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Old 11-17-2007, 01:05 PM
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blower engine question

Hi this is bambaking and I am building my first blower engine. I have a few questions that might be rookie questions but here we go, I am building a 468 pontiac motor with a 871 and 2 750 cfm carbs, can I still run power brakes and a vacuum advance distributer? None of my hotrod buddies ever had a blower motor and I get conflicting answers even from blower company tech lines. Any help from someone who has gone thru this or is well versed in blowers is greatly appreciated.
Thanks.

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Old 11-17-2007, 01:58 PM
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Vacuum power brakes will work the same. Hook up to the 3/8" vacuum port on the base of the carb(S).

Distributor vacuum advance will be a little different. Hook it up to the intake manifold under the blower. If there is no spot, you can drill and tap for a vacuum source fitting on the manifold. You'll actually want two taps on the manifold. 1 for the distributor vacuum advance and 1 for the carb(s) boost referenced power valves.

The distributor vacuum advance will need to be carefully dialed in to what the motor requires. (amount and rate of advance modulated by the amount of manifold vacuum at part throttle) Especially critical as the motor transitions at part throttle, from no boost/vacuum-light engine load to a partial/low boost condition in the manifoldwhile also at part throttle (rolling into the throttle). Then on to full manifold boost as you open the throttle(s) to full throttle.
As the boost comes up, the timing required drops off (quickly) You'll need a adjustable vacuum advance and probabily some small hose clamps to keep the vacuum hoses on. The blower/manifold boost will try to push the hoses off when the motor is under boost.
Do not connect the distributor vacuum advance to the carb above the blower.


Because most of us are naturally lazy and may be tempted to just run a stock vacuum advance from a N/A motor without bothering to dial the amount and rate of vac advance in for a blown motors requirements, many of the supercharger makers will tell you to no use vacuum advance.
For drag racing or marine use where you won't be under part throttle/low engine load its not necessary.
For many street use blown setups that use electronic boost spark retard to reduce the spark lead while under boost its not that critical.
But if you'd like a better set up street blower motor that gets good cruising fuel efficiency you want to employ a properly dialed in spark vacuum advance. If you set the distributor advance and vacuum advance up to work properly modulating the spark advance needs as the motor transitions from idle to part throttle/light load and on through to full throttle high boost, you can get by without a electronic boost spark retard. This takes some specific dialing in and fine tuning of the complete advance curve on your part.

The best most efficient set up for a blown street motor will employ mechanical and vacuum advance as well as boost spark retard.

Do you have a camshaft grind in mind for your blown Pontiac?

Last edited by F-BIRD'88; 11-17-2007 at 02:23 PM.
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Old 11-17-2007, 02:20 PM
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Good information from F-Bird'88, but do you need to go with vacum advance? As far as timing goes, though I'm manual with a BTM, which I've never brought into play, I'm running 20 deg initial and about 34 to 36 total, and have it all in at 2800 rpm.That's on a 383, 8-71 and two Demon 750 BC/s.

Good luck with the project, keep us posted.
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Old 11-17-2007, 02:34 PM
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Quote:
Originally Posted by 68NovaSS
Good information from F-Bird'88, but do you need to go with vacuum advance? As far as timing goes, though I'm manual with a BTM, which I've never brought into play, I'm running 20 deg initial and about 34 to 36 total, and have it all in at 2800 rpm.That's on a 383, 8-71 and two Demon 750 BC/s.

Good luck with the project, keep us posted.
if you're running high octane racing fuel, little or no spark retard is required.

If you have a very low compression ratio (7-8:1) and moderate blower boost
(10lbs or less) little or no spark boost retard is needed.

It will drive around fine but will drive just a little better with a little vacuum advance at part throttle when there is vacuum in the manifold (light engine load)

If your running pump gas (92 octane) and more than 7.5:1 compression with a good bit of boost, 6-7psi+) you want to employ boost retard to allow the most power/boost possible without detonation on pump gas.
Vacuum advance will help efficiency when cruising at low speed/low rpm/light engine load when there is manifold vacuum at part throttle.
if you're running a pretty radical combination with a very high converter stall and a lot of boost and rear gear ratio you're going to pass thru the point where vacuum advance would help, very quickly into a boost condition.

If when you cruise on the hyway, you have a lot of blower boost at part throttle you won;t need any vacuum advance.

Last edited by F-BIRD'88; 11-17-2007 at 02:53 PM.
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Old 11-17-2007, 06:26 PM
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blower engine question

Well thanks for all the good info. My set up will be running 7 to 8 lbs of boost and really want driveability on the street rather than big horsepower because this set up is for show more than go. I will look into the boost retard and see what is recomended. Thanks to all that responded it was very helpful.
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Old 11-17-2007, 08:10 PM
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You won't be into boost at cruising speed. I made a 50 mile cruise recently and got 10.5 mpg, and of course no boost at steady throttle. You'll see small boost accelerating normally (if there is such a thing in a blower car..), my setup, 8.4 comp, underdriven 8-71, Lunati fairly radical full roller produces 8-9 lbs at WOT. I've never employed boost retard nor have I detonated this motor.

As far as vacum/mechanical timing, maybe most of us are 'lazy', but it's not worth it, to me anyway, to mess with the variables involved when electronics are available, cost aside, and are efficient. My opinion only.
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