Low compression & stumped

Okay, I'm flummoxed. 67 BGT with rebuilt motor w/600 miles or close to it. Has a Crane 270 cam (valve lash set at Crane specs)and properly broken in (using EOS and Castrol). Oil pressure about 75 when at speed and no overheating issues....new "Eurospec" dist.
I took it to a local MGologist for some tweaking and he tells me he did a compression test and he is getting 75lbs. across the board. Did a "leakdown" test and no problems. I had told him I thought it was a bit doggish at low rpm but once at higher revs it seemed okay.
Short story long: my engine builder did admit to not using an offset key when installing the cam so I know it is at least a few degrees off.... My MGologist says the builder probably used wrong pistons or connecting rods and to take the motor apart and find the cause of the low compression.
I'm thinking he should try and determine if the cam is set right and to go from there...who knows, the cam could be off a tooth or two and affecting compression.
I'm going to assume his compression gauge is spot on....soooo, what could it be?
I'm not up for this chore but if I have to "yank it", won't be total waste of time as I painted ny engine compartment black (yellow car) so I can correct that sin.

Pooch

My Crane cam is running at 2 degress with an offset key. But
I do not see how even 6-8 degress could make enough difference on rotation to cause a dropped compression reading if the cam is close enough to let the engine run.

Deep dished pistions? Possibly. Stroke length? Also possibly.

How about re-testing with a different guage?

My Crane cam has a 4 degree key. You can lose a LOT of compression to improper valve timing, but I'd never suspect THAT much! How does it run?

600 miles is a little early to be making judgments on compression figures, i think.

It should have more than 75 psi compression immediately after a rebuild. Are you sure it's a Crane cam? I ask because they don't list a 270 cam, it sounds more like a number Piper uses. Being off the amount of an offset key will not cause compression that low.

You can get a close check on cam timing without tearing the engine down. Set number one cylinder to TDC compression stroke. Adjust the intake valve (valve #2 from the front) to .055" Turn the engine until the intake valve is beginning to open. The pulley timing mark should line up with the 10 degree pointer on the timing cover. This is correct for a stock cam but should be close for an aftermarket cam. If the timing is off a tooth on the crank gear the marks should be off considerably more than 10 degrees.

This procedure is described in the MGB shop manual or the Bentley manual copy. on page A.9.

Clifton

I would verify the compression readings myself. For example, were they taken with throttle wide open? If the throttle is closed, there is going to be less in the cylinder air to compress.

I'm not even sure that having the wrong pistons would cause that much of a drop in compression. I would be suspicious that the timing is off by a tooth. If the power is good at the high RPM (but not at low RPM) then this would suggest that the timing is retarded. Part of that may be due to the use of the standard key. The popular Crane street performance cam calls for 5 degrees of advance (a 2.5 degree key) if I recall. But that will only move the power band by a few hundred RPM. It would take being off by a tooth to make this much difference (I suspect).

However - and this may be it - if somebody really did use a 270 degree duration camshaft then you will require a much higher geometric compression ratio to get decent compression than you will with a milder camshaft. That is, normal pistons won't provide enough compression for a true racing camshaft. A real racing camshaft is also likely to be pretty sluggish below 3000 RPM. Most street camshafts are around 220 to 230 degrees from what I recall. I think 214 is the stock camshaft so 270 is pretty hot.

Where is Hap? - he will know what GCR is appropriate for a 270 duration camshaft. My guess is that it is about 12:1. (For comparison, the Crane street cam recommends 9.0 to 10.75:1 compression ratio for their street cam).

Finally, if this is really a 270 duration camshaft (specified at 0.05" lifter rise) then I don't think this is going to be a fun street engine.

First question would be is, was the cam degreed, if it was installed straight up, it should still run ok, just not making the most use of the cam, however it very easy to miss it a link of the chain and if so, that's like 16 degrees off, so even when just lining up the dots, the cam should still be degreed to confrim you are ok. On my average street rebuilds I use mostly the APT VP 12 cams advanced like three degrees and a deck the head .020", then normally puts a smile on the owner's face.

If deep dished pistons are used (18V) with an early head (Pre 18v with large combustion chambers) you will get low compression. Inversely if you used early pistons (pre 18V - flat top pistons) and the 18v cylinder head the compression would be higher to the point of pinging problems. So what pistons / block are being used with what head??

I'd do another compression test before jumping to conclusions.

Wiley typed "If deep dished pistons are used (18V) with an early head (Pre 18v with large combustion chambers) you will get low compression."

You will have a lower compression ratio, but not a significantly lower pressure reading on a compression tester.

Check for proper valve lash settings per the cam card or look up the cam or call Crane.

Recheck compresion readings doing proper procedure and no leaking at the plug hole. use the right reach tester end and o-ring.

Establish true TDC and compare it to the timing mark. (Has the crank dampener ring moved?).Then retime the motor at 32* TOTAL at around 3500 rpm or at full centrifugial advance.

You might get on the the engine assembler or look at your bill of materials on the work order and see exactly what pistons were ordered and used as well as the cam.

Road test the car.

Check valve events by establishing true TDC and noting intake opening BTDC and comparing with cam card or cam figures.

If all fails get the engine assembler to find the problem.

Thank you all for all the various scenarios......couple of things: I'm going to do a re-test of the compression using a different gauge and have my MGologist check the the cam timing and go from there. I can't go back to the engine assembler as he is very ill and for all intents and purposes, has put his wrenches down.
Once these results come back, I'll check back in....but, as questioned earlier, I do know it is a Crane cam as I bought it new from them.
The winters for the most part are real long here in New England and there is heat in the garage....all I need is time should the diagnosis be a tear down.

Stand by

I d like to know the answer to Jeff's question, how does it run? Specificall what RPM is it doggish?

Keith asked how does it run? It fires right up and runs fine....albeit as I said earlier "doggish" until higher rpms. I haven't driven it in a few weeks but I'd say it is "doggish" up to about 3000 -3200 rpm. After that, it seems to be fine....
My engine builder has years of experience (raced Lotuses and all sorts of Brit iron) and did a fantastic job on my Lotus Cortina rebuild. I would hope he would have caught any wrong pistons, rods etc going into the lump but he is very ill and looking back on it and knowing what I know now, he was not well when he did the work so anything is possible concerning something being overlooked. It is what it is and all I can do is work through it.

Pooch

You would think that if all the compressions were down to 75lb the car would be hard to start as 70lb usually shows up as a miss in an otherwise normal compression motor.
They are probably low but I would like to see a retest and if a little higher it would make solving the problem a little easier.