HP limit of TBI based system?

[RAMM]

Hello EFI guys. I have been contemplating building my own TBI centersection housing 4 injectors only. I would like to support about 620-640 hp on a conventional V8 engine. I believe 4 injectors rated between 80-90 lb/hr would work however I am not sure they can maintain a stable idle at the very low pulse width and or pressure required.

This leads me to my next question--Can I use an adjustable regulator to control pump pressure to have low PSI at idle and high PSI under load/rpm?

Is this just a bad idea all the way around or should I figure out how to house the traditional 1 injector /cylinder in the TB? At the moment I am just spit-ballin, any help would be appreciated. J.Rob

[Beagle]

I'll let the resident genius's answer this, there has been some talk lately of newer injectors turning on / off very quickly which I think is probably the key to idle.

From a curiosity standpoint, that would be a pretty big TB - are you sure you want four fat injectors screwing with the air flow? What kind of TB are you planning on using? Opinion here, but if I dropped coin for a 1000+ CFM TB, I wouldn't want anything stuck in the middle of it. Distribution would also alarm me, which may or may not be a concern.

What is your definition of low and high PSI? KB boost-a-pump kind of control takes the normal pressure (39psi , 13.8v) and cranks it up (50's, probably closer to 16v) via more voltage. There's a raging thread on here somewhere about reducing voltage, busted controllers, added heat, etc. for the pump. Most of the regulators I have seen are mechanical so there's not much changing at run-time. You get both sides of the pump controller camp here. Some guys say Yes, some guys say it's a good way to park yourself in the middle of nowhere. We were starting to talk about PWM control for the pump and then I drifted away from the conversation but I would think a PWM controller would be the way I would want to go if I were to do that.

[RAMM]

Thanks Beagle--Maybe I should be a little more clear about my idea of a TBI. I was/am seriously mulling around the idea of using a Holley 4150 1000 center section with custom machined "blocks " as metering blocks that house the injectors---Here's my brainwave idea----I was thinking about firing the injectors directly into the back of the booster cavity (annulars). Other than the actual volume of fuel and its subsequent control--I cannot think of a reason why this can't work reasonably well.

p.s. The only resource I lack is knowledge-I can manage the hard parts. Thanks again. J.Rob

[Matt Cramer]

Four 90 lb/hr injectors aren't much harder to control than eight 45 lb/hr ones. Possibly easier since they can be staged.

The biggest limiting factor on a TBI is likely to be fuel distribution; it may take a bit of tweaking to make sure it doesn't have the same issues as a carb on a manifold designed around some other carb design.

[Barry Donovan]

a regulator is supposed to just let that go back to return when injectors don't need it. same as any fuel system slowing down.

I am assuming you mean the injectors don't know how to slow down?

that is the resident genius queston to answer indeed.

[TheSilverBuick]

Four 90 lb/hr injectors aren't much harder to control than eight 45 lb/hr ones. Possibly easier since they can be staged.

The biggest limiting factor on a TBI is likely to be fuel distribution; it may take a bit of tweaking to make sure it doesn't have the same issues as a carb on a manifold designed around some other carb design.

Yup. Staged injection is the way to go IMO. Wire up two primary and then wire and program the other two to come one when needed.

[RAMM]

So you're saying to fire only 2 of the 4 injectors at idle/low rpm and bring the other 2 online as needed? J.Rob

[TheSilverBuick]

So you're saying to fire only 2 of the 4 injectors at idle/low rpm and bring the other 2 online as needed? J.Rob

Yup. As far as I know most EFI systems out there can do this. The only thing to keep in mind is the secondary's are probably on a progressive linkage so don't start opening up until the fronts are nearly all the way open, so you'd want to see what the TPS % opening is reading to decide when you want to bring in the second injectors. It could be greater or less than 50%.

I run a 2-bbl TBI with a 640 cfm throttle body on a Ford 428FE engine with 85lb/hr injectors with no issues idling. I would stage the injectors if I had the 4bbl TBI.

[TC]

Yup. Staged injection is the way to go IMO. Wire up two primary and then wire and program the other two to come one when needed.

Is that how Holley does it with their 4bbl TBI setups??

This kit is good for 600hp........

http://www.holley.com/550-402.asp

[dieselgeek]

600 HP is well past the limit I'd recommend for any TBI application. I've tried a number of different designs and they all get off the bus (distribution problems) in the 500-550hp range regardless of displacement.


I would definitely NOT recommend this especially to a customer, Jesse, I know you're thinking that EFI that looks like a carb *should* work - and I thought that too once - but trust me on this, you are headed down the path of Bad Idea. I wouldn't say that if I didn't care! :-)

Either way keep us posted. You can definitely run it with a megasquirt and staged as the guys here suggest.

[BKBridges]

Your single TBI set-up can/should be good to about 550hp or so in a manageable way. It will have all of the fuel distribution problems of the original manifold + additional ones associated with the injection issues...But Im going to join DG in the not reccomended catagory. The carb boosters are set to draw fuel under a vacuum... Pumping fuel into them (at EFI pressures)with injectors designed to squirt into a vacuum or ambient pressure may/will create metering issues unless you modify the heck out of the booster config. The MSD system is similar to your described set-up and they did a lot of engineering (with patents, just like FAST...) Better to copy the FAST TBI (as long as you dont sell it) and inject on top of plattens mounted under the blades. While I like making things, I like to start with "the good stuff". Why hide EFI...make it look good instead...let your multiport shine!
BKB

[CDMBill]

Jesus, is that pretty! That six pack illustrates what I think might be the next big thing, a really well functioning EFI induction set-up with Apple or whomever you admire level of industrial design. Rather than retro, a clean sheet manifold/TB/injector sensor set up that install on traditional SBC's or SBF's, LS's or whatever with the kind of execution pictured above could really be a signature lines of products.

[RAMM]

Well my mind is set and I'm gonna have to take the advice given here. I'm gonna go full MPFI-I'm thinking of injector placement and want to get them up fairly high--but aiming them will be the hard part. Thanks for the sage advice guys. J.Rob