Here are the numbers with my 462, 10.5:1, TA-413 (w/107 ICL) engine: 1 - 120 psi 3 - 120 5 - 117 7 - 115 2 - 133 4 - 128 6 - 117 8 - 115 These seem low to me but my previous 350 was in the 100 psi range, and it ran good too. The rear cylinders 7 & 8 were the hardest to get a seal on during the test, maybe the reason for lowest readings. Maximum variation with these numbers is 15.5%, not too bad I think. For the test all plugs were out, the engine was warm and the throttle opened. Cam spec.'s 234/344 @ .050 and .5/.5 lift on 113 LC. If I degreed the cam in to 109 ICL (vs 107) this increase my cranking compression right? About how much would 2 degrees change make in the pressure? Generally speaking I've heard 200 psi as a upper limit for the street and with forced induction a significantly lower cranking compression is recommended. I'm considering a moderate dose (100 - 150) of Nitrous next year. Which cylinders are typically the most lean with the SP-1 intake? Thanks...
Thanks guys, so if I go to say 105 ICL cranking compression will go up, but how much of a difference could I expect. Are you saying that #3 & #6 are typically the leaner cylinders with this intake? I will be doing a leak-down test shortly, again.
The problem with advancing the cam that far is you start to really stifle your upper horsepower. I would do the leak down test. Something doesn't sound right. Have you tried any oil in the spark plug hole when you do a compression check? If so, did the pressure come up? With 10.5:1 compression and that cam, you should have a lot of cylinder pressure. That actually might be on the hot side with pump gas, unless you're running AL heads.
I would guess he would either have a flat top piston in the hole with .040" deck clearance and 72cc heads (10.62) or a dished piston (22cc) set at 0 deck with 68cc heads (10.5:1) or something similar. Do any of those numbers fit?
The exact compression (below) was calculated from the following measurments I made myself: Bore 4.3425", stroke 3.9", compressed head gasket .038", cylinder head 67cc, piston 25cc., zero deck. These heads were shaved quite a bit before I got them. CR = 10.35:1 When I try the leak-down test I'll also try after adding a thimble of oil to the cylinder. Maybe I'll re-try the cranking compression in a cylinder as well to see the difference. What should I expect for leak-down ~5 - 10%? My old 350 was 1%!
How many miles on this motor? What prompted the compression test? When you do the leakdown test be sure to listen at the top of the carb and BOTH exhaust tips for leaking air. You'll also want to listen for leaks past the rings by listening at the oil fill hole. Those cranking compression numbers are low for your combo. A variation of 15.5%, IMHO, is right on the edge of acceptable/unacceptable. Depending on the rings used, age of the engine, history of overheating, etc, I like to see them no higher than 15%.
Just to confirm... It does sound low. My uncut heads & block with flat top pistons pulls 180-185 with no oil. Thats with a 118 cam installed straight up. Is this a fresh build, or did you just have some work done???
This new motor has 2500 miles on it. I was loosing some water slowly so I did a cranking compression and coolant system check. The coolant system revealed a leaky heater hose at the heater core. I by-passed the core and re-tested with good results.
cranking speed makes a difference in the pressure results. Do you have a slow-cranking engine? Yes, I also suggest a leak-down test.
Runs and starts great, never any smoke. It would be nice to have a screw-in cranking compression kit however. I think that may contribute to the lower readings for #7 & #8. I made an adapter for the leakdown tester (it has a "o"-ring) but does not have a internal valve in it to hold the cranking compression from the previous cycle. The kneedle goes up and back to zero on every compression stroke. Had to go back to the rubber taper and hand pressure to seal each hole.
".....but does not have a internal valve in it to hold the cranking compression from the previous cycle. The kneedle goes up and back to zero on every compression stroke. Had to go back to the rubber taper and hand pressure to seal each hole.........." There are your 2 problems. You need a screw in gauge as well as one that holds pressure between puffs.
That 462 i built for Rob had 200+psi with a 230 duration .050 installed on a 108 ICL with 10.5 comp. Maybe your gage is bad. See if you can barrow another gauge because if thats right something is off. That would be #'s for 9.0 comp. Did you put a degree wheel on the cam to check the duration #'s. Are you sure about the zero deck? If nothing else it sure will like the nitrous with that low of cylinder pressure. Leave the cam where it is. I want to say #4,5 are generally the lean runners. If the rings were that bad I would think you'd see smoke at WOT and you would have problems with oil leaks or oil blowing out of the breather. If I remember right you had a bit of a headache with this motor. Didn't you have to go through it after you first put it together? If it all checks out ok, don't worry about it. The nitrous will probably make more Hp out of the lower compression and give you better #'s than if you had 10.5 comp.
Those old rubber nipple compression gages are known to leak by. Definately retest with a screw in style tester.
Deck was 0.000 to 0.002 (recessed). Actual intake/exhaust duration was 235.3/244.5 deg at .050". Lift was slightly less than the .500" on the cam card. I'll look into a better gauge set-up.
O.K. there all comes a time when one of Dad's old tools needs to be replaced this was one of those times. Sad in a way but the new gauge put a smile back on my face. Here's the new numbers with a screw-in style adapter: #1 - 163 psi #3 - 165 #5 - 167 #7 - 165 #2 - 182 #4 - 176 #6 - 178 #8 - 167 Still some variation but in the range expected. Looking to get to the track on Sunday (Fayetteville) but I know I have a small vacuum leak at the carb. (idles somewhat rough at first start-up in the morning) I found it last night. It's on the passenger side of the Q-Jet around the rod location close to the back and close to the base. Not sure what can be done about that?
The more duration on your cam the lower your cranking compression 'cause the intake is still open as the piston is going up. This reversal is overcome as the engine revs because the valve closes and the air can not reverse in time. Static compression (measured with a guage) and dynamic compression (while actually running) can be very much different. This is the reason that long duration cams run rather lumpy because the air is being reversed back to the carb thereby confusing the vacuum signal.
I have about 200 lbs cranking compression. Same setup eccept my heads are about 75cc chambers, I got a little carried away when I unshrouded the valves. I have the Poston 105A cam with intake at 101*. I had bad pinging on pump gas untill I polished up the chambers and it went away. I don't know if its my home port job or the cam but I feel that my combination is strong to the 1/8th mile but kinda weak on the big end. I have dropped my ET alot by tuning but my MPH stays right about the 114-115 area. Tom
