ALX69RR440
Well-Known Member
From everything I’ve read all of the sparkplugs need to be removed to do a compression test. Does anyone know why? Could I remove one, test, put plug back in and move on to the next?
Compression Test
- Thread starter ALX69RR440
- Start date
69hemibeep
Sponge Bob Square Wheels
You can do one at a time, but its easier on the starter with them all out. I like to go all the way around the motor dry then if there are some low holes I do a wet test, so its better for me if all the plugs are out. If you do one at a time make sure to pull the coil wire.ALX69RR440 said:
A31PKG
Well-Known Member
Another reason is to obtain better data. By pulling the plugs the battery does not work quite as hard, & in turn, retains more of its power to provide more consistant results. Two more things: Be sure the battery is fully charged before you begin, and be sure to hold the accelerator wide open so the cylinders can have full & equal access to incoming air. Like Bob said be sure to disable the ignition system, easy to do by simply removing the coil wire from the center of the distributor cap and shorting it to ground with a jumper wire with alligator clips on each end.
A healthy engine should have cylinder compression readings within 10% of each other, although an engine with a 15% cylinder(s) will still likely run ok (not great, but ok). Hope this helps....
A healthy engine should have cylinder compression readings within 10% of each other, although an engine with a 15% cylinder(s) will still likely run ok (not great, but ok). Hope this helps....
ALX69RR440
Well-Known Member
Great, Thank you!
moparchris
Well-Known Member
I will throw my :cents: in here. I have a big block chevy in my Camaro that has a 40% split in between cylinders and has 60% leak down in several cylinders. It smokes a little on start up but otherwise runs perfect. It has been this way for years. The reason I say this is that if your doing a compression test because the engine has running issues then its a great tool for diagnosis, otherwise it can give false positives. I dont put much thought into doing a compression test on a healthy motor other than for reference in the future. Like they say in my proffession treat the patient not the cardiac monitor. Is it running rough?
69hemibeep
Sponge Bob Square Wheels
:soap: Dear Doctor. He was just asking how to do the test. Doctors of all people order all kinds of extra tests running up medical bills and subjecting us to radiation and chemicals, its no wonder healthcare is a mess. Umm what were we talking about :jester:moparchris said:
ALX69RR440
Well-Known Member
moparchris said:
Hi Chis,
No. It's pinging at low RPM (2500) under load. We've talked about this on this thread: viewtopic.php?f=6&t=6203
I’m just getting around to the compression test to determine if I need to use race fuel or I can get away with pump gas.
Well, I did the compression test (before I asked the question on this thread) one cylinder at the time with all of the sparkplugs in except for the one being tested and I didn’t have the throttle at WOT (I didn’t have a manual or new the firing order so I didn’t want to mess anything up)
I got these numbers:
1 = 160
2 = 160
3 = 155
4 = 160
5 = 160
6 = 165
7 = 160
8 = 165
So if these numbers are correct it looks like I should be able to use pump gas. The issue is that I’m still pinging at around 2500 RPM and if I retard the timing (12 initial) it seems to get worse.
I did this last weekend. The outside temp was in the 50’s.
I’m all in (34 total) at 2500 RPM. Is the curve wrong?
ALX69RR440
Well-Known Member
69hemibeep said:
LOL!
moparchris
Well-Known Member
:facepalm:69hemibeep said:
ALX69RR440 said:
The numbers you came up with are an excellent grouping. The numbers will be higher when you turn it over without plugs because of the increase in RPM. I let the gauge jump 4 times on each cylinder. You can do it a multitude of ways and come up with higher and higher numbers. The point is that the numbers are very close to each other and that is very good. The problem with rowdy stick cars is the lugging that can take place. A car with a 3000 stall will slip like a mother down low and not load the motor like a stick will. I use the 180 psi as bench mark for race gas. My Chevelle has 170 in each cylinder and pings like hell so I put 5 gallons of 110 leaded in it per tank which works perfectly. The fuel in SoCal is beyond crap and at 91 octane is actually lower because of all the additives. They calculate the octane by adding Motor and Research values and dividing by 2. Its kind like cholesterol, One number can skew the overall number and actually not have as high of an octane. This is why California fuel is so bad. Also the combustion chamber design plays a huge part in how the fuel burns inside. Unfortunately this is pretty hard to change, a head replacement or reconfiguring of your current chamber would help too. When you pull out a significant amount of timing it can actually make the engine work harder and exacerbate the problem. There is a sweet spot for the motor as far as timing goes and pulling too much timing can actually hurt. Jetting up can help cool the chamber but it is also a balancing act. That car is so well set up I think the best thing for you to do is put octane booster/lead additive in it or put some race fuel in it when you fill it up. I go through a 55 gallon drum every 8 months and it cost roughly $450. I think I put about 5000 a year on my car.
ALX69RR440
Well-Known Member
moparchris said::facepalm:69hemibeep said:
ALX69RR440 said:
The numbers you came up with are an excellent grouping. The numbers will be higher when you turn it over without plugs because of the increase in RPM. I let the gauge jump 4 times on each cylinder. You can do it a multitude of ways and come up with higher and higher numbers. The point is that the numbers are very close to each other and that is very good. The problem with rowdy stick cars is the lugging that can take place. A car with a 3000 stall will slip like a mother down low and not load the motor like a stick will. I use the 180 psi as bench mark for race gas. My Chevelle has 170 in each cylinder and pings like hell so I put 5 gallons of 110 leaded in it per tank which works perfectly. The fuel in SoCal is beyond crap and at 91 octane is actually lower because of all the additives. They calculate the octane by adding Motor and Research values and dividing by 2. Its kind like cholesterol, One number can skew the overall number and actually not have as high of an octane. This is why California fuel is so bad. Also the combustion chamber design plays a huge part in how the fuel burns inside. Unfortunately this is pretty hard to change, a head replacement or reconfiguring of your current chamber would help too. When you pull out a significant amount of timing it can actually make the engine work harder and exacerbate the problem. There is a sweet spot for the motor as far as timing goes and pulling too much timing can actually hurt. Jetting up can help cool the chamber but it is also a balancing act. That car is so well set up I think the best thing for you to do is put octane booster/lead additive in it or put some race fuel in it when you fill it up. I go through a 55 gallon drum every 8 months and it cost roughly $450. I think I put about 5000 a year on my car.
Thanks Chris. I'll go ahead and buy the race gas. So far that’s been the only way to get rid of the pinging. I put 5 gal of race gas and then top it off with 91 and it works great. I was just hoping to “de-tune” it a bit and get away with using just pump gas.
Where do you buy your race gas? And how do I get a 55gal drum full of gas down from my pickem up truck? Do they deliver?