4 barrel "Franken" TBI unit - Page 3 - Third Generation F-Body Message Boards

titchener · 03-20-2009, 11:36 AM

Quote:

Originally Posted by Jinmajay

My research to this point was that the TBI's with slight mod's would by around 400 cfm (800 total).

Jinmajay, 490 cfm is got to be pretty close to the correct number for a stock TBI as you can get around 290 HP or so out of them in stock form, which means they must be flowing somewhere around 490.

So it may be that 2 units is too much flow for your engine, but I'd try it first to see. One nice thing about tuning EFI systems is you'll be able to back out how much pressure drop you have across your intake system at various RPM's and this helps in figuring out if you have too much or not enough intake flow.

Its not the end of the world if 2x490 is too much for your engine, I think as I mentioned in an earlier post in this thread there are some 2.8L engine sized TBI units that use the same base plate and injector pods. I estimate they flow around 325cfm in stock trim, so if the 490's are too much you could use these instead.

I'm curious to see how well a system like this can work, so when you get to the tuning phase I'll help you as much as I can.

I'm assuming your stock 350 injectors are the ones that typically flow 61 lbs/hr @ 12psi.

Each HP requires about .5 lbs/hr of fuel, so to make 550 HP you need 550 x 0.5 = 275 lbs/hr of fuel delivery.

At the stock 12 psi assuming an 80% injector duty cycle your flow with 4 injectors would be-

4 x 61 x .8 = 195 lbs/hr

So you need to bump your fuel pressure up to make 275. The flow increases as the square root of the pressure increase, so the pressure is calculated as:

New pressure = 12psi x (275 * 275)/(195 * 195) = 24 psi

The stock injectors fortunately work fine at that higher pressure (some guys have successfully run them as high as 30psi), so you should be fine on that.

Your fuel pump needs to be able to supply at least about 170 liters/hour at that 24 psi.

Paul T.

This ad is not displayed to registered members.
Register your free account today and become a member on ThirdGen!

Jinmajay · 03-20-2009, 01:35 PM

Thanks for all that calculating. I did a similar run through on the Mega Squirt site and from what I remember, came out basically where you did. I did by an adjustable (ext) fuel regulator to put on the outlet side of the TBI's and it has the vacuum port adjust. I believe this will help out a lot at idle.
Poo, poo on the smaller throttle bodies!

Got to make the big ones work. I did not increase the diameter of bore any. I have the original 4.3 CPI fuel pump I plan to use as I just replaced it and it doesn't have that many miles on it. I believe they put out 60 psi and I’m not sure of the GMP flow. I figured I would hook it up and watch the fuel pressure gauge to see if it drops off at near WOT.

Yes, they are the 61 lb injectors. I got a flow matched set that have been serviced from the injector man.

Thanks and we'll be talking.

Ronny · 03-20-2009, 01:59 PM

Consider placing the vac line for VAFPR to a manifold vac source. I placed a threaded nipple where the brake booster attaches on starboard side rear of manifold.

I believe the logic is that changes in vac may be less abrupt. However when a GM VAFPR was used many ran line to the TBI unit for vac. Do not T it into map vac source for sure. That must be dedicated.

Jinmajay · 03-31-2009, 12:59 PM

I know this is about tbi but I wanted to show my custom oil pan I made to fit the stroker crank.

I would like to start a post of my own but I'm not sure hw it woudl be received being the engine is going into a Safari van.

mroth2008 · 07-18-2009, 08:34 PM

nice I want to see this finished already

comp · 07-19-2009, 07:00 PM

Quote:

Originally Posted by mroth2008

nice I want to see this finished already

i'm still waiting to

Jinmajay · 07-20-2009, 07:40 AM

I'm waiting on my transmission tools from Pro build automatics. Not to worry, I will need some help with filling in the tables. You all will be the first to know.