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Old 03-04-2012, 08:12 AM
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6-71 Newbee

I haven’t worked with 6-71 Blowers ever and might need a little Help (understatement of 2012!)
I am purchasing this 427 SBC Kit from White Performance for my 1965 C-10 project. (http://www.ebay.com/itm/200689847016...84.m1438.l2649) i have contacted them and they will be able to get the CR to between 8:0 and 8.7:1 which should be OK for the blower i am told. It will have Dart Pro 1 215 Heads (2.08 1.60) and Dart Block (4.0 x 4.125) with all forged components they will provide Steel damper and second Crank Keyway for blower setup. I have done a Cam request worksheet on Comp Cams Website, this hasn’t been received yet. I have also purchased 2 600cfm VAC Sec Boost referenced Carbs (rebuilt)
The blower came with SBC Manifold, and polished front drive assembly (Has snout with pulley flange), nothing else.
I am not sure how to tell if it is a 6-71, or 8-71.
There is a 3/8” brass flare fitting on top of the front drive case, looks like maybe has something to do with lubrication, I would believe that as good of shape this blower is in that it would have the sealed bearings and not need engine oil (it was on a street rod 5 years ago, I need to know what it is for.
I was told by a friend that the boost referenced 600 CFM Vac Sec Holley carbs were not a good choice by a friend. He says the accelerator pump is too small, and I will have a hard time tuning the secondaries for the blower. (I thought I could purchase the larger Holley 50cc pumps for this?)
I am not sure what belt system to use for this 3” or 8mm what's the difference? (Mostly Street Use, occasional Strip)
I am also not sure how much Boost to use for this engine, so I don’t know what pulley size to buy.
I know I need to get my Power Brake Vac from the blower top, but where? does the typical carb plate have provisions for this?
I was also wondering if anyone has had any luck cobbling together Blower Drive parts on E-Bay (need to Save $$$ if possible)
Thanks for your help……..

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Old 03-04-2012, 08:53 AM
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6-71 is 15" without bearing plates included sealed bearings in the back , run gear grease in front, not full enough to get on bottom gear aways. run a good head gasket the mls gaskets are good, far as carbs and other street stuff lots of good guys on here for that info, myself I'm a beleiver in the afb /edelbrock you can bolt them on right out of the box ( done it ran 151 at 9.09 out of the box 750's) so I won't argue that point no boost referencing needed with modereate amount of boost , and easy linkage no matter which carb I would not recommend progressive linkage
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Old 03-04-2012, 09:42 AM
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Looks like your long block is pretty good. You must make sure of the compression. The pistons shown are too high. You will need a deep dish for the 427. They should be supplying them as you noted.

The motor will most likely be externally balanced so check the damper for a weighted side. You really do not need this heavy damper hanging on the end of the crank. A blower hub with the external balance is plenty good for a blown motor. Just make sure of the balance method. Many long stroke cranks are balanced internally with "mallory metal" which is far heavier than steel. Our top fuel stroker motors have a couple pounds of it.

Bantam already took care of the blower length. The 427 will required close to 1:1 or possibly a slight OD to give about 6 psi boost to start. I go with a 8mm metric belt system as there are many more ratios available plus it looks more "race like". You also can set the pulley system up so you don't have a giant wagon wheel top pulley. You will have to play with the belt and pulley charts to get it right.

Fill the gear case up with 90 wt gear lube. I think Bantam means lube and not grease. If there is a small triangular plate on the gear case fill it up to the bottom of the hole behind this plate. The blower will run much quieter. I know the whine is cool but after hours on the road will be obnoxious. At least to me. Not sure what the fitting is.....picture needed. There may be a sight plug somewhere on the front cover.

The Holley vac ref 600 vac sec carbs will work ok. I've used these before but you do have to be careful stepping on it while under partial load. This sets up a situation where you are past the majority of the acc pump and now are calling for full throttle. The sec start tripping in and fuel flow has to start very quickly. This is where the DP carb works well. It's a tuning thing and you will have to work with it. Get the Holley book with the cutaway 4500 carb on the cover. Try Amazon. Personally I'd like the 750 dp boost ref as a lot of us use. These have 4 corner idle and metering blocks rater than plates. Very tunable. Plus you can get QFT metering blocks with adjustable air bleeds. You can do the 50cc pump but get the book so you understand how the thing works.

Bantam likes the ED carbs..fine each to their own. haha. Can't argue with his success.

Definitely do not use progressive linkage. Get the correct ball bearing type. Try Good Vibrations Racing. Pretty good prices and they have the stuff.

Do get a nice scoop. They also have the BDS scoop with air cleaners. It's the big bore opening rather than the weak ones with the screen in the front. haha

As for patching the drive system I would not recommend it unless you know exactly what you need. There are many options and combinations. I'd just go to Don Hampton ( takes a while to get but it is good), Blowershop or others and get a complete system. It will be nice and work correctly with a minimum of fuss. Figure out how many v-ee belts you need first.

If you are going to street this don't get too wild of cam. Comp should come up with a pretty good custom grind. They did for me although I have not installed it yet. A hydraulic roller would be best.

I'm sure you will have many other questions but we can help.
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Old 03-04-2012, 10:00 AM
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thanks bentwings on the grease (wife was yakking before going to church) best excuse I can come up with . Yes if it is a gmc blower it will have the plate do as mr wings stated on filling, also if it is gmc measure the case were your carb adapter goes to see if it is small bore or large bore that is a big deal to estimate boost. 3/4 large 7/8 small. As I like nostalgic the 1/2 pitch is what I like. And I might add again if it is gmc they never made a 8-71 . have fun nothing better than blown . and wings and some others on here can get you going if you listen. just remember them edelbrocks are trouble free.
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Old 03-05-2012, 09:21 AM
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If it were me, I would have them machine the crank for dual keyways.

You 'll need at least -31cc dish pistons, -34cc would be better. the lower the compression the better.
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Old 03-05-2012, 11:49 AM
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Your getting alot of incomplete advice and definitive answers for a variable situation as far as boost and compression ratio. The lower the compression ratio the higher the boost you can run. Bds has a good chart on this page http://www.blowerdriveservice.com/techcharts.php that gives you lots of information on this but the quick version for your engine on 91 octane is at 9:1 you can run 6lbs, at 8.5:1 you can run 7lbs, at 8:1 you can run 8lbs, and at 7.5:1 you can run 10 lbs. Its always a good idea to leave a little bit of a safety margin though and run a pound or two less boost. Also all of these are fine compression ratios for blower motors and you will always gain more power by going with a lower compression and more boost.
So now what you need to do is determine how much boost you want to run and if you can get that low of compression on your budget. Compression is determined by your quench( gasket thickness and deck height), head volume, piston volume, bore and stroke, not just piston dish. This is why I get irritated when people throw out answers like you need 33cc dish pistons when they don't know how much boost you are running or your head volume. Not even the most knowledgeable engine builder can tell you what compression ratio you will end up with without all these factors. you need to determine what your compression ratio will be unless you are buying your heads block and rotating assembly from a reputable person that will tell you what the compression ratio will be for the combination of those three.
The easiest way for you to figure your compression is to go to http://www.summitracing.com/expertad...on-Calculator/ and put in all your numbers. Your bore is 4.125. Your stroke is 4. Your cylinder head volume will depend on what head you get and id recommend getting a 72-76cc head to help bring down your compression ratio. Your deck height and gasket thickness are a little bit more complicated. You want your combined gasket thickness and deck height to be between .04 and .06 and not over .06 if at all possible. Although you can go higher your engine will be more prone to detonation due to the quench effect. You will get your deck height from your engine builder or you just measure from the top of the piston at top dead center to the bottom of the block deck height. For the sake of making things simple though I would just put .025 in for gasket thickness and deck height. This will give you your compression ratio and allow you to make sure your combination will work as opposed to listening to someone else.
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Old 03-05-2012, 12:23 PM
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For the other questions Yes you use gear oil filled to the bottom of the triangle plate. For your vacuum lines you pull your brakes and crank ventilation from the carbs but you pull your boost reference boost gauge and vacuum advance from the intake manifold if you have these. An 8mm belt has rounded teeth where as a 1/2in belt has square teeth, the 8mm belt will take more power and be a little more efficient but the 1/2in. will make more whine. Both will work fine for you. Both the edelbrock and holley will work fine if properly tuned. In my opinion a holley blower carb with factory blower carb jetting will be a little more likely to just work out of the box. However holley does not offer supercharger carbs with vacuum secondaries so unless you are going to spend the money on new holley supercharger carbs or are prepared to tune or have some one tune your carbs go with the edelbrocks.
Finally although you can use a balancer on a blower engine however it is not needed because the giant blower belt does the job of absorbing the harmonics. Its recommended to just get a steel crank hub from bds dyers or Hamptons so you have less weight hanging off the front of an already stressed crank nose from all the upward force of the blower belt. Yes you can get deals on eBay on drive parts but because their are so few parts on ebay and so many people that want them they tend to get bid up quickly and you lose almost all the savings when you factor in having to pay shipping on each individual item. The only two parts that i was able to save a reasonable amount was on the pulley's and belt.
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Old 03-05-2012, 12:40 PM
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Ahh, here we go, nobody else knows anything, so...open the doors and let 'em in!
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Old 03-05-2012, 03:43 PM
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A 6-71 @ 1-1 is a good starting point atop a free breathing 427 sbc.If its more street than strip..8.5/1 static cr on pump swill is more than doable with the correct for your engine valve timing events.I say this because you typically drive around under vacuum,,and only make boost when you crack the throttle and feed the blower some air.Stacked up positive manifold pressure does not happen until the blower supplys more charge than the engine can injest.
You should cam this 427 with the exhaust side in mind..keep the overlap down with a wide lsa and let the blower work its magic on the intake side.
Fwiw..I think you are not taking advantage of the roots by restricting it with 600 cfm carbs.Roots blowers hate inlet restriction..they do a better job of pushing vs pulling.
Just to add..pretty sure you will find that 80-90 is commonly used to lube the front gears.The whine you typically hear is the air being forced out between the belt and the pulleys.I like the belt loose on start up and then set it after a good heat soak.1 inch of deflection side to side on the drive side hot will not tax the front main bearing shell imo.
Hope some of this helps.
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Old 03-05-2012, 05:56 PM
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A roots blower driven 1:1 on a 400 is 11.6#, on a 454 it's 8.5#'s. I estimate on a 427 it will be about 10#'s, that's a lot of boost for a street motor, even more so for a budget or "cobbled together" setup, used parts, etc. I wouldn't go that high. Weiand, with their street 6-71 and 8-71's, are setup to deliver 5 to 7 pounds. If you haven't experienced 7 pounds of blown power from a big or small block, it's a lot of power, and relatively safe. Something else to consider is a static c.r. of 9:1, 10 pounds of boost will make your effective c.r. right at 14:1 at boost. You would be wise to consider a boost timing controller and methanol/water injection system. Steel heads are even more of an issue with detonation, and quench is still important, .045 is good.

I also agree, 600 cfm carbs are going to chock a BB blower motor. I run two 750 BG/BC's, mechanical secondaries, on my 8-71 383 without any problems.
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Old 03-05-2012, 06:20 PM
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Blower Answers

firsy all can say is WOW!!!

Thanks for all the information!!!

i have quite a few answers that have stimulated my new Roots SC Brain cells.

i am in Brazil right now in between citys and wanted to check in on this post,,,, i only have a few minutes here at the airport to respond. so i have
to be brief.

I feel that the carbs i have selected (E-Bay 365.00) will be best re-sold and use the cash to partially finace some 750 DP with 1:1 linkage. i will be looking into purchasing a complete Crank to Blower drive system that will not require so much time getting things to line up.... i am all about saving dough, but not at the expense of poor operatianal tunning nightmares. it seems that i hav e a GREAT support group, with years of experience here in this forum,,, my only issue is i am not at home and have scetchy internet coverage for the next week or so until i get back.. i will have to be making decisions and opening the check book then. please keep the imformation coming, i can be more responsive when i get back home to Galveston Bay,,, 77518...

Wow! got a lot of great info to digest on my flight!

Fat Bastard....
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Old 03-05-2012, 06:58 PM
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I have heard from more than 1 that run blowers not meaning everyday bolt the blower on and go people ,that quench is not that big of factor . before coming unglued research a little. What gets me this isn't rocket science a blower is the simplest by far to run. far as the carbs read street supercharging they got a section on it by a pro. says holleys are nothing but a bandaid carb. AFB/ edelbrocks are a bolt on out of the box guaranteed you can't over carb a blower get the biggest damn ones they got and put it on and enjoy. far as used parts get the right ones just as good , **** the worst U.S. used is better than new china. Build a engine with common sense and knowledge don't take a fortune.
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Old 03-05-2012, 08:16 PM
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Quote:
Originally Posted by FatBastardsHotRodShp
...i can be more responsive when i get back home to Galveston Bay,,, 77518...

Wow! got a lot of great info to digest on my flight!

Fat Bastard....
A little off topic, but small world, I visit Galveston Island several times during the summer from Georgetown, North of Austin, normally stay at Dellanera RV park right on the gulf.

Quote from 1932Bantam:

"...guaranteed you can't over carb a blower get the biggest damn ones they got and put it on and enjoy."

That's not true, bad information/advice.
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Last edited by 68NovaSS; 03-05-2012 at 08:24 PM.
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