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carb size on roots blower

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APOLLO

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Discussion Starter • #1 •
What will a motor do on the dyno that is using a roots blower with small carbs? I was using the formula from BDS to see what CFM will be needed with the boost being used. I'm thinking is it will tip over on the dyno. Run out of air being pulled through the carbs. Hope I'm asking this right!! Not sure what it will do. Thanks
 
gn7

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#2 •
A little too small may not hurt max power as much as you think. In actual use it will hurt acceleration more than top speed unless they are stupid small. A roots will pull air thru a carb better than you might think. But if they are too small, you may see them go rich at higher Rs. Not from lack air flow, but the extreme pressure drop will go outside the carb's calibrations.
Blowers do like their air flow.



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SoldHondaBoughtHondo

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#4 •
Saw a 540 10-71 deal at a local shop...3- 850's
 
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APOLLO

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Discussion Starter • #5 •
For instance. On my 525 bbc have two 750's. I put about 4 to 6 pounds of boost to it and the dyno showed it was pulling like 1300 and something of cfm. Can't seem to find the sheet but I believe it was something like that. Probable wrong with that number but I'll use that as a example. If it was 1300, Than that means I have 200 cfm left on the table. If I spin the blower any harder it will go past the cfm that is left on the table. This is where is was wondering what will happen. I say on the dyno is because I can see it I guess.

Formula being used is this. [(DXR)/3456]X[(B/14.7)+1] = C

D=cubic inch displacement of the motor
R=maximum engine RPM
B=maximum blower boost
C=maximum CFM required
 
M

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#9 •
With a roots blower, I would rather ''error'' to the large size with carbs. On a N/A street engine, i would rather error to the small side.

Its pretty hard to overcarb a roots, within reason. I had a little 250 B&M with a single 1050 carb. Switching to a pair of twin 850's on top, netted a little power increase, even though, it was way more carb than the engine required.
 
SoldHondaBoughtHondo

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#10 • (Edited)
APOLLO said:
Willing to bet he had three carbs just for the wow factor!
Click to expand...
Gary Kincaid builds nothing just for the wow factor:wink2:


Mild Thunder said:
With a roots blower, I would rather ''error'' to the large size with carbs. On a N/A street engine, i would rather error to the small side.

Its pretty hard to overcarb a roots, within reason. I had a little 250 B&M with a single 1050 carb. Switching to a pair of twin 850's on top, netted a little power increase, even though, it was way more carb than the engine required.
Click to expand...
I used to work for a guy who had a 350 in a mini truck. 6-71 @ 1:1 and 2 1050's (Rick King) Truck ran perfect, idled @ 1k if it was stone cold or up to temp and got 16mpg....But then again RK puts 660's on a blown bb and it ran pretty good to. Hows about an m-62 eaton on a 1100 cc motorcycle motor....with a q-jet. Motorcycle motors are over carbed even without boost.
Basic old school 2 valve inline 4 will have the bore of the carb pretty close to the diameter of the intake valve.
 
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Sleeper CP

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Sleeper CP

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gn7

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#15 •
C

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#16 •
carbs

Ths BDS site also states that going 30% oversize on the carbs is ok and I have gone from 2x 850's to 2x 1250's but with a 8-71 at 10% OD on a 440" first with choked up iron heads and now Edelbrock Victors ported.My air temp dropped 30*F also.I hope this info helps some.
 
gn7

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#17 •
1bdhondo

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#18 •
My hondo had a 482" with iron world products heads , 8-71, two 1150 Dom spinning 8100-8200
Motor ran great carbs were not to big.... But it was a drag deal although I did use it at the lake a little bit.
 
SoldHondaBoughtHondo

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#19 •
gn7 said:
Last time I saw that engine it was in a Hallett Vector.
Today, 3-850s is a wow factor.
Click to expand...
Local guy, older build, like 12+ years ago?

Oh wow, 3 850's makes 44 more hp than the dominators?:rolleyes:
 
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gn7

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#20 • (Edited)
SoldHondaBoughtHondo said:
Local guy, older build, like 12+ years ago?

Oh wow, 3 850's makes 44 more hp than the dominators?:rolleyes:
Click to expand...
Wrong Dominators.
Don't think I have considered this for a blown e85 deal. With bigger 4150s than 850s too. Not worth a spit.
In would have done it if for no other reason than the possible intimidation and wow factor. Other cheaper and better ways to make MORE power. Today, its bling at its finest.



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