FMU advice with 280cc injectors

[Streetwize]

On my 00 JRSC SI 10PSI I'm finally upgrading the injectors to a 280CC Prelude set I bought from CROM (THANKS).

Right now I'm at 40/52 and which by my calcs would equate to around 30/43 as the new base setting which should give me about the same A/F at idle and a slightly richer mid range which is what I need. I know the JR 5:1 is going to be way too high with 10psi boost and at present with the 240s it runs fine but I know it's already fat. Again according to my flow rate calcs and assuming my new base settings are pretty close, I'll really only need about a 1.5-2:1 rate of rise to get me to a 12-12.5 A/F at WOT. What is the best FMU to use? I'm assuming the Cartech, but which one and can I only by them direct from Cartech?

In the mean time I may put in a flow restrictor in the vaccuum tube to limit ("TRICK") the rate of rise with the JR until I get a new FMU. I figure I can cut the 10psi boost pressure seen by the FMU to a little less than 5psi "seen" by the diaphragm to get my WOT settings roughly equivalent to what they are now. I'm an electrical engineer and I work with several clever Process engineers that aren't very busy right now, It'll give them something to do.

[Holden]

I have my cartech set to a 2.6:1 rise rate and let me tell you, it was not easy to get it to do this and still react fast enough.

It took a combination of an adjustable restrictor and a larger, bored out check valve.

With the rise rates you want, it may be even harder. You may have better luck if you install multiple bleeder valves on the FPR.

Maybe there are other FPR's out there that would be better at providing such a low rise rate. Does Vortech sell a 2:1 disc?

[Streetwize]

Greg,

Thanks, I checked out the related info on your site. Very informative!!

I know I'm going to have to play with it a bit. I'm hoping to get some input from someone with an 8 lb GSR with 310's to compare notes.

I'm starting to figure out that you don't need as much fuel or as low A/F ratios on Blown DOHCs as many people think. I've worked on Blown Hemi Chryslers before and I'm finding there are a lot of similarities in what both these motors like and dislike. The relationship of the combustion chamber to the piston dome, flame travel and cylinder filling are pretty proportional.

[Streetwize]

Okay, the 290 Prelude injectors are in, My idle pressure is 32 and since I knocked down the JR rate-of-rise to approx 3:1 by increaseing the throughput (larger orifices)the motor is running really sweet. I don't think I'll need a Hondata now although with 22 years of tuning experience I'd like to play with one. The motor is richer from about -5"HG throughout the low boost (where it felt a little lean before and new lb/hr calc's bear this out)and it seems to be right around the perfect A/F on top 'cause it's pulling faster through the gears and the pipe is cleaner (only slightly sooty,ie, safely rich).

Wize