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  #31 (permalink)  
Old 01-10-2013, 08:19 AM
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A roots boost curve is pretty flat. This thread is technically misleading.

There are recommendations for what blower size to use from the manufacturer.

The whole runner matching to the blower displacement is a little misguided. A blower works well on a already good running combination.

Then dividing the total cfm requirement by 8, for the magic runner length, is definitly different.

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  #32 (permalink)  
Old 01-10-2013, 08:29 AM
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Quote:
Originally Posted by spinn View Post
A roots boost curve is pretty flat. This thread is technically misleading.

There are recommendations for what blower size to use from the manufacturer.

The whole runner matching to the blower displacement is a little misguided. A blower works well on a already good running combination.

Then dividing the total cfm requirement by 8, for the magic runner length, is definitly different.
In general gear/screw pumps have a much flatter curve than a centrifugal pump and one of the reasons they may be chosen for a particular application, BUT advances in housing and turbine design have increased the efficiency islands of centrifugal pumps. They all have pump curves though, here's one for a supercharger I was looking at using: http://www.eaton.com/ecm/groups/publ.../ct_128485.gif

As for the cfm/8 thing I'm not sure where that came from, but as you stated the engine should be built in a very similar manner to an NA one with respect to the heads.
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  #33 (permalink)  
Old 01-10-2013, 03:01 PM
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So show or link to the curves you're talking about.
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  #34 (permalink)  
Old 01-10-2013, 03:04 PM
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Originally Posted by cobalt327 View Post
So show or link to the curves you're talking about.
The link I provided in my previous post is a pump curve for an eaton M90 screw type supercharger. While each manufacturer will vary they will all look similar to that one, just a typical pump curve.
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  #35 (permalink)  
Old 01-10-2013, 03:08 PM
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Explain it's use so the link has relevance.
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  #36 (permalink)  
Old 01-10-2013, 03:19 PM
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right side is pressure ratio (as compared to 1 atm),
bottom is technically volumetric flow (it should be mass flow) at 1 atm,
the green lines are blower drive speed
and the varrying purple fields are the blower efficency islands.

you'd size it the same as you would the compressor side of a turbo, plot your desired flow and pressure ratio at various RPM and search for a blower where your line goes through the middle of the highest efficiency islands.
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  #37 (permalink)  
Old 01-10-2013, 04:04 PM
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Hopefully that will be of some use to the OP. But the same plot/curve of a Roots blower will not magically make an 8-71 a better choice for a street 350 SBC. So while this may be semi-interesting in an abstract way, in the case of a Roots blower- for a street application, on an engine of a known displacement- this is basically useless, other than to confirm what is already known and understood by anyone who's worked on and built Roots supercharged engines. And Roots blowers are what I was talking about, as I said already.

What I see yet again is a "Hey look at me, I read books (or more like surf the internet)!" More than I do any real world hands-on, "I know because I do" information. But when you have no real world experience to speak of, this type of smoke screen is all there is for you to showcase.

You seem to find a way of contradicting every thread you post to- that's the only reason I ever see you post- to disagree w/some small point or detail To nit pick in other words. And you have to take something out of context to even be able to do that- like showing a screw curve when my subject was PD. In the end, while some people may find that entertaining, I don't. You're more trouble than you're worth.

Last edited by cobalt327; 01-10-2013 at 04:10 PM.
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  #38 (permalink)  
Old 01-10-2013, 04:28 PM
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  #39 (permalink)  
Old 01-10-2013, 04:33 PM
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Originally Posted by 1Gary View Post
I think the core of this is given the constraints of pump gas being a given and as a example end user X has a short block 355 and has chosen a 671 with a under drive now needs to chose a set of heads and a cam.What does he look to buy?. Let's say the boost level is 7 PSI.He wants to match the heads as closely as he can given the boost level and well for a lack of a better term the CFM rating.What size intake runner/CFM heads??.
Actually heads and cam choice is a bit irrelevant to blower choice. What we look at is finding a blower that gives us the boost we wish for on that engine..A blower on a stock engine can give a good preformance increase specially if youlive in the rock mountains a mile up..Generally on a stock engine limiting the blower to 5-7 psi allows a usable combo that still runs on pump gas and keeps its pistons intact..

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  #40 (permalink)  
Old 01-10-2013, 04:54 PM
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Was the original question to find out if your blower will run out of room at top end. The answer is no but can become unstable if its spun to fast and temps go up.

When at idle the blower will be putting out a lot less cfm but the engine will be using alot less. Now at redline the the engine will need more but tue blower will be putting out more. Now how to match the curve i think thats been gone over. Call the manufacturer.

For all practical purposes most blowers and turbos are very over sized. Most turbos will put out 50 pounds of boost. Most blowers are also capable with some big numbers. Usally oversized turbos and blowers produce more volume with less heat so they will make more power. So go big or go home still applies.
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  #41 (permalink)  
Old 01-10-2013, 05:28 PM
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just because a turbo or blower can make 50psi doesn't mean it should- it may be way out of its efficiency range and not be moving more air even though its at a higher pressure.

As for reading books, yes I read a lot of books to become a licensed engineer who has speced a hell of a lot of different pumps on everything from chrysler plants, electric generating gas turbines, to waste water pumps moving some of the nastiest **** you've ever imagined.

There is the correct way to spec a pump and your way. and FWIW an 8-71 MAY be ideal for a certain sbc 350ci engine combo- it depends on the needs and desired outcome.
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  #42 (permalink)  
Old 01-10-2013, 05:50 PM
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Originally Posted by ap72 View Post
just because a turbo or blower can make 50psi doesn't mean it should- it may be way out of its efficiency range and not be moving more air even though its at a higher pressure.

As for reading books, yes I read a lot of books to become a licensed engineer who has speced a hell of a lot of different pumps on everything from chrysler plants, electric generating gas turbines, to waste water pumps moving some of the nastiest **** you've ever imagined.

There is the correct way to spec a pump and your way. and FWIW an 8-71 MAY be ideal for a certain sbc 350ci engine combo- it depends on the needs and desired outcome.
Uh... Well for my part thats my point even the cheapest stock turbo systems will way outflow the engines they are mated to. Also that rate increases with rpm. If a turbo can make more boost it has to have something to blow agaisnt so yeah it will be making more cfm than the engine needs a lot more. And yes it will be bad air or turbulent because of impeller speed at the tips will be way to high at that flow rate.

Most car applicatins are based on charge temps. Bigger wheel will produce more air at a lower charge temp. Thus more power. That is the most important factor when picking a turbo. Roots style blowers used a different method for compressing the air but to some extend they will be grossly over sized as well when performing well. Also roots blowers must be matched up on the pwoer it takes to turn them. Some can eat 100hp just to make it go around so many things need to be figured out when selecting a blower.

Twin turbos are used on v8 because a single would be so big it would be hard to place and slow to spool up. So two twins is best for most big power apps for v8. Under 500 hp can be had easily with a single turbo. An over sized cold side will give you more power with the same amount of boost.
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  #43 (permalink)  
Old 01-10-2013, 06:28 PM
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Wrong? At least I'm in good company.

As far ar oversized, there's a similar concept about carbs, that they need to be "X" cfm for a given engine size/rpm. But you know what? In the REAL world, that often goes out the window along w/the other hot air.

A low rpm Pontiac shouldn't need a big carb- but even an 455 that peaks below what many BBC engines do, still like that big carb. Has to do w/the quality of the flow, not necessarily the quantity. And like carbs, there are other things that do not always hold true to form.

Last edited by cobalt327; 01-10-2013 at 06:34 PM.
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  #44 (permalink)  
Old 01-10-2013, 07:19 PM
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Well then am I to guess a Whipple blower is that much better over a Roots because it is a flat line CFM from idle to top end??.
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  #45 (permalink)  
Old 01-10-2013, 07:24 PM
How fast is fast enough?
 
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Originally Posted by 1Gary View Post
Well then am I to guess a Whipple blower is that much better over a Roots because it is a flat line CFM from idle to top end??.
no it doesn't, not even close.
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