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Re: [T3] Injector interchangability?

On 25 Nov 2003 at 23:43, David V.N. wrote:

> As an aside ,Jim,Ive found some of your comments made a ways back on the
> maximum sizes of engine that D-jet could support.They don't jibe with other
> opinions I've read in some respects. I know from reading this list for a while
> that you know as much as some posters from the shoptalk forum and was
> wondering about your opinions/advice on the subject. 

It would help me a bit to know which comments you're thinking of. As far as 
general thoughts about going to a bigger engine with the D-Jet, I don't have 
any experience with this. I'm just guessing that Bosch didn't build in large 
amounts of "headroom" when they designed the system. As a good design criteria 
I'd guess that +20% design ceiling would be a fair way to make sure that the 
design could accommodate sample-to-sample variations in production. For this 
reason I generally suggest that a 10% increase in displacement should be safe.

As for swapping in various other components to try to achieve the performance 
of the donor engine, I think that approach is na?ve. Yes, you can probably put 
in larger injectors and that may well fix things up where the OEM injectors 
leaned out but at the expense of running rich everywhere else.

I understand that just setting up to run rich is common practice in many 
circles, but I dislike it for many reasons. It wastes gas and money. It 
pollutes our environment. It causes excess wear and tear on the engine.

The system is too complicated for us to just swap in components arbitrarily. 
The injectors, pressure sensor, brain, and throttle body all have to work 
together to get the right fuel/air ratio. I see a lot of suggestions to swap to 
a different injector, or different pressure sensor, but no one looks at the 
throttle body and the pressure drop vs. air flow across the different throttle 
bodies. In my opinion, It MIGHT be possible to upgrade our FI to that of a 
different car IF we also upgraded the throttle body either by machining out the 
throat diameter to that of the other car, or by using the throttle body from 
that car.

All this could certainly be done by someone determined enough and with enough 
time and money to do some real tests, but I don't think anyone other than Bosch 
has bothered. It's just too easy to throw extra gas at the problem and say 
"that works" without really knowing or caring about the consequences.

Just my opinion....

Jim Adney
Madison, WI 53711-3054

List info at http://www.vwtype3.org/list | mailto:gregm@vwtype3.org

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