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Old 06-04-2002, 07:47 PM
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Post Building a Blown Chevy 350

I've got a fresh 350 with flat top pistons, a GM factory LT 4 hot cam,(hyd. roller) and a set of ported World products S/R torquer heads (they were 67 CC but I polished the chambers, and I have to check the CC's). I think the static compression will be about 9 or 9.5 to 1. I have a 6:71 Moneyham Blower that I just have to use because this motor is going into my tube chassis Pro Street `37 Chevy Coupe. I've heard that too much static compression when installing a blower won't work for too long. I can run at 12% under driven, but I heard that a blower works best when over driven. How much static compression is too much? I want to run the car some and do a couple of local street cruises with it. I also have a set of new 450 Holleys that the guy gave me with the blower. Can I get away wuth the 450's or blow some more dough on a set of 600's? How about a set of 600 Edelbrock performers instead? What ratio gear should I use with 31 " tall tires? I don't plan on any highway travel at all. It has an Olds 5:13 w/spool now. Anyone got a 4:10 they want to get rid of? Who's got an opinion on the subject?

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Old 06-05-2002, 12:22 AM
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for a street motor on gasoline you will actually get more power from a motor witk compression arround the 9:1 mark and running the blower at lower at lower RPM, this is due to the fact that the roots style blower isnt very adiabaticly efficient, meaning that a consitererable amount if the energy that is used do deive the blower results in heating of the air instead of compressing it. Because of thir an onerdriven blower will heat the intake charge excessivly, resulting in detonation if you were to run your motor in pump gas.

now this is not the case with an alkohol motor as the methanol cools very efficiently, but with a bas burning motor you are better off going the route you allready planned.

Dont let anybody tell you that the boost number you are running is important either. if you had say 12psi boost on a 350 sb with mild cam and stock heads, and then changed your heads to somthing with better flow and udes a better cam, your boost would drop to mabe 6psi, but youd still be makeing the same Hp. as the boost with a roots is not a measure of how much air you are pumping into the motor but of the restriction in the intake side of your motor.

id say stay arround 12% under till you sort everything out and then turn up your ratio till yoou are getting 9-11 psi of boost, which is about as high as you can go without heating the intake charge too much.

and if you are still not sattisfied with the arround 550 hp you should be malkeing, then change to a roller cam with more lift and duration. and thn turn boost back up to 9-11 pounds

hope it helps
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Old 06-05-2002, 01:59 PM
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For a blower motor, you want to make sure the cam lobe seperation is around 112-116. If your running a cam with around 108 lobe seperation it opens and closes the valves to fast and youll loose power.
With the compression ratio at around 9.5 to one, Id only run around 6-8 pounds of boost. To much and it will start detonating and youll have a meltdown wich will be a lot of work and hassle.
I think BDS suggest no more than 12 to 1 static comp.
As the gearing goes, it depends on how much you drive and what tranny your running. 5.13 is way to much. Blowers make incredible tourque, so you dont have to have a huge deep gear. Id go 3.91-4.10. And definatly loose the spool for the street. First sharp corner with a cop around and he will nab you for squalling the tires or worse, you will break something because the spool locks up the tires completely and as you turn both the tires turn and its a nightmare on the rear end.
Goodluck. HG
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Old 06-05-2002, 02:31 PM
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Sounds like your combination is alright, having the lower compression will allow you to turn the blower closer to 1:1 which is better from a efficiency standpoint. Running more overdrive in the blower and less compression in the heads is desirable because the blower is a better (more efficient) compressor of air than a piston is when you are taking volume flow into consideration. This is important when you are racing because you want to extract every ounce of power from the fuel you can. In your situation I wouldn't worry about it because I doubt you would see even 15 HP difference by reducing the compression ratio just to run more boost. Look at it this way, it doesn't matter how you build compression it's just that using the blower to do it is more efficient because it moves volumes of air better than a set of pistons and valves can. For the street this is a moot point anyway.

Leave the engine the way it is and dial in your blower drive ratio to reach peak boost at peak torque rpm. The blower lasts longer being underdriven anyway. You might consider leaving the 5.13 gears, although you might have a few lawsuits from all the passenger necks your going to snap. I would lose the spool though, totally unpractical for the street and wears tires like mad. I would try your combination the way it is before I changed anything and spend your time and money on getting the timing and retard coming in correctly and dialing in the dual 450 Holleys which are all the carb you will need for that engine. Just build what you have and enjoy it before you decide to out-engineer yourself. You are going to have fun!!!!
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Old 06-05-2002, 08:31 PM
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What these guys are telling you is about right. You only want to run about 9:1 compression and 6-9 pounds of boost.
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