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Old 07-15-2007, 11:48 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, 12:54 PM
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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, 05:30 PM
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3500 stall but a 3:42 gear.
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Old 07-15-2007, 05:59 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, 07:22 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, 09:32 PM
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do yall have E85?......its 105 octane
Shane
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Old 07-15-2007, 10:28 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, 10:33 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, 09:28 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, 11:03 PM
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I wished i left my two peice cover on now
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Old 07-17-2007, 08:57 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, 10:16 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, 11:28 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, 12:43 PM
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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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Old 07-18-2007, 12:07 AM
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i am going to try different ideas that all of you have mentioned. Gears may be one of the greatest factors in the whole deal. The motor is designed to turn very high. but the duration is killing bottom end I am going to leave on transbrake next to see what happens.
But there is another thing. I also live near a highway and thats why i have deterred from the 4.11 gears. I can drive the ride down the road. highway speed limits are 65 mph which puts me right out of the stall when driving 70+ mph. I also plan on driving on the intersate to other tracks in the southeast, atlanta, huntsville, steele, montgomery, and really just have a lot of fun while in college. Future plans are leaning towards a more powerful 406 with 235cc intake runner rhs heads, "already in the works"
the lower lift cam and the heads that i have now, I thought they would work together. because at .500 lift they were flowed at 256 cfm-in. 200 cfm-ex. and peak flow numbers 262 at .600
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