Solutions for too much compression

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Valiant360420

Valiant360420

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Hey now.
So this is a short block that I purchased from the Mopar Performance catalog about 17 years ago. I think it was advertised as “9.5 to 1” compression ratio. I put a set of rebuilt 360 heads on it and yes, I took the time to measure the head volume and deck height and all, and unfortunately found that the actual cr was really something like 9.7 to 1 at least. I put it together along with a Mopar Performance cam ( not sure of which part number, but probably a mild street Performance option) and have been driving it off and on since. Now I am finally messing around with things and trying to get the thing more dialed in, I find that if I put any timing advance in the thing more than about 3 initial, 22 total, or so, I get pinging. I just got a compression tester and measured about 180 psi , which seems to verify that my cr is a little high.
What to do..?
Bigger cam to lower cylinder pressure?
Just set this retarded timing and run it?
Try to recurve my advance to adapt to the situation?

Additional info:

Timing mark was verified with a piston stop.

I have verified that the cam is not off, by measuring the rocker movement at overlap ( 360 crank degrees from TDC )
Am using an old school strobe timing light, not dial back.

Running 91 octane Premium gas.

Holley 750 vacuume secondary with “stock” jets and such.

Lastly...
The engine actually runs pretty damb good. If I advance the timing, it really likes it off the line, but then I do get some pinging. Especially on initial stomp, so I thought the vacuume advance had something to do with it... but even with it disconnected I still have pinging.
 
Last edited:
I think you have something seriously wrong in your measurements. Either the compression ratio is much higher, or the cam is out of time, or the timing mark is wrong. You ARE running decent fuel, right? How hot does it run?
 
Valiant360420 said:
Hey now.
So this is a short block that I purchased from the Mopar Performance catalog about 17 years ago. I think it was advertised as “9.5 to 1” compression ratio. I put a set of rebuilt 360 heads on it and yes, I took the time to measure the head volume and deck height and all, and unfortunately found that the actual cr was really something like 9.7 to 1 at least. I put it together along with a Mopar Performance cam ( not sure of which part number, but probably a mild street Performance option) and have been driving it off and on since. Now I am finally messing around with things and trying to get the thing more dialed in, I find that if I put any timing advance in the thing more than about 3 initial, 22 total, or so, I get pinging. I just got a compression tester and measured about 180 psi , which seems to verify that my cr is a little high.
What to do..?
Bigger cam to lower cylinder pressure?
Just set this retarded timing and run it?
Try to recurve my advance to adapt to the situation?
Click to expand...


Did you write down all the volumes? If so, can you post them?
 
67Dart273 said:
I think you have something seriously wrong in your measurements. Either the compression ratio is much higher, or the cam is out of time, or the timing mark is wrong. You ARE running decent fuel, right? How hot does it run?
Click to expand...
I am about to try to double check the basic cam timing as well as I can without taking the front of the motor apart. Yes, premium gas only. The motor runs cool enough. Around 190 degrees. I checked the timing mark on my balancer and verified it is pretty darn close, if not dead on.
 
3 initial? Dang! 22 total?!?!

I run 18-20 initial and totals between engines varies between 32-36 depending on the head. Your cam timing sounds out of wack.
 
yellow rose said:
Did you write down all the volumes? If so, can you post them?
Click to expand...
Shoot
I can’t seem to locate my notebook right now, but I know I went over my math several times, 17 years ago, and again lately. Doesn’t mean that I didn’t screw it up of course. I will post them when I find them.
Thanks.
 
rumblefish360 said:
3 initial? Dang! 22 total?!?!

I run 18-20 initial and totals between engines varies between 32-36 depending on the head. Your cam timing sounds out of wack.
Click to expand...
That’s what I’m going to look at next. I degree’d it when I put it in, but maybe I screwed up. I used to think my timing mark was off, but I verified it is pretty close by observing the #1 piston.
 
Valiant360420 said:
Hey now.
So this is a short block that I purchased from the Mopar Performance catalog about 17 years ago. I think it was advertised as “9.5 to 1” compression ratio. I put a set of rebuilt 360 heads on it and yes, I took the time to measure the head volume and deck height and all, and unfortunately found that the actual cr was really something like 9.7 to 1 at least. I put it together along with a Mopar Performance cam ( not sure of which part number, but probably a mild street Performance option) and have been driving it off and on since. Now I am finally messing around with things and trying to get the thing more dialed in, I find that if I put any timing advance in the thing more than about 3 initial, 22 total, or so, I get pinging. I just got a compression tester and measured about 180 psi , which seems to verify that my cr is a little high.
What to do..?
Bigger cam to lower cylinder pressure?
Just set this retarded timing and run it?
Try to recurve my advance to adapt to the situation?
Click to expand...
What octane do you run?
I can run 91 octane and just get by with 185psi @25 degrees total timing, with a .030 quench distance
22 total ,180psi and open non quench, adds up. Try premium and or booster..then lock the distributor out at 22 btdc. No advance just 22 degrees 24-7..it will run better like that than with 3 btdc.
 
67Dart273 said:
I think you have something seriously wrong in your measurements. Either the compression ratio is much higher, or the cam is out of time, or the timing mark is wrong. You ARE running decent fuel, right? How hot does it run?
Click to expand...
It sounds close to what he thinks it is if the closing is early, like 45 or 48.
9.8 or so
 
Assuming all the other potential errors mentioned by others above, cam timing, distributor timing, slipped balancer, etc. turn out to be okay.......if you have open chamber heads, in other words no quench to lose, a fatter set of head gaskets could solve the problem.
 
You can get a thicker headgasket. But before I did that I'd rule out the timing and other stuff mentioned
 
Did you check no. 1 on the balancer using a piston stop?.......If the outer ring of the balancer slipped you could be way off on the timing....
 
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What does it have and what have you done in terms of tuning the carburetor?

edit:
Also - what is the timing curve (when does it start and end)?
 
MOPAROFFICIAL said:
What octane do you run?
I can run 91 octane and just get by with 185psi @25 degrees total timing, with a .030 quench distance
22 total ,180psi and open non quench, adds up. Try premium and or booster..then lock the distributor out at 22 btdc. No advance just 22 degrees 24-7..it will run better like that than with 3 btdc.
Click to expand...
Are you suggesting locking in the mechanical advance and using no vacuume advance? I am receiving my advance limiter disk thing on Friday, so I will give it a try. I previously tried to limit the dist advance to only about 10 degrees (by jimmy rigging the heavy spring from extending) and using more initial, and it did seem to run better.
 
I'm seeing a lot of red flags here..

1. WHat fuel are you running? YOu should be running premium pump fuel.

2. Are you using vacum advance?

3. WHEN does the timing come in

4. What car is this in and what is the gear ratio

5. stock or auto

6. What converter?

180 PSI is nothing crazy nor is 9.7:1.

To remedy this you can run a wider LSA than you are running.

Beyond that, you need to adress the above questions to see what the issue is and how to remedy it further.
 
#1 You don't have to high of compression.
#2 with a mild performance cam you should be around 18-22° initial and 34-36° all in...
Finding out why #2 isn't the case would be my #1 priority...
While throwing bigger head gaskets and cams would lower your compression that's not your problem. Definitely not your carburetor and likely not your distributor...
Right now I'm starting to question your timing light? Is it a dial back and you're not reading the dial?...
 
A simple 1/4" hole ($2) drilled through the top of each piston will reduce compression.
 
j par said:
#1 You don't have to high of compression.
#2 with a mild performance cam you should be around 18-22° initial and 34-36° all in...
Finding out why #2 isn't the case would be my #1 priority...
While throwing bigger head gaskets and cams would lower your compression that's not your problem. Definitely not your carburetor and likely not your distributor...
Right now I'm starting to question your timing light? Is it a dial back and you're not reading the dial?...
Click to expand...

i'd HIGHLY recommend the OP NOT use a dial back light.
 
roccodart440 said:
MSD specifically says not to use a dial back light
Click to expand...
I wonder if we are on to something?
Without knowing, i connected my dial back to a msd. Right off the hop i knew something was awry.
Then the interweb told me my dial back was useless.
 
I've mentioned this some and gone further. I do NOT generally like nor do I depend (entirely) on "dial up" lights. Mark the balancer instead. Way back in the 70-89's I sold auto parts for about 15 years. Back in those days, there was a couple of repair services for things like instrumentation and timing lights and such. I got involved with as many as 4 or 5 timing lights that were simply not accurate or dependable

I still have my old Penske light I bought new back in the 70's, and recently bought it's twin at a garage sale a few years ago

Like this one

Vintage-Sears-Penske-DC-Inductive-Timing-Light-2442115-_1.jpg
 
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