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  #16 (permalink)  
Old 01-09-2013, 11:19 AM
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A couple fragmented thoughts...

Besides the usual rules of thumb and whatever formulas there may be for matching the blower size to the engine, the thing that has to be kept in mind is the blower should be matched so there's no need to under or over drive it more than a few percent.

Even though a blower can be overdriven to produce more boost, the more it's overdriven the less efficient it becomes, much of it from the air being heated. Conversely underdriving a blower can cause the engine to see no appreciable boost until the engine is too far up in the rev range to give any real benefit.

Being as how there are only so many blower capacities to choose from, it becomes evident that the best results will usually span two blower sizes for any given application. Instead of trying to use the CFM output of the blower, it's easier to use the potential boost produced by a blower at a given drive ratio to match it to the engine displacement. All the manufacturers have charts and such for this. Because in the end, it's the amount of boost that's going to be the limiting factor. Too much boost is same as too much CR- the fuel's detonation resistance limits how much we can use.

Unless the blower is sized wrong, it won't max out before the engine peaks, rpm-wise. If it does, the pulley size can be changed. If that goes beyond about 10% or so, the blower may be the wrong size to begin with. This doesn't include the mini-blowers- they are WAY overdriven, like in the 2:1 range.

Links to blower info.

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  #17 (permalink)  
Old 01-09-2013, 11:34 AM
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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??.
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Old 01-09-2013, 11:35 AM
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Originally Posted by cobalt327 View Post
Besides the usual rules of thumb and whatever formulas there may be for matching the blower size to the engine, the thing that has to be kept in mind is the blower should be matched so there's no need to under or over drive it more than a few percent.

Even though a blower can be overdriven to produce more boost, the more it's overdriven the less efficient it becomes, much of it from the air being heated. Conversely underdriving a blower can cause the engine to see no appreciable boost until the engine is too far up in the rev range to give any real benefit.

Being as how there are only so many blower capacities to choose from, it becomes evident that the best results will usually span two blower sizes for any given application. Instead of trying to use the CFM output of the blower, it's easier to use the potential boost produced by a blower at a given drive ratio to match it to the engine displacement. All the manufacturers have charts and such for this. Because in the end, it's the amount of boost that's going to be the limiting factor. Too much boost is same as too much CR- the fuel's detonation resistance limits how much we can use.

Unless the blower is sized wrong, it won't max out before the engine peaks, rpm-wise. If it does, the pulley size can be changed. If that goes beyond about 10% or so, the blower may be the wrong size to begin with. This doesn't include the mini-blowers- they are WAY overdriven, like in the 2:1 range.

Links to blower info.

you're making a very wrong presumption that all blowers are designed to work at 0-6000 (or so) RPM. Many factory cars equipped with superchargers overdrive them in factory condition. Under/Over drive has pretty much no relevance- blower drive speed does though and it needs to match the conditions a blower is designed for.


Drive speeds are usually included on the pump curve charts as well.

So again, JUST LIKE IN A TURBO APPLICATION use the pump curve to select the blower that best meets your needs, and as Cobalt pointed out you need to pay attention to drivespeed and match your pulleys to provide what the blower is designed for.
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  #19 (permalink)  
Old 01-09-2013, 11:39 AM
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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??.
size your heads the same as you would for a NA application. On a typical street 350 that means you'll probably want around a 200cc port and around 270cfm on some good heads if you're looking at a 6500RPM redline.

and forget about boost psi for a minute and think about air quantity, what you're really concerned with is lb/min, not psi. For a given lb/hr you actually want the psi as low as possible- this will indicate the rest of the engine is working well. A high boost level actually indicates a high restriction and not much more.
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Old 01-09-2013, 01:21 PM
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size your heads the same as you would for a NA application. On a typical street 350 that means you'll probably want around a 200cc port and around 270cfm on some good heads if you're looking at a 6500RPM redline.

and forget about boost psi for a minute and think about air quantity, what you're really concerned with is lb/min, not psi. For a given lb/hr you actually want the psi as low as possible- this will indicate the rest of the engine is working well. A high boost level actually indicates a high restriction and not much more.
Yeah-the air quantity is what I am trying to describe in the term CFM.So if a guy has a constant PSI of 7 lbs,he needs to look harder at the engine parts to see why the engine isn't accepting the boost level.WOT should show a dip in PSI then.
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Old 01-09-2013, 01:32 PM
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It is kind of amazing no one has developed a electronic variable under drive that would adjust to throttle position,load and RPMs. Well at least I don't know of anyone that has. I think that would broaden the output of the blower.Say a 871 that would normally be used to a 500 inch engine that has that potential on a 355 under driven and as the need arises changing the under drive ratio.Of course taking into account the pump gas we have to work with.
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  #22 (permalink)  
Old 01-09-2013, 01:35 PM
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Originally Posted by 1Gary View Post
Yeah-the air quantity is what I am trying to describe in the term CFM.So if a guy has a constant PSI of 7 lbs,he needs to look harder at the engine parts to see why the engine isn't accepting the boost level.WOT should show a dip in PSI then.
cfm is flow, quantity (mass flow) is measured in lbs/min, lbs/hr, kg/second etc. It's a mass unit over a time unit.

as for boost at WOT that depends. It may temporarily spike up or down depending on where you measure the pressure and where the throttle plate is located.

What do you mean by "accepting the boost level"? As your engine breathes easier a given blower at the same speed will produce less boost, but the same (or slightly better) mass flow. In this case boost levels going down is a GOOD THING.
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Old 01-09-2013, 01:38 PM
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Originally Posted by 1Gary View Post
It is kind of amazing no one has developed a electronic variable under drive that would adjust to throttle position,load and RPMs. Well at least I don't know of anyone that has. I think that would broaden the output of the blower.Say a 871 that would normally be used to a 500 inch engine that has that potential on a 355 under driven and as the need arises changing the under drive ratio.Of course taking into account the pump gas we have to work with.
now you're looking at a variable vane/compressor on a exhaust housing for a turbo. Which is in current use and has been for years. The trend is moving from blowers to turbos because they simply work better, there's nothing wrong with a blower but the only advantage it has over a turbo is packaging- and even that is arguable.
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Old 01-09-2013, 02:01 PM
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Actually turbos are easier on the bottom end and have the sleeper value.I have on some of the diesels heard turbos top out and what I mean about that is a turbo going beyond their output. Not very pretty sight.
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Old 01-09-2013, 06:18 PM
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Im interested in turbos now.So much power to be had. I see that even the carb blow through systems are getting pretty good. The sleeper value is great.If someone drives away from you then no big deal.If someone drives away from you and you have a blower sticking out of the hood ,then OMG how embarrassing.
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  #26 (permalink)  
Old 01-09-2013, 07:47 PM
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What is a trend I have seen,is twin turbos because they can be had out of pick and pulls.I'd guess the downside of that is it would be critical to have matching sides.Not that I mean matching turbos,that is a given,but matching performance output out of both sides.
In either case,blowers or turbos,I don't think the misfortune of a mistake backfire into the intake or the exhaust with turbos can be afforded without serious consequences.
I do vaguely remember a story of a street blower on a rod yrs ago on a cruise night blowing up at a hot dog stand hurting a number of people with shrapnel.
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Old 01-09-2013, 08:44 PM
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What is a trend I have seen,is twin turbos because they can be had out of pick and pulls.I'd guess the downside of that is it would be critical to have matching sides.Not that I mean matching turbos,that is a given,but matching performance output out of both sides.
In either case,blowers or turbos,I don't think the misfortune of a mistake backfire into the intake or the exhaust with turbos can be afforded without serious consequences.
I do vaguely remember a story of a street blower on a rod yrs ago on a cruise night blowing up at a hot dog stand hurting a number of people with shrapnel.
Should they be matched and optimized- yes. Is it required? Not at all. Even if your efficiency is terrible you're still moving a large amount of air and fuel and will still make pretty big power numbers.

Buying a single properly matched turbo is pretty common now though, you can get one for well less than a grand and it can be built to match your engine, not just whatever is available at a scrap yard.
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Old 01-09-2013, 09:13 PM
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One foundation in turbos that I remember back from the days when I owned a Corvair spider is the turbos must have good clean oil.That the heat from turbos requires more frequent oil changes.
Is the turbo guys more sensitive about high volume or high pressure oil pumps and look to use a increased size oil pan??.
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  #29 (permalink)  
Old 01-10-2013, 01:52 AM
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Originally Posted by ap72 View Post
you're making a very wrong presumption that all blowers are designed to work at 0-6000 (or so) RPM. Many factory cars equipped with superchargers overdrive them in factory condition. Under/Over drive has pretty much no relevance- blower drive speed does though and it needs to match the conditions a blower is designed for.


Drive speeds are usually included on the pump curve charts as well.

So again, JUST LIKE IN A TURBO APPLICATION use the pump curve to select the blower that best meets your needs, and as Cobalt pointed out you need to pay attention to drivespeed and match your pulleys to provide what the blower is designed for.
YOU are making a very wrong assumption that I was talking about "factory cars equipped w/superchargers". I may be wrong but because 1Gary has posted to several Roots blower threads and has mentioned "street blowers", I guessed he was interested info on the common sizes of Roots-type blowers used on SBC street/strip engines.

"Under/Over drive" certainly DOES have "relevance"- it's what controls the blower drive speed, after all. If you stop trying to nit pick and think outside the textbooks for a minute, you might realize that 'under-/overdrive' can also relate to the percentage of difference from the optimum ratio- whatever that optimum ratio might be.

As I mentioned about the mini blower's high optimum ratio, it could said to be too overdriven when it's at a 2.5:1 ratio. A bigger Roots blower may be said to be too overdriven when it's at a 1.25:1 ratio (both arbitrary figures).

When you say "pump curve" when referring to a Roots blower, exactly what do you mean? Because Roots-type positive displacement blowers basically have a flat boost curve. So what other criteria would you be talking about for this pump curve that would be a better guide than using boost/drive ratio/displacement?


Last edited by cobalt327; 01-10-2013 at 01:59 AM. Reason: Add graph.
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Old 01-10-2013, 08:06 AM
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Originally Posted by cobalt327 View Post
YOU are making a very wrong assumption that I was talking about "factory cars equipped w/superchargers". I may be wrong but because 1Gary has posted to several Roots blower threads and has mentioned "street blowers", I guessed he was interested info on the common sizes of Roots-type blowers used on SBC street/strip engines.

"Under/Over drive" certainly DOES have "relevance"- it's what controls the blower drive speed, after all. If you stop trying to nit pick and think outside the textbooks for a minute, you might realize that 'under-/overdrive' can also relate to the percentage of difference from the optimum ratio- whatever that optimum ratio might be.

As I mentioned about the mini blower's high optimum ratio, it could said to be too overdriven when it's at a 2.5:1 ratio. A bigger Roots blower may be said to be too overdriven when it's at a 1.25:1 ratio (both arbitrary figures).

When you say "pump curve" when referring to a Roots blower, exactly what do you mean? Because Roots-type positive displacement blowers basically have a flat boost curve. So what other criteria would you be talking about for this pump curve that would be a better guide than using boost/drive ratio/displacement?

The curve isn't flat, its "flatter", the reason its curved is because it is more efficient at some speeds than others. Also, this graph shows the boost pressure on a particular engine, NOT the pump curve. The engine has a huge effect on the curves you gave.

And I was referring to the commonly accepted meanings of under/overdrive on roots blowers, not the ones you just conceived in your head this morning.

As for keeping out of books- if you opened a text book or catalog you'd see that all the information is there making this conversation irrelevant- for anyone that can find a book that is.

Last edited by ap72; 01-10-2013 at 08:18 AM.
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