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Re: [Shop-talk] An off shoot of the Toyota debacle.....

To: "Nolan" <>, "shop-talk" <>
Subject: Re: [Shop-talk] An off shoot of the Toyota debacle.....
From: "Eric J Russell" <>
Date: Sat, 6 Feb 2010 21:17:24 -0500
Since we're drifting off topic I've decided to chime in.

I've driven lots of gasoline engined vehicles and some diesel engined 
vehicles. Although not exactly apples to apples I can compare a gas engine 
3/4 ton pick up w manual transmission & gas engine (Chevy C-20) to a 3/4 ton 
pick up w manual transmission & diesel engine (Ford F250).

The gas engine pick ups have much more 'engine braking'. When I first bought 
a diesel engined pick up (83 Ford F-250) I wondered if something was wrong 
because it didn't slow down as much as I expected when I'd let off the 
'throttle'* or down shifted.

* A gas engine is throttled. Closing the throttle causes the intake vacuum 
to rise and the braking effect is the engine working against the vacuum. A 
diesel engine does not have a throttle. The intake is wide open. The 
'throttle' pedal actually varies how much fuel is injected. Taking your foot 
off the pedal merely reduces/stops fuel injection. The intake remains wide 
open (no intake vacuum) **unless** the engine is equipped with a 'Jake's 
Brake'. See:

<quote>In a gasoline engine, some engine braking is provided during 
closed-throttle operation due to the work required to maintain intake 
manifold vacuum, the balance coming from internal friction of the engine 
itself. Diesel engines, however, do not throttle air intake and hence do not 
provide engine braking from throttling losses.</quote>

Eric Russell
Mebane, NC
----- Original Message ----- 
From: "Nolan"

> You're right, it's not 100% and 0%.  There is some force that can go the
> other way.  I don't know the effectiveness ratio.  I do know it's not
> much.  I've never driven a gasoline engined vehicle that had much engine
> braking, even with a manual transmission.  The engine is way too low in
> compression to give much.  A diesel, yes.  That's a different critter.
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