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Old 07-15-2007, 10:48 AM
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cranking compression

Hey guys i want some opinion. I have built a bracket motor for the street and strip. Its a 355 small block chevy. 11.4:1 compression. I have designed it for pump gas but my cranking compression isn't very high. Its 170-175 psi across all cylinders. I put one of doug herberts solid lift camshafts: RPM Range: 3500-7500 Advertised Duration: 310/320Duration
@.050: 260/272Lift: .510/.535Lash-Lobe Center: .025-112c
The motor seems to run good but it seems not to have as much power as it did before i changed the heads, cam, and intake. I added 190cc intake runner aluminum heads and larger intake than my old victor Jr.
I may go back to the Jr. But i had old factory iron on the motor.
Also when running open headers does that kill power? Long tube fenderwell headers for s10. All of these products are in a s10 blazer that I made a pass at 12.84 right off the trailer when i put it together. I can't tune it on the street because i live down town on main street. No traction issues. Any suggestions, I'll answer questions and take any suggestions

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Old 07-15-2007, 11:54 AM
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whats your stall speed? you wont start making the best power until 3500 up because thats where your powerband starts at.what gear ratio are you running
Shane
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Old 07-15-2007, 04:30 PM
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3500 stall but a 3:42 gear.
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Old 07-15-2007, 04:59 PM
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In my opinion, you need another full point of static compression ratio to make that cam work properly, or you could back down on the cam with your present scr. Problem will be that if you make much more cylinder pressure than you're making now, the motor may not operate on pump gas.
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Old 07-15-2007, 06:22 PM
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I met the minimum compression ratio for the cam. should it make decent power with that amount of cylinder pressure. I have debated on changing the cam for more lift and less duration and also bring up cylinder pressure. But want that get into the mixed gas more along the lines of race gas
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Old 07-15-2007, 08:32 PM
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do yall have E85?......its 105 octane
Shane
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Old 07-15-2007, 09:28 PM
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"should it make decent power with that amount of cylinder pressure."

Yes.
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Old 07-15-2007, 09:33 PM
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I run a very similar combination, to get it to ET you will need more stall
and lower gears.
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Old 07-16-2007, 07:16 PM
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You should be running a 4.56:1 rear gear or more and a 4000+ stall speed.
You need to find the sweet spot of your cam timing. Optimium intake closing point. How do you do this? you start by doing a compression test while moving the camshaft in the motor to find the intake closing point (cam advance) that creates the most cylinder pressure on the compression test.
The "sweet spot" is going to be the cam timing spot (advance) somewhere between where your cam is located now and that point you found on the compression test. The sweet spot cannot be found in a book or by someones therory alone. It is the spot that your motor delivers the best performance over the quarter mile (best et.) It is generally between 100 and 109 intake lobe centerline.
I suggest somewhere around 102 to 107 intake centerline. You can only find this sweet spot by experimentation (moving the cam timing 2 deg at a time and track testing)
but first you have to determine the point (amount of cam advance) that creates the most cylinder pressure on the compression test and work from there. You don;t even have to degree in the cam to do this (unless you want to know that the numbers are) the compression guage will tell you where you need to be. this is your starting point for testing.
I would lock out the distributor mechanical advance and use 32 to 36deg timing. once you've determined what your motor needs for a intake closing point to achieve maximum running performance you'll be much better equiped to pick a new cam grind if after dialing in what you have yo still feel and different cam is required.
All this will no doubt give ya something to do at the track besides checking out all the babes. You'll become an expert at tearing off and reinstallng the water pump and timing cover.

What was your MPH?
Just to avoid possible engine damage I recomend you mix your fuel during testing at least 25% with 110octane unleaded racing gas. Once you've determined the best cam timing point for max performance you can experiment with fuel octane. This is much cheaper in the long run than replacing a damaged motor. Most tracks sell racing fuel.
I suggest making at least 3 passes on each cam timing test point.
Your 60ft times, 1/8th et and MPH, 1/4 et and mph will devulge a ton of info to help you dial it all in. Do not change the ignition timing or jetting. just the cam timing.

Last edited by F-BIRD'88; 07-16-2007 at 07:41 PM.
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Old 07-16-2007, 08:28 PM
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I've heard the degreeing for compression story a few times. I know one old timer that bought a 68 302 Z-28 new and he said it was so soggy down low he advanced the cam untill he got the best compression and it really woke the motor up. He won a lot of street races back in the day from what I hear.
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Old 07-16-2007, 10:03 PM
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I wished i left my two peice cover on now
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Old 07-17-2007, 07:57 AM
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not enough gear.

too much cam.

too much intake.




install a 4.11 lsd, use a cam with 244 degrees duration at 0.050", and use a victor jr or performer rpm intake.
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Old 07-17-2007, 09:16 AM
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Chris.... where in Alabama??

Are you running 1/8 or 1/4.
As stated above... you at the least need more gear. 4.10/11 or a 4.56

What rear do you have in the truck?
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Old 07-17-2007, 10:28 AM
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Quote:
Originally Posted by chrismac
Hey guys i want some opinion. I have built a bracket motor for the street and strip. Its a 355 small block chevy. 11.4:1 compression. I have designed it for pump gas but my cranking compression isn't very high. Its 170-175 psi across all cylinders. I put one of doug herberts solid lift camshafts: RPM Range: 3500-7500 Advertised Duration: 310/320Duration
@.050: 260/272Lift: .510/.535Lash-Lobe Center: .025-112c
The motor seems to run good but it seems not to have as much power as it did before i changed the heads, cam, and intake. I added 190cc intake runner aluminum heads and larger intake than my old victor Jr.
I may go back to the Jr. But i had old factory iron on the motor.
Also when running open headers does that kill power? Long tube fenderwell headers for s10. All of these products are in a s10 blazer that I made a pass at 12.84 right off the trailer when i put it together. I can't tune it on the street because i live down town on main street. No traction issues. Any suggestions, I'll answer questions and take any suggestions
No traction issues, I have a hard time with that on my 350 powered S-15, it has nothing but traction issues, I can light 'em anytime, anywhere by breathing heavily on the gas pedal.

Chets cam is an old design with a lot of duration and not a great deal of lift. While the LSA is spread out it still has a lot of overlap and a late closing intake. This results in a lot of mixture reversion which greatly reduces dynamic cylinder pressure in the lower to mid RPM band. While you may be running the minimum specified compression ratio, the cylinders are under filling and there just isn't enough molecules in there to make much power. The compression you have may be pump unleaded premium happy, but this cam should really run a couple three ratios higher on leaded racing fuel to get the bottom end response your missing.

You can goof around with a smaller port intake as that may recover some mixture velocity, that will beef up the bottom end some. But basically this engine is overcammed and under comressed for available premium unleaded fuel.
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Old 07-17-2007, 11:43 AM
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The cam has an inordinate amount of exhaust duration given what would normally be speced out with current cylinder heads. It also does not have any lift relative to the duration. So maybe not the "best" choice in grinds.

What I would do is simply put the truck on the chassis dyno with an A/F probe in the tailpipe and see where you are. You will see your power curve, be able to judge the converter you need to run and then be able to calc out what rear gear you'll need to change to. If your bottom end can't take 7500+ RPM then all is moot as that's where you are going to end up at.

Dial the fueling in, dial in the best total timing, get some gears, maybe up the converter to stall at peak torque and see what happens.
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