Carb CFM vs TBI CFM (vs TB MPFI CFM)

Hey folks,
I am evaluating my options for my mild 496" stroker and can provide motor details if relevant to my questions...

Can anyone tell me if the CFM of a TBI EFI system should be equal to that of the carburetor it is replacing? I'm eying the Fast EZ 2.0 kit and Fitech offerings right now, but trying to gather info. Fast TB is 1000 cfm whereas the Fitech GO Efi 8 is 900cfm. Is cfm from efi to carb an apples to apples comparison?

Fitech is looking real appealing right now, but I am wondering if the CFM will starve my engine.

Neither will starve your engine unless it is truly something special.

Alright, how about a slightly more specific question? Does anyone know if an Edelbrock 440 Victor EFI intake manifold will fit underneath a stock '70 Cuda hood with a fast/fitech style throttle body on top and air filter?

First, TBI should flow the same or more as the carb you would have chosen.

The FITech style throttle bodies appear to be close in height to a carb, about 4". An edelbrock victor intake is about 1/4" or so taller than a performer RPM intake, which guys are getting to fit with drop base air cleaners.

Daytona Turbo,

Thank you for the reply. That was my concern. I was looking at both the Fitech Go Efi 8 and then their MeanStreet Efi for 800hp (similar to the Go EFI 8, but without timing control and support for power adders - unneeded for my application). I *think* Ken said that the MeanStreet throttle body and the Go EFI 8 was 850CFM. They have a Multiport system coming out that is a 900CFM throttle body that wouldn't cost much more than their EFI 8 kit. I could be all backwards at this point witht he CFM numbers though.

Originally I was looking at Fast EZ Efi 2.0, but all the horror stories had me second guessing that option. Whether its due to installer error or something else, I'm not sure?

I would go with the largest throttle body offered. air flow shouldn't be your limiting factor. its important in a carb only to get the fuel flowing out of the bowls. in an efi system, the injectors fire no matter what so don't limit your HP potential and get a small throttle body. also you have a 496, I don't know if I would run a dominator on there at 2000 cfm if you want to drive it on the street. you could even get away with a 1200cfm version. any larger than that and part throttle driveability will suffer.

It is possible to go too big with EFI just like you can on a carbed engine.

When I moved the turbo 440 from a factory Cobra throttle body to the 1600 cfm Accufab I felt a distinct loss of throttle response.

I heard from a couple of sources when EFI was showing up on production vehicles that EFI had to run larger CFM throttle bodies to be equal with a carb.

I know when snowmobiles and motorcycles went to EFI it was the same thing: 46mm throttle bodies to replace 38 to 40mm carbs.

Take that for what its' worth, and perhaps things have changed.

I have the Fast 2.0 on my 512 stroker street engine. It runs great. I have put close to 2,000 miles on it so are and have had no problems with it. I believe most of the horror stories you have heard about are due to installation errors.

I have an MPFI system on my 340 and I'm using a Super Victor EFI manifold and a Holley 1000 cfm throttle body that's carb flange. The throttle response is great, even though it has non-progressive linkage.

Thanks for the reply! I guess I need to figure out if I could even fit that intake under the hood with a big block. Still not too sure about the cfm between the two systems, but it sounds like it's an apples to oranges comparison, unless I misunderstood the prior replies.

Any one else have any positive experiences with the Fast 2.0 system and/or any installation tips?

Forget about the CFM of the throttle body and look at what HP the system will support. The number and size of the injectors is more of a determining factor as well as the pump volume to support the injector size.

Case in point, "I believe" the FAST EFI throttle bodies are the same size for the EZ efi and the 2.0 systems - but the 2.0 uses 4 more injectors to up the HP handling capacity. Likewise with the FiTech systems.

If you want the RIGHT answers, you should ask "moparrich" who is a member here and the FAST EFI guru http://www.fastmanefi.com/

the fast efi system is good. for your size of an engine you can use a large throttle body.

in a carb because of the venturi effect of drawing in fuel, you have to limit the cfm so you get good atomization and a strong signal to pull fuel from the bowls.

in efi you don't have to worry about this as the injectors will atomize the fuel and deliver the proper amount not matter what cfm your throttle body is.

in terms of cfm:
1. small cfm will limit horsepower and give really good throttle response
2. too large of cfm will not add any more hp than your engine can put out, but part throttle drivability may suffer as well as fuel mileage.
3. the just right zone. good throttle response, just enough cfm to not limit hp, with tuning can achieve decent mileage.

yes on fuel injected engine, the injector size is more critical than throttle body cfm. to large of injectors, and your low rpm will suffer, as well as driveability. too small and you lose hp and can run lean.

I run an XFI system on a B 496 in a '70 Challenger with flat hood. With the short throttle body, M1 single plane and mild drop base air cleaner, it just fits. I believe the standard Cuda hood is bulged which should help clearance... still the Victor is higher, the RB is higher and with a taller TBI throttle body not sure it will fit. With a short port injection TB it probably would fit.

As many have posted, going bigger (to a point) on the TB does not hurt drivability with FI like it does with a carb. My non progressive throttle body has great throttle response... almost too good. Crack the throttle at lower RPM, all 4 barrels open and you are moving. A progressive TB would be smoother and also easier to tune the acceleration enrichment.