Technical GMC Supercharger .......how much power to drive one

I'm thinking seriously of putting a 6-71 on an engine.........but haven't decided whether to make it functional or just a dummy. The engine it would go on will have pretty decent power (450/500) naturally aspirated. The car will be pretty light. While I know if I set it up properly, I should be able to add an honest 100 hp or more.
The problem is that I will have to always run premium gas, and gas mileage will go down. I realize that the amount of overdrive/underdrive will affect how many HP it takes to drive a working supercharger. I am thinking that it will take roughly 50 additional HP to drive a functional blower......so I would have to make 150hp extra to realize a 100 hp gain.
I'd like to hear from some of the people who run these blowers on the street and the pros and cons of doing so. I already have a pretty decent 6-71 sitting on the workbench.......where should I go from here?

 

It takes hardly any power at all to turn a blower, when the throttle is closed...but when it's open, and making boost, it takes significant power to turn the blower. How much? good question, and it's hard to answer. The important thing is that the added power from the boost is always more than it takes to turn the blower.

I run 87 in my blown 427 pretty often, but not when I'm racing--then I run premium. It has about 8.5:1 compression, and aluminum heads, and makes somewhere around 6-8 psi boost. Mileage is nothing to write home about, but not much worse than I get towing it behind a gas powered truck.

Put the blower on, have fun, you only live once.

 

Thanks Jim, I didn't know you could run regular with a blower for daily driving. I plan to run a lower comp (not firmed up yet) and was thinking about 9:1 but lots of cubic inches. That helps me a lot.

 

NO...simple answer !

As you may...figure, as the engine rpm climbs, so does the load on the blower !
As I understand, it requires about 1000hp to turn a 10-71 blower on a Nitro engine. This number is a few years old...possibly more today

Obviously, you won't be running nitro, or a 10-71 blower.
BUT, as I said, as the rpm climbs so does the load on the blower.
Also consider -
1. How the blower is built. Teflon on the lobes, nylon on the ends of the rotors ?
2. How tight are the rotors to the case, and each other ?
3. What will you be using for overdrive ?
4. How much boost do you want to see ?

As you can see here...if the blower is a quality, tight build, it will require more power to turn ! As the overdrive goes up, it will require more engine power to turn !

A loose rotor to case number, the easier it will be on the engine, BUT, it will require more overdrive to build boost ! So, you are fighting the blower build vs. boost pressure.

Your BEST bet...is to contact a super charger builder and ask THEM...your specific questions.

Mike

 

Man the days of fake blowers went out with bell bottom pants in the last century. All of my cars have superchargers heck even put a 671 on the flathead in my deuce and drove the hell out of it. The additional power realized from the blower deal as well as the looks factor is hard to beat. Power it takes to turn the blower depends on a lot of things blower drive ratios, weight of the ride, how many carbs opening, and the rear gear ratio to name a few. Could be 50 HP however if it is that's at the very high end of the engine RPM .
The blower is only riding along until the carbs open so the gas milage and the premium fuel needed is only a consideration until you want to use the option that being the load pedal. Your results may vary it all depends on how its being used.
Ronnieroadster

 

As I always say "if you start with (more than) enough hp, then it's all good"!!!

 

Simple Answer = the HP to compress the CFM to the boost required and the efficiency of the blower
- moderate boost and engine size approx 125 to 150 HP 36fordguy

 

Get this book. It explains a great deal.

 

Do not do a fake blower! I've put together 3 Roots packages, and for me it's hard to beat. The drag race tie in is undeniable, the sound of it whining away out there, the look that says pure power, and throttle response when you hammer it never gets old. They say a typical street 6-71 eats between 50-100 HP, but if you put together any kind of decent engine, you will gain at least 125 horse over the loss, more likely closer to 200. Yes, they eat some fuel, and they do demand some dedicated good parts, but the first time you stand on it, you'll understand.

 

Don't over think it. There is no need to spin yourself down the theory rabbit hole. Once you decide for sure you want to do it, give us a holler and we can get you pointed in the right direction. Blowers are cool looking, work great and are much more simple that what you might think.

Here is my pile that has almost 100K street miles with a blower and the only drag race I ever lost was to Squirrel.



-Abone.

 

Did they mention how much power the blower was adding to the engine's output at 17 psi boost?

Like he says, don't overthink it, just put the blower on and have fun

 

A friend of mine worked for Ak Miller back in the day. One of his customers was Dr. Nathan Ostich (the flying caduceus, first jet powered LSR). He was always playing around with his cars. On time my friend, Al Bradshaw, put together a blown hemi for the doctor. They were trying to get by the parasitic load of the blower so the blower was run by a Harley engine. Al said it worked, but the timing ***ociation said they had to be in a two engine cl*** and that put them out of any compe***ion.

Ray Brock, the Flying Caduceus, and Al Bradshaw.

 

So I went back and read this again. What I posted earlier does not represent reality. Apparently this Drag Boat racer Bob Brownwell who ran Arias powered blown gas flat bottoms built a blower dyno to try and test the very question the OP asks. They put an 8-71 on his blower dyno and hooked up a stroked 365 cid sbc w/one 4 bbl. headers and such to turn the blower. When they closed one or two of the air box valves to make boost it stopped the engine. Next he hooked up a 427 BBC which could only hold 5-6 Lbs. with the 8-71 race blower before bogging down. Finally he hooked up the 454. They went on to test different blowers at different intake temps to test adiabatic efficiency. They were definitely over thinking this but hey sure you can just slap one of these things on anything but armed with the knowledge of those who came before us can result in a lot more fun.

 

My old 392 makes 610HP at 5800 RPM and it gets around 10 MPG on the street... This huffer did come off an old GMC 671 diesel truck.... Runs @ 6% under makes 8LBS of boost and uses 91 Octane pump gas... I run 32 degrees mechanical advance all in by 2000 RPM....

 

Many years ago John Force team ( Austin Coil ),,built a supercharger dyno just for this purpose.
If I remember correctly,,,,they found that it took around 300 HP to drive their blowers .
But,,,,that was a 14/71,,,,with Nylatron on the rotors and a super tight case tolerance,,,,,,,putting out enough boost to lift the heads off most engines .
Think about it,,,,,300 hp is a lot of load just to drive a pump,,,,,but it creates much more than it uses .
And,,,that’s at full boost,,,,,,Fuel engines only run at idle or full throttle,,,,,,and I think they idle at about 2500 .
With the overdrive at around 50 %,,,,,,engine turning about 8000,,,how fast are those rotors turning ?

( Edit ,,,,I want to add,,,,,all of my numbers are only generalizations that are not written in stone .
All teams use slightly different levels of overdrive,,,,different engine redlines and these things are always evolving .
I am not an expert by any means,,,,,,but ,,,,even the experts can’t agree on an accurate number themselves,,,LoL .
And some figures are purposely misrepresented just to keep people guessing .)

On a street car,,,you will never know that a 6/71 is using any power at all .
Don’t even think about using an empty case,,,,,go all out or don’t go at all .

Tommy

 

This blower stuff is of particular interest to me as I have been collecting parts for another 327 (actually a 331). I have a Dyers built 6-71, a complete 2" Isky drive, an Edelbrock B10 manifold, Crower 6.125 Rods and JE dished pistons. Sort of a nostalgia deal. According to Dyers and I am sure others, a street blower on gasoline does not have or need rotor seals. FYI, If this is going on a SBC the weakest link is probably going to be the crank snout. From what I have heard 7-8 lbs boost on the street is about the limit for pump gas and that's going to be under driven with a 6-71. Check out the BDS site as well there's more good info there.

What ever you do, don't run a fake blower.

 

The man that really did the default blower testing was Norm Drazy with PSI blowers. He did more from a do***ented view point than anyone before him. Here's what I have learned: the efficiency of the blower is directly related to how loose it is. A wore out unit back washes a substantial amount. That heats the air charge and leads to increased detonation. A properly clearanced unit is your friend. The real reason that fuel motors require so much horsepower to drive them in the amount of fuel that runs through them. Liquid is not easily compressible. Look at a hydraulic cylinder and see how little it moves to go from 100 psi to 2500 psi. Same thing with the tremendous fuel volumes that blown fuel and blown alky run. If you look at all of the modern blowers, you will see a pie cut opening on the bottom. That was where Norm found out the air was coming out in his testing. An open bottom gmc has more back pressure against the full length of the rotors, decreasing the efficiency of the unit. If you have an 8.5 comp engine with a boost retard hiding on the ignition, you will find out how long the rest of the parts will last. The tuning of the carbs has to be correct. There are numerous carbs on the market that will allow you to be real close without having to learn why they sell prepared blower carbs. The amount of underdrive will determine how many problems you will have. I believe you will read over and over that 8 lbs. is a safe limit on most street engines. We all know that the modern electronic controlled and intercooled engines handle way more. But this is the HAMB and you want a hot rod that sounds and acts like one. To me that means when you stop on the throttle, serious sounds come out hopefully with some tire smoke. I'm sure you are aware of this but you need good engine parts to do this and be happy. Good luck with your decision.

 

You can run a lot more than 8 psi boost on the street on pump gas, if you set up the engine right.

Should be a fun mill once you get it going!

 

Don't know what your current setup is engine-wise, but if it's truly making 450-500hp currently, I'm surprised you're not already using hi-octane fuel. A clapped out 350 with 9:1 compression won't make that. What's your setup (displacement, heads, compression ratio, etc.)? There are charts online that will tell you what your final compression will be under boost, that's what you need to be concerned with and go from there. The blown street engines I've built had fairly large combustion chambers (like 72cc) and around 8:1 static compression. Under boost (<8 lbs) it would end up around 11:1.

I think you will like how the supercharged engine performs a lot better than your current setup with a couple carbs stacked atop an empty blower case that acts like a giant, empty plenum causing all kinds of turbulence and fueling disruptions.

 

My understanding is that the open-bottom casings provide no compression in the casing. As the inlet valves open, the boosted cylinder pressure cause reversion. The reversion leads to a loss of efficiency. The pie-shaped casings provide a degree of compression in the casing, removing the potential for reversion and hence increasing efficiency.

I could be wrong though.

Cheers,
Harv

 

Roots type blowers are inefficient so they add lots of heat relative to the amount of boost (the heat increasing risk of detonation), they are often used without intercooling, they give boost from low rpm when engines are most sensitive to detonation limiting how much boost you can use at higher rpm when the engine really could have handled more... Besides that, the roots blowers aren't great at producing very much boost, if I remember it right the long rotors tend to bend under the load from the boost pressure.

They look good, they're traditional, there's lots of accessories and knowlege available, so using them is fairly simple. But if someone is looking for maximum performance and fuel economy a roots blower with carb(s) on top won't be the ideal choice. Sure, they are used for some very high power dragracing, but that's more a result of the rules forcing it rather than being the best way to produce that amount of power.

 

Hanging out on the HAMB is kind of contrary to getting maximum performance and fuel economy....so a Roots blower with a couple of carbs on top is the perfect setup, here!

 

Think of it this way. The blower is the first of two air pumps in series. The second pump is the engine itself, on its intake and compression strokes.

On a blown engine, the function of these two pumps together is the same as that of the intake and compression strokes of an unblown engine on their own. That is, to fill the combustion chamber with mixture at a certain specific pressure. All engines have pumping losses; on a blown engine you're sharing the pumping losses between the blower and the engine. The total pumping losses on a blown engine will be greater, but they shouldn't be materially different from those on a correspondingly bigger engine.

It is said that there's no subs***ute for cubic inches. That doesn't mean that the only source of cubic inches is the engine itself. A blower is a supplementary source of cubic inches ― or to be really correct, m*** of mixture. The point of the exercise is to get a greater m*** of mixture into the engine, not mixture at a greater pressure. You still have the same pressure limitations as before, and the same implications for detonation etc. That is why you go for a lower static CR, not because the blower is somehow going to blow the engine up like a balloon until it pops. You're adding back the CR you take away, through the additional m*** of mixture supplied by the blower.

People have the idea that the additional demands made of blown engines are directly due to boost pressure. This misconception is especially common in the turbo world, even a**** people who really ought to know better. The additional demands are in fact due to increased bmep caused by increased mixture m***: you're making a small engine burn the same mixture m*** a much bigger unblown engine would burn.

 

I believe this is the right way to think about it. The blower will add dynamic compression ratio to whatever static compression ratio you start with. The fuel used and its ability to suppress detonation will limit the amount of overdrive or underdrive as well as ignition timing the engine will tolerate. Compressing the air/fuel charge will raise IAT further reducing the fuel's resistance to detonation as the fuel density goes down.

Installing a 6-71 on a SBC is not the same as installing one on BBC or some other engine design. The displacement of the engine and the blower obviously have to be considered when selecting drive ratio and in turn the resulting dynamic compression ratio. Some combustion chamber designs are also better than others.

 

You made some good points, so I think its best for me to explain a little further.

The engine in question is a 500 cu in Cadillac. They come with basically 2 different combustion chamber sizes. I have several sets of each size (76cc and 120cc). One set of 76cc heads already has some professional porting and larger valves and springs installed. Right now they are the ones I am considering, but I think I could make the larger chamber pretty similar in flow if need be.
I have aftermarket forged rods, roller rockers and roller lifters. Forged pistons will be used in it if I use the blower. I do not have the cam of choice yet as that depends largely on blower/no blower and the cam manufacturers actually getting some roller cores. I will do a complete blueprint of the engine when I ***emble it. I plan for about 5500 rpms to be on the safe side.

Fuel delivery is another problem. I have a Boogieman intake that is known to have fuel distribution that is not always good. Here is a picture of the intake.
The top plate in the picture does not come with it and it is an open plenum under the plate. There has been some experimentation (by others) at reducing plenum size and insuring better fuel distribution. I will be making my own plate to fit the manifold, so I can make it for whatever I want to put on top of the manifold. I believe I have the fuel distribution problem solved so I won't get into details about that. Its basically now about airflow only and whether plenum size (especially with a dummy blower) is a problem. With an operational blower, I don't think there would be any problem.

So, my basic set up is going to be 500+ cubic inches with some decent flowing heads and a roller cam and headers. Thats why I feel that 450/500 hp before adding a blower is reasonable. It will be backed up by a Tremec 6 speed manual transmission, so idle speed and stall speed will not come into play.

I appreciate all the inputs that I am getting from you guys.

 

^^^^^This! And for many reasons.

 

And that's nothing to ashamed of. Nice looking ride.

 

I wish the bell bottoms would make a comeback....

 

BDS has a dedicated blower manifold for the 500 Cad. 1800 bucks, which is a big chunk, but if you are going to supercharge a Cadillac expect to write checks.

-Abone.