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Supercharger fact or myth???

5K views 31 replies 11 participants last post by  camaroman7d 
#1 ·
I have heard that you have to re-tune, so to say, your engine about every weekend or all the time if you run a supercharger, is this true? And if im being too blunt then i'll explain more: i hear from half the people that you have to adjust the timing, rockers, and lifters just about all the time if i were to run a 6-71 or 8-71 Weiand street supercharger on a 427 (which that shouldnt matter all that much i think), and the other half says that you should only have to do it around every due tune-up. Superchargers are kinda complicated for me, along with gearing and drivetrain, but i kinda figured that the later was true and that you could do it with the tune-up. I guess i should get a "Supercharger Bible" but that cost money, this is free, :D God i love this forum, you cant beat free. But if anyone could help me out, id appreciate it.

thanks,
classic
 
#2 ·
even so I haven`t delt with superchargers, I would think tuning it often would be called for, since with that much cash and power invested under the hood, if it goes lean disaster can result and fast. as far as adjusting the rockers go, if the valve train uses hydraulic flat tappets or hydraulic roller tappets, there would be no need to adjust them often, once you have them adjusted there set unless a rocker arm nut backs off. if running solid tappets, flat or roller, then the need to adjust them often would arise, but it wouldn`t matter what was on the engine be it a turbo, supercharger or natraully asperated, the valve train don`t know whats doing the breathing of the engine. while it is true you can do some adjustments with hydraulic cams and make up some difference in how the engine performs, it`s not as much as it would be with a solid cam. If I were running a supercharger, I would tune it quite often, since I`d want to keep my engine intact, but since it wouldn`t be a daily driven vehicle, I would only do routine checks before I ran it, timing, fuel pressure, float settings, plug readings etc. I wouldn`t run a supercharger on the street with a carb, I`d rather spend my time doing other things than constatly tuning it, if it were a fuel injected vehicle, like a camaro or mustang with computerized spark and fuel management I wouldn`t worry about it since the computer does the work for you.
 
#3 ·
You don't have to re-tune Superchargers that often. If they get setup right the first time it should be just like any other carburetor setup. If you have access to a wide band 02 sensor and a dyno then you might want to tune about twice a year, once for the cold months and once for the warm months. Unless it just gets to running bad and needs a tune. The only time you should re-tune is during drastic weather changes Summer / Winter and if you add more boost to the mix.

If you've got to re-tune your carburetors on superchargers that often you might want to look at the carb manufacturer and not the supercharger.
 
#4 ·
Basically what "some people" have told you is NOT true. There is no need to tune them any more than any other car. I have only had mine together about a month, it was easy to tune (maybe I just got lucky).

I have yet to put in on the dyno, but that's where you can get all the fine tuning if you want to drain every bit of power out of it.

Once you get it tuned it should stay tuned just as with any type of car. The key is monitoring all aspects of the engine (water temp, oil temp, oil pressure, boost, fuel pressure) These things will usually tell you when something is going wrong.

I agree with Jamnut about the tuning for time of year/outside temperature but, this applies to all carbureted performance cars.

As far as solid cams, you don't have to adjust them that often. Just make sure you run good nuts and locks. You should check the lash every oil change (or every other). If you run good oil and good nuts and rockers., you will see that when you check them they rarely are out of adjustment.

Royce
 
#5 ·
is it also true that the efficiency of the supercharger goes down after a few months due to the temp of the engine/supercharger mix? Cause i heard that the supercharger raises the temperature drastically, and that the only way to guarantee that the supercharger will run efficiently is to get an intercooler. And what would be the average temp of an engine, a 427, with a 6-71/8-71 supercharger, would anybody know???

thanks,
classic
 
#6 ·
The efficiency of a blown motor is degraded by heating the intake air during the compression process of the blower. The efficiency loss is dependent upon how hard the blower has to work to achieve the desired boost. You mentioned 6-71 and 8-71 blowers for a 427 cubic inch motor. An 8-71 blower running at 1:1 with the crank will make an estimated 12 lbs. boost on that size motor. A large bore 6-71 will have to run 7.5% overdrive to make the same boost and a small bore 6-71 will have to run at 27.5% overdrive to make the 12 lbs. of boost. So...from an efficiency perspective, you'd be better off running the 8-71 to make the desired boost on that size of motor. The slower you spin the blower to make the boost, the less heat you are putting in the engine. It is not true that the efficiency goes down over a few months due to the temp, it is that way out of the box. The efficiency of a blower will degrade with time due to wear, but wouldn't be noticeable over a couple of months unless something was drastically wrong.

It is not true that you need an inter-cooler to run a blower. The need for an inter-cooler is dependent upon your set up and vehicle use. Obviously, if you were trying to squeeze the most hp out of a motor for drag racing, an inter-cooler should be considered as any other enhancement modification.

A properly set-up blower motor won't run any hotter than a normally aspired motor with a properly functioning cooling system.

Go to different blower companies on the web, they have a wealth of free information. Try http://www.blowerdriveservice.com/ and just browse their different links. I'm sure other blower companies have good info as well.

As far as injected versus a carburetor, it's not any different than the same decision on a normally aspired motor. What do you prefer and better yet, what can you afford? Blower motors need to be run a little richer to avoid detonation. The ideal A/F ratio of 14.6 is too lean for a blower. As discussed earlier, no additional tuning unless it is set-up improperly and you are "chasing" the set-up. The only difference I see, is to make sure the blower belt is in good shape cause it won't run without it (and carry a spare).
 
#9 ·
In the book it says that a blower(unless super hi horse race motor) that you will have no troubles with supercharged motors. You get better gas mileage at cruise rpm's and it starts easier to. You cant beat em for looks and performance. HG
 
#11 · (Edited)
A supercharger increases gas mileage when HOT making boost. The reason is because the rotors spin and mix the air/fuel better. This promotes a more efficient burn. Now if you are on the pedal you can forget that, you will make more power and have WAY less gas mileage. I get 10mpg with my 8-71 blown small block with two 750 double pumpers. I got about the same with my previous naturally aspirated engine, single carb 3 less cubic inches, same car.

Royce

Edit: I meant to say NOT where I typed in HOT above. (NOT making boost).
 
#12 ·
The cooler the incoming charge and/or the higher the compression ratio, the more thermally efficient an engine will be. An automotive engine is simply a heat engine and energy extraction efficiency is a function of temperature difference in the cycle. The bigger the difference between the lowest inlet temperature and peak combustion temperatures, the higher the efficiency. Blowers do heat the incoming charge but that is after the cold air has been ingested to the system so that heat is useable. The higher overall efficiency comes from the higher compression-higher peak temperature in the cycle.
 
#13 ·
Oh mommy, this conversation is getting over my head. I thought superchargers were essentially always producing boost, as opposed to turbos.

Are we talking about engine efficiency or fuel efficiency? I'd buy that a SC improves engine efficiency, but that doesn't necessarily translate to better fuel efficiency, does it? I mean, I've heard V8s are more efficient engines then 4 cylinders, but obviously they still burn more fuel.

My very simplistic way of looking at this boils down to A/F ratios. If the SC stuffs more air into the cylinders each cycle, more fuel needs to be added also. Where has my logic gone wrong? (seriously)
 
#14 ·
LOL

No blown engines are not always making boost. I am talking roots type blowers here. I can drive my car all day and not make boost (actually make vacuum).

In order to make boost you have to have the throttle blades open. Boost is the measurement of backpressure in the manifold (really not an accurate way to judge performance but, that's a whole other discussion). There will be no boost/backpressure as long as the blades are closed or only open wide enough to satisfy the engine, once you open them all the way up then the blower has more air/fuel to work with and will force more into the same space, this creates boost/backpressure. This is a way over simplified version. Blower speed also has to be take into effect, at slower speeds the blower turn slower and therefore doesn't move as much air.

Now if you have very restrictive heads/intake you will see more boost and more often as the backpressure/boost will build easier, this does not mean you will make more power. For example an engine making 12 lbs of boost with very small restrictive heads will not neccessarily out perform an engine of the same size making 8 lbs of boost with very good flowing heads. Flow is still king when trying to make power naturally aspirated or forced induction.

Royce
 
#15 ·
There are two efficiencies associated with IC engines. Volumetric Efficiency (VE) refers to the amount of fuel/air charge actually introduced into a cylinder vs. the actual capacity of the cylinder. A naturally aspirated engine will always have a VE lower than 100% since it depends on creation of a vacuum to fill the cylinder and there will always be a small amount of exhaust gasses left after the exhaust stroke. A super charged engine can have a VE of over 100% since the exhaust gasses can be totally blown out of the cylinder with positive pressure during valve overlap and since the cylinder is being fed fuel and air under pressure.

A high VE doesn't necessarily translate into high Thermal Efficiency (TE) which relates to how well the fuel energy is converted to mechanical energy. An IC engine, being a heat engine, generates higher TE as the compression ration goes up. VE is helpful there too IF it is controlled and raw fuel is not blown out the exhaust. A super charger can increase TE (fuel efficiency) buy increasing compression and peak temperature and by increased VE if it isn't working with a radical cam that allows fuel to be blown through the engine.
 
#16 ·
I have to laugh about the better gas mileage statement. That is such an old wise tale. It's not true at all. I'm not trying to start a fight or debate on this, but this was the hot gossip statement in the 80's in fact I remember seeing a Dyers Blower flyer almost claiming the same thing. Dryers claimed better gas mileage because you could run a taller gear ratio because of the extra power. Blowers do mix the fuel air mixture better but thats at rpms 4K and above. Not to mention a blower takes anywhere from 10-65hp to operate. You also have 200lbs extra weight between the intake, extra carb, blower, special harmonic balancer and pulleys. If you want better gas mileage take the blower off.
 
#17 ·
I am getting the exact same mileage on the highway as I was before the blown engine 10mpg. Around town my mileage is far worse than before 3mpg. I don't think anyone would install a blower to gain gas mileage. I have about 200-250 more horsepower than before and get the same mileage, I don't think you can do that without a blower (assuming carbs).

If you had 200lbs of extra weight I don't know what your blower was made of. My 8-71 weighs about 75lbs the intake weighs 37lbs the extra carb maybe 10lbs, I am running a crank hub so I lost weight there probably about a 6-10 lb. loss. I would say the whole setup weighs about 125lbs.. I did have to upgrade my front springs (I had old cut small block springs before)

You are 100% correct that the blowers take HP to turn, I think a 700HP blown engine will get better mileage than a 700HP N/A engine. I would be wiling to bet on that.

They still advertise on most (if not all) blower sites that gas mileage will be better when not making boost (better than a N/A engine).

If you have a blown engine I don't think you built it for mileage to begin with.

Royce
 
#18 ·
Realize all of these arguments are theoretical. True it takes horsepower to run a blower but it puts out more power than it consumes. A blower/cam system designed strictly for higher thermal efficiency (better gas mileage) wouldn't be very exciting. Witness that the popular -71 series roots blower comes stock on GMC diesel engines for the express purpose of increasing thermal efficiency. Conversely a performance blower is designed for maximum volumetric efficiency at the expense of thermal efficiency. It is of secondary importance to a racer that he get good gas mileage but it is all important that he get maximum horsepower. Engine design is a compromise, you can't have everything in one engine.
 
#20 ·
I think I need a SC book. Back to the SC running no boost though for a sec. Isn't the SC always TRYING to make boost (maybe it doesn't actually produce any because the throttle is closed) since its hooked to the engine? There's no clutch or anything for the SC, right? Do they turn 'on' at a certain RPM? And are we talking about a draw through or blow through SC setup? Or does it matter?

I agree SC are meant for power, not fuel efficiency. But that's why I've always been onboard turbos. I thought they only really spin up when needed, so the fuel efficiency shouldn't be affected when it isn't boosting. But I wasn't aware superchargers had any feature like that.
 
#21 ·
Superchargers and Turbos work basically the same way, one is belt driven one is exhaust gas driven. They both do the same thing.

You can't make boost if the throttle plates aren't open. They do not "turn on" or off. If you gradually open the throttle with little or no load you probably won't make boost. If you snap the throttle open it will make more air available for the supercharger to compress and then you will have boost.

Of course depending on how fast you spin the blower this will effect the amount of boost that is made and at what RPM etc... There are several factors and this is all part of the tune up, you adjust your drive ratio to make boost when and where you want it and how much.

Royce
 
#22 ·
Blowers make more power only under boost. 95% your running your motor with regular vaccum as normaly aspe. vehical. It takes power to turn the blower while your just crusing and idleing. This is kinda of like saying my 4cly toyota gets better gas mileage with the A/C on then my 600horsepower v8. A true statement but its not reason behind it. While crusing the blower takes horsepower to run. It may seem as your getting the same GPH but Im sure there are changes made to your motor like a blower cam, blower intake, ect. As far as weight you could be correct for a chevy, the blowers for fords weight alot more I dont know what kind of manifold a chevy uses but the Ford intakes are gigantic since they have to lift the blower over the distributor, and BDS's harmonic balancer is a weight monster at 30lbs. not to mention the heat a blower adds to the intake charge. I dont believe for a min they improve gas mileage in any situation. I would bet your gains are from other areas, maybe from a blower cam, porting, exhaust.
 
#23 ·
I wasnt meaning in any way that you would build a blower motor for gas mileage, just that the old B&M article said they increased mileage by a few miles per gallon with one. It was based on the fact that you need less foot in the throttle to get power with a blower.
Push your foot in the throttle and all gas savings goes out the tail pipe. HG
 
#24 ·
I never said my gas mileage improved, I am saying it did not get any worse on the highway. It is terrible around town and idling. The exhaust is the same, the heads are the same, the trans and rear end are the same (even the tires). I am not trying to convince you that blowers are great for fuel economy, you don't build a blown engine for fuel economy (in a Hot Rod).

I will say that my engine makes a lot of power even when I am making vacuum and that surprised me I didn't expect any big power until it stated making boost. It makes huge amounts of torque and I have only seen boost two times so far once I saw 7 and once 10lbs. I think the fact that I have a very good flowing intake and heads has something to do with this.

I think we agree general fuel consumption will be worse with a blower. Though I think blowers may be more efficient at highway speeds, when not making boost.

Royce
 
#25 ·
Hemmi,

What alot of people dont realize is that these companys do months of testing to get the figuers and readings they want. I am sure it is completely possible to throw a blower on a motor and make changes with adding .003in clearances and drive ratio's to make a motor come up with better gas mileage. In the real world it is far from the truth. The slower you turn the blower the less power it needs for cruzing but then again less over all psi. Run a blower 40% under driven and set it up for 3-4psi over all and it may help. I just think its misleading, and in all the years I have delt with blowers I have never seen a situation in which a motor gets better gas mileage with a 6-71 or 8-71. If you think you got your motor running that way Royce good job two thumbs up.

Ben
 
#26 ·
Your right on that. LOL
Do you guys like the looks of the 250 powerchargers? Its smaller, but you can choose between one or two carbs and its in between the 6-71 and the 174 units. Heres a pic of one. If it shows up that is. Letme know. HG

http://www.geocities.com/mikey797979/supercharger/blower1.gif
 
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