Reply With QuoteI heard, with a warm motor, and carb off you should divide it by 14.7.
So 190/14.7= 12.95.
Not sure if it's right.
Did a compression check on the motor in my Advantage and its running around 125 - 135. The motor is supposed to be 9-1. I would like to start the Rogers project that I bought on E-bay last year and decided to do a compression check on it. The first cylinder I check was 190, the rest were 180+ except for one at 150
.Am I correct in taking the 135 dividing it by 9 =15 lbs, then taking 180 divided by 15 = 12 to 1 comp. ratio, or is there some kind of sliding scale?
I heard, with a warm motor, and carb off you should divide it by 14.7.
So 190/14.7= 12.95.
Not sure if it's right.
Thanks Tony - The next step is to have the motor torn down to see what the internals look like. This was probably a pretty good runner back in its day. High compression motor, magnito, duel quads, ss impeller, droop, nozel reducer,and adjustable ride plate.
Is it possible to covert psi to a compression ratio without knowing all the variables such as cam timing? I have SBC in my boat at about 9.something to 1 and it has between 135 and 145 per cylinder, used to have more (not much about 10 psi) until I put in a bigger cam. My Mustang with a more conservative cam and about 10 to 1 has about 200 psi per cylider when doing a compression check.
Next question for the experts. Number 6 cylinder spark plug builds up alot of carbon and I also have alot of blow by on the right bank. One of the reasons I did the compression check is that I thought I would find that cylinder to be weak, and perhaps the rings didn't seat,but it was one of the 135's. Could the crap on the plug not be related to the excesive crankcase pressure,maybe two sepperate nonrelated problems? Motor was rebuilt using a new performer rpm cam which I had been running the same profile before for the last 8 years. Since the rebuild I am only turning 5000 rpm's, before I was getting 5250-5300 rpms. The only other change made was putting a holley 850 on it,replacing the edelbrock 750. Any opinions?
Next question for the experts. Number 6 cylinder spark plug builds up alot of carbon and I also have alot of blow by on the right bank. One of the reasons I did the compression check is that I thought I would find that cylinder to be weak, and perhaps the rings didn't seat,but it was one of the 135's. Could the crap on the plug not be related to the excesive crankcase pressure,maybe two sepperate nonrelated problems? Motor was rebuilt using a new performer rpm cam which I had been running the same profile before for the last 8 years. Since the rebuild I am only turning 5000 rpm's, before I was getting 5250-5300 rpms. The only other change made was putting a holley 850 on it,replacing the edelbrock 750. Any opinions?
How big is this motor?........First, the 850 is way too much fuel, so the motor is fat. Lean it back down with the 750 and the Rs should come back.......Second, the carbon build up is probably from a bad valve guide/seal. A more radical cam has more overlap that will distort the compression reading, down. Since the overlap allows some incoming air/fuel to escape at running speeds, alot escapes at cranking speed. Another thing to consider when it comes to compression ratios and detonation is piston speed. At cranking speed the air is compressed rather slowly, at high RPM the air/fuel is compressed very quickly, creating heat, creating detonation. Hope this helps........MP
PS Btw, barometric air pressure at sea level is 14.7 PSI....If doing a compression test at sea level divide the results by 14.7 to determine comp ratio. +/- 5% or so......
How big is this motor?........First, the 850 is way too much fuel, so the motor is fat. Lean it back down with the 750 and the Rs should come back.......Second, the carbon build up is probably from a bad valve guide/seal. A more radical cam has more overlap that will distort the compression reading, down. Since the overlap allows some incoming air/fuel to escape at running speeds, alot escapes at cranking speed. Another thing to consider when it comes to compression ratios and detonation is piston speed. At cranking speed the air is compressed rather slowly, at high RPM the air/fuel is compressed very quickly, creating heat, creating detonation. Hope this helps........MP
PS Btw, barometric air pressure at sea level is 14.7 PSI....If doing a compression test at sea level divide the results by 14.7 to determine comp ratio. +/- 5% or so......
Thanks Moneypitt, Motor is 454 30 over, got rid of the truck heads and found some 1965 396 heads and put severe duty valves in it. Bought the complete Edelbrock Performer RPM package, back in the mid 90's, cam, lifters, springs, timing chain,intake and 750 carb. This cam has a .560 int/ .573 ext lift, duration @.050 is 240/246. Motor has only 9-1 compression and as I recall the only other things changed at that time were forged pistons and rollers rockers. Last year, I spun a bearing, so instead of turning the crank I ordered a new one from summit, along W/ the 850 holley and Crane gold rollers. Come to think of it my old cast rollers were 1.73 and the Cranes are the std. 1.7, is that much of a difference? It sounds like w/ the carb situation bigger is not always better and the 750 was matched with the rest of the package. I also bought the MSD setup but have not put it in yet, will this make a big difference? Right now it still has the Mallory w/ the electronic conversion.
Thanks Moneypitt, Motor is 454 30 over, got rid of the truck heads and found some 1965 396 heads and put severe duty valves in it. Bought the complete Edelbrock Performer RPM package, back in the mid 90's, cam, lifters, springs, timing chain,intake and 750 carb. This cam has a .560 int/ .573 ext lift, duration @.050 is 240/246. Motor has only 9-1 compression and as I recall the only other things changed at that time were forged pistons and rollers rockers. Last year, I spun a bearing, so instead of turning the crank I ordered a new one from summit, along W/ the 850 holley and Crane gold rollers. Come to think of it my old cast rollers were 1.73 and the Cranes are the std. 1.7, is that much of a difference? It sounds like w/ the carb situation bigger is not always better and the 750 was matched with the rest of the package. I also bought the MSD setup but have not put it in yet, will this make a big difference? Right now it still has the Mallory w/ the electronic conversion.
The difference in rocker ratio is minimal......You hit the nail on the head when you mentioned the 750 being matched to the rest of the package. Edelbrock has been doing this stuff for 60 years or more. Their kits are well researched and developed. Try the 750 again and let us know what happened.....I may get some arguement here, but spark is spark. If the motor aint skipping or shooting ducks an ignition change with a simular timing curve won't show any substantual difference. Re evaluating your existing timing curve MAY make a difference, but not until the engine is happy with the amount of fuel you're feeding it.......You will get mixed input if you change too much at once. Change the carb back, test.........Then change the timing curve, re test....Then try a different ignition, test...etc etc. That way you can evaluate each change and know what, if any, improvement was gained.......or lost...........You can always go back to the way it was, IF you know how it was..............You need a GPS and an accurate tach, preferably with a memory. Also consider weather conditions, altitude. Oh, AND WRITE IT DOWN.....Record the conditions and the results after each change/test. Big changes don't always result in big differences, and little changes could...........Start a notebook, write it down...............MP
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