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Can comeone explain to me why comparable motors could run with an efi stealth ram that flows 275cfm, however you would need a 650 cfm carburetor on the same motor?
Why are the cfm numbers so different.
__________________ 355ci motor, stock bottom end with forged pistons, dart irong eagle 180cc runner 64cc combustion chamber heads, 224/230 xe comp cam, stealth ram, LT1 BBK TB, double roller timing chain, aluminum valve covers, smog pump delete and more
275 cfm ratings on the stealth ram ARE FOR EACH INTAKE RUNNER
a CARB feeds all 8 RUNNERS
your ENGINE only pulls from one cylinder at a time but you need to supply enought for about two to three cylinders in the plenum,to not be restrictive
stock 48mm approximately 650cfm
52mm approximately 780cfm
58mm approximately 950cfm
oval blade mono approximately 1150cfm
that of course ASSUMES the plenum and MAF allow the full flow which they won,t in most cases
keep in mind your never going to run into a restriction in the throttle body untill the
intake manifold flow, heads flow , cam lift and durration,and exhaust are significantly upgraded, and that will require drive train changes like a high stall converter and 3.73:1 rear gear and higher compression ratio in the engine
another factor most guys don,t understand...
flow is rated at 1.5 inches, increase the depression and the flow jumps, thats why a stock TPI throttle body can feed a 454 big block, that tb that flows in the mid 600cfm can flow 800 cfm with a bigger engine and cam putting a larger drop on pressure in the plenum
Intakes and cylinder head ports are flow tested at a 28" pressure drop.
Carburators (4bbl) are rated and flowed at a much lower pressure drop 1.5" very different.
2bbls are flowed at 3"
EFI throttle bodies are rated the same as 4bbl carbs 1.5"
So.... a 750cfm carb when flowed at the cylinder head port standard of 28" will flow 3240cfm.
okay so im going to need a little help putting all of that together
grumpy check the sig, its a speed density motor with stock tb, stealth ram intake, dart iron eagle 180cc heads, and a .50 224/230 cam. so its got some mods.
I have an LT1 BBK 52mm tb that i need to figure out how to convert to work with my tpi setup, have been told i just need to bore out one sensor hole or something to the like. Just hoping im gonna pick up at least 10 hp with it, but i know my heads are the biggest restriction right now
Intakes and cylinder head ports are flow tested at a 28" pressure drop.
Carburators (4bbl) are rated and flowed at a much lower pressure drop 1.5" very different.
2bbls are flowed at 3"
EFI throttle bodies are rated the same as 4bbl carbs 1.5"
So.... a 750cfm carb when flowed at the cylinder head port standard of 28" will flow 3240cfm.
This is partially correct, what is left out is that the heads & intake at 28" is of water. It is a pressure drop of 28" H2O.
Throttle bodies and carbs are measured at pressure drops in inches of mercury. So the 1.5" and 3" of drop is in inches of mercury.
Which brings us to the next part:
1 pound per square inch (psi) = 2.04 inches mercury (in Hg) = 27.7 inches water (in H2O)
Doing a little rounding 28" of water approximately equals 2" of mercury. That puts the 740 CFM 4bbl carb at 1.5" Hg at 866 CFM at 28" H2O.
omg, how do you guys know/learn and understand this stuff, i am so lost lmao
you can be a total and complete grease monkey and have performed a billion engine trans and other swaps for over a decade and still not know what all this stuff means, such as myself for example
crazy stuff,
yall are gods imho, lol
nice thread here
__________________ IM or text me 217 671 7792 i don't check forums often!! fastr68 on yahoo msngr anytime! 1982 Trans Am HT WS7 - i have no F-body cars any more- i still do have many '82 to 84 AND '85-up bird and camaro parts and parting out 1991 S10 also all pieces blue colorCo-Founder of the Early Bird Club(82-84)
Don't forget also, that the flow into an engine isn't a smooth, steady, constant thing; it's a series of peaks and valleys. Each cyl getting its gulp of air is a peak, which can be MUCH higher than the "steady state average" that you calculate from CID, RPM, & VE. The carb or TB must allow that PEAK in the flow, or else it will be a restriction.
The intake plenum helps average out the bumps in the flow. The smaller the intake plenum, the greater the difference between peak and average; and as a result, the GREATER the carb or TB CFM needs to be, compared to the "steady state" calculation. Which is why an engine with a giant single-plane intake needs a smaller carb than the same engine would, if it had a dual-plane with a small plenum.
Most port FI systems have a VERY LARGE plenum, and consequently, the flow through the TB is relatively smooth, and lacks any peaks that exceed the "steady state" value by too much.
The difference in head flow CFM numbers and carb CFM rating numbers, due to the the difference in the pressure measuring method, is less than 15%. Not a super huge discrepenacy.
__________________ Numquam ponenda est pluralitas sine necessitate. — William of Ockham, from Quaestiones et decisiones in quattuor libros Sententiarum Petri Lombardi
Roughly paraphrased into modern English, and applied to figuring out what's wrong with your car:
The simplest explanation that fits all the facts is probably the right one.
