This is a quote from another forum I'm on...

If it's a centrifugal then it's always creating boost and heating up the air in front of the throttle plate, which is more drag on the motor at part throttle. You only see the boost on your gauge when you step on the throttle because your gauge is connected to your intake manifold. I'll bet if you port it from the outlet of the centrifugal supercharger you'll see 20#s of boost while cruising down the road at 2000RPM.


This guy is arguing with me that there is no way possible that I get higher highway gas mileage with a s/c than without...in spite of what I think I have observed. Since I don't have enough expert knowledge to argue particulars, I was wondering if this statement seemed a little odd to anyone else. If my s/c made 20psi at anytime I'd be very concerned about the possibilities for a blasted engine...but maybe it does and I don't know it...

Any help out there?

 

What was the question?

There's definitely a difference in pressure before and after the throttle blade. At idle I show a vacuum, but I have positive pressure in front of the throttle.

The BOV releases excess pressure (and heat).

 

Is it true that it's putting out 20 psi at the s/c outlet? Seems a bit high to me, but like I said I'm not an expert so I don't know. According to this guy, heat is a major factor in why it'll get lower gas mileage, BTW.

 

That's curious. At WOT with the BOV closed, the boost is only 8#s. "Centrifugal" refers to increasing boost as rpms increase. At idle with the BOV open, the best he can claim is some positive pressure in the runner but 20#s? No way. Not posiible. There is no vacuum in the runner but certainly no real boost either.

 

"This guy is arguing with me that there is no way possible that I get higher highway gas mileage with a s/c than without..."

For what its worth, I got better gas mileage my trip back from LPE after having Maggie installed than I did on the way out.

 

It's hard to truely tell since I do more WOT blasts than ever before, but my mpg is at least the same if not 1-2 mpg better. Still around 18 mpg in town.

As far as 20#s of boost at the blower outlet, think of the restriction of air flow that would require. My guess is he is exaggerating or on crack or both.


[Modified by drcoffee, 12:42 PM 5/12/2004]

 

Most I've talked to have had gas mileage increase...


[Modified by SleepieAce, 1:20 PM 5/12/2004]

 

The guy is an idiot...assuming your IC is 70% efficient your IAT at normal cruise...lets say 80* ambient then you are going to be 80* at normal cruise and maybe 100-110 IAT at WOT. It is not like you are at WOT/Max Boost all the time...even I could never achieve that..and I have a lead foot.

The only thing that could possibly happen is a little more drag because you essentially have another accessory to drive..but you are the best judge..if you are cruising your car around and averaging good mileage and your wallet and gas guage reflect what your averages off the DIC are stating...the F that guy...he is just an idiot.

I drove to Andy's shop on 1 tank of gas and can just make it there from my house and be on Empty...that has not changed with the FI or NA.

But on short trips I get close to no gas mileage..because I am always getting into it.

 

If you had #20 of boost at idle behind the throtlle blades your back tires would go wildly out of control the very second you even so mucch as cracked the throttle.. like an on-off swith for a 250hp. hit of nitrous at idle..

 

That would be insane not to mention the motor would probably blow up. Like I said the guy is an idiot....a complete ignorant idiot....hard to argue with people so stupid...there is no such thing as a stupid question...only stupid people. Yes I can be stupid sometimes.

 

The guy does not know what he is talking about. There is very little (if any) positive pressure in the plumbing under non aggressive driving. The BOV relives the majority of it and the design of the impeller itself keeps flow very low until high RPMs are achieved. Remember, the scroll and impeller of a centrifugal unit do not provide positive displacement and require high speed to overcome backflow. This guy seems to think it works like the positive displacement air compressor you have in your garage.

You can prove the heat issue by looking at IATs. Mine are actually lower than they were before I put the blower on (now very close to ambient). IATs can go up a bit in normal driving without an intercooler present, but even that won’t have much impact on gas mileage in hwy conditions.

One thing to consider when talking about MPG. The MPG info on the DIC is not very accurate (especially when you swap injectors and do custom tuning…). I’m pretty sure that # is based on pulse widths and the injector flow rates (not how much fuel has been used out of the tank). I watched my MPG #s go way up on the DIC when I raised the fuel pressure. The car was actually burning way more fuel than the computer thought it was. Anyway, I’m sure you get the idea.

Overall, I think the gas mileage stays about the same with the typical blower/stock motor setup.

 

When people saying thing like that. He don't have a clue or understand how forced induction work. Or just shooting his mouth bla bla....... You are wasting your time argu with him.

Bruce

 

15 years ago most kits or custom set ups didn't have BOV's and that was true. At 60 MPH, with your engine at say 2000 RPM, you would have a pressure on the throttle valve equal to the full blower pressure at 2000 RPM (engine speed) plus whatever parasitic drag the blower added to the drive belt load. This would reduce mileage.
But not much-I don't think the boost from a centrifugal at 2000 RPM would read on most of our meters. At 90 MPH in 3rd gear (still part throttle), you might have 3-5 PSI heating the air upstream of the throttle plate.
Modern kits with BOV's just vent the air from the blower at part throttle to ambient and the HP used in that process is relatively little.
I don't see where an increase in MPG would come from, with the installation of a blower, except the tuning after the blower install.
Roy