the test was done. I am still waiting for leak down numbers

[77carrerabt]

Originally Posted by T77911S

is it a MM rebuild? wait, it has 3k miles on it, cant be.

how much?

from what I understand everything was re built with the case split etc. I do not know what a MM build is.
sorry

[77carrerabt]

Originally Posted by dad911

They are fairly even, which is a good thing. If it was me, I'd do a leakdown test, a fresh engine should be under 2% across the board.

I am still waiting for leak down numbers. If I pursue this car I will do my own leak down and compression test.

[77carrerabt]

Originally Posted by Joe Bob

How was the test done? Warm, cold, WOT?

I am not sure how the test was done.

[77carrerabt]

Originally Posted by cgarr

Those tests don't mean a lot except that they are around the same. You can change that number by changing the cam overlap. Do you know where it's set?


Sent from my iPhone using Tapatalk

I do not know where the cam over lap is set. I do know that a friend of mine had over lap that was out of spec. After doing a improper compression test and getting low numbers he reset his over lap to spec (somewhere in the middle) and performed a proper compression test his numbers did not change much.

[john walker's workshop]

145 to 150 is normal on those. readings will vary a bit between different gauges.

[T77911S]

who did the rebuild? MM=motor meister

[911_Dude]

I have a '75S with the same problem. I rebuilt the engine 10 yrs ago, but it only has about 7000 miles on it. I installed Solex cams and Webers. The cam timing was spot on when I put it together, but Im only getting about 135psi, even across all the cylinders. I didnt do a compression check until a few months ago. I was a little dissapointed, but it runs strong. Measured with engine warm/hot, at WOT. Im not sure if I need to investigate further or not. Also, Im not sure how some people get such high numbers. Shouldnt the theoretical pressure readings be approx 14 psi x Compression Ratio?

[77carrerabt]

Originally Posted by 911_Dude

I have a '75S with the same problem. I rebuilt the engine 10 yrs ago, but it only has about 7000 miles on it. I installed Solex cams and Webers. The cam timing was spot on when I put it together, but Im only getting about 135psi, even across all the cylinders. I didnt do a compression check until a few months ago. I was a little dissapointed, but it runs strong. Measured with engine warm/hot, at WOT. Im not sure if I need to investigate further or not. Also, Im not sure how some people get such high numbers. Shouldnt the theoretical pressure readings be approx 14 psi x Compression Ratio?

I think a lot of guys have this problem on their cars. My friends old 74 with no rebuild has readings of 135 - 145.
I am going to pass on this one and get another. Thanks for all the help guys. Thanks for your information also John.

[Trackrash]

I would not buy a recently rebuilt motor unless I knew what shop did the work and what piston an cylinders where used.

If the receipt says new Mahle piston and cylinders I would not hesitate to buy it. I would probably pass on anything else.

[911pcars]

Originally Posted by 77carrerabt

I am looking at buying a 75 911 with records indicating a rebuild less than 3K ago. The compression numbers on the engine are all between 135 - 145. If I am not mistaken, should they not be significantly higher? I thought they should be around 160 - 170 range. The pistons, cylinders and everything else where changed and the owner has documents showing all work yet the numbers seem low to me. Should I pass on this or take a chance? Thanks for any help.

Here are some factors that may affect the compression test results:

- Cranking speed - Make sure the battery is fully charged and all spark plugs removed. If the newly rebuilt engine is "tight", cranking speed may be lower than normal.

- Valve timing - more aggressive cams have more overlap and may require additional compression strokes to register a "normal" reading

- Altitude - Higher elevations reduce the compression pressure

- Compression ratio - Compression pressures are affected by the compression ratio. What is it?

Do not adjust the cam timing to improve the compression pressure. That's counterproductive as well as retarded (as it relates to valve timing), but you know what I mean.

Sherwood

[VFR750]

Since an "S" cam gives high end power, at the expence of low end torque, wouldn't you expect low speed compression checks to come out on the low end of the spectrum??