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  #1  
Old 02-26-2005
Bill usn-1's Avatar
Bill usn-1 Bill usn-1 is offline
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Lightbulb FI needs. Let's do the math

There seems to be a lot of questions as to what parts and what size TB to use.
So I think we should all try to use the math available to us and lets prove it all out for ourselves. Then when it's done I will make it a sticky.

What I want to do is figure the CFM for a given CI and RPM. Then we can calculate the injector size needed and we can even adjust the fuel pressure to see the effects on the theoretical inj size.
So when we are done, everyone should know exactly what size TB and inj they really need and if they change something..how it will affect it.

I recommend people start to do a little google seach for specifics. Take a look at some of the links on the FI links page.

So first is CFM.

As many of us know, the consumption of an engine can be described with the equation {(CIDxRPM)/3456} * VE. CID means the cubic inch displacement of your motor, RPM is the max RPM reached, and VE means the volumetric efficiency. VE on a stockish motor is usually 60-70%, where as professional full race motors can reach close to and even a little above 100%.

So lets start with the biggest and work our way down.

392CI at 4000 rpm with a volume eff of 85%. I think that's pretty generous.

392 X 4000/ 3456 = 453.70 cfm X .85 = 385.64 CFM

So what if you want to spin it up to 5000 rpm?

392 X 5000/ 3456 = 567.13 cfm X .85 = 482.06 cfm.

So as you can see..500 cfm should run the motor fine and 600 cfm would be more than enough.
Now keep in mind that this is for stock motors or a little better. that's why I choose the .85 VE. Oh I guess I could mention about a 1% loss due to the fuel being sprayed into the air stream.

So now you can figure it for a 345, 304, 196, 152

Let's try to keep this simple. I don't think we need to get to the engineers level of manifold and head runner eff or the thousands of other things that effect the flow!


So next we will need to figure out the fuel requirements to feed the motor.
You need to keep in mind that we will use 2 injectors when we calculate the fuel.
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  #2  
Old 02-26-2005
Hank Hank is offline
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Re: FI needs. Let's do the math

So what your saying (once again),is that the smaller more common TB will be fine on my 392? Thanks Hank.
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  #3  
Old 02-26-2005
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Re: FI needs. Let's do the math

Well Hank this is an all hands learning thread!!!
Have you found the flow numbers for the SBC TB yet?
There are some real flow numbers out there!!

If no one finds them, I will do a search and post some i know of.
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Last edited by Bill usn-1; 02-26-2005 at 11:46 AM.
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  #4  
Old 02-26-2005
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Re: FI needs. Let's do the math

Well according to a TBI performance mod. company, the stock TB only flows about 480 cfm max. Hank. : idunno :
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  #5  
Old 02-26-2005
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Re: FI needs. Let's do the math

Quote:
Originally Posted by Hank
Well according to a TBI performance mod. company, the stock TB only flows about 480 cfm max. Hank. : idunno :
Here's one I found on fullsizechevy.com
thread

Quote:
i just got done doing a flow rating on a GM TBI for:
4.3 - 5.0 - 5.7L TBI applications
the blue line represents a throttle body in full trim (injectors, injector
pod) no air cleaner.
the red line represents a throttle body in full trim with and injector pod
spacer installed (injectors, injector spacer and injector pod) no air cleaner.
the green line represents a throttle body with nothing installed.
the column on the left is air-flow in cubic feet per minute
the row underneath the graph is throttle position rated in volts from
a stock TPS.
the test was run with 1000cfm @ 28.00" temp 68.5
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  #6  
Old 02-26-2005
Hank Hank is offline
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Re: FI needs. Let's do the math

They may be understating the CFM, to help sell there TB performance mods. Hank.

Last edited by Hank; 02-26-2005 at 01:09 PM.
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  #7  
Old 03-01-2005
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Re: FI needs. Let's do the math

Here's another set of numbers I found for the SBC TB.

Quote:
Good info in this thread. I'd like to add a little more including the flow testing on the stock TBI. As above with the flowbench the TBI was flowed w/o the injector pod. Here are both values:

Stock 4.3/5.0/5.7 2bbl TBI complete -- 574.1 cfm (dry)

Stock 4.3/5.0/5.7 2bbl TBI w/o injectors -- 584.7 cfm

The following airflow tests were performed on the University of Northwestern Ohio's SuperFlow SF600 Flow Bench. All CFM values are corrected for airflow at 28 inches of water.
:
Quote:
Stock single bore TB (42.8 mm or 1.68" inside diam bore)
one injector tower gasket @13.6"
285 CFM (each), 570 CFM together.

Stock single bore TB (42.8 mm or 1.68" inside diam bore)
two injector tower gaskets @13.6"
286 CFM (each), 572 CFM together.
This one, 572 CFM together, is the closest to a stock Fcar 2bore TBI

Stock TB, injector tower REMOVED for study @13.6" - 288 CFM (each) total, 576 CFM together.

stock single bore TB bored to 2" inside diam
two injector tower gaskets @13.6"
333 CFM (each), 666 CFM together.

stock single bore TB bored to 2.130" inside diam
two injector tower gaskets @13.6"
379 CFM (each), 758 CFM together.

Holley dual bore 2" inside diam TB
670 cfm (their rating, unknown depression)

Notes

1. the extra injector tower gasket (stacking them vertically to raise the injectors) makes a slight difference on the 2" and 2.13 TBs, but makes almost no difference in airflow on the stock-size TB's. You'll have to find the orig posts on the CF Vault to see the before/after comparo of single v double gaskets.

2. Removing the injector tower means removing both the injectors and the injector mounting. There is almost no difference in airflow, so the injectors/tower really don't impede the airflow at all, contrary to what's often seen on TGO.

So the above measurements refute a mythical "improvements" made by TGO TBI owners: raising the injector to increase airflow past the injector.

To compare other depressions (aka the pressure differential across the TB when it was flowed):

new_flow = old_flow*sqrt(newdp/olddp)

where newdp and olddp are the new and old depressions (depression is the height of the column of water used in the test manometer). Depression is a form of a pressure measurement, rather than a vacuum measurement -- so take care not to mix units of pressure and vacuum.

Lastly, more depression means more pressure difference and hence greater flow. The above equation is an approximation along a flow streamline, so it's useful but it's not perfect. HTH.
So given the numbers: the airflow should not be a limiting factor in selecting a SBC to power a stock IH motor.

So what about the injector size required?
Anyone find anything yet?
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Last edited by Bill usn-1; 03-01-2005 at 02:26 AM.
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  #8  
Old 03-01-2005
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Re: FI needs. Let's do the math

Fuel limit:

hp = (#injectors)*(#fuel/hr)/BSFC

hp = 2*(65lb/hr)/.50 (lb/hr/hp)
= 130/.45 = 260 hp at the flywheel

math link
math link 2

What's the HP rating of a stock 392?
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  #9  
Old 03-01-2005
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Re: FI needs. Let's do the math

I wish I was bored in Italy! : thumbsup

I did bump into this information on injectors and hp.
http://turbonation.com/injector.htm
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Old 03-01-2005
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Re: FI needs. Let's do the math

I got no SCOUT and I got no HOUSE!!!!

: mad : : banghead
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Old 03-01-2005
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Re: FI needs. Let's do the math

I feel your pain! : angry:

It'll be there soon! Who needs a house... : thumbsup
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  #12  
Old 03-01-2005
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Re: FI needs. Let's do the math

Quote:
Originally Posted by Eagle-Mark
It'll be there soon! Who needs a house... : thumbsup
Yeah, you can live in your Scout, but you can't drive your house : idunno : : idunno :




































: thumbsup : thumbsup
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Old 03-02-2005
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Re: FI needs. Let's do the math

Quote:
Originally Posted by Bill usn-1
What's the HP rating of a stock 392?
Depends on the year, but spec according to the service manuals is about 195 net and 235 gross. I can get the exact figures when I get home tonight.

FWIW, My Ford CFI TB has 1.5" bores and a pair of ~48lb (477-483cc/min, depending on whose spec you believe) injectors. I've got the fuel pressure running 53psi, which brings them effectively to about 53lb/hr. THey're just barely big enough, but work for daily driving. I just have to remember not to mash the gas hard when it's warming up. Once it gets to operating temp and the warmup enrichment drops out they're fine.

HTH,
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  #14  
Old 03-02-2005
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Re: FI needs. Let's do the math

So lets go with the biggest possible!!
235hp
hp = (#injectors)*(#fuel/hr)/BSFC
hp = 2*(55lb/hr)/.50 (lb/hr/hp)

If we look at standard 55lb gm injectors

hp= (2*55)/.50
hp=110/.5
hp=220


So at stock 13psi the stock 350- 55lb injectors are good to 220hp

So now we need to know what pressure we need to run so the 55lb inj will feed 235hp!
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  #15  
Old 03-02-2005
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Re: FI needs. Let's do the math

So if we take the new pressure/old pressure.

lets say 15psi/13psi= 1.154
now we take the sqrt of that=1.074
This is the amount of change.
so we can now multiply the 55lb/hr inj X the 1.074 = 59.08lb/hr

hp = 2*(59lb/hr)/.50 (lb/hr/hp)
hp = 236

So we have now learned that a 55lb/hr inj operated at 15psi is capable of operating a 235 hp 392 on a small block chevy TB.


The is at 100% duty cycle.
To calculate at a more reasonable 80% duty cycle we need to multiply by .8 than add a little more pressure to compensate.

So we can all now see that if the sbc TB is capable of running a 235hp 392 than it will obviously run a 345 or a 304 of the same HP.

What we need to do is recalculate the base pulse width that is needed to be set in the chip for each size engine and inj size!

Later!!!
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  #16  
Old 03-03-2005
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Re: FI needs. Let's do the math

for those interested, here is a calculator for checking the change in fuel flow for any change in pressure.
FI flow calculator

all automotive calculators
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  #17  
Old 03-03-2005
David - WI David - WI is offline
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Re: FI needs. Let's do the math

Okay... but how do you equate the engine's airflow requirements to the TBI's flow at 28" of water (as tested)? The engine will never pull 28" of vacuum.

The 28" of water standard is great for comparing two components... but I'm not certain that those airflow ratings are "valid" for choosing a carb/TBI unit to fit a given engine.

According to David Vizard's "How to build horsepower" you need approximately 1.8 to 2.0 cfm per cubic inch of engine displacement "when power is an important priority".

Of course, this is based on having a decent exhaust system, camshaft, manifold, etc.

The "stock" 196 IH motor that I entered into the Engine Analyzer only flows 202 cfm at it's peak... but it also only has a peak volumetric efficiency of 67% and it's down to 60% by the time it flows 202 cfm! : innocent

The restrictive exhaust, poor port flow, mild cam probably limit the flow rather than the carb I entered (306 cfm calculated airflow at 28" vacuum).

According to the program, a stock 4.3L Chevy making 211hp flows 355 cfm at 90.5% VE... 262ci through a 500cfm @ 28" rated throttle body at 4800 rpm. About 1.91 "rated" cfm per cubic inch of displacement.

Just wondering if using the 28" of vacuum flow number is valid. David
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Old 03-03-2005
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Re: FI needs. Let's do the math

Don't know.
I am not an engineer and I don't have access to a flow bench.
So I found numerous articles on the web concerning the flow of the SBC TBI and flow in general.

There are more than enough calculators out there to determine the required flow for a given CI/rpm.

There are also just as many debates just like yours, challenging the validity of the data.

So all I can say is find the one you like and think is right.
I have read numerous and read thru a lot of the debates.
i came to the conclusion that for a stock farm engine like ours, "close is good enough"

I am not building a 100-105% VE motor.
If someone is then I'm sure they have the smarts or the money to pay someone to calculate the requirements for them.
And they probably won't be discussing TBI flow to power it!

As I stated at the top of the thread,
Quote:
Let's try to keep this simple. I don't think we need to get to the engineers level of manifold and head runner eff or the thousands of other things that effect the flow!
I have seen recommendations from 1.3 to your 1.9 cfm/ci.
I just use the math that I found! : idunno :

I have also seen guys interchange measuring system within an equation and really come up with some good numbers!

All of this info was only for a reference.
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  #19  
Old 03-04-2005
David - WI David - WI is offline
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Re: FI needs. Let's do the math

You don't need a flow bench or an engineering degree.

Using the math from your first post, a 5.7L (350) Chevy truck motor would "require" only 430 actual cfm at 5,000 rpm... but the stock TBI was supposedly rated to flow 670 cfm @20.4 inches of vacuum.

The 392 in your example requires 52 cfm more than my 350 Chevy did... so a 500 cfm would strangle the thing.
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Old 03-04-2005
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Re: FI needs. Let's do the math

Not sure I follow.
The stock 350 requires 405 cfm at 4000rpm.
the stock 392 requires 453 cfm at 4000 rpm.
This is at 100% VE which a stock motor is not.
so actual CFM required would be less.

Now the stock small block TB is listed as flowing 570 cfm.
The BBC is rated at 670 cfm.

So if the 392 would only require 453cfm max then why won't the sbc TB rated at 570cfm be adequate enough to feed it?

Now if the IH motor is only at 60%VE like yours I would think the little ford TB that Will is using on his would be plenty of air also. : idunno :
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  #21  
Old 03-04-2005
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Re: FI needs. Let's do the math

Quote:
Originally Posted by David - WI
You don't need a flow bench or an engineering degree.

Using the math from your first post, a 5.7L (350) Chevy truck motor would "require" only 430 actual cfm at 5,000 rpm... but the stock TBI was supposedly rated to flow 670 cfm @20.4 inches of vacuum.

The 392 in your example requires 52 cfm more than my 350 Chevy did... so a 500 cfm would strangle the thing.
You really don't need to do the math. It's mostly bogus for real world stuff anyhow, since to take all the myriad of interrelated variables into acount you'd need a Cray or some similar supercomputer to complete the calc's within a halfway reasonable time.

The accepted method over on the MS board is to find/use parts from an OEM engine of similar construction and power level. IE, if you're wanting to put TBI on a 200 hp V8, you use injectors and TB(s) from a 200 hp V8. If you are building a 2500 hp V12 Merlin, you figure out 16 injectors to feed the beast, and then how to plumb them. :) BTW, there's a guy over in England attempting to do just that right now. : beer :

The factory engineers have to build a bit of fudge factor into a production configuration in any case. For us DIY types (that don't have to worry about warranty issues :) ), generally if you're within 10% of the OEM application you're close enough.

As Bill noted, I'm running a Ford CFI TB on a 392 in my daily driver '72 1010 Travelall. It has 1.5" bores, the same size as the 350 CFM Holley 2300 2bbl that IH used on these motors from the factory, so it should flow enough air. The problem I run into is the original application for this TBI unit is only 160 hp @ 38 psi fuel pressure. :( So the injectors are really too small for my ~195 hp stock 392. To make up for some of this, I'm running them at 53 psi, and could go up to 60 if I needed too. But even so, they're just BARELY big enough. ~46lb/hr at 38psi, and 53lbs/hr at 53psi.

I wonder, are the stock GM TBI injectors rated at 43psi/1 Bar (standard) or at the 13 psi they're normally run at? I'd think it would have to be at 13, since they wouldn't flow anywhere close to enough for 200 hp if they were rated at 43 psi.

Since the GM's TBI's bores are bigger, and the injectors are already good for 190 hp on the stock engine, there's no reason to believe it isn't big enough to feed a stock IH V392. A modified one might be another story. :)

HTH,
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  #22  
Old 03-05-2005
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Bill usn-1 Bill usn-1 is offline
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Re: FI needs. Let's do the math

All GM TBI inj were flow rated at 13psi.
The problem was the regulators could be anywhere from 11-14psi so the ECM did the fine tuning for the system!
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  #23  
Old 08-08-2005
David - WI David - WI is offline
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Re: FI needs. Let's do the math

[QUOTE=Bill usn-1]
-snip-
So if the 392 would only require 453cfm max then why won't the sbc TB rated at 570cfm be adequate enough to feed it?
-snip-
QUOTE]

Because the TBI is "rated" at 20.4 inches of vacuum... which is way more than the engine will actually produce at WOT.

That's why you can't just take "theoretical" engine CFM numbers and match them up to "rated" flow numbers... the TBI won't actually flow the "rated" numbers on a real engine.

If you want to be certain that the TBI won't be the power restriction... you need about 1.8 to 2.0 "rated" CFM per cubic inch of displacement on a "performance" engine.

FWIW, the stock TBI was barely adequate for the stock 5.7L (350ci) truck engine. Even simple mods like a free flowing exhuast and a performance catalytic convertor allowed the engine to flow more than the stock TBI could handle... which is why we started using the TBI "spacers".

In 450,000 miles I ran several different "cat-back" exhaust sytems and two different hi-performance catalysts on the same truck... I also gutted the air intake silencer and ran premium fuel with 5 degrees of additional ignition advance. For anyone who cares... my experience was that the stock SB TBI was right at it's flow limit with ~210hp on a 350ci engine... that's 1.63cfm per cubic inch according to your airflow numbers.

As soon as you improved the exhaust flow... the TBI became the most restrictive part of the system.

Based on that, bare minimum flow for the big IH would be 392 x 1.62 = 638 "rated" cfm... meaning the big block 670cfm unit is the one you want. That would put you right at 1.7cfm per cubic inch; enough for a "slightly-better-than-stock" performing engine like my 350 Chevy.

I don't want to argue about this, I just hate to see people spend a bunch of money on a SB TBI and then choke the big IH and be disappointed.

David
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  #24  
Old 08-08-2005
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Re: FI needs. Let's do the math

How do you account for the IH engineers only running a 2 barrel with 1.5" bores? I assume Wills data is correct since he chose to run the smaller bore ford TB on his 392.
Did they not know any better?
The SBC is 1 11/16.
I assume a hole is a hole and whether it be a carb or a TB, the size of the hole will flow the same on either.

So If IH used a 1.5 bore and the SBC is 1 11/16...I would assume the bigger hole would flow more air?

I don't know, I'm not an engineer. Your saying the SBC will choke it. Is the 1.5 bore correct?

I'm not saying the BBC TB wouldn't be preferred but I don't see anything wrong with running the SBC TB on a stock 392 either. I have done a couple and they ran extremely well. But maybe there was more.

Maybe we need some more chassis dyno runs!!!
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Old 08-10-2005
David - WI David - WI is offline
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Re: FI needs. Let's do the math

I thought a stock 2V 392 IH only made 180hp and the 4V made 196hp?

We had some of each, and you could definitely tell the difference in performance; but that would be 9% more power (which is huge to me... a 9% improvement usually costs a lot of money).

I guess I was assuming that people who spent the money on an EFI system would be looking for more power than a "stock" 2-barrel 392 IHC would produce... but that may not always be the case, I suppose.

I'm really not saying it won't "work" on a 392. It would probably work fine in normal driving and also be very drivable; but I think it will quickly become a flow restriction that will limit power as the rpm's go up. Perhaps as much as the stock 2-barrel carb did?

If you add headers, dual-exhaust, change camshafts, add stiffer valve springs, etc... I think the 5.7L throttle-body will really disappoint. It would suck to build a 240hp IHC 392 and have it only actually make 180hp because the TBI was too small. : angry:

Anyway... I might be wrong, but that's how I would explain the stock 2V.
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