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Discussion Starter #1
Is there any real reason to run specifically 8 or 16 injectors in the top of a roots blower? I figure that the fuel is all going to swirled around so much inside the roots blower that there's no point in setting up my Motec as a sequential ECU, and for the same reason does it really matter how many or how big your injectors are?
As long as you have sufficient flow to support the power level, and sufficient control of the injector pulse widths to get the car to idle, is there any more to it?

This is a pump gas street car with one fuel cell and no plans to run methanol or E85 with staged injectors.

Thanks!
 

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i cant answer your question that your asking , but just a fyi you saying about the fuel swirling around the blower , you would be surprised how accurate them things throw fuel into each port , hence why we change hat nozzles to cool cylinders down etc on our blown motors ...

Dan
 

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8 injectors. More space less cables less money less work. the rest you have already figured out your self. what type of throttle/injector are you going to use?
 

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Discussion Starter #4
I have some 96lb Holley low impedance injectors, Motec M48, and I'll probably go with a 90mm Accufab throttle body as it's worked so well on my other motor.
Going to fabricate a plenum that tapers from almost nothing at the front, and opens up to a rear facing throttle body so that I can fit it all under a 4" cowl hood.

Dan - that's a really interesting point. I didn't realize that it was as accurate as that. Might tune it as a sequential deal after all then...
 

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Roots distribution accuracy,

I don't know what kind of manifold you are running but on most non setback roots motors distribution is anything but good. Lots of guys with roots blown hemis run shorter rods on the front two cylinders because even juggling port jets they are unable to weaken the front two holes enough so that they can optimize the timing on the rear six without reducing the efficiency of the front two with reduced compression.

PS these guys are not running electronic timing boxs.
 

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90 mm will choke it if you want more than 600 hp. go 2 of those throttles and it will have enough air. let one of them open before the other and it will be easier to handle on the street. sounds like a good plenum design to have a smoth looking hood.
 

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Discussion Starter #7
I'm really confused about throttle body size...

I know that my 90mm Accufab is good for well over 800hp on my YS Vortech SBF.

George from Accufab said on another thread here that the cfm of a throttle body is doubled in a blow through or draw through forced induction motor. That being the case, a 90mm is good for around 2700cfm. That should be enough for up to 1000hp, shouldn't it?
 

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The roots blower sucks through the throttle body so you need alot of area. Turbos and Centrifugal Blowers force the air through so a smaller one works ok.

I don't know who said roots top injectors are good but there is a reason we run down nozzels under the blower.
 

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Discussion Starter #11
WJ - that was definitely my first thought, but it unfortunately won't fit under the cowl hood without cutting up and re-profiling the leading edge.

I was thinking of a design similar to this - where it's only a couple of inches tall at the front of the plenum, but tapers back to whatever size/flow throttle bodies are needed
 

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You're going to choke the shit out of that motor with something like what you have pictured, if that's where you intend to mount the throttle body.

Hilborn has stack injectors meant for the tops of blowers... 2 and 4 port uprights are common, but they also have single stacks in a multitude of sizes. I used to run a single 4-5/8" stack, and then swapped to a double of the same.

Pics of single and double are here: http://www.onebadgmc.com/index.php?option=com_content&view=article&id=17&Itemid=20

Hildborn told me flat out that the single stack in a vacuum condition would supply 1500 CFM. It choked the crap out of my 462 with the 8-71 on it. When I added the second stack (in the photos with the hood), it made so much more power that it drove through the converter. On the second pass with the twin stacks, I over rev'd it and smashed the valves into the pistons.

I don't know how much vacuum is at the top of the blower inlet, with the throttle bodies at WFO, but I don't necessarily believe that you can double a throttle body's volume with vacuum. Pressure now, as in a blow through setup, is another story. Flow of gasses or liquids can be calculated pretty easily based on the density of material versus the orifice size.

The other thing for you to keep in mind in doing all of this is air flow into the blower. About the only guys I know who run hats backwards on blowers are the boat guys, and they only do it because the motor is backwards in some boat applications.

Shrinking the plenum just above the front of the rotors is bad enough, but then you're going to make the air do a 180 before it enters the rotors???

Cut a hole in the front of your cowl and make it look nice. Otherwise, you're gonna kill power.
 
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