why not use a centrifugal supercharger instead of a turbo? i know turbos are cheaper but i was curious to why we could(nt) use em...

 

It's because the turbos are easier to mount. There's really nothing that makes using a supercharger impossible, ya just gotta figure out how to mount it.

 

A centrifugal blower also requires rpm to build good boost, the 300 doesn't like that much. You would have to use a drive pulley combination that was difficult to package to get the blower rpm up high enough to work well. Turbos also make much more power for a given boost level.

 

o iigh, makes sense i guess.

 

Turbochargers are as efficient as 85% because there is NO load on the crankshaft..They use WASTE (Exhaust gasses) to create the boost NO additional load on the engine...


ftwfred

 

Unfortunately the turbo restricts the exhaust putting a different kind of load on the engine but a load nevertheless. Nothing is free...

I would think a belt driven supercharger could be rigged to work also.

Thanks for using FTE!





-

 

85% for the turbo versus 50% for the "Roots" type supercharger, The turbo is a far better deal, Efficiency wise.(In my opinion)
Both units can be "Sized".. A turbo sized for boost at high RPM suffers boost "LAG" at low RPM, with very little exhaust restriction at high RPM or WOT conditions.. One sized for boost at low RPM does not suffer the boost "LAG", has absolutely wonderful low end torque, However the exhaust restriction kills the power curve at high RPM.

A belt driven superchargers power band can be raised or lowered throughout the RPM range of an engine through the use of different drive ratios for the blower. A lower drive ratio would not provided maximum boost until higher RPM, but would not load the engine as much as one with a high drive ratio, Wich would provide maximum boost at LOWER RPM levels, With the cost of a greater load on the engine..

Although I do not know which one would last longer in an everyday driver??

How many miles are these Lightning and Harley Davidson trucks with blowers on them lasting before blower trouble??

There are not many FACTORY GAS Turbocharged engines out there (For Trucks) that I know of....Turbos aren't used much and I wonder why??


I Would like to know what kind of miles the Lightnings and Harley Davidson trucks are averaging before blower trouble...

ftwfred

:7

 

The roots is a positive displacement blower. What that means is that it takes a certain amount of air from one side and pushes it to the other. If the other side cannot hold that much air, it gets compressed. A centrifugal supercharger is not a positive displacement blower. It takes the air (a centrifugal supercharger is effectively the compressor of a turbo belt driven instead of turbine driven) and spins it at realllly high speeds which compresses it, then shoots it out the scroll. This is more adiabatically efficient than the positive displacement type. 85% efficiency for a turbo/supercharger compressor is a bit optomistic for most turbos, but they are still 20%+ more efficient than a simple roots type blower.

The limitation on a turbo as to how much lag you've got really is the compressor. You don't want to run into low efficiency and/or the surge line, so you've got to size your turbine properly to skirt those issues. Contrary to some peoples belief, a small turbine is not a bad thing at all with a properly sized wastegate and essentially keeps max backpressure low.

Lets look at some numbers....
Lets take a 5l engine and 10psi. I'm not going to do the lb/min calculation myself, i'll just use the calculator at Ray Halls Turbochargers.....

If you wanted to run up to 5000rpm at 10psi, you could use a T61 compressor. If you wanted to remain above 70% efficiency at 10psi, you could run from 2800rpm to 5000rpm. If you wanted to keep your limit down to where it is now at ~3600rpm, using a T04E 40trim, you could run from about 1600rpm to 3600rpm at full boost. Using a T04E 50trim you could run full boost at 1800-4600rpm... All with over 70% adiabatic efficiency.

Lets take that last compressor and say its on a supercharger. Lets say we want that 10psi at 4800rpm. Judging by the compressor map you've got to be turning about 87000 rpm at the turbine. That means we need an 18.125:1 step up ratio. Here is a quick little map of Motor RPM to Compressor RPM:

Motor RPM Compressor RPM
1800 32625
2000 36250
2200 39875
2400 43500
2600 47125
2800 50750
3000 54375
3200 58000
3400 61625
3600 65250
3800 68875
4000 72500
4200 76125
4400 79750
4600 83375
4800 87000

Now, we could play around making marks everywhere showing where everything would be, but how 'bout we just take the 49704rpm mark. Thats pretty darn close to the 2800rpm motor mark. 10psi is about 1.7:1 pressure ratio. Look at the map and tell me how in the world you can get 10psi when the compressor is turning ~50000rpm. It can't.

So would you trade off 800rpm of higher pressure (figuring 800rpm idle, and somewhere in the 1600rpm range before the turbo pushes more boost than the supercharger) for 2800 rpm of full boost goodness? Once the turbo kicks into full boost, it WILL be more efficient than a supercharger.... And thats not taking into account the HP required to turn a supercharger vs a turbo. All the engine has to turn in a turbo is a turbine, 4 bearings and a compressor, and thats done by exhaust gasses. A supercharger has to turn a belt, a high-ratio transmission and then 2 or 3 bearings an the compressor. Plus, the higher the boost, the more belt tension you're running, which can cause front main bearing issues.....


-=Whittey=-

 

whittey u are really knowledgeable, but can u dumb it down a little for me, cuz i have no idea what ur sayin

 

[updated:LAST EDITED ON 23-Dec-02 AT 02:17 PM (EST)]What i'm trying to say is that a turbo, once spooled, will outperform a supercharger because it is going to be at full boost through a much wider RPM range than a supercharger. A supercharger will have the advantage over the turbo at low rpm's (where the turbo is not spooled up). The question becomes, would you trade full boost over a 2800rpm range for a little boost at an 800rpm range.


-=Whittey=-

(EDIT) If you want, you can go to Ray Halls site and fire up that calculator of theirs and get the lb/min at each RPM range and line it up with the guesstimated RPM mark on the turbo map (also on their site) to find actual boost at the rpm's noted.

(EDIT2) On another forum I frequent, that same "Why aren't there many turbo gasoline motors in trucks" question. I think the general consensus was reliability. The exhaust gasses on a gasoline vehicle are generally much higher than on a diesel engine.

 

>I Would like to know what kind of miles the Lightnings and
>Harley Davidson trucks are averaging before blower
>trouble...
>
>ftwfred
>
>:7

I frequent the Lightning board and don't remember anyone ever saying they had blower trouble. Some guys are running 20+ psi boost and are in the mid 10's and low 11's in the quarter with no trouble. The biggest problem on the 5.4 S/C is kicking the rods out with lots of boost.

FWIW, my Lightning has full boost by about 2000 rpm and makes it until 5200 rpm (shift point).

Here's another plug for the turbo. A friend of mine has a 97 Z28 with the LT1 engine and 6 speed. In stock trim the car ran high 13's in the 1/4 at about 102 mph. Last week he finished his homemade turbo and took it to the track Saturday. It ran 11.93@128 on only 7 psi and spun the tires most of the way down the track. Other than the turbo the car is stock. If the car was set up for drag racing it would run low 10's and maybe even a high 9. It dyno'd 484 hp and 543 ft-lbs at the wheels at 7 psi. Most LT1's are in the 265/300 range stock.

 

im convinced on the idea of a turbo, but just wanted to know why supercharger werent used, well now i do. mMmMm i can see it now...EFI 300 w/ turbo, ported/polished head, different cam, dual flowmasters and header...can u say rice killer all packed into a 92-96 f-150 2wd 5 speed (t-5) lowered a little with 20"s...i think i have my futrue project truck...

 

Don't get me wrong, superchargers aren't bad in any way shape or form. No matter how strong of a bottom end you've got, there will always be lag. Also, due to the direct drive nature of a supercharger, you're still creating full boost when you're at part throttle. This is a good thing when you're off and on the throttle because when you open the throttle you've got instant boost so you get the snap of a N/A vehicle. With a turbo at part throttle, you're probably not building boost so you'll have to wait for it to spool, which (with a properly sized turbine and depending on the rpm of course) shouldn't take long at all. In any case, to some a turbo system looks good (me, heh) but most will agree that a nice polished roots-type blower is a good lookin thing....

Another thing we didn't speak on was a Lysholm supercharger. Whipple is the name many of you would know (sprintex is another). That uses screws to compress the charge (so it is internally compressed like a super/turbocharger) and is also a positive displacement (like the roots). It gets the best of both worlds. For low-medium boost levels, if lowend is important, i'd certainly go with a Lysholm. But for maximum boost (which I doubt anyone here is going to do) I would go turbos all the way.


-=Whittey=-

 

 

[updated:LAST EDITED ON 24-Dec-02 AT 02:06 PM (EST)]If I was turbocharging the 300 I wouldn't use a headers, I'd use the EFI manifolds and two small turbos. With a header the turbo is a long ways from the exhaust port which causes lag. You wan't the turbo as close to the exhaust valve as possible.