Outer Limits 4x4 Board

Most Diesels do not have a butterfly throttle as you know it, acceleration is achieved by more fuel being injected into the chamber. So the super charger is still giving the engine positive feed as it will be setup to put more air in than the displacement of the engine requires regardless of throttle position. Air volume out of the super charger is only changed by engine speed, temperature and restriction to flow.

chimpboy wrote:

SilverBulletBM wrote:Wouldnt you get engine braking anyway if you've got your foot off the throttle? Your not putting more fuel in so there wouldnt be any accel, thats the way ive always looked at it anyway.

I thought engine braking was also a result of the closed throttle making the engine work much harder on the intake cycle, ie it's drag due to the air intake being highly constricted. So if you have a supercharger in theory it is going to overrule that effect a bit.

But I guess it comes down to how much difference it makes in practice.

You also have the added mechanical work involved in turning the motor over, driving the supercharger and compressing the additional air in the cylinders..

i was under the impression that if you run a super or turbocharger with a carby, you will in effect increase the amount of fuel going into the engine whilst engine breaking, thus pick up speed.

However, as most cars that use a super ot turbocharger will have injectors, the engine breaking is acheived by reducing the amount of fuel going in, thus reducing power.

Think of most trucks. they all have turbos, deisels and still are able to use engine breaking, before they need to switch on the exhaust breaks.

An engine uses the rotational motion of the firing to compress a cylinder, if you remove the fuel, you reduce the amount of power being produced to compress

But hey, just m2c

TheBigBoy wrote:Couldnt you make the same arguement with a turbo (exhaust enduced supercharger) as to when your braking and the revs are still up your creating exhaust pressure that is still adding boost?

No. Turbo's and superchargers are chalk and cheese.

Turbo's run off spent exhaust gases and heat (from the gases). When you take your foot off the throttle, you are cutting 99% of the fuel, hence removing the elements that push through the exhaust turbine in the first place. Thats why in a turbocharged car, you can coast/cruise at (eg.) 2500RPM and the turbo will not be creating any boost. As another "rough" example, I can free-rev my 180B to 6500RPM and still create little/no boost. The turbine might be spinning flat out at 70,000RPM, but because there's no load on the engine (and no extra fuel), there is less spent gasses being expelled.

As said, superchargers operate at an exact relation to engine RPM. Having the supercharger produce no boost at idle would be ideal, but then it comes to the whole money side of things. To get no boost at idle and still have decent power figures through a given rev range, you would need to buy a specific supercharger with a specific trim.

Hope that's easy enough to understand.

On a side note, this is what NOT to to when sizing air pumps for forced induction: http://www.youtube.com/watch?v=WpTa6h78n2I

1BAR (14.5psi) on a free revving motor (no load) is just plain scary. The turbo will probably max out at 2000RPM under load, and then hole some pistons, possibly grab a main bearing or two....

Oooo Very good. Yep, I understand that just fine. Very tricky stuff.

Well said Coxy. Good example.

I take it the Toyota super chargers with clutches would be too small for a TD42?

Depends how much pressure you want to run. Find out how many CFM they are rated at.

KIWI - It was enough to boost a 4.5 litre. There where 2 toyota superchargers available, a smaller 1 and a BIG 1 .

The toyota superchargers were designed to go on a 1.6L for the small one and a 2L motor for the larger one. I really wouldnt waste my time with them, and when are you goin to ever want it turned off?
The mustang ones wont have the clutch, but will be able to boost higher with a better effeicency = more power!