Before i spend 160 dollars on a 350 tbi setup, intake and throttle body does anyone no what the cfm rating is for that throttle body, because i am sticking it on a 350 crate motor with headrers stock heads and a .425/.454 lift cam just wanna make sure the tbi will work?Thanks fellas

 

WOM is 475-500 cfm @ 3"/Hg.

The Holley is 2" and they claim 670 cfm @ 3"/Hg. The stock one is 1-11/16" so it's gotta be less than that.

BTW, don't get that flow rating mixed up with a 4V carb. 4V carbs are flowed @ 1.5"/Hg.

So 500 cfm @ 3"/Hg is equal to 353.6 cfm @ 1.5"/Hg.

Which works out kinda neet for a 305.

According to the formula

CID x Max RPM
3456
Then multiply that answer by VE% and that Equals = Recommended CFM


305 x 5000
3456

Equals 441.26
Multiplied by 80% (or .8) = 353.00 CFM

I guess GM knew the minumum they needed to do the job.

BTW, 80% VE (volumetric efficiency) is about average for a stock SBC.

 

okay - so i'm kinda perplexed guys. In short, the holley 670 I bought doesn't flow 670cfm, but more something to the tune of 500cfm correct? Because if this is true...it seems like TBI is a waste. And dont flame me for this - i've struggled through a lot w/my tbi, but i never knew it was this limiting with flow.

TRP

 

yes tbi is restrictive by nature. It was never intended to be a "high performance" induction system. Not to say you cant make power with it but other induction systems, like carb, for instance, are way better suited for high output engines. Of coarse, you also get worse fuel economy and less reliability then you would with tbi. Its all about trade offs.

Holley rates their 1-11/16 tbis as flowing 400 cfm @ 3 inHg (most four cyl. need more air then this), so this gives a good comparison of how the stock tbi stacks up to the larger holley tbi.

 

so will a 350 tbi unit work with my setup or will i be wasting my money!...

 

Go with the Holley 670 or the GM 454 TBI unit.

 

i would personally say buy a carb. lol. I got the 670 holley, the matching intake, an aftermarket ignition - and its kind of a waste because everything is TBI specific. I could have spent the money on a bitchin' carb setup.

TRP

 

how do i determine max rpm in the formula is it based on the cam rpm or other factors? thanks for the input!

 

I'll just be one to say, I don't really trust/follow those formulas. How much does the stock twin 48mm TPI T/B flow ~520 cfm? Yet GM put it on there 502 ramjet crate engine.

In Formula SAE competition 600cc engines are restricted to a 20mm hole. 20mm hole = 314 mm^2 engine makes 60-75 hp NA

Stock TBI = 2 X 42.8 mm holes = 2877 mm^2 area

I'm sure the area doesn't equate exactly to flow (and don't want to look it up right now), but your looking at 8 times the size. See what I'm saying. Your dirty airfilter probably causes just as much of a restriction.

Sure the TBI probably becomes a small restriction above 300 hp, but so does a single 3" exhaust at 6000 rpm.

I think most people's problem with slapping a TBI on a crate engine or any modified engine is the tuning. TBI is a speed density system and will run horribly if the computer is not tuned to the engine it's on, specifically the cam and heads creating most of the engine's requirements.

 

so last question will a 454 tbi unit be too much for a 350 small block with stcoke heads, hedman headers, and a .424-.454 lift cam cause i just dont want t o over fuel the engine if yea no what i mean!

 

The big difference between the tbi adn the ramjet is the large plenum volume that the ramjet has. All that extra air inside helps to buffer the pulses that result when each cyl. goes to intake air. The less plenum volume you have, the larger the bores have to be to be able to cope with the surging air flow. Why do you think most q-jets flow around 750-800 cfm? Its because of the small volume that the stock intakes have.

 

oh, and about the FSAE cars, alot of those teams use very large intakes. My friend was showing me the composite intake he made for the sae car and the intake is absolutly huge inside. He was saying that if they didnt use such a large intake, they wouldnt be able to make nearly as much power as they do now.

 

Yeah, I forgot about plenum volume, the FSAE cars do have much greater plenum volume to displacement ratio.

Anyways, I think the stock 350 TBI throttle body will be alright for this application. I just don't think people should believe those formulas like they are 100% accurate or if you have a little less cfm rating it will kill you power completely.


Example of a formula being way off is the trap speed/rwhp formula. My car weighs ~3500+lbs with me in it and ran 14.7s at 93 mph. rwhp formula says 220 rwhp, Dynojet says 178 rwhp. That's a 20% error.