13 replies to this topic

[stevegrabba]

Hi Guys,

 

I recently discovered that the fuel injectors on my 7 port were a massive 480cc, and usually fitted to a Vauxhaull Zafira VXR.  So based on some advice and a few formula's I changed them down to 220cc.

 

When changing from one injector size to the next, how linear is the relationship between injector and a fueling compared to the next?   For instance in this case I'm moving to an injector which is near half the size of the prior injector.  So could I literally double all my pulse rate values in the theory 480/220  is x 2.18....and hence multiply all values by 2.18 throughout the whole map.  Then tweak individual points as necessary using my AFR gauge readings?

 

Any help or suggestions would be much appreciated thanks!

[MIGLIACARS]

is the car boosted or n/a

[stevegrabba]

No, normally aspirated...

[MIGLIACARS]

what make are the old injectors

[stevegrabba]

what make are the old injectors

 

Bosch...both old and new are.

[MIGLIACARS]

pico

[stevegrabba]

pico

 Yes, pico

[MIGLIACARS]

what have u done with the old ones

[stevegrabba]

what have u done with the old ones

Danced around them pagan style, naked in the kitchen!!

 

.....sorry mate but I was hoping and waiting for some input on the original question??

[retired]

The answer is that it's a good starting point but isn't entirely accurate especially if you currently have low pulse widths.

 

The reason being that the injectors have a "dead time" which, very simplistically, is the time they aren't flowing fuel because of the way they open and close on the applied electric signal.

 

I say simplistic because it's not a simple single quoted time (not with a decent injector anyway) but is affected by battery voltage and is usually non-linear on the initial opening especially when short pulse widths are used.

 

The datasheets should have the information but different manufacturers tend to quote it in different ways or even for different injectors by one manufacturer depending on their intended use.

 

So if you had an injector that was running a 5mS pulse width (at a given point in the map) and it had an inherent 1mS dead time, it would only be flowing fuel for the equivalent of 4mS.

If you then went for an injector half the size and it also had a 1mS dead time you would want a 9mS pulse (at that point in the map), not 10mS, in order to get 8mS of fuel flow. Even that is simplistic because of the non-linear bit.

 

The smaller the pulse widths are, the more the dead time comes in to play when trying to do comparisons.

[stevegrabba]

Retired - Thank you!

 

Makes absolute sense, and I hadn't even considered the effect which the battery could have also.  It is a good an healthy battery, as far as the eye can see and both the voltmeter and ammeter can show.

 

Are the injector dead times known or given by the maker...by size, injector model etc etc?

 

 

I'm going to give it a try firstly by multiplying all existing values by 2.18, and noting the AFR readings at different rpms, and different TPS values.

 

I'll report how I get on just in case it helps others.

 

 

Cheers for your help!!

[stevegrabba]

A little extra info here about injector 'dead time' and how to correct/account for the 'dead time'.....

 

Injector dead time setting   https://www.hamotors...-dead-time.html

 

Once I have transferred my new fueling map over, and given it a try, I'll see what values the ECU is giving me and report back, in case it's useful for others also. 

[retired]

 

Are the injector dead times known or given by the maker...by size, injector model etc etc?

 

 

For the mainstream manufacturers, yes but you might have to search quite hard on the web to find a genuine datasheet.

 

And, as I mentioned, some manufacturers (like Bosch) give the data in different ways between injector types (ie, simple data for standard road car injectors, and more detail for performance aftermarket stuff).

 

Here's an example of the ones I use....  https://www.rcfuelin...atency_Time.pdf

 

Those are for my turbo Mini running MS2 port injection siamese code (injects only when the inlet valves are open) hence the very large size as my maximum pulse width is limited to 6mS (to avoid valve overlap on the siamese ports).

 

But the table just shows how much the dead time varies with battery voltage and fuel pressure.

 

Your ECU should allow you to put in the battery voltage compensation values and if you've got a manifold referenced pressure regulator (rather than a stupid rising rate one) the pressure should be constant relative to the inlet manifold anyway.

[stevegrabba]

 

 

 

Your ECU should allow you to put in the battery voltage compensation values and if you've got a manifold referenced pressure regulator (rather than a stupid rising rate one) the pressure should be constant relative to the inlet manifold anyway.

 

 

Yes it does thanks.  I am collating the readings as we speak, to then enter the correct correction values.  I really appreciate your help thanks....I've learned a lot this week about fuel injection.