Throttle body questions

[Spittybug]

When I did my EFI conversion I had to pick out what throttle body to use since I went with a Peugeot manifold. I decided that it would be smart to stick pretty close to the stock bore diameter which is 2 x 46 mm. I ended up putting on a 2 x 48 mm LT1 Camaro throttle body. While it works fine through most of the range, I find it very difficult to get power down at the low end (off the line) and have to play around a lot with my fueling, idle, advance and all of the other parameters. This is to be expected since I'm always tinkering, but I'm learning that my setup may be making my life more difficult than it needs to be.

I found an optimal throttle body size calculator that uses this formula: SQRT(((154.2)*(#cylinders)*(stroke in inches)*(rpm at max hp)*(bore in inches)^2)/67547) Our engines have 6 cylinders, a stroke of 2.87 inches, max hp at 5500 and a bore of 3.58 inches. This yields an optimal throttle body diameter of 52.6 mm (2176 mm2 area). Too small and you rob the engine of air. Too big and the air moves too slowly and you lose power. Our stock 2 x 46 mm throttle body gives us an area of 3324 mm2, or about 53% more than optimal! WOW. WHY? I theorize that several factors further limit the air movement into the cylinders and pull the effective airflow down; air deflector plate, the 180* turns at the W pipe, the rough surfaces of the unpolished manifold and the somewhat restrictive exhaust system.

In my setup, I don't have the air deflector plate, my path to cylinders is much shorter and smoother and I have no crazy 180* turns in it. Stock exhaust however. Add in the fact that I'm 66% larger than the optimal butterfly area and I think I have a case of insufficient air velocity at low RPMs. I suppose a simple test would be to blank off one of my two bores, get down to 1810 mm2 area as a result (17% below optimal) and see what that does at low RPMs. It should only become restrictive near the peak power area of 5500 RPM I would imagine. Making a blank that doesn't get sucked into the engine will be the challenge..... If this proves to be the case, it looks like I should be searching for a single bore, ~56 mm range throttle body. Anybody know any good little donor cars for something that size?

Logic flaws? Suggestions?

[Bitsyncmaster]

Can't you just blank the one side by making a gasket with that thick black gasket stock? Just don't cut out the one side.

[Josh]

Interested to see the derivation of that formula. You can find a formula on the internet but its not necessarily right.

[BABIS]

I don't know if this helps, but racing engines ( even the racing prvs on certain Alpines ) have a single throttle body for each cylinders...

[Spittybug]

jschatte
Interested to see the derivation of that formula. You can find a formula on the internet but its not necessarily right.



"90% of the stuff you see on the internet isn't true" - Abraham Lincoln.


Factor to consider:
The size of the throttle body controls the velocity of the air. Smaller throttle bodies make the air go faster, larger throttle bodies make the air move slower. Since the velocity of the air is limited to the speed of sound, which is 767.58 mph (miles per hour) or 67547.4 fpm (feet per minute), a throttle body which is too small could limit air flow in the engine. If throttle body size is too large for a particular engine, the air will move too slowly and reduce power.

The HiPerMath equation is:
throttle body air velocity in fpm = (4.317)(2.616)(number of cylinders)(2)(stroke)(rpm)(3.14159)(bore / 2)2 / ((throttle body size / 25.4)2)/4)(3.14159)(12)(10)
which reduces to:
throttle body air velocity in fpm = (121.43)(number of cylinders)(stroke)(rpm)(bore)2 /throttle body size2

I will continue to research the topic and try to confirm the formula, but I just finished some testing. I removed one of the butterfly plates (and foolishly broke the little screws that hold them onto the shaft...ugh) and remounted back on car. A plastic aspirin bottle fit almost perfectly into the open bore with a wrap or two of tape. This blanked off 1/2 and had no chance of being sucked in since the shaft prevented it. I started it up and went to work on my fueling table in Megasquirt. Significantly lowered the fuel required sub 2000 or so RPMs. With a little bit of playing around I was able to get it to be much more responsive to throttle stab. I'm definitely on the right track, but kept getting interrupted by runaway idle..... When I'd stab and hit a fuel bin that was still too rich, I'd get a backfire out of the throttle body which would projectile eject the pill bottle. INSTANTLY the revs go through the roof (of course) and I had to kill the engine. My arm still hurts from the electric shock of impulsively yanking the high tension lead out of the distributor to kill it the first time it happened when I was manually working the throttle. Ouch. The fact that wide open throttle (on the pill bottle barrel only since the the other slams shut) really, really made the RPMs go up fast with no hesitation confirms for me that I ought to be able to get to where I want to with a more permanent setup.

Whether I source a different throttle body or simply blank this one off in a more permanent manner, I think I'm going this direction ASAP.

[DMC5180]

"90% of the stuff you see on the internet isn't true" - Abraham Lincoln.


Which means 90% of what you, me or anyone else says on here is total BS.:devil:

Good luck sifting through all the misinformation out there.

[Josh]

When I get a spare minute I will look at the math. Seems reasonable when you broke it down. Any equation with factors in it I do not like using. Units are lost and you don't know where the numbers that created the factor came from. If that makes any sense.

[David T]

When I get a spare minute I will look at the math. Seems reasonable when you broke it down. Any equation with factors in it I do not like using. Units are lost and you don't know where the numbers that created the factor came from. If that makes any sense.

Typically formulas like that are only to get you into the "ballpark" so to speak. Having a large bore is not what you want for low air flow conditions but a small bore is no good for high flows. You need to find a compromise which is one reason you see a double bore throttle body. There are many other engineering tricks to make a small bore look like a big bore and vice-versa. This whole subject is very complicated with air volumes, flow paths, resonance and such. The cross-fire is another factor (the order each cylinder fires in). Experimentation is the only way but it can be expensive unless you have access to a junkyard and can get parts very cheap. Even then you have to be creative. Usually best to research what others have done and build on that. BTW, I was taught to make the units and factors work and then you try plugging in numbers otherwise all you get out of an equation is useless if the units are not right. And follow your decimal points so you are not off an order of magnitude. Be careful all of your units are in the same measurement system, ie, do not mix up metric and English. That's how they lost a Mars lander!
David Teitelbaum

[Josh]

Typically formulas like that are only to get you into the "ballpark" so to speak. Having a large bore is not what you want for low air flow conditions but a small bore is no good for high flows. You need to find a compromise which is one reason you see a double bore throttle body. There are many other engineering tricks to make a small bore look like a big bore and vice-versa. This whole subject is very complicated with air volumes, flow paths, resonance and such. The cross-fire is another factor (the order each cylinder fires in). Experimentation is the only way but it can be expensive unless you have access to a junkyard and can get parts very cheap. Even then you have to be creative. Usually best to research what others have done and build on that. BTW, I was taught to make the units and factors work and then you try plugging in numbers otherwise all you get out of an equation is useless if the units are not right. And follow your decimal points so you are not off an order of magnitude. Be careful all of your units are in the same measurement system, ie, do not mix up metric and English. That's how they lost a Mars lander!
David Teitelbaum

Awesome input as usual, thanks David! I saw a double bore throttle body on a 302 f150 at the junkyard the other day; I was wondering why they were used. Checking my units is something that has been beaten to death at school it is second nature for me. That is why I questioned the equation originally.

[WelmoedJ]

Very usefull information!

While you're going to experiment, consider using / making a spacer of which you can vary the port openings, alike the shutter of a camera.
Combined with the possibility to change the opening without removing the spacer, it would give you sufficient port sizes to use while finding the correct opening for optimal air flow.
Or perhaps simpler and cheaper, a spacer with something like a throttle plate to restrict air flow?

The disadvantage is most likely the money-side and air leaks.
Otherwise make a spacer with the same opening as you have now and inlays each a bit smaller than the previous inlay.
Disadvantage of the latter is the need to remove the carb each time you want to use another inlay.

Happy experimenting :)

[Bitsyncmaster]

The formula may change when you use a double throttle body as opposed to a single. Just a guess.

[dustybarn]

Maybe a throttle body from a GM 122 2.2 engine would be about the right size. They had port injection rather than TBI after about 1992. There should be enough of those around on J-cars and S-10s that finding one shouldn't be too difficult.

[Spittybug]

For a guy like me that likes to question why, learn new concepts and then try applying them, this forum is a great resource. There are so many like minded individuals working on little improvements to our cars and offering them as purchasable items, how-to's or simply as inspiration for others. It's not just about cheap fixes or "making it work". I think that's where some people ran into forum-friction in the past................

I think today's business will include making a thin metal backside gasket/blanker so that I can effectively close off one of the barrels from the backside. A thin sheet of roofing tin perhaps. I'll RTV it to the backside of the throttle body and pass the mounting screws through it. The original gasket can then be used on the manifold side. This will allow me to get on the road and do some tuning with Megasquirt's autotuning function. It is a closed loop O2 sensor-based algorithm that adjusts the fueling on the fly. No more projectile ejection of pill bottles!

I suspect that there will be limited application for those with stock setups. I think the TB is oversized for the reasons previously enumerated. In my case however, the improved flow and the fact that I was bigger than stock is leading me down this path.

Welmoed, I like your idea of a camera aperture-style of butterfly! That would be cool and could very accurately adjust air flow. Hugely difficult to invent and keep clean however.

[David T]

Using a plate or an aperture is very bad from an airflow prospective. You need a smooth transition or you get turbulent flow and vortices that will reduce the airflow. OK for experimenting but not ideal.
David Teitelbaum

[Spittybug]

Well, well, well.... 10 minutes of easy (no stomping) tuning with the car around the neighborhood and up on the highway (>88 mph but still no time travel) and I'm super pleased. No hesitation from low RPM, much better response and a very smooth idle once warm. I didn't notice any starvation at any RPM. I love it when a theory bears fruit! :Headspin:

I did what I said I would; a thin aluminum plate across the back of the TB with a hole cut for the remaining bore, the other blanked off. I also had to blank off one of the channels for idle bypass from the now unused bore, but that was easy. I used Permatex gasket maker and attached the plate well and there is nothing that can dislodge or otherwise get sucked into the motor. I mounted it right back on the manifold with the original gasket. The air flow for the remaining bore is totally unobstructed and the same as it was before.

Next step is to turn on my acceleration enhancement and adjust that curve. This is the extra boost of gas that comes in when you step on the gas transitioning from one cell to another on the table. It's adjustable so that minor acceleration only gets a minor amount of additional gas while a stomp (WOT) gets more. It's a bit tricky to get just right, but when you do the feeling is nice. The closed loop idle is working well; I set it for 800 when warmed up. Variance is +/- 20 rpm at the most I'd say. A/C kicks on and the idle motor opens a bit to respond. Nice.

[Bitsyncmaster]

Always is great when you test an idea and it works. Even is good if it makes it worse since that give you more data to find out why.

Are you going to keep the blocked off TB or look for a smaller one?

[Spittybug]

Always is great when you test an idea and it works. Even is good if it makes it worse since that give you more data to find out why.

Are you going to keep the blocked off TB or look for a smaller one?

Given the fact that it is a good quality blanking and I won't have to mess with making an adapter and finding/buying a TB, I'm going to just run with this and be done with it. Nothing temporary about it.
I love the scientific methodology!

[Spittybug]

I finally got around to adjusting my spark advance table over the last couple of days and as I was dialing the fuel table in afterwards, I developed a surging idle. Even after evening out the fuel and the timing at the idle area, it would still do it. Well, I turned off the closed loop idle and the oscillations stopped. By going closed loop it was just opening and closing....... Now, with the idle valve completely closed off, the only air getting to the engine is via the one barrel (the other is blanked off). I noticed a discernible change to the noise coming from the engine as the air velocity through the throttle increased. Power dropped too.

I have to do some more testing with the one vs. two barrels as a result. I think the closed loop idle may be the culprit all along. The idle air port is sized proportionally to the dual barrel throttle body. I already know that I was somewhat too large using both barrels, then giving the idle valve opportunity to open up even more makes it worse. I'm going to leave the idle valve set to warm up only and control idle simply with the spark and fuel (which I know how to do quiet well). I think the engine is going to want its other throttle bore back as a result! Maybe I'll just slowly but surely drill out a larger and larger bore in the blanker plate and find the best mix of volume and velocity.

Those of you going EFI with the stock setup don't have this level of tweaking to do, but what I'm learning is worth volumes.....

[Spittybug]

First drive in a few weeks. Yup, once warm and the idle valve is totally closed, the engine gasps for air when accelerating since I'm only on one barrel. It's weird to see; instead of the manifold vacuum decreasing as I open up the throttle, the restricted air flow actually can't keep up and the engine vacuum INCREASES. Not a good situation. Sends me way lean since the fuel bins in that part of the table aren't optimized. I've got the TB on the bench and will be drilling a hole in the blanked off side. I still think I want to have some restriction, but I'm going to go at it progressively and test as I go.

[Spittybug]

Drilled a 21mm diameter hole in the blanker plate for the second barrel. Total area is now 2156 mm2, just a hair shy of the "optimal". Will remount TB and do some testing.

[Spittybug]

Yup... materially better. I still think it needs a bit more, so I've now drilled out to 25.5mm which gives me a combined 2320 mm2. Still well short of the stock setup at 3324 mm2. I'll try this tomorrow and see how it feels.

I need to play with the idle again as a result of all this. Goal is to have the strongest manifold vacuum at the target idle RPM. I'm not quite as strong as I was (when bores were bigger) and I need to understand why.