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Old 01-09-2013, 08:29 AM
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Blowers-CFM??.

I'm being curious.Here is the question.We know street blowers are limited by pump gas somewhat.So let's say we are running 5 to 7 PSI.But doesn't a blower have a power curve in terms of CFM??. I mean to say as the blower spins faster it has I am guessing a power curve in CFM at a constant PSI that tapers off in the high RMP's.And if you building a blower motor you would want to match the blower's CFM power curve with cam timing,head intake runner size maybe just a tad smaller than the blower's max CFM,and matching intake the same?. That maybe the CFM is more important than the PSI?.

I'm not a blower guy and by posting this thread I am hoping for a intelligent informative non-combative discussion.

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Old 01-09-2013, 09:11 AM
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We control the blower out put by either overdriving or underdriving the blower..then we use a wastegate or popoff valve to limit the amount of boost the engine sees so we do not overboost the engine resulting in a damaged engine..In some perfect world the blower would be exactly matched to the engine but that just does not happen..

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Old 01-09-2013, 09:49 AM
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so how does a person know what size of blower to purchase if it does not have a cfm rating? I would think a 540 needs a different blower than the famous 305?
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Old 01-09-2013, 09:57 AM
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If you are talking GM blowers we compare the category of blower to the engine we propose to put it on. The 502 would require an 8/71 blower and a 305 woudl fall into either a 4/71 or 6/71..kinda crude but works. For a turbo just see how big an engine it came off of and go from there..that is how we do it in the hotrod world..If designing from scratch then that discussion is probably beyond the scope for most of us..

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Old 01-09-2013, 10:08 AM
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turbos use a formula or a graph.The sizes for turbos are many and the do give CFM and boost pressures. Heat factors become the variables.look at a garret sight and a calculator tells you what size turbo(s) to use based on boost/hp,just like a gear/speed calculator.
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Old 01-09-2013, 10:10 AM
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True story..fellow needs to do some homework when getting serious about installing a blower or turbo..

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Old 01-09-2013, 11:14 AM
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How to figure CFM with a blower.....

From Blower Drive Service......
"The size of carb(s) or CFM required for a given application can be calculated by the following formula A: {(CID x RPM) 3456} x {Boost 14.7) + 1} = CFM required. The amount of CFM required will determine carburetor size and quantity. If you try to use a carb with less CFM than required, performance and economy may be greatly reduced. Bigger is not always better when selecting carbs that are 30% over what is required, you may encounter problems in fuel distribution."

So, let's do one.....we'll use 8 lbs on a 355 @6000 rpm's....
355 x 6000 / 3456 = 616
8 / 14.7 + 1 = 1.544
616 x 1.544 = 951 CFM
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Old 01-09-2013, 11:36 AM
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Well if we chose a blower on the basis roughly the size of the engine and we know that blower's CFM is a constant,but we can vary knowing that CFM,heads and a cam that runs parallels the blower's power curve.

I am assuming first that blowers operate on a power curve because of a CFM output.Slower speeds less CFM and middle range a greater CFM and then the fastest speed or top end the CFM output tapering off.

I do understand the under drive concept.Also the blow off plate or blow off valves.
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Old 01-09-2013, 11:41 AM
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Quote:
Originally Posted by techinspector1 View Post
How to figure CFM with a blower.....

From Blower Drive Service......
"The size of carb(s) or CFM required for a given application can be calculated by the following formula A: {(CID x RPM) 3456} x {Boost 14.7) + 1} = CFM required. The amount of CFM required will determine carburetor size and quantity. If you try to use a carb with less CFM than required, performance and economy may be greatly reduced. Bigger is not always better when selecting carbs that are 30% over what is required, you may encounter problems in fuel distribution."

So, let's do one.....we'll use 8 lbs on a 355 @6000 rpm's....
355 x 6000 / 3456 = 616
8 / 14.7 + 1 = 1.544
616 x 1.544 = 951 CFM
Kewl.Tech then at 951CFM we would want say intake runners that are 118 CFM??.
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Old 01-09-2013, 11:56 AM
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1gary,I think when you edited your post from 225 cfm you were going to 218 not 118?
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Old 01-09-2013, 12:05 PM
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I am also guessing that as each intake valve opens consuming the boost there has to be enough PSI/CFM to cover that loss.That if we graft the CFM while the blower was operating we would see a vary erratic line of loss to regaining the CFM.Humm.Maybe we don't want to try to match the intake runners because we would consume too much of a blower's CFM output.
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Old 01-09-2013, 12:06 PM
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Quote:
Originally Posted by 1Gary View Post
Well if we chose a blower on the basis roughly the size of the engine and we know that blower's CFM is a constant,but we can vary knowing that CFM,heads and a cam that runs parallels the blower's power curve.

I am assuming first that blowers operate on a power curve because of a CFM output.Slower speeds less CFM and middle range a greater CFM and then the fastest speed or top end the CFM output tapering off.

I do understand the under drive concept.Also the blow off plate or blow off valves.
like a turbo blowers also have pump curves based on air mass, pressure, and efficiency. It's pretty much the exact same thing as sizing a turbo and most major blower manufacturers can supply you this pump curve for each blower. Like modifications done to turbine housing or wheels sometimes a "rule of thumb" must be used to take an existing curve for a known blower and apply it to a very similar blower (same model but perhaps slightly modified).

Once you learn how to use a pump curve it's really quite simple. (BTW these curves are the same type of thing you'll see for an oil pump, water pump, fuel pump, etc.)
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Old 01-09-2013, 12:09 PM
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Originally Posted by 1Gary View Post
I am also guessing that as each intake valve opens consuming the boost there has to be enough PSI/CFM to cover that loss.That if we graft the CFM while the blower was operating we would see a vary erratic line of loss to regaining the CFM.Humm.Maybe we don't want to try to match the intake runners because we would consume too much of a blower's CFM output.
you're looking at it backwards. In general if you reduce the restriction you increase the flow, lower the pressure, and increase the total power as your blower efficiency usually goes up slightly (yes even with less boost).

Of course if a blower is built for high boost levels (like 45psi) then it may LOSE efficiency at lower boost levels and decrease power. That's the idea of properly sizing one just like you size a turbo- keep it in the highest efficiency range possible for a given air mass.
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Old 01-09-2013, 12:10 PM
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Quote:
Originally Posted by vinniekq2 View Post
1gary,I think when you edited your post from 225 cfm you were going to 218 not 118?
Vinnie-I was looking at 951/8=118 and change.
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Old 01-09-2013, 12:19 PM
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!gary,I was looking from the point of horse power.220 cfm port making enough flow for a 400 horse power engine normally aspirated times 1.5 from the blower,making now 600 horse power needing a 900 plus cfm carb to accomodate the air flow required

all numbers rounded off to simplify discussion
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