Compression test cranking pressure
09-20-2018, 09:43 AM
#1
Racer
Thread Starter
Compression test cranking pressure
I’ve got the new engine mostly broken in and the cranking pressure is higher than expected. So I’m curious what others with modified Gen I smallblocks are seeing. My calculated static CR is 10.9:1 and my dynamic CR is 8.6:1. I was expecting to see about 185-190 lbs of cranking pressure, but I’m seeing up 210 lbs of pressure.
09-20-2018, 09:52 AM
#2
Burning Brakes
I’ve got the new engine mostly broken in and the cranking pressure is higher than expected. So I’m curious what others with modified Gen I smallblocks are seeing. My calculated static CR is 10.9:1 and my dynamic CR is 8.6:1. I was expecting to see about 185-190 lbs of cranking pressure, but I’m seeing up 210 lbs of pressure.
09-20-2018, 09:58 AM
#3
Racer
Thread Starter
I used the cam specs to calculate pressures prior to buying the cam. All engine specs were confirmed during engine assembly. The cam was degreed during installation. And even things like barometric pressure were taken into account with calculations and during testing.
Last edited by Drothgeb; 09-20-2018 at 09:59 AM.
09-20-2018, 10:00 AM
#5
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JIM
09-20-2018, 10:05 AM
#6
Safety Car
Just wondering if your static CR calculations took into account the piston deck height, actual combustion chamber volume and actual compressed head gasket thickness. Asking because the (NOM) 427 in my '65 ued stock L-72 pistons, stock L-72 cam, but had oval port heads, 0.018 steel shim head gaskets and I never checked the piston deck height. A stock L-72 should show about 150 psi cranking pressure. Mine had 240 all around.
09-20-2018, 10:24 AM
#7
Racer
Thread Starter
Ya know, it cranks fine at 15 degrees initial timing. But I’ll bet that changes if it gets hot.
Cam was degreed during installation.
Yes everything was accounted for. I even CC’d the heads and pistons to confirm combustion chamber size during assembly. To illustrate how **** I am, I even installed and removed one head gasket to confirm it’s compressed thickness.
Specs are 60cc Trick Flow Heads, 7cc .060” Wiseco pistons, .000” deck height, .039” head gasket, 6” rods, 3.48” crank, Howard’s 110885 cam IVC @64.5 degrees ATDC.
I designed the engine to be right on the edge of using pump gas at 36 degrees total timing. So far there’s no pinging or knocking. I’m just surprised how high the cranking pressure is.
Pretty much what he said......cam and where it's installed as far as intake lobe centerline will make the difference. Shorter duration will typically increase pressure. The wider LSA typically retards intake lobe so it will reduce pressure somewhat....but that's all dependent on the actual cam lobes and ICL. You can advance any cam enough to increase pressure.
JIM
JIM
Just wondering if your static CR calculations took into account the piston deck height, actual combustion chamber volume and actual compressed head gasket thickness. Asking because the (NOM) 427 in my '65 ued stock L-72 pistons, stock L-72 cam, but had oval port heads, 0.018 steel shim head gaskets and I never checked the piston deck height. A stock L-72 should show about 150 psi cranking pressure. Mine had 240 all around.
Specs are 60cc Trick Flow Heads, 7cc .060” Wiseco pistons, .000” deck height, .039” head gasket, 6” rods, 3.48” crank, Howard’s 110885 cam IVC @64.5 degrees ATDC.
I designed the engine to be right on the edge of using pump gas at 36 degrees total timing. So far there’s no pinging or knocking. I’m just surprised how high the cranking pressure is.
09-20-2018, 10:26 AM
#8
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Cranking compression pressure is a function of SCR, the point where the inlet valve closes, and cranking RPM. The later the IVC and slower the cranking RPM the greater the reversion and lower the pressure reading.
You can't rely on DCR to compute cranking pressure. That's because there is no common definition of IVC, so different calculators yield different results.
As long as the readings across all cylinders don't vary by much, the engine is healthy.
Duke
You can't rely on DCR to compute cranking pressure. That's because there is no common definition of IVC, so different calculators yield different results.
As long as the readings across all cylinders don't vary by much, the engine is healthy.
Duke
09-20-2018, 03:44 PM
#9
Le Mans Master
As long as it runs well, doesn't ping and starts ok, why worry about it? Its gonna be a torquey little sucker for sure. I bet its super crisp and responsive.
FWIW, my 496 cranks 210-215 psi and has for 24k miles, I've had no problems running it on pump gas as long as I keep the timing around 33* total and limit the vacuum advance to 4.5*.
FWIW, my 496 cranks 210-215 psi and has for 24k miles, I've had no problems running it on pump gas as long as I keep the timing around 33* total and limit the vacuum advance to 4.5*.
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Drothgeb (09-20-2018)
09-21-2018, 07:11 AM
#11
Race Director
Might have been a unique experience on my part and maybe I did something wrong in running the test but back in the 90's I ran two series of compression checks on my 427 several weeks apart and the results came out in entirely different ranges - one set was well over 200 psi and the other was in the 180 psi range I think. Otherwise the cylinders were all reasonably close together. I just struck it up to the engine was slightly different temp, air density a bit different, battery strength a bit off on one - I don't know. Maybe I adjusted the timing a little between tests. I do know they were run with the carb wired open and the engine spun over until the gage stabilized. Just saying you might run another series of tests and get results in a different range.
09-21-2018, 04:02 PM
#12
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The tired 383 stroker in my '61 has 195 psi in every hole, along with 30% leakdown. Runs like a scalded cat and doesn't ping. I'm sure the PSI will be above 200 after I freshen it up. No worries. Sure has crisp throttle response!
09-21-2018, 11:48 PM
#13
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I’ve got the new engine mostly broken in and the cranking pressure is higher than expected. So I’m curious what others with modified Gen I smallblocks are seeing. My calculated static CR is 10.9:1 and my dynamic CR is 8.6:1. I was expecting to see about 185-190 lbs of cranking pressure, but I’m seeing up 210 lbs of pressure.
I like to blab about engine builds as much as anyone here. Just that many threads like these are something only a palm reader could help with.
09-22-2018, 08:05 AM
#15
Racer
Thread Starter
I don't think the compression results of my "target" crate motor 350 could possibly be of help with yours but with stock dish pistons, 67cc heads, 272*/110lsa cam pressure was about 180psi I think but have since lost those results. But it is what it is. You claim you measured and calculated everything correctly but results where about 10% off expected. So I don't know what you want us to explain? None of us where there when you measured everything nor do we know what you measured with. What can we help you with here? Or do we just embarrass ourselves trying to explain the in-explainable?
I like to blab about engine builds as much as anyone here. Just that many threads like these are something only a palm reader could help with.
I like to blab about engine builds as much as anyone here. Just that many threads like these are something only a palm reader could help with.
Not really looking for an explanation, help figuring it out or, for you to blab about your build. Just looking for some comparisons with real data. I was hoping to hear like.... I built for a DCR of 8.xx and ended up with 190psi, or 180, or 200. Other useful info would be the timing you’ve run at xxx amount of cylinder pressure.
Your comments on having 180psi would have been helpful, but without knowing your actual piston dish size, specs on your cam and etc, I can’t do any useful calculations. That’s why I just mentioned DCR, I was trying to keep it as simple as possible. Do you happen to know your DCR?
I keep notebooks on all of my builds. I keep track of calculations, specs and, expected and actual results. It’s not as important with this particular build. But I also build with multiple stages of turbos and sometimes multiple stages of intercooling. With those builds, calculations are a big part of deciding which turbos and how much intercooling use with with a particular pump output and injector. I know all the calculating and measuring is a bit much for some of you, but it’s how I do it. And, I have 3 more engines to build over the winter. Any useful data from this build, or, from any of you, will help with those builds
As I mentioned, it’s a Howard’s 110885. Cam specs are 278/278 (225/225@.050”), 110 LSA, 106 CL. Actual measured IVC measured while degreeing the cam was 64.5 degrees. The other engine specs are also back in post #7.
09-22-2018, 03:49 PM
#16
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Designing a build for a DCR of 8.5 - 8.6:1 with aluminum heads is pretty typical. I was just curious what others are seeing as a cranking pressure with other similar builds. And yes, I already know that the DCR is not the only determining factor in cylinder pressure, but it is a big part of it.
Not really looking for an explanation, help figuring it out or, for you to blab about your build. Just looking for some comparisons with real data. I was hoping to hear like.... I built for a DCR of 8.xx and ended up with 190psi, or 180, or 200. Other useful info would be the timing you’ve run at xxx amount of cylinder pressure.
Your comments on having 180psi would have been helpful, but without knowing your actual piston dish size, specs on your cam and etc, I can’t do any useful calculations. That’s why I just mentioned DCR, I was trying to keep it as simple as possible. Do you happen to know your DCR?
I keep notebooks on all of my builds. I keep track of calculations, specs and, expected and actual results. It’s not as important with this particular build. But I also build with multiple stages of turbos and sometimes multiple stages of intercooling. With those builds, calculations are a big part of deciding which turbos and how much intercooling use with with a particular pump output and injector. I know all the calculating and measuring is a bit much for some of you, but it’s how I do it. And, I have 3 more engines to build over the winter. Any useful data from this build, or, from any of you, will help with those builds
As I mentioned, it’s a Howard’s 110885. Cam specs are 278/278 (225/225@.050”), 110 LSA, 106 CL. Actual measured IVC measured while degreeing the cam was 64.5 degrees. The other engine specs are also back in post #7.
BTW I never did find a "correct" part number for the pistons of my 350 "target" motor. And attempts to measure the piston volumes with the block in the car failed. But the results using a 0.015" steel shim gasket was more than satisfactory. So that's my eblaboration and I will wear that on my sleeve for an achievement.
I'm out of this train wreck.
. 
09-22-2018, 08:51 PM
#17
Instructor
I’ve got the new engine mostly broken in and the cranking pressure is higher than expected. So I’m curious what others with modified Gen I smallblocks are seeing. My calculated static CR is 10.9:1 and my dynamic CR is 8.6:1. I was expecting to see about 185-190 lbs of cranking pressure, but I’m seeing up 210 lbs of pressure.
09-23-2018, 12:39 AM
#19
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First...I'll give my take on DCR for others that might be following. Yes..it is absolutely a real thing.....and something all engines are built to achieve...sorta of. As Duke mentioned...there are several calculators out there giving varying results. But the true amount of cylinder fill is a wildly variable thing and if you have a very efficient induction tract...it's going to get real full real fast...as opposed to something not so great. Combos with high calculated SCR and DCR and operating on less octane than you would expect...are telling you things aren't quite as efficient as you would expect...or tune is compensating. I never bother to calculate DCR because I can make a 13 to 1 motor show well on it and indicate pump gas with cam juggling...but I know it will be a LOT different when my foot is on the floor and the good heads start filling the cylinders. Best comparison is to think of the old Busch NASCAR engines. 9.0 compression, incredible heads, smaller cams....and you better have really good fuel in it because once it was rolling it made incredible cylinder pressure with the great intake tract. Even with a tiny carb.
Look back at the old cams sold in the 70's/80's to reduce cylinder pressure on the bad boy engines to run on pump gas.
I'll say of you have 210 PSI and it's able to take full timing....then it should be nice and crisp. A 225* cam in at 106 ICL isn't that big even in a 360 so that's contributing to increased pressure.
I'm out also. Too much arrogance.
JIM
Look back at the old cams sold in the 70's/80's to reduce cylinder pressure on the bad boy engines to run on pump gas.
I'll say of you have 210 PSI and it's able to take full timing....then it should be nice and crisp. A 225* cam in at 106 ICL isn't that big even in a 360 so that's contributing to increased pressure.
I'm out also. Too much arrogance.
JIM