JustinL

A bunch of guys on my local board are telling me that they can build 8psi of boost revving their engine in neutral. I'm pretty sure my 951 can't build much boost at all. What is the difference between the engines that allow this?

Justin

porshhhh951

boost is directly in conjuction with load....you cant really build much load in neutral so there for you cant really build much boost.If i were to rev it to red line i could proly build a little boost but not much.,...nothing worth mentioning.Besides what does it matter........revving your engine at the light is a ricer thing.Better to make sure you cant build boost while driving.lol.=)

Sami951

Good question.. I've seen some ricer cars that do infact put out a dump valve PSSShT sound when they rev it in neutral, indicating there was at least some boost... can't see that really happening in a 951, no clue why they have it and we don't - maybe they've got more responsive turbos?

Duke

Sami, the dump valve can recirculate/vent air even if it's not under boost.
For example, if you open up your throttle to the max at neutral - the air will flow into the engine at a certain velocity.

Close the throttle rapidly and there's a big change in vacuum and the valve vents the air that was going through the intake pipes towards the throttle body.

shortyboy

i build boost in neutral but that is near redline,like 6k rpm.my hks bov is really loud at that engine speed.i think the reason why other engines can bild boost sooner is because of their displacement(correct me if im wrong?)we got a big 2.5L 4 banger,other competition has got 2.0 and such.ive also tried neutral dropping and shifting into 3rd,no sense doing cause by the time you build a little boost and shift into gear youll bog a bit.HTH

Laust Pedersen

Isn't is simply a question of where the boost is measured? Depending on the characteristics of the BOV you can build boost before the throttle plate, while you cannot build boost after the throttle plate (all at no load). Any semi healthy naturally aspirated engine can easily spin to the redline (and beyond) in neutral. Try to measure the boost before the throttle plate and you'll get a much different picture.
Laust

Danno

You can also look at it in the reverse way. That is, what kind of power is required to free-rev an engine to redline. Only about 70-lbs of metal to spin around a relatively small circle. Might only require 5-10hp to rev that engine to redline in neutral and you won't even have to crack open the throttle anymore than 50%. Thus you won't develope any more power than needed or else you'll be through redline before you know it. I betcha if you completely floor the engine fully in neutral and hold it for about 5-seconds, you'll see some boost...

Chas

those other people also have those little super-quick spool hotsides (the ones on all the 1.8T engines...Volks and Audi), so they'd get a little more pull from the turbo by just revving it.

And as previously stated ... when you rev it, you go from open throttle body to closed throttle body really fast...and the quick change in AIRFLOW (a lot to a little) will make ANY bov sing

...if you are able to build significant boost in neutral, I'd see it as a bad thing...too much internal resistance in your engine, too heavy flywheel, etc.

JustinL

That's pretty much what I'd figured. My guess was that they either have gigantic flywheels with a huge moment of inertia or a little tiny turbo. Either that or they're bouncing it off the limiter for a while as the boost builds. I was just blown away that they can get so much boost from free revving.

Thanks guys.

Justin

toddk911

Maybe with a VATN turbo