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CFM for a moderate size pport on a 12a?

Old 10-24-04, 12:49 AM
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CFM for a moderate size pport on a 12a?

What kind of CFM would a moderately large peripheral port on a 12a flow? Say power peak around 9000-9500-ish rpm, just a ballpark estimate. Around 600cfm?
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Old 10-24-04, 02:37 AM
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Are you asking about the estimated cfm flow rate of the block, or the cfm rating of the carb?
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Old 10-24-04, 02:56 AM
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What CFM carb would be required to run at optimal efficiency is what I'm getting at.
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Old 10-24-04, 12:46 PM
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Ballpark around 650cfm carb rating. The 48 IDA Weber or Racing Beat 51 IDA Weber would probably be a good choice, as this is mostly what I have seen on 12A PP engines. I don't remember seeing any 12A PP engines with a Holley, as the 2-port pp engines tend to use 2-barrel carbs rather than 4-barrels.

The RB carb selector may help:
http://wwww.racingbeat.com/PDF/intakeguide.pdf

I am moving this thread to the race car tech forum to see if there is any better input.
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Old 10-24-04, 03:40 PM
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Any particular reason for the 2 barrel carbs as opposed to 4 barrel?

The Webers look like an awesome carb, but you never see them on ebay and they're almost $800 new, you can get holly 650's all day on ebay for $50. Even if the Weber made 15hp more, it aint worth it for that kind of cost difference, and fabrication of an intake for either shouldnt be too difficult.

Thanks for your help btw
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Old 10-24-04, 08:00 PM
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Originally Posted by Raptor13x
Any particular reason for the 2 barrel carbs as opposed to 4 barrel?
Yes, a 4-barrel carb has 4 holes, and a pp engine has 2 holes. Four is more than two. Two is the same as two.

You may have noticed that "driveability" and a broad power range are related to the number of holes in the engine. A pp engine has two holes, a stock port engine has 4 holes, and the ultimate full-range NA street engine has 6 holes. More holes allow finer control of air through the engine by controlling the velocity of the air at a given flow rate. Best performance is gained by maintaining the air at a high enough velocity to keep up the momentum, but not at such a high velocity where drag becomes excessive. The downside is that more holes means more surface area for a given total cross-sectional area, which increases the toll of surface friction, so ultimate power is hindered by having more holes (in theory). It's one of those give-take things, where it depends on whether you want better peak power or a better overall power range.

Originally Posted by Raptor13x
The Webers look like an awesome carb, but you never see them on ebay and they're almost $800 new, you can get holly 650's all day on ebay for $50. Even if the Weber made 15hp more, it aint worth it for that kind of cost difference, and fabrication of an intake for either shouldnt be too difficult.
Yes, and you never see Ferraris and Lamborghinis on Ebay, and they are very expensive new, but you see used Hondas all day.

This may help:
http://tom.pratchios.net/Rx-7/facts6.html
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Old 10-25-04, 08:06 AM
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Evil Aviator is right.

If you have to make the transition from a 4 barrel carb to a 2 port engine, then the flow velocity will get screwed up where you make the transisiton (either the flow velocity will have to slow down and then speed back up, or it will have to make turns.)

Currently, the best carb setup for PP motors is to take a 48mm Weber IDA and bore it out even larger, as much as the carb casting will allow. The reason a 51 Weber IDA makes good power on a PP motor is because you can start out with a huge velocity stack, accelerate the flow thru a 55mm upper section, on into a 46mm venturi, and gradually slow the air flow back down thru the 51 throttle, then direct connect it to the intake manifold, which is a piece of 51mm pipe, the intake makes a gradual 45 degree turn and dumps into the peripheral port. Just a nice straight down shot into the engine chamber with no sudden changes which screw up air flow.

Why does a modified Weber cost close to $1000, and a Holley is so cheap?
tuneable venturies: Weber yes ---- Holley no
tuneable air jets: Weber yes ---- Holley no
tuneable idle jets: Weber yes ---- Holley no
tuneable emulsion tubes: Weber yes --- Holley no
float resists cornering g's: Weber yes --- Holley no

You can tune ALL aspects of a Weber IDA - this is why they cost so much. I have been running an IDA for the last 10 years, and it has never failed me at the race track. It is a very reliable racing carb.

With a Holley carb, all you can do is change the main fuel jet. The Holley has a power valve to try and bandaid the problem of leaning out on top end. You cannot easily tune the fuel delivery at all rpm. This is why a Holley is so cheap.

Last edited by speedturn; 10-25-04 at 08:16 AM.
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Old 11-07-04, 11:27 AM
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If You are serious about biulding a Peripheral Port engine, then You should not be concerned about the cost of a Weber carburator.
In the full scope of things, the Weber will probably be one of the CHEAPEST parts of You're engine, not to mention the Housings, Hi-speed Rotating assembly, and Dry sump system for the RPM. you're talking about. Also, You MUST run Individual runners to each port.
in order to obtail correct fuel distribution, and avoild velosity issues. Besudes, have You considered the cost of fabricating an intake manifold for a Holley to a PP. engine ?
Regards,

GT1-20b
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Old 11-07-04, 09:40 PM
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weber will work the best for your application... gene berg 58mm will work even better, but if your concerned with price stick with the weber..........

holley on a p-port=no good, plain and simple
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Old 11-07-04, 09:47 PM
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How about get a 51IDA Throttle body, and make it EFI? Tune all day long to your hearts content
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