360 Cylinder Pressures

-
KevC

KevC

Active Member
Joined
Mar 15, 2013
Messages
40
Reaction score
36
Location
Nottingham, england
Hi All

I have a question (and I think I know the answers) but I would appreciate your opinions.
My 73 Duster 360 has a few issues I want to deal with.
1) It refuses to spin the wheels
2) Its averaging 14mpg
3) It seems to be breathing a little heavy
other than that it runs perfectly.

I know little about the motor.
It has a lumpy Comp Cam (unknown)
Edelbrock Performer Intake
Edelbrock Carb
TTI Headers
Torque Converter unknown
727 Trans
3.23 Gears

It comes on hard around 2500 rpm once moving.

We did a compression test and found 2 cylinders at 75 psi and the rest in the 80's with the 3 highest being 87 psi.
My first thoughts then is that the engine needs replacing or rebuilding.
I understand the high lift cam and low compression will rob cylinder pressure, but how much?
Could it just be that the cam is not right for the engine?

I am temped to throw a standard cam in and see.
Any thoughts greatly appreciated.
Thanks
Kev
 
did you do a wet cylinder compression test also.

After you do a cylinder "dry" then put a 2-3 squirts of oil in the cylinder and retest it to see if the results get any better. If a low cylinder increases when wet, then it is in the rings, if a low cylinder doesn't get any better, it's probably in the valves leaking.

Start with a wet AND dry compression test and go from there...
 
It's a stock motor with a large cam not building cylinder pressure.
 
What is your scale on the compression tester?
Is it in PSI?
 
Something sounds weird on that test. Been my experience that its hard to even get that low of cylinder pressure to even fire. Is there any chance that the valves are being held open slightly? Weird all cylinders are that low.
 
Hi Guys

Thanks for your comments, it does seem odd that all are that low.
I do believe that unknown lumpy cam could be part of the problem, but I'll redo the test with the squirt of oil and let you know the results.

Thanks again.

Kev
 
The unknown lumpy cam IS the problem. A cylinder will fire all the way down to about 40 PSI as long as it will HOLD compression. you will never feel a skip, because there won't be one.

Static cylinder pressure is really a moot point, other than to be used as a basis for comparison to other cylinders. People just don't understand that. When a cylinder fires, compression pressure goes WAY beyond the static compression read on the compression gauge.

As long as there's enough static compression for the mixture to light off, the rest is history. You will of course notice a weak engine, but a smooth running one as long as the cylinder isn't dead.

I surmise that if you either advance the camshaft timing or re-cam it and degree the new cam, it will run like a completely different engine. The camshaft now is the entire problem.

In stock form, these engines have somewhere under 8:1 compression. They probably have around 125 PSI in stock form in good condition. Couple that with a large camshaft that has a late intake valve closing event that does not allow good cylinder pressure to be built and the result is what you have.

The book gives a 10% variation between cylinders. You are right around that threshold. Consistency is what counts in a compression test. Although it's nice to see high numbers, what you've recorded indicates your engine is probably in good physical condition for what it is. It needs some changing around to become something that delivers better performance.

My advice would be something like the stock 340 cam, or maybe a 256 Lunati VooDoo grind. Anything bigger IMO will be too big, unless you do something to increase your compression ratio. With a better matched cam and properly degreed, that thing will probably run like a fire is lit under it. It is simply suffering from mismatched parts.
 
Thanks RusrtRatRod for a brilliant explanation.

I will re-cam with something closer to stock and let you know how it goes.

Thanks again

Kev
 
Hi Everyone and Happy New Year!

Just thought I would post an update now I have started the cam swop.
The cam that I have removed is a Comp Cams item from the Thumpr series, part number: 20-600-4, with an Intake duration of 279 and an Exhaust duration of 296, and a rev range of 2000-5800, which confirms this (coupled with the low compression of the 360) is the cause of my problems.
Looking forward to getting the new Xtreme Energy cam installed in the next couple of weeks, and then I will update you again, hopefully with my first tyre frying stories!!

Cheers
Kev
 
Which one did you choose? Just cause it has a snappy name won't make it run good.
 
KevC said:
Hi Everyone and Happy New Year!

Just thought I would post an update now I have started the cam swop.
The cam that I have removed is a Comp Cams item from the Thumpr series, part number: 20-600-4, with an Intake duration of 279 and an Exhaust duration of 296, and a rev range of 2000-5800, which confirms this (coupled with the low compression of the 360) is the cause of my problems.
Looking forward to getting the new Xtreme Energy cam installed in the next couple of weeks, and then I will update you again, hopefully with my first tyre frying stories!!

Cheers
Kev
Click to expand...

I'll throw a diagree in here the cam IS NOT the problem. Maybe the install of the cam is the issue, ICL, or a crummy ignition timing set up.

Where was the cam ICL installed and what was the intial timing and total numbers?
 
When doing the compression test, is the throttle all the way open?
 
Whether the throttle is open or closed is not a deal breaker. Sure, it might make a difference, but what's important is to make sure you d them all the same. Whether open or closed. I've done it both ways and noticed no difference at all.
 
RustyRatRod said:
Whether the throttle is open or closed is not a deal breaker. Sure, it might make a difference, but what's important is to make sure you d them all the same. Whether open or closed. I've done it both ways and noticed no difference at all.
Click to expand...

me too...three and four
 
May I chuck in my two bobs worth. I have just complete my first Mopar small block build in 40 years a qualified mechanic. My 360 had standard low comp dished top pistons .125" down the bore - that is 1/8"! That plus open chamber heads is why you have no compression to speak of. I have now installed KB190 pistons that have a small dome and a higher pin position and with .027" copper gaskets I have 165 PSI cranking dry. This is with a meaty ST 21 Racer Brown solid camshaft.
Cheers
 
You must log in or register to reply here.
-

Back
Top