Calcs Involved: EFI Throttle Bore Sizing?

[StrangeRanger]

What does this have to do with Buffy, Faith, 7 of 9, Ta'Pol and Aeryn Sun?
Oh, wait, this was a T/B thread, wasn't it.
Well, there's throttle bodies and other kinds of bodies as well.

 

[xctasy]

Well, it is a hardcore session of course. The same as any one elses, but distilled, hi-proof, and ever so much more so.

SR said:-

The 302 uses a 50mm T/B, the 300 a 43 or 44, I forget which. The ratios of their areas is approx. 1.3 which is the approx. ratio fo their HP and torque peaks. There's some kind of an optimum velocity thing happening there, don'tcha think?

Crictical air velocity is 200 feet per second according to Ricardo, and almost everyone else since the 30's. If the air speed is lower at wide open throttle, it will gain power at the expesne of low-speed torque. If its above 200 feet per sec, the car will gain torque in the lower regions, but loose it in the high speed zone.The Throttle bodies are not to be sized to drop the air velocity below this speed if at all posible.


However, analysis is a funny thing. An 85 250 EFI Falcon has 5.14 sq inches of area, or 0.035718 sq ft, on an engine reving to 4500 rpm. Cfm demand is about 277 cfm. Since Q=v/a, the air speed is about 129 feet per second. But remember, the throttle is a flow limiter, and it have 12 inches of torque ram affect. Its intake runner volume is about 2 liters, or 121 cubic inches, almost 50% of the total swept volume.


So I surmise if you intake runner length is long, and the volume is fairly small (<than 50%), then you can go for an air speed at maximum revs of less than 200 feet per second.

I'd suggest everyone pool there candidate engines MAF size, runner length, and maximum revs, and then see what the sweet spot is.

A 225 5.0 HO has a 50 mm throttle body, and has 0.021135sq ft of area. It revs to 5500 rpm or so. Air demand is about 408 cfm. So air speed through the throttle is about 322 feet per second.


On a 300 EFI, it lookes like about 277 feet per second for a 75% ve engine reving to 4000 rpm with a 43 mm throttle body.

On a twin throttle body 300 EFI, I'd say 173 cfm for a hot I6 reving to 5000 rpm lookes just fine.

The 4.0 V6's have some funny throttle bodies that don't flow well, as I stated earlier. There is a 'gate valve' co-efficient due to the smoothness or otherwise of the throttle body. I've seen road car HSV Holdens that use one 80 mm throttle bodies, and only reving to 5500 rpm. But on race cars, and auxilary 80 mm throttle body allows the engine to rev to 7500 rpm! 660 cfm deamand on a 100% VE EFI engine, with 450 hp, and 100 feet per minute at max revs. It did have a Calpak Delco, and DFV/DFX-style injector trumpets like the latest 5.4 DOHC Mustang Mach 1 engine.

See, doubling up lookes like a neat idea.

 

[MustangSix]

I'm don't think that velocity thru the TB is can be regarded the same as as if it were carrying fuel in the airstream. In a carb, you need to manage air velocity as it passes thru in order to properly feed and atomize fuel, so sizing is critical to proper performance. Not an issue with EFI. The same formulas don't apply. You can put a way-too-big TB on an EFI engine and it will still run ok because the TB is not mixing fuel for you. For example, a 1000 CFM TB on a port EFI 2 liter engine would work because the engine management is only providing fuel as a function of actual air flow as determined by MAP or MAF.

Most port EFI systems are runners fed by a plenum fed by a TB. Any velocity gained in TB sizing would simply be lost as the air enters the plenum volume. Look at velocity in that part of the intake for other reasons, such as intake noise abatement. Making the airflow go mach could quell some noise issues at the cost of some restriction, like in the airbox intake air horn. A properly sized TB would not create as much noise.

In my 1000 CFM 2 liter example above, you may have additional issues with throttle tip in, sensitivity, matching MAF, and feel, so sizing does play a part. Also, off-idle transisitons might play a factor in TB sizing, though engine management can mask a lot of things like that.

TB size is still an issue is at WOT. If the TB is not allowing the plenum to reach zero manifold vacuum at WOT at max power, it is too small, but that is not air velocity, per se.

Now, if you go to an IR system with separate throttle blades, that's a different story.

 

[xctasy]

The total cfm and velocity isn't the same at the throttle body as at the combustion chamber inlet for sure. But a throttle body is just a flow limiter in the same way a carb is. I don't agree with the idea that EFI is any different to a carb.

I'll cover your Locost 200 X-flow machine as 250 hp at 6500 rpm fullhouse version with an independant runner carb engine. The I'll compare it with a 2-bbl version doing 250 hp at 6500rpm.

Clearly, any independent runner EFI or carb system will operate with minimal wetted perimeter, and can be sized ideally to suit the application. (The ideal size of an individual runner carb or throttle body is always larger than a combined throttle body or carb. Eg a 250 hp 200 cube X-flow reving to 6500rpm will need 38 mm carbs or injectors to flow the 275 cfm needed, a total area of 10.5 sq inches. The peak velocity is in fact only 74 feet per second, well below critical but still great for power at those revs.

These IR set ups take less fuel to operate than one 325cfm 2-bbl carb which would only have about 4 sq inches of carb area. But the air speed is 194 feet per second. The cfm flow of a six 38 mm carbs is over 250 cfm at 1.5"Hg per barrell. Your Locust has 1500 cfm of carb flow only giving 250 hp, verses a hot shoe 325 cfm carb doing 250 hp under extreme duty. All through the power range, the Mikunis wil,have better power and progression than a 2-bbl 325 cfm carb. This is because the air flow is less turbulent in a IR set up.

Air is air. The old flow principles transcend new technology. Austrailan Phil Irving, in his book Tunning For Speed, stated that a couple of Amal, Lectron or Mikunis have an optimum size for each cc per cylinder and rev range.The premise is to promote less than 200 feet per second air speeds. His work with the Repco Brabham port injection followed his years of carb work.

Today, everyone in reace circles are, in my opinion, looking in the wrong direction. Throttle valves near the intake runner are a daft idea on an EFI engine because it promotes turbulance. Use of a MAP sensor and correctly sized throttle body ensures that turbulent flow is minised. The fake EFI Weber carbs work only because its cheeper to build an IR unit on a Weber intake tahn make a production car MAP single or dual throttle body injection system. Chevy's Gen III is an example of what all EFI systems should be modeled off.

In practice, David Vizard and others have found that injection air flow must be smooth and approaching what they call 'sub-critical flow'. That is, no eddies can form becasue of a savage turbulent flow condition. If you have long intake runners with large volume to attenuate rough air flow, then you can go right up to a small throttle body which promotes high air speed without causing problems. If its just air flowing, there is no liquid to displace or cool air, so that so-called 200 feet per second intake air speed could be a greater amount.

David Vizard found in situations where the intake runner velocity is high, placing a restrictive flat plate close to the air inlet stabilises air flow and creates more power than an open plenumb.

In sumary, I feel happy that 200 feet per second or less is the flow number to aim for at maximum revs, and that we should size a throttle body to suit, and try an go for a smooth entry.

 

[SuperMag]

Air is air. The old flow principles transcend new technology.

My dear man, I would have thought you, of all people, would appreciate the difference between dry flow and wet flow. But that's another discussion for another day (and I certainly don't claim to be an authority on either ).

But logically speaking... on a vehicle so decidedly proletariat as a pickup truck, I wouldn't think they'd give much consideration beyond the necessary air flow to support the given level of HP (1.4 CFM/HP?) at whatever flow velocity is conducive to the best torque.

 

[xctasy]

I know where your at SM. I work with propane whic displaces air as its introduced. It displaces up to 30% of total dry cfm flow. So a 4-bbl carb flowing 500 cfm dry neads to flow 650 cfm to deliver propane in liquid form.

In an EFI environ, there is no flow displacement at all. The air flow figures for a Holley or any carb are dry, not wet. So apples are with apples.


I understood that best mean power is supposed to happen at about 200 to 250 feet per second acording to Ricardo. Best mean torque is likely to be at a higher, turbulent flow speed.

I'd not like to put a figure on flow displacement in a carb. I've heard 17% at 12.5:1 air/ fuel, but unless I sit down and work it out, I have no idea.

Hmm, wheres me pen and calculator?

 

[StrangeRanger]

The edition of Ricardo that I read (1953) said that maximum VE occurred at an intake velocity of 140 FPS (80%) and was in the acceptable range from 40 (70%) all the way to 240 FPS (60%).

On an EFI 300 with a 1.6" (40.6mm) diameter intake runner 140 FPS = 2908 RPM. 40 FPS = 831 RPM, 240 = 4985 RPM. This squares pretty closely with the real world performance of the 300, although for a variety of factors its peak VE is in the range of 65-70%, meaning that the high end VE would drop below the acceptable level long before 4985 RPM. With some headwork though....

That all occurs DOWNSTREAM of the plenum though. The T/B is obviously upstream of the plenum. Since the 3 cylinders on each half of the plenum operate at almost zero overlap, maintaining the same 200 FPS velocity would require a T/B diameter only slightly larger so 44 mm seems plausible.

A stock 302 truck engine uses a 1.69" intake valve diameter. That would require an intake runner diameter of 1.51" (I don't have one of those to measure though) and would hit 140 FPS at 3200 RPM (assuming a slightly higher VE of 75%) and hits 40 FPS @ 14 RPM and 240 FPS @ 5488 RPM. Again this seems to square pretty well with the real world performance of the engine.

Each port of the T/B would have to support 4 cylinders which operate with significant overlap, so it would have to be proportionately quite a bit larger. In playing with the numbers, if you assume that a 302 T/B needs 33% more area to accomodate the overlap involved in the 4th cylinder and a reduction in area of (1.51/1.6)^2 to accomodate the theoretically smaller runner, then you get a diameter of about 52mm rather than the 50mm which Ford uses. It would be interesting to measure the runner diameters of a 302 intake manifold to see what is really happening. Anybody have access?

 

[MustangSix]

Not sure on a 302, but the stock runners on my crossflow are only 1.4" in diameter and are round. In stock form they are also badly mismatched between the plenum, the injector rail, and the head, further impeding airflow.

At full power, I suspect the airflow limit is runner flow, not the TB. Even if the TB were the size of a trashcan lid, the plenum was the size of a bathtub, and velocity thru the TB was zero, the runners still would react the same way and the engine would still pull the same numbers.

I think 75% VE is on the low side for even a stock 5.0. Probably closer to 80-85%. I know that a well matched EFI system will exceed 100% on some cylinder heads.

 

[StrangeRanger]

The 302 truck heads have very small valves compared to the Mustang heads and moreso compared to aftermarket heads. That has to limit VE. That's why I'm curious as to runner diameter for a truck EFI manifold. Either the manifold or the valve could be the limiting factor.I'd like to get some sort of an idea about the effect of T/B sizing.

 

[J.R.]

The Ford Cologne 2.9L V6 in the Bronco II,Ranger, et.al. is very similar to the to the Cologne 2.9L engine in the Ford('Merkur' in North America)Scorpio. The work on the Scorpio done in Europe showed that the dual T/Bs provided better power than a single large T/B. Driving the two 'same, but different' engines here is like night & day.

My Scorpio 2.9L(>350,000miles, still stock & purring like a kitten) is a willing performer at any speed up to the factory 5800rpm redline. Typical Ranger 2.9L isn't overly smooth at any rpm over idle, and its single TB feeds a much longer plenum and runner system than the stubby short runners of the Scorpio. (I'll bet the torque curve is flatter/wider on the Scorpio anyway). This comparison does NOT seem to fit what most of us would expect.

As for my single turbo 300, I'm planning to use dual T/Bs, but possibly from a 351 or 460. Still trying to figure out the upper EFI intake manifold shape to take in the air after the air-to-water intercooler has helped lower temps. Haven't found mandrel-bent aluminum tubing, either. Still lookin'!


ABS tubes, heat curved & glued to phenolic or other composite flanges, then bolted to lower intake, and throttle bodies, could work also, and keep the intake air from re-heating as much as in metal tubing. Will have to do some fabrication & testing.

J.R.