I picked up a 671 GMC blower and need to know the best pulley tooth count, It will sit on top of a stock 350 chev 1970 truck 4 bolt block and 441 stock 76cc heads 9.0 compression ratio. steel SFI balancer. I want to run 1/2 pitch belt and pulleys as they are cheaper and my boost will be under driven. from what I've researched I should run no more than 5-6 lbs of boost, probably 5 lbs. to keep the effective compression ratio at or below 12.0 Some of the charts I've seen seem to conflict with each other or Im reading them wrong. Im gathering that I should be 10% under driven thus using a 40 tooth top pulley and 36 tooth bottom pulley. does this sound correct?
sounds like a good starting point. Your comp. is on the high side for a blown gas motor though. Choose your cam wisely or have one cut for your setup .
Yes, you are in the ballpark with your figures. You can figure the drive yourself by taking the difference in the tooth count in this case 4 teeth (40 - 36 = 4 and divide by 36) tooth count of the bottom pulley. if you have a larger pulley on the blower you will be underdriving the blower, and the opposite if you reverse the pulleys. At 12 to 1 you will probably have a problem with detonation on pump gas. If you don't hammer on it constantly you can make it live.
It seems to me like your numbers for boost and underdrive are right. I had been doing similar figures for my project and came up with about the same numbers. If you were looking at the same charts I was for boost level, some charts are for purpose built superchargers with teflon stips on the rotors, and they likely are building boost more efficiently than a converted GMC blower.
Ran a stock block motor for a time with a blower . Perhaps we horsed it a bit to much but in a short period of time cracked 3 piston skirts (found them in the base). Had to rebuild it with forged pistons . Just a warning, the cast pistons will not take much abuse . It ran great after that for a long time.
Would it be better to go a tooth or two down? if I went to 39 top and 35 bottom that would bring me to 10.2 underdrive. and if I went 38 top and 34 bottom that would bring down to 10.5 underdrive. but is it even going to see any worthy power increase at theses levels?
I think a critical issue is going to be whether you have a large bore of small bore 6-71. The boost level would seem to vary a lot. I will have to look at my book, but if I remember right, small bores have a case thickness of 3/4" and large bores have a case thickness of 1/2" http://www.blowerdriveservice.com/techcharts.php
I ran a setup similar to what you're talking about for awhile. 350 Chev with unknown internals. Am guessing it was all cast as it was a 70's Camaro motor. 76cc heads, SFI spec damper, big mechanical fuel pump, 1/2 fuel line, Pertronics 2 igniter in an old Mallory housing, with headers and 3" pipes. 1/2 pitch drive belt with 38 top and 34 bottom pulleys. Never saw more than 6lbs on the boost guage. It was no powerhouse, but certainly more fun than when naturally aspirated.
My 70 350 sbc is rated at 255hp, your engine was probably about the same as a natty, how much increase in HP or total HP do you think you made if you were a good guesser? and what octane gas did you use?
get some dished forged blower pistons, then you don't worry too much about the drive ratio, as long as you're around 10-20% underdriven you should be ok. but you probably want a steel crank too. If you keep the rpm down then the stock rods should be ok, although it's a good idea to get them resized with good bolts.
HP guess ..... no idea, but not alot. Ran it on BP Ultimate, which is 98 octane down here. Was really just part of the learning curve while funds were short.
between the vertical ribs on the side of the case. You should just be able to stick you fingers in there and feel it. 1/2" feels real different that 3/4" I'm looking for a reference now to double check my numbers. Hopefully someone will chime in to verify.
I thought all the small bore's were 6V71's and large bore were 6-71's edit- upon further investigation the 6-71 can be ether. one way to differentiate is the cross bars on the top if all have a machined surface then its a small bore, if 2 of em are rounded unmachined then its a big bore , that is if they haven't been cut out yet. mine is a big bore.
According to the Ganahl book: no need for fancy piston/rod/crank at 5-6 pounds of boost. The problem will come when you are addicted to boost and change the pulleys....
There are big and small bore 6-71s. Not to be confused with a V. The amount of boost difference on a street motor is nominal though.
OK I need to put the brakes on for a second, I just realized my gears are on the back of my blower and none on the front! has a funny back cover and the normal front cover. Im assuming I can just flip the rotors and gears to the front and be ok I hope
sounds like you're gonna get yours going before me/mine - would like you to keep me posted on your progress. I'll bring the beer when you fire it up!
I recommend Pat Ganahl's book on Street Supercharging. I think I memorized a large part of it while planning for the day I could put my 671 into my 32. Mine runs 7.5:1 static compression with 19% under-driven blower. Makes for a nice street setup. Lots of noise and looks but very streetable with no overheating. The engine was built as a blower motor so has forged pistons, Pink rods steel crank, roller rockers etc. I only see about 5-6 lbs of boost when I get on it but lots of low end torque. I keep thinking about taking it to 10% under but it ain't broke so I ain't fixing it.
"Now its time to put this huffer back together.Before we go any further I should mention that a blower could be accidentally put together backwards. Match up your mounting bolts with your manifold to make sure of the bolt pattern. The bolt pattern is offset." Quoted from Rob Kozaks thread. I think I'll be ok, probably will have to bring the bearing plate around to the front with rotors and gears too being the rebuild kits use different front and rear bearings. I.ll check into that book too. I remember the fellow I got it from said he thought it was off an International, I'm guessing these 671's were used on more than just GM products, mine is red and most Ive seen are a puke green color.
Im a long long way of turning the key, car still looks like my avatar and I havent even started on the frame yet-its too dang cold out, so im doing some indoor stuff like buying unknown parts
Here is what happens when you get blower happy without using the right parts I did this a few weeks ago. BBC454 cast everything, 5LBS on a 671
damn! so that's why I spent all that money 20 yrs ago to build a strong, mild blown 454 still running the same forged pistons I bought way back when. Lotsa miles on them.
Yeah.. I knew this engine was going to go ( already been sleeved twice )... I'm working on a forged build now.
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