Cranking pressure

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BarryC

BarryC

'74 Duster 360
Joined
Jun 30, 2008
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Leitchfield, KY
What is a good cranking pressure for a low compression 360? Mine is running around 130 psi. I was wondering if that is normal or on the low side.
 
It's been over 20 yrs. since I had a stock 360 but if I remember right mine was over 150 and it had over 120,000 miles on it so yours sounds pretty low. How many miles on it and has the timing chain and gears ever been replaced? The stock timing sets were notorious for the top nylon gear coming apart causing alot of slack in the chain which retards the cam timing and will lower the cranking compression.
 
yeah, it was replaced a couple of years ago. I installed a extreme energy 268 comp cam with a comp timing chain aligned straight up. The pistons are stock replacement. It was rebuilt about 15 years ago, but only has about 20,000-25,000 miles on it. Was going to pull out this winter to clean up and was wondering if I should check into anything else.
 
BarryC said:
yeah, it was replaced a couple of years ago. I installed a extreme energy 268 comp cam with a comp timing chain aligned straight up. The pistons are stock replacement. It was rebuilt about 15 years ago, but only has about 20,000-25,000 miles on it. Was going to pull out this winter to clean up and was wondering if I should check into anything else.
Click to expand...

Ok that changes things. I just ran the figures on my compression calculator which also calculates what your compression reading will be and it came up just a little over 130 so it sounds like your ok. The bigger cam mixed with the low compression equals lower compression readings. How does it run with that cam? It seems a little big for the low compression those engines have.
 
it dont run that bad. It is lacking a little on bottom end torque, but it is better than it was with the purple shaft 268/272, .450/.455 that I did have in it. What can I do cheap to it to help it more?? I have 3.91 gears and a fairbanks 3000 converter that goes about 2600 against the brakes. Timing is at 36 total and it is all the way in at about 1200-1500 rpm.

Thanks,
Barry.
 
The low compression is the biggest hinderance for low end torque so your not going to overcome that cheap. The dynamic compression is probably in the very low 7's range. That don't make allot of torque. You might be able to run more timing since the compression is so low. I'd try adding 2 degrees at a time to see if it gets any better. What's the intake and exhaust setup?
 
There are three things that will bump compression.

1. Mill heads (If you go this way, the intake has to be milled too.)
2. Thinner head gasket.
3 As I recall, the stock 360 piston was dished. If you have stock pistons, go to a flat top. (If you go this way, check piston to valve clearance.)

Most owner built street engines, that I have seen, that are not restorations, are over carbureted. They all are crummy on the bottom end and don't wake up until 2500-3000 rpm. Low end throttle response will improve with a smaller carburetor. Use this formula to calculate ideal carb size. (CID x Max RPM)/3456 This will give you the ideal size at 100% efficiency.
The typical HP street engine goes 80-85% efficiency. Multiply the formula result by .85 to get the ideal flow needed for you application. If the number doesn't come up with anything available, round up.
 
You can advance the cam timming a couple of degrees. That is cheap and it will pump up the compression some. Not a cure-all but it will help. Intake closing is the most important event in cam timmimg and closing it earlier will 'trap' more in the cylinder for more low end torque. As has been stated before bump up the timming 2 degrees at a time until it pings and then back it off two. Good luck.

Terry
 
Forgot to mention,...when advancing cam timming you MUST check piston to valve clearance. But with low compression pistons you should be ok,..but check it anyway.
 
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