Inliners International Forums General Discussion Hi Performance cfm on efi

Does a Holley 1000 cfm 4 barrel TBI works fine on a 250?

If anything, that would be overkill for a mild NA 250. What sort of power/ engine combo are you looking for? John

Something about 450-480 range in NA engine. With a 298 Iskenderian cam, burning alcohol with 15:1 compres ratio.
What kind of injectors should i use?

it would be best for you to contact holley on the injector type and size. tom

Maybe it´s too much for this 250 set up????

Mighty6 uses a 4 barrel TBI in his Pick-up. Don´t know for sure what size it is, but it´s huge! Remember that Mighty6 have a 292 with 12 port aluminum head and a lot of nitrous....

Here's a formula that might help guide you in the right direction.

CFM= (max rpm x cid) / 3456

So, if your redline is 7000rpm with a 250cid engine,
your max CFM is ...... 1,750,000 divided by 3456 ... or.... 506.365 CFM. That's not to say you need to only run a 500 cfm throttle body. It's just a guideline.

My book states this is for a turbocharged engine, but it will give you an idea. The 0.55 figure is the "maximum load brake specific fuel consumption (bsfc) of a typical turbocharged engine."

pounds of fuel per hour per injector=

(expected bhp x 0.55) / number of injectors

so... 500bhp x 0.55 = 275

275/ 6 (I'm guessing you're going to run 6 injectors) = 45.833 lbs an hour per injector

Like I said, I just posting this as a guide so when you talk to a fuel injector company y'all will be on the same page. Good luck. John

one more thing to keep in mind is that you will need a much larger injector with alcohol than for gasoline. tom

then it changes the volumetric efficiency, doens't?
Than, should we use the same formula?

Anyway, thanks for the formula. But actually i saw some things at the dyno that don't go straight with it. Like a 780 cfm Holley quadrijet works better than a 650 one. And the engine was reving 7.400 as the hp peak, so to stay on the cam, we rev it to 7.550 and both power and torque values were bigger with the 780.
I think this formula is a kinda guide, but its not the bottom line for sure

Thanks anyway

seiscanecos74, et al.,

Just for fun, I ran a simplified computer simulation of your engine and got the following results for the various carburetors and throttle bodies, including a perfect one with no pressure loss at any flow.

CFM at 28" H2O, HP at 7400 rpm, sea level
No Losses.........519
1000................508
780.................501
650.................492
500.................474

How close were your dyno test results to the 9 HP gain predicted for going from a 650 to 780 Holley? I would not expect the absolute numbers to be correct, but the trends should be good.

Yeah Hoyt

These numbers are very similar to what we found. The difference between 650 and 780, with out any other modification, was 12 hp. And after we've changed the carb position, mounting it with the bowls perperdicular to the block, the power rose 3 hp.

I agree that formulas and text books are just a starting place. As an example, on my 7200 RPM Ford 300 bracket motor I have tried (3) 350 Holleys, (3) 350 Holleys taper-bored to use 500 base plates, (3) 500 Holleys, and (3) 500 Holleys modified and blueprinted to flow 540 cfm each. Each time I increased carb size the car ran quicker. Some racing buddies who run one of the factory Ford billett aluminum racing heads (8900 RPM) are looking at three Dominators split in half to try. Textbooks would say we are crazy.

Maybe we are, but not crazy enough not to use what works.

Frenchtown Flyer You must be wrong about books NOT always being right because many years ago a gentleman proved on paper that under perfect conditions that it would be impossible for a wheel driven car to exceed a figure near 180Mph in a 1/4 mile(I don't remember the exact speed) and history proved he was right until about a month later when a hot rodder that had failed to read his "proven facts" went faster. I often wonder if he still feels stupid.

It all depends what textbooks you read. All of my textbooks tell me that if I reduce the pressure loss of the flow through the carburetors the volumetric efficiency and the horsepower will increase. If the carburetor "rating" is 500 CFM at 28" of water pressure loss, the engine is losing about 7% or more (9% in the example I worked out above) of its potential power if the rpm and displacement are asking for 500 cfm. In the jet aircraft business, inlets are oftened designed to cause no more 1 to 2% pressure loss at max airflow.

Just keep making the holes bigger until you don't go any faster.

Hoyt When you reach that point (Making Holes bigger) you just figure out a way to flow more air then you make the holes bigger and go still faster. In my above post I was making fun of a guy that proved something beyond a shadow of a doubt that was wrong about a month later. One thing all these years of playing with engines has showed me is nothing is ever as good as it can get because tomorrow someone will make it better by doing something that just couldn't work but did. Like a wise old aircraft engineer told me once " you can make a 2x4 fly if you put enough thrust on it".

Once again a agree that textbooks are just a guide. If they weren't so, everebody would still be using any 3 setup carb or even a quadrijet that flowed 500 cfm in our 250 Chevy engines and nothing else would work properly no matter what they've done on the rest of it.
I saw in some book once that you need to consider the engine purpose and multiply thus the whole cfm formula for a coefficient of correction.
For exemple for street engine which has 85% of volumetric efficiency:

CFM = ((peak rpm * cid)/3456)*.85

And for a track engine that has 100 to 110% volumetric efficiency:

CFM = ((peak rpm * cid)/3456)*1 or 1.1

So this coefficient modifies the whole history. Isn't it?

How the hell, after modifing an engine, you will know the exact volumetric efficiency percentage? Well, a bench flow maybe. But it gives you percentual values?

And how can an engine has 110% of efficiency? It means a machine with no loses and working above its own capacity. Is it possible?

seiscanecos74

The good textbooks do explain how to measure and predict the volumetric efficiency of high performance racing engines. A good way to measure the volumetric efficiency is with a “nonintrusive” airflow meter in the engine’s air intake system, with the engine running at full, wide-open throttle. The measured airflow, converted to SCFM at a given rpm, is then divided by the airflow calculated from the basic rpm X displacement equation:

CFMgeometric = ((peak rpm * cid)/3456).

Volumetric Efficiency = CFMmeasured / CFMgeometric

Where one gets the nonintrusive airflow meter for an engine that did not come with one is another question. The high-tech dyno shops should have one.

Predicting volumetric efficiency is not easy. In addition to knowing the steady flow rate versus pressure drop table obtained from flow bench tests of the carburetors (or fuel injection throttle body), intake manifold, and heads, the effect of cam timing and the wave dynamics in the intake runners and exhaust headers is a big factor, as we all know. Volumetric efficiencies well above 100% can be obtained, even with flow losses, if the cam timing, valve lift, and wave dynamics are properly tuned.

A good example of how to predict volumetric efficiency for racing engines is in the textbook “Design Techniques for Engine Manifolds”, published by the SAE. One of the examples in this book shows performance data for a Formula I engine, where a volumetric efficiency of about 120% was predicted at 10,000 rpm, and 122% was measured. A link to the book’s cover and the example is below. The mathematics in the book can get pretty complicated and a computer is needed to actually make the predictions. It might be easier and more fun to do the tuning on the dyno.
Volumetric Efficiency example

Wave Action textbook

If my engine is a stock Chevy 250 with a dual tube headers exhaust, with probably a 85% of volumetric efficiency, according to the equation "CFM = ((peak rpm * cid)/3456)*.85", it will have to rev 9758 rpm reach 600cfm. But my engine revs only about 5500-6000 rpm.

So, I'll never de able to run a stock Chevy 250 with a Clifford intake for a 4-barrel Holley 600 cfm??? I'm not happy now...

I was planning to do a nice work in my heads before install the carb&intake setup, but now I'm confused... The camshaft will affect directly the Volumetric Effiency or not?
What modifications will I have to do to run this setup?
Does someone ever reached 600+ cfm on a 250, especially reving only about 5500-6000 rpm?

|o| Help!

Chevy 250 & 261,

With a Holley 600, a decent cam, a good head, and headers, I would bet that your volumetric efficiency will be closer to 1.00 than 0.85. In any event, I think that your engine will run fine with the Holley 600. At 6000 rpm and a volumetric efficiency of 0.95, your 250 will want about 412 CFM, about 69% of the rated at flow of the carburetor at 28” of H20 pressure loss. Thus you will see only about 13” of H20 pressure loss, as the pressure loss through a carburetor (or just about any aerodynamic obstruction) varies with the square of the flow rate. At low speed, part-throttle operation, you will be just running off of the primaries, which should be just fine. The only conditions that could be a problem would be low-speed and wide-open-throttle, but with a decent transmission and driving technique you should seldom be in that position.

Hoyt, thanks. I'm kindo of happier now. I'll buy next week the intake manifold.
Well... At low-speed, high gear and wide-open-throttle there will be a problem? I think you're right... It won't be so "streetable" then...
Can a bigger accelerator pump (50cc) or bigger size nozzles (squirters) solve this problem and improve throttle response at this conditions above?
Another question is that if a spacer can help a little improving cfm at low rpm conditions? How can a choose the carb spacer thickness?
Thanks a lot!

Chevy 250 & 261,

As stock49 points out in the numerical examples in his post in the triple choke thread, stock49\'s example , there is not much one can do if the airflow demand is low and the venturi area is large. If you have the throttle wide open, you probably want to accelerate, so you might as well downshift and get the rpm up, or back off on the throttle and just run off of the primaries.

I'm not sure if the carb spacer would help. Some method to richen the mixture might help, but the additional fuel would need to be very precisely added. Sounds like an EFI system. There is a limit to what can be done with a fairly simple carburetor; but at least you are not at the mercy of a computer and a bunch of sensors and controllers. I may be showing a little bias here! A little experimentation is always fun and sometimes can lead to something better.