240 PSI Cranking pressure?? - Team Camaro Tech
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post #1 of 13 (permalink) Old Dec 19th, 00, 06:09 PM Thread Starter
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Just did a compression test on my 383 and the readings averaged 240 PSI per cylinder. Does this sound correct? When I performed this test I removed all spark plugs and pinned the carburetor in the "wide open" mode with the air cleaner removed. For some odd reason I keep thinking that this cranking pressure is higher than it should be. Did I not perform this test correctly or am I missing something?
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post #2 of 13 (permalink) Old Dec 19th, 00, 07:13 PM
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Sounds very high. I saw it once when a guy put an rv cam in a 12-1 377 small-block; didn't like the "large" cam that was in there. The thing pinged itself to death (literally)!!!

If the gauge is accurate that is very high. Engine specs?

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post #3 of 13 (permalink) Old Dec 19th, 00, 07:30 PM
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Jody! We are practically neighbors!! Have you checked out Camaro Generations?? Check out the link to our club website below and email direct if you like...
See ya!
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post #4 of 13 (permalink) Old Dec 19th, 00, 11:50 PM
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Borrow some compression gauge or test another car. If your car is 240.... OUCH!!!!
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post #5 of 13 (permalink) Old Dec 20th, 00, 02:39 AM
 
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Isn't it supposed to be around 160/175 psi for the street??? Which comes in about 9.5 to 1 dynamic CR?? Or is that static CR??
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post #6 of 13 (permalink) Old Dec 20th, 00, 06:11 AM
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While this is a high reading it is very possible. I have a 350, bored .030 with flat top pistons, 64cc heads, Performer cam, and 1.6 to 1 rockers. I get readings of 215 and 220 per cylinder. Runs fine on mid grade gas. With your stroker your readings with a similiar type cam would be higher. Have to listen very close for detonation at wide open throttle in the mid range though.
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post #7 of 13 (permalink) Old Dec 20th, 00, 06:51 AM
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You did the test right, 240 is pretty high for a street engine. As suggested above I would try a different gauge first to see if you get the same readings. I am in Houston also, I live SE of the city in Seabrook. Let me know if you are close by and you can borrow mine.

What are your engine specs? Deck height? Chamber size? Flat tops? Domes? Head gasket thickness? Steel or alum heads? Cam specs? Cam advanced or retarded?

A mild cam with a wide lobe center will give higher readings. Retarding the camshaft will usually lower the cranking compression.

180 is a good number for a street motor but there are a lot of variables.

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[This message has been edited by squarles (edited 12-20-2000).]
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post #8 of 13 (permalink) Old Dec 20th, 00, 09:19 AM Thread Starter
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Thanks to everyone who responded-I appreciate it!

Here's the scoop: Tested the gauge on my Nissan Pick up (You got me! I drive a rice-burner) and the average readings were 140 PSI. I bought the gauge less than a year ago. I believe the gauge is accurate.

I recently switched from a solid roller Chet Herbert grind, specs: .550"/.550" 280 Deg. @ .050 on a 110 LDA, to a more streetable .443"/.465" 224/230 Deg @ .050 112LDA. With the solid roller, the car would not idle under 1000 RPM and it was a real bear to drive around. No noticeable torque until close to 4K RPM-This was a problem because I could never drive in 6th gear without the car literally lurching itself to death. If you tried to drive in OD it would rattle your fillings out of your head.. So, I switched to a smaller cam. I also had to switch heads because my other hads had triple valve springs in them which would not work with my new hydraulic cam. I am now running "Fuelie" style 186 castings that I pocket ported. These castings usually are 64CC heads but mine have been angle milled about .040" so I am estimating to be between 60-62CC now. I am running flat tops with a raised compression surface/deck (top of piston) of .100". Block is just a .030" over 350. The reason that I did the compression test is because after switching cams and heads I have a back fire through the carburetor any time the engine revs above 3500RPM. I wanted to eliminate any cylinder issues before pressing on. I think that I did not break in the cam properly and that it is "dissolving" inside the motor. I have started the engine with the valve covers off and find that exhaust rocker on #2 cylinder is moving far less than any of the others on that bank. After adjusting this valve for lash my ticking went away. I then drove the car for 20 minutes and the ticking was back again.. I checked #2 again and now find there is more "lash" present.. Either I am losing that lobe or that lifter is collapsed. Being that it is a brand new cam & lifters, I can only assume that I did a bad job of breaking the cam in and it is time for a new cam and lifters.. Such is life with a modified car.. Back to my catalogs-Comp Cams I will be calling you soon!
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post #9 of 13 (permalink) Old Dec 20th, 00, 09:42 AM
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It's almost definite that you lost a lobe. There's really nothing else that would cause what you're describing. When picking a new cam call the manufacturer and tell them about your compression, etc. I have a .150 dome piston with 70cc heads and it's about 11.5-1 compression. I would say yours is higher.

Did it detonate at all? Or did you have a chance to drive it hard before losing the lobe? A different cam will go a long way towards being able to run this thing without race gas.

The lobe should be the cause of the backfire; I would say that's fixed with a new cam. Just be honest with the cam guy regarding what you want to accomplish; race gas, pump gas, etc. It depends on what you can live with.

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post #10 of 13 (permalink) Old Dec 20th, 00, 02:56 PM Thread Starter
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Again, thanks to all who responded.

I agree with above posts. I will be tearing the engine down once more to replace the cam and lifters.. I thought I was doing well to have sold my solid roller cam, lifters, and heads for a large profit, but it looks like doing a cam replacement twice is quickly eating into that profit. Live and learn I guess. Of all the motors that I have rebuilt I have never had a cam go away like this. I guess it was just my time.
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post #11 of 13 (permalink) Old Dec 21st, 00, 10:41 AM
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I calculated you compression ratio using the following:
4.030" bore
3.75" stroke
62cc chamber
-6cc(estimated) for the .100 dome pistons
.039" gasket thickness
pistons .020" below the deck
This calculates to 12.47:1! It's going to be very hard to get this motor to pull way down in sixth gear on pump gas without detonation. Switching to 76cc chambers lowers it to 10.5:1.
Stephen


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post #12 of 13 (permalink) Old Dec 23rd, 00, 08:12 PM
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Hate to say it ,but between you almost unavoidible detonation problem and half your cam floating around your motor i'd say you should start saving for more than a cam.
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post #13 of 13 (permalink) Old Dec 27th, 00, 10:43 AM
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hmmm, detonation city, hope it's not trashed. I did a similar 400 long ago, 64cc heads, flat tops, but the cam was 108 centerlines, and cranking compression around 185. It was a bear to make that thing stop pinging. You need to come half way back on both the cam and the heads. 70cc edelbrocks would be good on a 400, and a cam with enough overlap to kill some of the pressure at low rpm's. And tune it real careful like, start rich and retarded, and go easy till you get it right. I just love 400's!!!



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