Surprising piston compression test results on my mystery 340

[Loa]

Hello,

The 74 Charger (I know this board is for cudas and challengers, but the engines are the same!) I bought last year is a mishmash of many cars that have probably been wrecked. The front is a 74, the back is a 73 and the engine has a 72 stamp on the block.

When I did a compression test on each cylinder to check if they were ok, I was surprised: I got 4 at 155lbs, 2 at 150lbs and 2 at 160lbs.

The information I had was that values above 150 meant higher compression ratios more in line with pre-1972 340s.

Is it possible that I have a 1972 block but someone switched the heads?

Thanks.

[xtopfuel]

Cranking pressure has nothing to do with static pressure
You have cranking compression and static compression, there is no way to determine CR through compression test

[Beekeeper]

On the plus side, you have consistent numbers between cylinders which indicates things are healthy. Clean it up and run it. I never get that lucky. I usually get motors that require total rebuilds. Haha

[734406pk]

On the plus side, you have consistent numbers between cylinders which indicates things are healthy. Clean it up and run it. I never get that lucky. I usually get motors that require total rebuilds. Haha

  And these are "engine cold" compression readings? Not Bad!

[Loa]

Yes, the engine was cold.

I'm glad they're all good, but I was surprised that they were so high.

[734406pk]

Do you know what camshaft is installed in this engine by chance?

[1 Wild R/T]

You can see 120 PSI with an 11-1 engine or you can see 150 with a 8.5-1 engine... Camshaft duration & especially intake closing point have a huge effect on cylinder pressure....  Can't build pressure if there's an escape path for the air........

[Beekeeper]

Get that baby running nice and open it up on the road. You'll get a real good idea if it's a dog or not. If you're plan is to rebuild it before getting it on the road, then I wouldn't put too much effort into figuring out what components you have. Easier to do that as you tear it down in the garage.

[Loa]

I don't know what the cam shaft is: with this car it could be anything.

I'm not planning on a full rebuild.

It runs well on the road, but I only have a Challenger with a 340 from 1970 using 3.73 gears to compare it with. Given that the charger has a 3.23 gear in the rear, it's hard to compare the two cars. 

Then again, the Challenger seems to have instant power while the Charger only comes alive at around 3000 rpms. It won't spin the wheels at all while the Challenger does it easy.

That could be the diff or the engine. Not sure.

[chargerdon]

I have a rebuilt 360.   It has stock rom 76 heads...and a mopar purple 508 cam.. (292 adv duration).   A cold compression test shows 168-172 in every cylinder...   Thats cold...  i think cold actually will show higher compression than hot.. 

It runs very strong.

[734406pk]

I don't know what the cam shaft is: with this car it could be anything.

I'm not planning on a full rebuild.

It runs well on the road, but I only have a Challenger with a 340 from 1970 using 3.73 gears to compare it with. Given that the charger has a 3.23 gear in the rear, it's hard to compare the two cars. 

Then again, the Challenger seems to have instant power while the Charger only comes alive at around 3000 rpms. It won't spin the wheels at all while the Challenger does it easy.

That could be the diff or the engine. Not sure.

The Charger is heavier than the Challenger. If you are happy with the performance of the 340 in the B body, then enjoy!

[Loa]

If you are happy with the performance of the 340 in the B body, then enjoy!

I will, but I'd rather make sure that I'm getting all that it's capable of doing now, rather than waiting a few years until some good mechanic tells me that the engine/carb wasn't working well.

[734406pk]

Do you know what the stall speed is on your current converter installed on the Charger? (Foot on the brake and full throttle, try not to do a burn out, read the rpm) It may be quite low on a '74.