dont forget you also have to give the motor fuel to turn the blower too, that takes up power too. People forget about that, thats why Xlb injectors work in a turbo app and the same Xlb injectors may not work in the supercharged app because it has to feed the motor the extra fuel for turning the blower. Make sense?

i'm a little tired and it wasn't the greatest example.

Also every motor has a specific bsfc, not everyone is going tbe the same

 

I know.. that is why I was using a BSFC of 0.6. I never thought it could suprass the 0.6 margin... after using a 0.65 I see that I am out of fuel...

 

I respectfully disagree with the notion that superchargers absorb large amounts of crankshaft power.

You see, it takes power to compress the inlet air. Agreed. However, the air in the cylinder is under pressure during the intake stroke. This turns the intake stroke into a low-pressure power stroke. So, the most of the power (some is lost to internal friction in the supercharger) it takes to spin the supercharger is recovered during the intake stroke.

On turbochargers, to have to put more work into the exhaust stroke since that is what powers the turbocharger.


Everyone seems to think that the turbochargers do not take power to turn. Nothing is free.

Mike

 

They take more or less the same power to do the same thing...

There is nothing like a free lunch.. that's all well known.. lets not turn this into a turbo vs supercharger thing again...

But I do wonder why are turbo's lately making more power than superchargers?

Anyways.. the bsfc I think is a bit high at 0.65. How can you improve that?

 

i never said they absorb "large" amounts of crankshaft power, i was simply stating that when comparing a turbocharged motor versus a supercharged motor, in my experience and in many other professional experience/opinion supercharged motors use more fuel than a turbocharger motor. Now you seem to be an engineer, i am not. Possibly you can explain this to me without use getting too off topic but i am simply stating what in many experiences happens. That is all, take it for what it is worth

This sounds like a sound theory but it many cases all the power cannot be recovered due to the size of the supercharger. If you are using a smaller more efficient centrifugal supercharger it is possible to try and recover the power, yes, but as you get larger the power absorbtion curve is not linear. Call up Jim Summers (who now works @ ATI and used to be ATI's poster boy with the ASSC "big red" pro 5.0 car) and ask him how much horsepower those big blowers were taking up.


Agreed, nothing is free. There are frictional losses in the turbo, high pressure in the exhaust eats up horsepower also (to an extent). I am not saying this, all i was stating is that when you compare the fueling needs for a supercharged car versus a turbocharged car the supercharged car needs more fuel. From the explanations i have been told is mainly because of the belt, it is constantly putting the blower in a pressurized state even at lower idle speeds, but do to the bypass and/or blowoff valve it will relieve the pressure from getting into the intake whereas in a turbocharger the turbo isn't always making boost, it is dependant on load rather than rpm/load like a supercharger is.

Like highlander said, let's keep this thread on subject. If you would like to discuss this more (which i wouldn't mind, i always try to learn new things and sound theorys email me )

------
Highlander, one old adage that any car junky could tell you that will help solve your problem is "give the motor what it wants." If you motor is desiring more fuel than other similar combos, give it more fuel. Kinda like ignition timing, give the motor what it wants to build power. When all else fails and all the "theories" and examples of somebody elses car aren't matching up, just go by what i said above it usually fixes the problem *shrug*

Think about it though, how many times do you hear of X using one type of fuel system on one car with great results and Y using the same type of fuel system on the exact same combo and there just seems to not be enough fuel? I have read it online lots of times and i have heard @ the machinist's/engine shop even more.

Hope you can get it fixed

 

IROC, this is Highlander's thread and he's apparently interested also, so I don't think he'll mind us discussing this here.

If you tell me that a supercharger has a higher BSFC than turbo at full throttle, then I'll assume that's true since I have no data stating otherwise.



I would like to offer the following explanation, however.

Turbocharged: Both the intake and exhaust manifolds are pressurized. So, during valve overlap, exhaust backpressure (used to spin the turbine wheel) keeps intake air and fuel from short-circuiting into the exhaust. Turbo motors are similar to NA motors in that respect, so the BSFC is similar.

Supercharged: The intake is at a higher pressure than the exhaust. So, during valve overlap, intake air/fuel short-circuit into the exhaust. Therefore, for X amount of fuel, a certain percentage is wasted and not turned into work (hp). You must inject extra fuel to account for this.

Just a theory. . .


The company that I work for owns a large 2-cycle natural gas engine that makes 600 hp at 300 rpm (10,500 ft-lb of torque). I am currently looking at installing a turbocharger to make it run leaner and lower emissions. I inquired about blowers, only to find that blowers allow fresh air and fuel to escape to the exhaust, hurting emissions. The turbocharger automatically limits the amount of exhaust escaping, so a higher amount of air is "trapped" in the cylinder.

Mike

 

I dont mind and it will end up anwering the question in a way...

Remember that the emissions will be highly dependant on the CAM... when you mount a supercharger you mount a cam with little overlap!! as opposed maybe to a reverse split cam on a turbo to keep the exhaust from comming in...

Are those engines you are using are going to be powering a MegaWatt power plant???

I've seen those and yes.. a turbo always works!!!

Anyways...

the only advantage is the power down low.. .but the other day I was questioning such a power down low!!! I raced an SS LS1 with a 125 shot and all he did was spin the tires... wouldn't that happen with the turbo car more often?? the turbo would be nice when you are pulling or something... or as said earlier, when you shift, that you will have the same boost and with a supercharger you will go down.. but the power curve on the supercharger up top is almost flat too.. .so.. its not all boost.. its also how the engine performs...

I agree that with the nitrous you get instant power down low and its good if you have good tires.. but with street tires??? anyways... we both spun on first and second.. when I hit 3rd... a big kick on my a$$ and I was gone for about 10 cars from 60-120 or so...

 

i was under the impression that NEITHER supercharged or turbocharged cams have a lot of overlap....most of the time you either have some very little overlap or even possibly negative overlap in either situation, but the duration numbers are different. For instance you use more exhaust duration with a supercharged car to help get all the charge outta the combustion chamber faster and in a turbocharged motor it is more like an n/a single pattern cam or even some more intake duration than exhaust IF you have a pretty efficient exhaust system and/or a poor flowing intake but the overlap issue is usually the same in either siuation: little or no overlap

highlander....true turbo boost comes on low and hard and stays for the duration of the rpm band, usually, every application is different and yes this causes tuning a turbo car harder since it isn't linear like a blower car

but

turbo boost can be manipulated to become linear and/or at different parts of the rpm band by exhaust housing size and cam design and even electronic boost controllers.

Think about a low rpm torquey nitrous car with a heavy 250shot using a progressive controller versus none SIMILAR to a small exhaust housing turbo set for high psi with an electronic boost controller

 

Centrifugal blowers take large amounts of hp to drive. I have seen (and believe) figures of 50-100hp for a high boost setup on a V8. Ever wonder why an 8 or 10 rib belt or even a cog setup is needed to drive these things?. A psotive displacement blower make take even more hp to turn, depending on the setup. Turbos decrease pumping efficiency somewhat and therefore do absorb a small amount of hp. Most of it is "free" hp though - the energy to turn the impeller comes mostly from the turbo acting as a "heat engine". The turbo turns energy in the form of both pressure and heat in the exhaust gasses into mechanical energy. The portion from the heat is "free", the portion from the pressure is not.

Cams are a whole different ballgame for a turbo and a centrifugal. Don't have time to go into it now.

Rich Krause

 

Has anyone ever made a comparisson between the 2 systems in the same car and same boost and airflow???

 

89ProchargedIROC, 99.999% of cams ground since 1940 have a significant amount of overlap. For instance, let's look at Rich Krause's cam:

214/224 @ .050" or 266/276 seat timing LSA 114
This cam has negative 9 degrees of overlap when measured at .050" lift. However, this cam has positive 43 degrees (!) of actual overlap. That's almost 1/8 of a crank revolution!

Another common supercharger cam is the Comp 305 on a 114.
This cam is:

220/230 @ .050" or 281/290 seat timing LSA 114
This cam has negative 3 degrees of overlap when measured at .050" lift. But, it has positive 58 degrees (!!) of actual overlap.

So you can see that these popular supercharger grinds actually have a significant amount of overlap.

Check out this website for an overlap calculator:

http://cosmik.org/calculators.htm

Mike