Individual Throttle Bodies - Installing Chokes

[3point141]

Simple question:

Can a typical Individual Runner Intake (ITB) such as a Jenvey (et al) be choked down to suit the demands of an engine in the same way as a performance carburetor?

I am thinking of the chokes you routinely fit to models such as Weber IDA's to "Trim" the available CFM to suit the engine.

Background to this: I have obtained a "Kinsler EFI" ITB system for a V8 (LS engine) I am building. I calculate that the air/fuel demand of the motor is lower than the nominal diameter of the throttle plates fitted (Ø58mm, whereas I need Ø48mm). I am hoping that by reducing (choking) the diameter down to Ø48 upstream of the throttle plate, like an IDA/DCOE carb, the CFM capacity of my intake will be matched to my engine.

Any help or insights are appreciated.

[mk2cossie]

Whenever Dave Walker has tested different sized throttle bodies in PPC magazine, there's been no lose of low down torque with bigger than necessary chokes
So I would say you should be fine without having to restrict the inlet sizes of them if they are too big

[3point141]

Thanks for the reply.

However, am I correct in thinking you may be refering to length of ram-tube? I've never heard of anyone choking an ITB before, even DW, hence the question. (if i'm wrong, I stand corrected..) However engine guru Graham Bell would have a lot to say regarding too big a carb/ITB system if fitted to an engine.

Essentially, the diameter of an intake runner "system" dictates the RPM range at which maximum torque will be made (all other engine peramaters being catered for) whereas runner length alters the character of the curve,"tipping" it either way to extend torque below or above that point.

Basically, doing the calculations, I estimate using Ø58mm throttles as they are would pin my maximum torque point for the engine at around 7650rpm!! Muchos Torqua Unavalibolios!!

Fundamentally, I know how carbs respond to chokes and how bad the motor will run if they are too large. It's just there is apparently no information out there relating to choking down an ITB intake in the same way.

[penski]

You don't need to choke ITBs! A carb's choke is required to adjust the air/fuel ratio; with throttle bodies, the ECU alters it by measuring the airflow and adjusting the amount of fuel injected.

*n

[3point141]

I believe the problem is a little trickier than that..
AFR can be corrected by tuning the jets. Experience says you can set up a carb that's way too big for an engine & attain correct AFR across the rev range and get really bad throttle response & poor torque figures low down in the rev range.
AFR being correct, for a N/A motor, its cam phasing and intake VELOCITY that makes the torque & hence the power. Velocity is mainly a function of intake restriction & diameter.
No ammount of ECU control can make up for loss of intake velocity.

[slater]

Its just not critical on an injected engine. The fuel only enters further down stream so port velocity is of little consequence at the throttle. On a carb you need to maintain good velocity all the way down or you get fuel dropping out in the manifold at low RPM.

The main consideration of ITB size is really down to throttle response. Bigger the body the less fine control you have (although this can be compensated for by your linkage design of course.)

[penski]

I think that drawing parallels between carbs and ITBs is confusing yourself.

*n

[slater]

No your confused between the cold start kind of choke and the kind of coke that creates a depression in the carb so it actually works in the first place (i think!)

[penski]

Mar 3, 2015 20:31:52 GMT slater said:

No your confused between the cold start kind of choke and the kind of coke that creates a depression in the carb so it actually works in the first place (i think!)

My bad; is it more towards lowering air pressure within the carb?

*n

[3point141]

To clarify, the choke (not the cold-starting one) in an IDA/DCOE carb is there to help the intake achieve good charge velocity at a desired rev range. It has almost zero to do with fuelling.

Velocity of the intake charge makes torque due to the kinetic effect of the air travelling at high speed along the intake. This is why at the "sweet-spot" where your engine makes highest torque, the Volumetric Efficiency of your engine (for a tuned engine) is above 1.0. This figure supports the fact that net average pressure across the valve is above atmospheric.
This effect is purely due to fluid dynamics, hence why the carb and ITB could (I think) be treated equally.
Also, I am considering only conditions at what all across the rev-range, therefore ignoring effects of the throttle plate.

How a carb & EFI achieve correct AFR, I believe, is irrelevant.

[hfstuart]

Mar 3, 2015 21:16:48 GMT 3point141 said:

To clarify, the choke (not the cold-starting one) in an IDA/DCOE carb is there to help the intake achieve good charge velocity at a desired rev range. It has almost zero to do with fuelling.

The choke size on a weber has quite a lot to do with fuelling, it's primary purpose being to increase the pressure differential (ie decrease the pressure in the carb) in order to help draw more fuel from the main jet. Use a choke too close to the barrel diameter such as a 36 choke in a 40DCOE and the venturi effect is much less possibly leading to very poor running at low rpm.
A secondary consideration is balancing the volume of air required for maximum power with getting good airflow speed at lower rpm in order to maintain good fuel atomisation. The aim with carbs is to use the smallest choke that will give the maximum power output so you get good low speed / part throttle response and fine control of the mixture.

Your first replier was correct - tests show that a larger throttle body than 'standard' is not detrimental other than in the loss of some fine throttle control. You can conclude from that that there is no advantage from choking the throttle body.

NB My 2.0 Lancia Twin Cam if fitted with 45DCOE would have 34 or 36mm chokes. I run 45mm ITBs, peak torque is at 3200rpm and it will pull part or full throttle from 1200rpm in top. It doesn't suffer from having the ITBs 10mm 'too big'

[3point141]

Mar 3, 2015 21:51:20 GMT hfstuart said:

Your first replier was correct - tests show that a larger throttle body than 'standard' is not detrimental other than in the loss of some fine throttle control. You can conclude from that that there is no advantage from choking the throttle body.

I'd really like to see the tests, it's just that everything I read on the matter says exactly the opposite! (Graham Bell's book 4-stroke engine tuning / Speed-Talk forum) It's not that the system won't operate, even without obvious "holes" in the torque curve, but compared with what the engine might be capable of providing, if the system was optimised... that's what I'm hoping to achieve.

I totally appreciate your thoughts on fuel atomisation, which does separate the needs of a carb from EFI with regards how each achieve this, and you're right, velocity is important to achieve & maintain this. But, from the stand-point of achieving good volumetric efficiency across a broad engine operating range, I still believe that throttle-body diameter (control of velocity) is as just as important as optimising valve events.

Thinks I'm going to have to do some more digging....

Keep the thoughts coming!  

[slater]

But the choke is only in a carb to create a depression across the float bowl and suck out fuel. Otherwise it just creates a restriction.


Port tuning or pulse tuning is what your getting at i think which is more down to length (volume) of inlet tract than having a choke in the system. A benefit of injection is that you can have a nice funnel shaped inlet that gradually speeds the air up rather than an abrupt restriction.

[Clamity]

^ as well as inlet tract(s) length, also total plenum volume inboard of any restrictions, e.g. the air filter / afm sensor. The whole thing is a resonant system.

[3point141]

Mar 4, 2015 10:45:48 GMT slater said:

But the choke is only in a carb to create a depression across the float bowl and suck out fuel. Otherwise it just creates a restriction.

Half right. The venturi does the job of getting fuel into the air-stream, but the diameter of the choke is specifically suited to the maximum airflow of the engine. Looking at it that way, you cannot point to the choke and call it a restriction. The choke will flow all the CFM your engine needs, it's the diameter of the carb body either side of the choke which is at odds with air-flow.

Ideally, the air intake should be sized to just meet engine airflow demand, any more just hurts velocity & decreases power. You're absolutely right about gradual cone taper along the whole length of the intake from bell mouth to valve, but air, like all fluids prefers constant rate of acceleration. Having too large ITB does little to help the air accelerate along that portion of the intake, so could be placing the emphasis elsewhere, where rate of acceleration of the air could be too abrupt. Result of this could be introducing turbulence & sudden reflections of energy back up the intake, reducing the energy we are trying to build.

I think I'm gathering a picture of what the answer might look like & unfortunately, perhaps, by comparing an ITB with carb could have muddled the question.

[RetroWarwicK]

the general consenus is that you go smaller if fitting ITBs (Dave Walkers words, not mine). So if you have 45 webers currently, a 40-42mm ITB would be ideal. Fuel injection in its nature is more forgiving as you can fine map the fuel and ignition delivery, so as already said, going with 45 ITB's would work but with a slight loss of fine throttle control.

on my 1998cc 16V redtop, i have 45mm hayabusa TB's which are a bit too big for street use truth be told. would be ideal for racing though

i suspect 45mm ITBs on a 5.7 V8 would work fine. you'ed need to look into trumpet length as that effects the torque curve

[3point141]

Mar 4, 2015 11:40:05 GMT RetroWarwicK said:

i suspect 45mm ITBs on a 5.7 V8 would work fine. you'ed need to look into trumpet length as that effects the torque curve

I calculated I need Ø48mm for optimum performance... however, I have Ø58mm TB's to work with.   Only 10mm... but a CSA increase of 43%!!!

The engine is a stroker 408 cu.in (6.7L) but that's still waaay larger than I'd like to run.   

[RetroWarwicK]

jenvey seem to suggest a tapered 58 to 45mm TB or 60 to 45mm for LS engines, so you are in the zone

www.compsystems.com.au/index.php/latest-news/9-news-products/44-jenvey-itb-s-for-the-whole-ls-range

[3point141]

Mar 4, 2015 15:25:58 GMT RetroWarwicK said:

jenvey seem to suggest a tapered 58 to 45mm TB or 60 to 45mm for LS engines, so you are in the zone

www.compsystems.com.au/index.php/latest-news/9-news-products/44-jenvey-itb-s-for-the-whole-ls-range

Thanks for this link, I'll pursue my investigation further with the guys at Jenvey. Thanks for the input.  

[nomad]

Mar 4, 2015 11:26:59 GMT Clamity said:

^ as well as inlet tract(s) length, also total plenum volume inboard of any restrictions, e.g. the air filter / afm sensor. The whole thing is a resonant system.

Not my area but resonant system and tuned length / standing wave harmonics used to be important on tuned carb motors... Just saying