6-71 Blower: Two Lobe Rotors, Straight vs Helical - Hot Rod Forum : Hotrodders Bulletin Board
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Old 04-11-2006, 12:56 AM
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6-71 Blower: Two Lobe Rotors, Straight vs Helical

I tried a search but couldn't find any info. Are there 2 lobe rotors available for a 6-71? Straight and/or helical?

My theory is that a 2 lobe would pump more air per revolution and increase boost without having to step up to an 8-71. I think it would increase pulsing in the intake but don't know if it's an acceptable compromise. I'm not that concerned with idle quality.

I have the same issue with straight lobes. Helical reduces pulsing but increases side load on the bearings. Helical would still be my 1st choice if they're available. What are my options?

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Old 04-11-2006, 03:12 PM
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In this months Street Rodder their is a article about it I believe. See if you can't grab a copy of that.
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Old 04-11-2006, 06:30 PM
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I think you are mixing up a few things. A 6-71 no matter which rotors should move the same amount of air per revolution, this is what makes it a 6-71, same for an 8-71 (of course there are variables). You also have to look into air leakage. I'm not sure what goals you have or what size engine you are working with so, without that it is hard (impossible) to give any accurate advice. If you are thinking by running straight rotors you will be able to make the same boost and run the blower slower, I don't think it will work tht way. Share a few more details and maybe someone will have the answers. I don't have time right now to go look up the different rotor types, advantages and dis-advantages. I will try to take a few minutes later to look that up.

Royce
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Old 04-12-2006, 12:25 AM
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Thanks, I'll see if I can find that Street Rodder mag.

Published figures state that a 6-71 blower displaces 411 ci per revolution. With twin, 3 lobe rotors that should mean the area between the lobes that is trapped against the inside of the case is 68.5 ci. (411 divided by 6) It's not the lobes themselves, but rather the gap between them that determines displacement.

By comparison, the gap between a 2 lobe rotor is much larger and displaces more volume. If that volume measures 102.75 ci, then multiplying by 4 gaps would equal 411 ci, exactly the same as a 3 lobe. If the space is anything larger than 102.75, and that's what I'm thinking, obviously it would pump more than 411 ci per revolution.

It seems to me, the greater the number of lobes, the smaller the gaps and the less air it pumps. Like I said, it's just a theory, that's why I'm asking.

"You also have to look into air leakage."

Yeah, I thought about that too.

The engine will eventually be a 499ci small block stroker. At 30% OD the 6-71 will max out at around 12psi. I don't know how much rpm the blower can handle and I'm not sure if this will be enough to break the 1000HP barrier.

I'm not thinking straight rotors will give me any more boost. My reason for mentioning it is that if I had to fabricate 2 lobe rotors from scratch, it would be easier than helical ones. I have yet to see any available for a 6-71.
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Old 04-12-2006, 06:02 AM
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Why not just go with the 8-71? I think Camaroman posted here a little while back that your probably better off upsizing the blower and turning it slower to reduce parasitic loss form the drive not to mention bearing wear and possibly less heat to the air.
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Old 04-16-2006, 05:09 PM
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Here's a 6-71 down the page with straight, 2 lobe rotors.
http://www.chevyhiperformance.com/techarticles/49378/
It's a B & M Mega Blower. Unfortunately I haven't been able to find much information about it. BMRacing.com doesn't list superchargers on their site.
Other places have it listed as a 420 Mega Blower. I'm not sure if that means it displaces 420ci or if it's just a number pulled out of a hat. If it's the displacement, then this 2 lobe design doesn't appear significantly larger than a 3 lobe rotor.

Just for additional information I already have a brand new Weiand 6-71 that came with the project car, the movie car I'm replicating came with a 6-71, and the 8-71's 6% larger displacement didn't really impress me. An 8-71 sounds much bigger than a 6-71 but the rotors are only 1" longer and it displaces 436ci vs 411ci per revolution according to Holley/Weiand stats.
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Old 04-16-2006, 08:12 PM
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Quote:
Originally Posted by Brian S
Thanks, I'll see if I can find that Street Rodder mag.

Published figures state that a 6-71 blower displaces 411 ci per revolution. With twin, 3 lobe rotors that should mean the area between the lobes that is trapped against the inside of the case is 68.5 ci. (411 divided by 6) It's not the lobes themselves, but rather the gap between them that determines displacement.

By comparison, the gap between a 2 lobe rotor is much larger and displaces more volume. If that volume measures 102.75 ci, then multiplying by 4 gaps would equal 411 ci, exactly the same as a 3 lobe. If the space is anything larger than 102.75, and that's what I'm thinking, obviously it would pump more than 411 ci per revolution.

It seems to me, the greater the number of lobes, the smaller the gaps and the less air it pumps. Like I said, it's just a theory, that's why I'm asking.

"You also have to look into air leakage."

Yeah, I thought about that too.

The engine will eventually be a 499ci small block stroker. At 30% OD the 6-71 will max out at around 12psi. I don't know how much rpm the blower can handle and I'm not sure if this will be enough to break the 1000HP barrier.

I'm not thinking straight rotors will give me any more boost. My reason for mentioning it is that if I had to fabricate 2 lobe rotors from scratch, it would be easier than helical ones. I have yet to see any available for a 6-71.

Just remember a 411 ci blower will support a 822 cu inch engine driven 1/1. So if you overdrive it 30 %..... WOW.
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Old 04-16-2006, 10:16 PM
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Brian,
Finish doing your research. You are correct it is easier to make 2 lobe rotors. With that being teh case what do you suppose the reason blower companies don't use them? Let's assume your theory is correct and each lobe moves more air, that would also require more power to turn. It's like comparing a fine tooth gear to a coarse tooth gear, with one takes less power to turn? Which one runs smoother? I am not sure why you are trying to re-invent the wheel. I highly doubt you will pull off making your own rotors, blower companies couldn't even make their own 3 lobe rotors until fairly recently.

The B&M Mega blowers are no longer made. Also Holley bought Weiand and B&M they are now all Weiand blowers.

There is a significant difference between a 6-71 and an 8-71. Blower speed has a large effect on power and heat. I won't argue or try to change anyones mind. I've been down this road before. If you don't think I know what I'm talking about pick up the phone and call all the blower shops, see what they will tell you. Overdriving that 6-71 30%, I sure hope you plan on using race fuel. It will create plenty of heat. Sure a 6-71 can make big boost, it will just hve to work harder to do it, the harder you work it the more heat you get, the more heat you get the less power you make and the higher your chances of detonation. All boost is not equal, cool boost makes power hot boost, looks pretty on the gauge but, doesn't make power.

How are you going to make a 499ci small block? I assume it's not a Chevy. What kind of block are you using?

Royce
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Old 04-16-2006, 10:48 PM
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"The engine will eventually be a 499ci small block stroker. At 30% OD the 6-71 will max out at around 12psi."

According to Blower Drive Service, a large bore 6-71 driven 30% over will pressurize a 500 cid motor to 18 psi.
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Old 04-17-2006, 01:45 AM
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"According to Blower Drive Service, a large bore 6-71 driven 30% over will pressurize a 500 cid motor to 18 psi."

Thanks, I verified those numbers too but I'm not quite sure what they mean by small bore vs large bore 6-71.

My numbers were based on the download from Holley's site
http://holley.com/data/TechService/T...ech%20Info.pdf
There it states approx 12.5psi on a 502 @ 30% OD with a Weiand 6-71. I wouldn't mind the 18 though!
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Old 04-17-2006, 06:00 PM
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There's a large bore or standard bore 6-71. The larger bore moves more air. I don;t have the exact differnces in front of me. Also you can't really compare a BDS (or other custom) blower to a standard off teh shelf Weiand. The tolerances are different as is the cases (custom cases are typically stiifer, less air leakage).

Once again just because you "can" overdrive the blower 30% or even 80% doesn't mean it's the best way to make power. Pump gas will only take so much heat. People get to caught up on boost, it takes boost and FLOW to make power.

I am still curious of what type of small block you are building to displace 499ci?
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Old 04-25-2006, 02:59 AM
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Street Rodder Magazine info

I was finally able to pick up the June Street Rodder so I'll repeat some of the things they mentioned.

Below a picture of a pair of rotors that appear identical to the B & M ones I linked to earlier it read,

"These are the straight, two-lobe rotors from a Weiand 6-71 blower. Their CAD/CAM design allows for tight tolerances, which results in maximum boost and efficiency. While the 3-lobe rotors are preferred in competitive applications, the difference is undetectable on the street with mild boost."

An excerpt further down the article under Blower Selection reads,

"Weiand's 6-71 and many others use a pair of straight two-lobe rotors. As you might guess, there are arguments put forth for both designs. Two lobe-rotors are said to move more air per revolution, and the straight rotors don't push the air to one end of the case like the three-lobe helix design rotors do. On the other side of the debate are those who say the two-lobe rotors create pulses in the intake manifold and they allow internal leakage that reduces boost. The fact is all these arguments have merit."

So it doesn't appear my theory is a far off base as some would have you believe. Blower companies DO use them. Even though I can machine my own rotors, doubt it all you want, I can't reinvent something that already exists.

Is it worth pursuing? It wouldn't appear so at this point. It looks like the slight increase in pumping volume would be offset by internal leaks at high boost pressures.

I'll have to check with BDS someday to find out if they make an 8-71 with the same 3 rib external appearance as the std. 6-71s. If not, their large bore 6-71s should get me to the 15-18psi I'm looking for.

PS: I'm not running pump gas. The 499 was a typo, it's a 495. Custom 400 Ford block 4.185 bore X 4.5 billet stroker crank. I'm buying parts as budget allows but total costs expected to exceed $25K.
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Old 04-25-2006, 06:21 AM
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One thing to think about is that most 2 lobe straight rotor blowers are small. For example the B&M forced induction 144ci or 177ci blowers are fairly small blowers when compared to the size of a 6-71 or 8-71. There is a reason behind this. When a straight lobe blower becomes larger somthing interesting starts to happen. When the blower starts to build pressure, it actually builds pressure in huge pulses. Harmonics or frequency of the pressure pulses become a serious issue. This is why helicoil rotors were designed. What I dont know is why the pulses caused problems in larger blowers. I would have to guess that either the pulses created problems on the bearings or maybe the pulses created problems with clearances on the rotors from the off and on of pulses.
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