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Old 10-31-2009, 06:21 AM
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More boost less horse power?

heres my problem forged 383 sb 8.8 comp 671 with 2 edelbrock 650cfm carbs. about 7 lb. boost made 475rwhp in 69 camaro with 700r4 3000stall. i flipped the pullys 38/34 to 34/38 boost went to 12lb rwhp dropped to 360? running leaner as well after rejetting carbs to max regapped the plugs got it up to 435rwhp, any help on this one?

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Old 10-31-2009, 09:54 AM
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In rough numbers, you've gone from about 10% underdrive to about 12% overdrive.

1. It takes more HP to spin the blower harder.

2. The 12 PSI air is hotter than the 7 PSI air.

To get a flavor for the HP required to turn the blower, research the blower displacement and use the crude formula; Flow X PSI divided by 1500 = HP requied.

This doesn't address other factors including air temp, fuel specific gravity, the effect of the amount of fuel induced cooling to the output air, ignition timing, etc., etc.

Austin Coil has a blower dyno. It's unlikely a blower dyno would be avaialble to you and I since the cost is high and the market to recover cost and make a profit is marginal.... very marginal. It would also create a marketing hurdle for blower sellers as they (might) would be forced to test and publish results

Last edited by Old Rotor Flap; 10-31-2009 at 02:27 PM.
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Old 10-31-2009, 02:16 PM
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If your tests were done on pump gas the motor was most likely detonating on the dyno with the high boost blower drive ratio. Requiring excessively retarded spark timing. Re run the test on 110 octane race gas or methanol and restore the spark timing.

Water/methanol injection is a alternative to get the fuel octane and charge cooling required for max power from this hi boost+ hi cr combo.

In order to run 12 psi boost with overdriven blower pulley ratio (without the excessive spark retard nessessary to avoid detontaion) the engine should have a 7.5:1 mechanical cr. 8.8:1 is too high for the boost/drive ratio and pump gas.


Do a compression test. The head gasket may be leaking. Detonation is a major cause of this.

Hopfully you have not yet damaged the motor.
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Old 11-01-2009, 07:07 AM
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Thanks. Sorry did not mention i have a meth. injection set up also have boost retard on the ignition pulling 1 degree per lb of boost. last test after checking & re-gaping plugs got back almost 100 hp. still less than 7lb boost best hp test. had not considered the head gasket but don't think that is an issue. after talking with a couple of guys who run blowers the consensus seems to be i may have exceeded the fuel system capacity? I have a 140gph pump with a stock fuel line regulator is set at 8lb fuel pressure. i appreciate the help
Frank
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Old 11-01-2009, 12:45 PM
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Also possible spinning your tires on the dyno.
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