loco@ttidcb.UUCP (CM Dev. Group ) (04/23/85)
>Well, they sort of do the same thing but the major difference is that >a supercharger forces outside air down the engine's throat (in a carburated >engine, the supercharger sits above the carb), Not on a "jimmy" I've ever seen. The blower bolts almost directly to the head via a small adapter manafold. Ones that use carbs have them mounted right on top of the blower, as if it were a manifold. Since carbs require vacuum to meter fuel, I don't see how it could be any other way. -Pete- ------------------------------------ | | | "Jura nyy ryfr snvyf, shpx vg!" | | | ------------------------------------
phil@osiris.UUCP (Philip Kos) (04/25/85)
It looks like efforts are being made to dispel some of the mistaken
notions about the operation of {turbo,super}chargers, but there are
still a few floating around so I thought I'd put in my 50 cents
worth (inflation, y'know).
A *charger will work on either side of the carburetor. The pressure
differential which causes fuel to flow throught the carburetor jets
is created somewhat artificially in blow-through installations by
using the increased air pressure on the other side of the fuel, i.e.
venting the carb fuel bowl to the *charger output rather than to the
ambient atmospheric pressure.
The decision to use a blow-through or a draw-through setup is based
on whether the *charger can stand backfiring through it. If it's a
blow-through design, it doesn't usually have to; if it's a draw-
through, it does.
The blow-through design also tends to be a little cheaper, because
the plumbing between the *charger and the carb doesn't have to be
fuel-resistant, while it does in the draw-through design.
Other design factors which are affected by the choice:
+ Blow-through designs allow the carb to be much closer to the
intake ports, which decreases the possibility of fuel preci-
pitating out of the mixture.
+ Draw-through designs feed the mixture through the compression
process, which helps some in atomizing the fuel (as above,
less precipitation).
+ Intercooled blow-through designs (assuming the intercooler is
between the *charger and the carb) present cooler air to the
carb, which may or may not help atomization (can anybody give
me help here? I'm just stating this as a difference, because
I don't know whether it would be an advantage or a disadvantage).
+ Intercooled draw-through designs may worsen the precipitation
problem - again, I'd like some more input.
There are lots of little details that determine the difference in per-
formance between the two kinds of *chargers. The main functional dif-
ference is that superchargers are *always* on, while turbos have to be
"wound up" before they start boosting. The two most common solutions
to turbo lag are (a) decreasing the mass of the turbine, and (b) pro-
viding a "closed loop" to keep the turbine spinning quickly, and bypas-
sing the turbine at small throttle openings. Many V-engine turbo
applications (particularly in racing engines) use option a with two
small turbos, which is easily done because the exhaust ports are located
on opposite sides of the block.
Anyway. This has gotten pretty long-winded (long-fingered?) so I'll
leave well enough alone. I'd appreciate hearing from anyone who could
give more information on the practical differences between blow-through
and draw-through designs.
Phil Kos
The Johns Hopkins Hospital
...!umcp-cs!aplvax!osiris!phildavew@shark.UUCP (Dave Williams) (04/25/85)
(Referring to superchargers vs turbochargers): >>Well, they sort of do the same thing but the major difference is that >>a supercharger forces outside air down the engine's throat (in a carburated >>engine, the supercharger sits above the carb), (Referring to a GMC roots type supercharger): >Not on a "jimmy" I've ever seen. The blower bolts almost directly to the >head via a small adapter manafold. Ones that use carbs have them mounted >right on top of the blower, as if it were a manifold. Since carbs require >vacuum to meter fuel, I don't see how it could be any other way. The relative position or type of fuel induction system has nothing to do with whether a system is a turbocharger or a supercharger. (see reprint of Popular Science article elsewhere in this newsgroup). Most drag racing engines using roots type blowers inject some or all of the fuel ahead of the supercharger as the denser fuel/air mixture helps seal the clearances in the blower and make it run more efficiently. The Paxton supercharger, which is an after market unit blows air through the existing carburetor on an engine. The carburetor must be placed inside a pressurized box or the carburetors rebuilt so that the carb doesn't leak from the increased internal pressure. Turbocharger systems can have the fuel inducted ahead of the unit or use port injection. In an earlier item I said that a roots type blower on a drag engine would eat up 35-50 HP just to drive it. Someone wrote me to ask if this figure was correct for a Honda V65 motorcycle conversion a company in Calif. is now selling. I was referring to a 400+ CI V8 running 12-15 lbs. boost. On a 65 CI engine intended for street use (~8-10 lbs boost), the power used to drive the unit would be substantially less. Finally, I was questioned by the same party as to my statement about turbo lag and rotor design, I will say that placing the turbo as close to the intake valves as possible and using the exhaust gases as close to the exhaust valves as possible will result in some improvement in turbo lag, the greatest improvements have come as the result of improved rotor and wastegate designs. design -- Dave Williams Tektronix, Inc. Engineering Computing Systems "The 6000 Family" "The workstations that made Wilsonville famous."
jlw@ariel.UUCP (J.WOOD) (04/26/85)
A couple of problems with Dave Williams response. 1. It's Rootes not roots for the positive pumping type of supercharger as user in GMC 2-stroke cycle Deisels. It was named for the inventor? or company in England which first used it. The Rootes group of Sunbeam fame finally went out of business. Well, becoming part of BL is the same thing, isn't it? BTW the bit in the 'Road Warrier' where Mad Max starts his blower is a farce. Without the blower running no air gets through since its a positive displacement pump. 2. When blowing over pressure air through a carbutrator, the entire carb \must/ be placed in a pressure box. The venturi effect works on the fact that due to Bernoulli pressure reduction gas is \pushed/ from the float bowl into the venturi. Therefore, the pressure in the float bowl must be higher than in the venturi. Joseph L. Wood, III AT&T Information Systems Laboratories, Holmdel (201) 834-3759 <ariel!>titania!jlw
davew@shark.UUCP (Dave Williams) (04/29/85)
In article <916@ariel.UUCP> jlw@ariel.UUCP (J.WOOD) writes: >A couple of problems with Dave Williams response. > >1. It's Rootes not roots for the positive pumping type of > supercharger as user in GMC 2-stroke cycle Deisels. > It was named for the inventor? or company in England > which first used it. The Rootes group of Sunbeam fame > finally went out of business. Well, becoming part of BL > is the same thing, isn't it? > BTW the bit in the 'Road Warrier' where Mad Max starts > his blower is a farce. Without the blower running > no air gets through since its a positive displacement > pump. > >2. When blowing over pressure air through a carbutrator, > the entire carb \must/ be placed in a pressure box. > The venturi effect works on the fact that due to > Bernoulli pressure reduction gas is \pushed/ from > the float bowl into the venturi. Therefore, the > pressure in the float bowl must be higher than in > the venturi. 1. While I may be wrong on the spelling of Rootes, at least I spelled carburetor correctly. :-) I never mentioned 'Road Warrior' (again, note spelling), since I never saw it and could care less. You are correct in stating that Rootes type puffer can not be cut in and out at will as the tricotial impellers impede the flow air. 2. Wrong Turbo Breath..... fuel is drawn from the float bowl to the venturi area due the pressure differential caused by the Bernoulli Effect. The pressure in the float bowl should be at the same pressure as the pressure ahead of the venturi. This can be done by pressurizing the carb. The carb can be modified without resorting to a pressure box around the carb. Examples of this would be the 1954 Kaiser Manhattan with a McCulloch supercharger mounted on its six cylinder engine. The Paxton units on late 50's Studebaker Golden Hawks and the 1957 Ford NASCAR engine using a Paxton unit. This will only work for moderate amounts boost (about 8 lbs. max.). The pressurized box is the better way to go. -- Dave Williams Tektronix, Inc. Engineering Computing Systems "The 6000 Family" "The workstations that made Wilsonville famous."