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sizeing of throttle bodies
02GF74 - 6/1/21 at 08:20 PM

Does anyone know of any online resuorces that describe how to select throttle body size (internal diameter)?

I guess what people do is look at power of the original engine and select TBs from an engine of similar power.

In which case the original 1200 cc carb'ed engine pumps out 145 @ 9,000 so taking TBs from another 1200 cc engine that gives 170 bhp @ 10,000 should work?

Are there any drawbacks with having too large TB when running at low revs, namely issues with fuel vaporisation into slow moving air?

If there is, would selecting smaller TBs, say from a 800 cc engine 110 bhp @ 10,500 but fitting larger volume fuel injectors be better?

[Edited on 6/1/21 by 02GF74]

sdh2903 - 6/1/21 at 08:30 PM

Bit of a guide here

https://www.jenvey.co.uk/support/faqs/nunc-sit-amet-lorem-at-neque-aliquam-volutpat-eget-in-nisi/

rusty nuts - 6/1/21 at 08:33 PM

When trying to work out what size I needed when converting my Xflow I found Dave Andrews , tuning Weber’s site very useful https://www.google.co.uk/search?q=dave+andrews+tuning+webers&ie=UTF-8&oe=UTF-8&hl=en-gb&client=safari
Hope this helps

Using throttle bodies that are too large may cause poor drivability at low throttle openings

02GF74 - 6/1/21 at 09:20 PM

quote:
Originally posted by rusty nuts
Using throttle bodies that are too large may cause poor drivability at low throttle openings


So how do manufacturers get round that? (the second engine referred to is Honda VFR 1200)

coyoteboy - 6/1/21 at 11:12 PM

I can't see any reason why large TB would be an issue for an injected engine other than throttle sensitivity, which could be eased by an appropriately cammed linkage. What am I missing? Of course that assumes the diam isn't greater than the runner diam.

r1_pete - 7/1/21 at 10:44 AM

quote:
Originally posted by coyoteboy
I can't see any reason why large TB would be an issue for an injected engine other than throttle sensitivity, which could be eased by an appropriately cammed linkage. What am I missing? Of course that assumes the diam isn't greater than the runner diam.


Its down to gas speed..

A small diameter tb = high gas speed = good torque = drivability

Large Diameter tb = slower gas speed = less torque but in theory better high end bhp

At low throttle openings keeping gas speed up is key to drivability, a big throttle valve is difficult to control, hence you see a lot of progressive throttle cams and drivability wedges.

coyoteboy - 7/1/21 at 03:03 PM

My caveat on not making the throttle larger CSA than the port for that reason.

Reducing a small section of an intake tract won't improve torque though, as the gas will expand back out and slow again.

[Edited on 7/1/21 by coyoteboy]

02GF74 - 7/1/21 at 08:12 PM

quote:
Originally posted by coyoteboy
My caveat on not making the throttle larger CSA than the port for that reason.


There is lots of clever physics going on here that is outside my area of expertise, whatever that may be, but I'm pretty certain that is not right, the length of the inlet tract that the TBs form a part plays a role.

If you try to breathe through a long garden hose, I'll bet you'll suffocate whereas if the hose was 2 cm long I'll bet you'll be fine. My explanation, and shoot me down if you want, is that the air in a tube acts as a spring. In the long hose, effort trying to inhale is mostly spent stretching the air spring with not much air getting into your lungs.

I would expect the CA area of TBs is larger than the port opening. Unfortunately I don't have any cylinder heads with TBs to measure up.

pigeondave - 8/1/21 at 10:19 AM

You also can loose a bit of midrange if the speed is too low.

When I changed from Carbs to ATPower throttle bodies I saw a little bit better top end, but a little less in the midrange.

Fuel consumption was massively improved. I also lost the whiff of petrol which followed me around

I didn't do the physical swap. I sent the engine back to Dunnell, so I'm guessing that we're comparing apples with apples.

Don't be too greedy chasing a headline figure unless you're racing it.

Jenko - 8/1/21 at 12:37 PM

As mentioned....main issue is just of idle. To large and will struggle to get a smooth just of throttle delivery.
Bike TB's of 38mm will do 200bhp. but size is certainly not quite as critical as it is with carbs.

SPYDER - 8/1/21 at 07:51 PM

When I went from 40's with 36mm chokes to 42mm GSXR throttle bodies torque went up between 16-18 ft.lbs throughout the range. Peak power was just shy of 200bhp. The 42mm bodies are fine at this power level but I was advised to go up to 46mm Hayabusa bodies as my new engine is specced for around 220bhp. To counteract the low throttle sensitivity I have fitted a larger, offset throttle quadrant which gives a rising rate effect, ie. more cable travel at lower openings.

02GF74 - 9/1/21 at 12:03 PM

thanks - good to know; you mean something like a snail cam below (I think I've gotthe shape right)

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coyoteboy - 9/1/21 at 11:58 PM

quote:
Originally posted by 02GF74
quote:
Originally posted by coyoteboy
My caveat on not making the throttle larger CSA than the port for that reason.


There is lots of clever physics going on here that is outside my area of expertise, whatever that may be, but I'm pretty certain that is not right, the length of the inlet tract that the TBs form a part plays a role.

If you try to breathe through a long garden hose, I'll bet you'll suffocate whereas if the hose was 2 cm long I'll bet you'll be fine. My explanation, and shoot me down if you want, is that the air in a tube acts as a spring. In the long hose, effort trying to inhale is mostly spent stretching the air spring with not much air getting into your lungs.

I would expect the CA area of TBs is larger than the port opening. Unfortunately I don't have any cylinder heads with TBs to measure up.


My comment may have been a little misleading. I think my point here is that the ITB diameter is only *part* of the entire intake runner for it, depending on where it is in the runner the ITB diameter may need to change slightly, but in essence you want to match the runner diameter *as a whole* (the ITB slightly larger makes sense to minimise the losses at the throttle plate). Ideally you want to pick your intake diameter depending on the port flow capacity (slightly larger, but only ~30%) and on any engine the correct runner diameter is going to be around the port diam (unless the OEM did something whacky for a reason). If your ITB is a large part of your runner length, then it will have significant effect on performance as it affects the gas velocity (and gas velocity is closely related to cylinder fill).

People saying "smaller diameter ITBs give you more torque" - well...mayyyybe... but only if it's in conjunction with a properly sized/length runner etc. Same with a large throttle - it might give you more power, but only if tied with an optimally sized runner, otherwise you may find the runner restricts more than the throttle and you just get the negatives of the poor low throttle control.

Point being, when all's said and done, in true engineering form "it's not that simple" - it's a whole system approach, and as usual you're better matching the whole system to the power levels realistically expected.

SPYDER - 10/1/21 at 04:35 PM

quote:
Originally posted by 02GF74
thanks - good to know; you mean something like a snail cam below (I think I've gotthe shape right)

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I made the quadrant and fitted it to the small existing one with screws. The axis of rotation is where the red dot is. The change of angle of the cable is catered for by the little rose joint.


02GF74 - 10/1/21 at 05:49 PM

Perfect, thanks for that. That's more extreme than what I thought, shouldn't be that hard to try different profiles.

rusty nuts - 10/1/21 at 07:39 PM

I was unable to fit a progressive throttle cam due to lack of space between my TBs so I had to make one that fits near the throttle pedal operated by a short cable which then pulls the main cable. Made slow speed driving so much more pleasant rather than jerking every time I hit a bump

RichardPH - 7/3/21 at 08:06 PM

Hi, just browsing the forum and came across this question.. I've spent many a happy hour with Megasquirt and a Renault engine in a Lotus Europa, in the process accumulating some information that may be relevant to this discussion.

I've played mainly with plenum and runner systems, these have to be matched to port spacing as near as possible, that tends to mean the donor engine will be of a similar capacity, that was the case in my conversion and it was an easy installation. The required throttle, manifold pressure and temperature sensors were all on the plenum/runner as removed from the donor (k-series), as was the stepper motor valve for idle set-up, injectors and a fuel regulator. Simple.

I've also played with bike ITBs, in my case they were chosen based upon similar power and came with with constant depression slides, sourced from a YZF600 R6 Yamaha. They required using a throttle position map rather than the manifold pressure driven plenum/runner setup. Took a little longer to set-up and guess what, absolutely no difference in performance.. ITBs are really only worth using for engines that produce max torque high in the rev range where the runners need to be short for optimum inlet pulse tuning. Waste of time below 6000rpm. Looks good, but not necessary.

Which brings me to something else that is absolutely worth its weight, Tunerstudio software for setting up the fuel map. It cost about £60 IIRC but was sooo useful from Megasquirt (also for Speeduino nowadays). You just get the engine running after a fashion, jump in and drive around with it set to 'fast change'. It updates your fuel map based upon what it sees coming out of the exhaust registering on the O2 sensor. It's really simple and a perfect excuse for an enthusiastic drive.

Here's the thing you need to get absolutely right, injector size. Be realistic about the power you expect to get and source injectors from an engine of the same power/speed. The reason it's so important is setting up the off-idle pickup performance. You'll have trouble with calibration if the injector is too large. This is due to the turndown in injection pulse duration at idle, it's crucial to keep that pulse as long as possible to avoid large errors that may exceed the desired duration.

Another key consideration for off-idle pickup is injection pulse timing, not all ECUs are equal in this respect, the Megasquirt 2 I used didn't have the facility, I soon learnt from those who know that this would have helped a lot. Speeduinos have it as standard I believe.

Any questions..

coyoteboy - 8/3/21 at 09:36 AM

Almost any engine can benefit from itbs but you need to have an inlet long enough to match the wavelength of the RPM you have. What you're saying is right in the sense that you need to match your parameters, no point fitting parts optimised for one state to an otherwise unoptimised engine.

I've been watching speeduino for a while but its lack of native 8 cyl sequential ruled it out for me. I think that the latest STM32 support effectively fixes that, I started designing a board for it.

RichardPH - 8/3/21 at 10:14 AM

quote:
Originally posted by coyoteboy
Almost any engine can benefit from itbs but you need to have an inlet long enough to match the wavelength of the RPM you have.


Apologies if I've misunderstood your words, but it's the other way around. ITBs come into their own at high revs, because the optimum tuned length of the inlet reduces as speed increases. Think music and the notes generated by long vs short wind pipes. Performance motorbikes are high revving beasts that need a short intake, if a plenum and runner system was used the plenum becomes the dominant space claim, exactly where on a bike it would get in the way [of the rider], hence the use of ITBs and an airbox/filter. More compact.

For a cooking car engine ITBs are an unnecessary step, just fit a plenum and runner system. Loads are available off the shelf very cheaply from your local breakers. No need to make things any more complicated than necessary.

Fit a high overlap cam and the lines get blurred, but the short intake length is only beneficial at high revs, one needs to think about driving style and assess whether it's worth the low down torque loss penalty.