this may be a stupid question, but i was just curious...

how come our cars don't generate boost when we are sitting at idle & revving our motors? shouldn't the turbo still spin & send compressed air to the intake manifold when revving but not moving?

i'm pretty sure MAF equpped cars do generate boost just by revving, which is why you can hear the bov's going off on mr2's, supras, ecplises, etc...what makes cars with map sensors any different?

 

Has nothing to do with the MAF vs. MAP sensor deal. The fact is, factory turbo'd cars have WAY smaller turbines which spool quickly enough that they CAN generate boost under no / light load.

The reason is when you rev in neutral, the engine uses very little fuel; if it's burning very little fuel, there is very little combustion and therefore not much energy in the exhaust. Under load however the exhaust has plenty of energy because you're burning tons of fuel.

 

and lots of fuel consumption = lots of expanding gas which builds pressure in the exhaust manifold which spins the turbine

 

this i did not know. i never really thought about it. good info!

 

My friend has a stock mr2 turbo he doesnt blow of when he revs...

 

well if its stock everything it shouldn't "blow-off" factory turbos cars have a compressor bypass valve. its just like a BOV, except it doesn't vent to the atmosphere (where it makes noise) instead the air is routed back into the intake tract (where it doesn't make any noise).

HTH


[Modified by BlueShadow, 12:58 AM 5/14/2002]

 

he has a blitz BOV?

 

Simply put: An engine under load is like a boat propeller in water, as opposed to a boat propeller spinning in open air. You don't see the boat move until you put the propeller into water. (This is a very simple illustration just to prove a point.)

Hot expanding exhaust gases (with a rich fuel mixture) have more mass and kinetic energy than cooler exhaust gases (with a leaner fuel mixture).

Make sense?

HTH


[Modified by Fluoxetine HCL, 8:04 PM 5/13/2002]

 

a twin turbo supra can blow off while reving in neutral

 

And thats why equal length headers don't add a thing. It's all about thermal energy, not flow.

 

but earl what about your equal lenght manifold? it must be a show item

 

then it's not stock now is it?

seriously, I dunno what to say.

 

hmmm, i'm pretty sure i've encountered modified supras & mr2's that still made enough boost to blow off, say when leaving a parking lot for instance, but have never heard a honda, even with a low budget junkyard turbo kit, blowoff in the same situation.

 

Whatcha talking about bubba?

On my car(1g dsm) with the stock turbo and stock bov routed back into the intake you can hear it blowing off while free revving in neutral. When I upgraded to a 16g I could still do it. When I upgraded to a 20g I could still hear it. Now I have a full T4 and I could free rev it all day with a big tial bov and It will not blow off.

Can you make the connection?

btw this is all free revving in neutral.


[Modified by Geovannie, 1:12 PM 5/14/2002]

 

<< he has the right idea.

 

Not my choice on that one.

 

mine spins at idle...t3/4 .63 ar...but my compresiion is pretty high too

 

my friend had a audi turbo(tiny) on his HX, and he could blow off in N, also my friend with a 87 300Z and a eclipse BOV can do it, but they are the only ones ive ever heard

 

Well I can blow off in neutral also, but it's a rather mild blow off as opposed to when under load. It's inevitable that an amount of air will be sent from the compressor to the TB even w/o load, so revving hard and then letting off the gas give the air nowhere to go but up out of the BOV..

Remember also, a BOV's setting (soft/hard) will effect it's ability to blow off under no load as well..


[Modified by VTC_CiViC, 10:39 AM 5/14/2002]

 

I've had 3 turbos. The first one, would blow off at 6K RPM+ in Neutral. It was a Turbonetics T3/T4 with a .63 A/R exhuast hous, .60 A/r Comp housing, and 57 Trim

The second one blew off at 4,500 RPM in Neutral! It was a very quick spooling t3/t4 turbo.

and this one that I currently have is the same as the first one, I have yet to try it...

 

This guy knows what he's talking about

But it does depend on your turbo size as well. If you have a tiny turbo that doesn't take a lot of exhaust energy to spool, it will blow off under no load.