260cc to 440cc injector req fuel

[280zedx]

I am running a version 2.2 fuel only MS and had it fairly well tuned around 9psi but was running out of injector around 5500rpm with stock 260cc's . I'm squirting water/meth and that sufficiently kept me rich till red line but i still wanted to get my duty cycles out of the +100%. I just got some nice 440s and was curious if anybody had changed injector sizes while running MS.

It calculates the new req fuel at 7.1 (before 12.0) but I was curious if the opening times or pwm changes?

Theoretically my map should be the exact same (req fuel amount might need a tweaking) shouldn't it?

[z-ya]

Mike and I have been exchanging PMs in this, but basically this how to upgrade injectors when using MS.

 

First thing to remeber is that not all injectors flow fuel at the same rate of change (flow vs. duty cycle). So two different 380cc/min inectors of different design will have a different flow vs. duty cycle plot. So when you upgrade to a higher flow rate injector, you cannot just change the "required fuel" setting in MT. You will need to make changes to the fuel map. Especially at the higher duty cycles (higher engine loads).

 

This is how I have upgraded in the past. Let's say we are upgrading from 270cc/min injectors to 380cc/min ones:

 

First, if you are using the calculated required fuel setting that MT does, do the same for the new injectors. If your required fuel constant is different than what MT calulates, then reduce the required fuel number by the percentage flow rate increase of the new injectors. This should get you close enough to get it started and idling.

 

Let the motor come up to temperature, and then adjust the required fuel constant until you get the idle as smooth as possible.

 

The math:

 

Flow rate % increase = 1 - (270/380) = 0.29 x 100 = 29%

 

I've done this upgrade, and the required fuel constants are as follows:

 

270cc/min = 11

380cc/min = 8

 

Required fuel constant increase = 1 - (8/11) = 0.27 x 100 = 27%

 

So you see that the required fuel constant comes out almost correct.

 

This is close enough to run it and drive it easy. The top end will be rich, but after tuning the top end, you will find that the injector opening time increase in the fuel map is not proportional to the required fuel constant increase across the fuel map.

 

The bottom line:

 

You will need to retune the fuel map for max power, but just scaling the required fuel constant will get you close enough for idle and cruising.

[280zedx]

great post Pete, that helps lots with getting me in the ballpark. thanks.

[twoeightythreez]

mine came to 7.0 using the program...6 437cc injectors into 168cid.

Mine came courtesy of 3 ford LTD's with 5.0 TBI.

[280zedx]

I finally got around to firing it up and 7.1 put the idle right where it was before. sweet. Now i just have to wait till the snow melts....