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Old 11-16-2012, 06:22 PM
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Supercharger 6-71 vs. 8-71

I have a Dart SHP block, Dart Iron Eagle heads, SBC 3.75 stroke, 4" bore, Comp cam .640 lift, 255 I - 262 E, 2.08 I - 1.6 E valves Mech solid rollor lift cam, 8.2-1 comp, Scat forged rotating assembly, This motor should handle a fair amount of boost.
For carbeuration I will run 2 650 cfm Quickfuel boost referenced carbs. I am ordering a blower from Supercharger USA and can't make up my mind between a 6 or 8-71, The car will use E85 fuel, which can handle about 11 - 12 lbs of boost. altitude is 2500 ft. On the 6-71 stage 3 billet, teflon I would need to overdrive 10 yet a 8-71 stage 1 or 2 (No teflon strips) billet it would be about a 1 - 1.
My concerns and reason for this post is to find out 1. Torque difference (if any?) 2. Temp drop with E85 and 1-1 ratio vs 10 overdrive. At 3600 RPM it would be 360 less rev w/ the 8-71 - At 7000 9Max RPM) it would be 700 RPM. I was told this difference should have little effect on the performance.
Is it better to not run teflon strips on the street? This car will go to the track and car shows. Mostly a wild toad street ride
1963 nova caged 18" tire 4.88 gear th350 3300 stall

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Old 11-16-2012, 06:51 PM
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if its show,use the 871. dont be shy with the carbs,use 2x 850s at least
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Old 11-16-2012, 06:54 PM
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Too much carb - for street - wanted to know about engine and air/fuel temp
Thanks
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Old 11-16-2012, 07:33 PM
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2 x 650s is not enough for N.A. if its show,use w/e you want. I use that cam in my n a street car
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Old 11-16-2012, 09:05 PM
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First of all, a 6-71 on a 383 running 10% overdriven is 15.5 pounds of boost, an 8-71 at 1:1 is 14.4 #'s.

Your motor is a 377 so you'd be somewhere in between.

Secondly, a 383 8-71 motor with two BG 750's isn't too much for the street, mine is very well mannered. 850's? depends on the build. 650's? Posing comes to mind.

I don't really understand what your concerned about with this temp thing, but I would shy away from teflon for a street blower, just from a maintenance stand point.
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Old 11-16-2012, 09:29 PM
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My concern is overheating on the street with too much overdrive heating the air in the supercharger and causing detonation E85 fuel runs cooler right? Anyone have a supercharger with E85 and what do you know about how it works.
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Old 11-16-2012, 09:31 PM
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Also, you don't need to come close to 7k rpm's, the roots blower makes a ton of torque off idle and at low rpm.

Whether it's a 6 or 8-71 isn't the issue with torgue, torque at 10 #'s of boost is what it is, given the same motor specs, regardless of what's bringing that boost on.
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Old 11-16-2012, 09:37 PM
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Quote:
Originally Posted by David 377 View Post
My concern is overheating on the street with too much overdrive heating the air in the supercharger and causing detonation E85 fuel runs cooler right? Anyone have a supercharger with E85 and what do you know about how it works.
You're going to heat the air regardless, a 6-71 overdriven will be spinning faster than the 8-71 at the same boost level. I wouldn't be too concerned about the difference in fuel charge temp at reasonable street rpm's, the 8-71 is capable of higher boost, but you don't really "need" it...did I say that??
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Old 11-16-2012, 09:51 PM
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Thanks - yet I'm still worried about the torque loss with the 8-71 how much cooler is 1:1 vs. 10 over - is it enough to be concerned or to help keep it streetable?
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Old 11-16-2012, 10:47 PM
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Just another thought, from a performance, durability standpoint I would seriouslly consider a modern supercharger. Whipple Industries
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Old 11-17-2012, 01:46 AM
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What are you talking about with "torque loss"?? A loss of torque due to fuel inlet temp or heating of the fuel by the blower?

That's waaay over thinking a blown application for the street. That said, I would assume the 6-71 overdriven will generally produce more heat.
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Old 11-17-2012, 09:43 AM
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I appreciate all the input yet alot of what I originally asked has not been discussed. E5 with superchargers - who can answer how much cooler it runs. 8-71 at 1:1 ratio vs 10 overdrive 6-71 - 411 per rev 6-71 - 436 per rev 8-71. How much torque loss will the 8-71 have with the extra air being forced at a lower 1:1.
This car is at 2500 altitude and running anywhere fro 40 degrees to 100 in summer
Bottom line looking for the proper application for this lil puppy motor!?
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Old 11-17-2012, 02:50 PM
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Check thihttp://www.performanceboats.com/pb-open-water/49891-e85-790hp-sbc-epa-carb-complient-supercharged-engine.htmls out!
I guess I found what I needed - Thanks everyone
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Old 11-18-2012, 01:48 PM
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6/71 due to its size . weight and it requires less bhp to run . if your bothered about heat introduce some water meth inj to bring the inlet charge down . the higher octane the better . think your spot on with your carbs for the street , if you can fit the blower cooler below even better . dont worry too much about the blower spining faster . wetten it up a tad more and it will be fine . torque wise there will be a difference for sure . 8/71 much better low end but 6/71 will be better mid range i feel and watch the 8/71 after 5000 rpm . thats why i would use the 671 as its safer and more suited to your engine size.

good luck
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Old 11-18-2012, 02:56 PM
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Quote:
Originally Posted by mustang 671 View Post
if your bothered about heat introduce some water meth inj to bring the inlet charge down.
The OP is gonna use E85, so I'd be more worried about icing up than I would heating up. You start whippin' alcohol around in a blower case and you can expect some very cold inlet temps.

When you see a crew member spraying WD40 or some other lubricant on the blower hat throttle blades and shafts on an alcohol car at the starting line, that's to help prevent the whole mess from freezing up solid.
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