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Author Topic:   4412 Power Valve Selection
RangeRover
Dirt Freak

Total posts: 279
posted March 02, 2005 05:35 AM  
OK guys, tell me if I went about this properly cause I'm going to go buy some parts soon. I put a hose barb fitting in place of a plug on my intake manifold and plugged in a vacuum guage. I fired up the motor and let it idle. This motor seems to want to idle at 1000 rpm where it shows about 6" of vacuum. If I raise the idle a bit, the vacuum raises to about 8 or 9" (I thought it was supposed to drop as engine speed rose?). If I lower the idle to about 850 or 900 rpm, the vacuum drops to 4" or 5" of vacuum. I played with the idle skrews and decided that the 1000 rpm idle with 6" of vacuum was the best setting (running a healthy cam- 246 @ .050/108 LC).

So, would I be OK to simply put a 4.5 powervalve??? Are my seemingly low vacuum numbers normal for this type of cam?

KPLugnut
Dirt Forum Champ
Total posts: 1019
posted March 02, 2005 06:55 AM  
Yes, 4.5 PV would be good for that application, and that's entirely normal for a vacuum reading and how it behaves. Keep in mind the vacuum WILL drop quick if you whack open the throttle, but just raising the idle slightly doesn't do the same thing as swinging the butterflies open (opening the "flood gates").

KP

RangeRover
Dirt Freak

Total posts: 279
posted March 02, 2005 08:59 AM  
Thanks KP. One more thing, with a relatively low PV like 4.5, is there some danger of running lean up high if the vacuum lingers just above that value and shuts the enrichment off? Or, is that where the right main jets comes in?

KPLugnut
Dirt Forum Champ
Total posts: 1019
posted March 02, 2005 09:16 AM  
The right main jet should take care of that as well as can be expected from a 2bbl application, as long as the carb is properly tuned throughout. The thing to remember is that if you HAVE to have the PV fuel supply in order to let the engine live (or not be way too lean) at top end, then you'd have to run a PV with REALLY high #'s. Manifold vacuum goes sky high in 2bbls at higher rpms, and the PV will always be shut no matter what after a certain point no matter how high a PV # you use.
From our experience, 2bbls do not lean out on top end, they actually go to the rich side because with such high vacuum, fuel starts getting pulled from everywhere (idle circuit reactivates, squirter pullover, etc etc).
Go with the 4.5 PV and jet accordingly and you'll be fine.

KP

waltonjr1
Dirt Forum Champ
Total posts: 516
posted March 02, 2005 09:29 AM  
I thought manifold vacuum drops after a certain rpm?


stockcar5
Dirt Forum Champ
Total posts: 578
posted March 02, 2005 10:59 AM  
not with a 2bbl. even with my big 2bbl the manifold vacuum increases as rpm increases. its worse with a 500 or 350 holley.

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nailngoracing
Dirt Maniac

Total posts: 170
posted March 05, 2005 10:43 PM  
We don't powervalves

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