compression test results.

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AdamR

AdamR

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I ran a compression test on my 340 this morning. Engine at operating temp, all plugs removed, Throttle wired wide open.

1 130psi
2 135psi
3 135psi
4 130psi
5 130psi
6 135psi
7 135psi
8 145psi

This is a 72 dated 340 short block. However it has a steel crank balancer on it and all my converters have been unbalanced so Im guessing it has a steel crank in it. Im not sure if early 72, 340s came with the forged crank or if they were all cast but I think the bottom end is all stock right down to the 8.5:1 pistons and stock cam.

Im also running a set of 4323345 casting heads wich were used on 1985 318 4bbls and 3604bbl police cars. I have read that the combustion chambers have been measured anywere from 74ccs to 80ccs wich is freekin huge.

Also all the plugs were clean except for 1,3 and 5 wich were black. Not sure if its oil or a jetting.

So what do you think ? Pull it, replace oil pump and check bearings ?
 
Those compression numbers aren't too bad especially if your running thoses huge chambers. The comp. ratio is probably somewhere around 7.5 to 1. As far as the plugs Is it a sooty black or oilly feeling? Why are thinking about replacing the oil pump and checking the bearings? Oil pressure problem? I haven't had much experience with 340's but I thought all from 71 up were cast crank.
 
Seems like the compression is good.
I would put a mechanical oil pressure gauge on just to check it against the electric one.
Then go from there tearing into the engine.
 
Thats on the list to do. Bolth my oil pressure and water temp guages are electric. I'll be changing them to mech. The motors coming out regardless of the out come. Gaskets are leaking, Timing chaim is sloppy. She needs some attention.
 
If you put the oil gauge in the car, remember to throw the plastic tubing that comes with them away. copper or some braided line will work, espesially if you plan on taking it to the track once in a while.
I would hate to see one of those lines break inside the car.
 
Rory had that happen. Hose broke and pour oil all over the inside of the car. Will that plastic crap pass tech even ?
 
those #s are pretty good... just about how my 318s were... i would pull it, make sure the wireing is good while the motors out, redo the gaskets, timing chain, oil pump and bearings. if the rest looks good slam it back together and in and go have fun...
 
Thats pretty much the plan. I have a set of heads for it too. I was told the ones I'm running stink. Not really anyworse then anyother 1.88 valve heads stock but they dont gain much from porting.

Its also going to get a new cam and a deep oil pan.
 
AdamR said:
Rory had that happen. Hose broke and pour oil all over the inside of the car. Will that plastic crap pass tech even ?
Click to expand...
Nope, Won't pass tech.
I can't see how they can even sell that stuff. Can you imagine getting sprayed with 200deg. oil while driving down the road? :ncool:
 
All 72 340's had a forged crank. The only difference between 71 and 72 340's was 72's had a smaller intake valve (1.88 vs 2.02) and the pistons were changed for lower compression. In 73 the cast cranks came out. Some early 73's still had the steel crank, though. Your compression numbers really don't seem all that bad. probably needs bearings.

don
 
I figure compression with these heads and a standard FelPro gasket (.039" thickness) is 7.95:1 at best. That explanes why the car has always been pretty soggy down low.

The Magnum heads and a thinner .025 gasket should put me at 9:1 wich would wake it up by itself. The trick is going to be picking the right cam so I'm not bleading off any of the pressure. I'd like to use a moderately sized solid cam. I was looking at the CompCams Magnum 296 wich is 246 @.050" but that may be a bit large.
 
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