Type IV 2056 NA ITB size

[TZepeSH]

I have been reading a lot trying to choose the right parts for Megasquirting the 2056 type4 I want to build. There is a lot of info on throttle body sizing, but I do not find any reliable site. Most of them deal with single throttle body calculation, not ITB. For example, from the calculation done according to this site http://www.hipermath.com/engines/throttle_body_size , the size of the single TB for my engine would be ~46mm. But all this is computed for passing all the necessary air for the motor through one TB. The volume of air through each ITBs would be smaller.
The valve size will be 42mm, and the heads will be able to flow maybe 170-180CFM. The whole engine at 6000rpm would only need 217CFM. So bigger TB than 42 would serve nothing, as the valve would be the restriction. I have seen another topic where on the same valve size were used 40mm ITBs. In this case, wouldn't the ITB become the restriction for the air flow, meaning the head/valve would flow more than the individual throttle body at WOT?
Any advice on chossing ITB size is highly appreciated.

[Clonebug]

Nate on the Type 3 forum has a thread (ProVolks T-3) and runs a ITB'd T-4 in his squaresh.

You could ask him what size he is running.

Ask Mario at thedubshop too......he can give you an idea.

[Jadewombat]

I'm pretty sure the heads would be the restrictive area in this case. There's no absolute number for size x equals flow y. Once you start making bends, adding intake runners, etc. things speed up and slow down quite a bit. If you ran 2 TBs 42mm, it would probably be in the 300-400cfm range I'm guessing or roughly comparable to the early rabbit TB listed below. Also, whatever number given for head flow is tested statically (valve open and a certain amount of lift), which of course is not the case on a running engine.

Ford Throttle Bodies-

Size Make CFM flowed @ 28in. of mercury

60mm Ford 526 cfm
65mm AccuFab 616 cfm
70mm BBK 726 cfm
75mm AccuFab 845 cfm
90mm AccuFab 1190 cfm
90mm AccuFab 1225 cfm

Solex Carbs.-

Solex 30 PICT 2 – no choke assembly
71.8cfm @ 20.4in. of mercury

Solex 28 PICT
64.65cfm @ 20.4in.

Single port manifold
96cfm @ 20.4in.

Weber progressive carb. manifold center section
181.5cfm @ 20.4in.

Weber prog. center section with DP carb. end castings
186cfm @ 20.4in.

Weber prog. center section with ’75 bug EFI end castings
175cfm @ 20.4in.

Watercooled VW Progressive TBs(Rabbit, Golf, etc.):

Early rabbit/scirocco 32/40mm(with insert)
280.76cfm @ 10in. of mercury
Primary only – 51.42cfm @ 20.4in.

Early rabbit/scirocco 36/45mm
349.87cfm @ 10in.
Primary only – 72.59cfm @ 20.4in.

Digifant/post-'85 throttle body 35/52mm
393.2cfm @ 10in.
Primary only – 60.5cfm @ 20.4in.
Digifant throttle body with stock intake pipe
337cfm @ 10in.
Digifant throttle body with 3” intake pipe
404cfm @ 10in.

[Dive!Dive!]

From Jenvey :

What is the best throttle body diameter ?

Factors influencing size are; Power output, RPM, cylinder head design, cylinder capacity, position of the throttle body in the inlet tract and position of the injector.

Choice of bore size is a balanced compromise resulting from the following;
1) A larger bore leads to lower flow resistance, but obeying the laws of diminishing returns.
2) A smaller bore leads to better throttle control and response (never underestimate) and improved fuel mixing.
3) The system should be considered in total - from (at least) trumpet flange to cylinder and proportioned accordingly.

Basic references for BHP per cylinder, assuming ca 120mm from butterfly to valve head and a max of 9,000 rpm are;
Up to 30 BHP - 30mm, up to 33 - 32mm, up to 39 - 35mm, up to 46 - 38mm, up to 51 - 40mm, up to 56 - 42mm
Up to 65 - 45mm, up to 74 - 48mm, up to 80 - 50mm, up to 87 - 52mm, up to 93 - 54mm.
These power figures may be increased by up to 10% in a purpose designed and well proportioned system.
As butterfly to valve distance increases, butterfly size will need to increase in proportion to system taper and vice versa.
Lower revving engines and those with injectors placed before the butterfly will generally accept a larger body.

[aircooledtechguy]

I understand that a person can get all caught-up in the math, theory, runner lengths, RPMs etc., etc. of choosing the "correct" ITB size. All that may be extra important if you are an OEM company or planning a spec race motor where getting absolute peak HP at a certain RPM range while dyno tuning is imperative. That is not 99% of us here. . .

ITBs of a given size, say 40mm, will out-flow a carb of the same size due to no restrictions like venturis, main jet stacks etc. Typically a 40mm carb works great on a 2056cc motor where you're turning up to 6KRPMs. Knowing that, I would consider 38mm ITBs and possibly 40s, but no higher for a 2056cc. I ran 45mm ITBs on my 2056cc for a year or so and they were just too large, but I knew that going in since they were destined for use on my 2316cc when I bought them. Even now a pair or 42mm ITBs would likely be just fine for my 2316cc.

I guess what I'm really saying is that you can make it into rocket science, but for most of us and what we're doing with our cars, it's really not.

[Dive!Dive!]

^^^^
I agree. In practice, I'm not aware of ITB's smaller than 38mm. According to Jenvey that will support up to 180hp on a 4pot motor - that would be a decent N/A 2056 I'd guess. I agree err on the small side.
I have 38mm (VW Speedshop ones) on my 1776 that makes 130hp and they are great

[TZepeSH]

Time to re-start the discussion. The engine configuration is fixed now: 2056, 42x36mm valves, ported, 163/86B camshaft, 5500-6000 rpm max, I expect around 140HP.
What size throttle bodies to use? I like very much Max Welton's build with Honda VFR ITBs. BUT, they are 36mm inside diameter. I'm afraid this would be too small. From the Jenvey site, they would fit (up to 39BHP/cylinder - 35mm), but the calculation is done for other engine parameters. Why I like them: easy to install, keep stok parts around the ITBs. For example a similar 2056 would work great with IDF44 and 36mm venturies, ITBs same size shall perform at least the same. Am I wrong?
I don't like to separate the ITBs and get to complicated flange adapters. Mainly because around here in my town there is nobody to mill some flanges like I have seen in other topics. Also I do not want to make my own fuel injection rail, I would rather use a stock one. And it will get very expensive.
What I read around here is that most of the people ar using too big ITBs (But I don't understand if that is 40mm, 42mm, 45mm or something else).
BUT, if 36mm ITBs are too small, I will go to al the trouble to pun on bigger ones. What size, 38mm, 40mm?

I am looking forward to any suggestion. I am at a loss on this subject.

[TZepeSH]

The topic with the TB size is still open.

I read a bit more on intake design, and I'll try to summarize what I found. Based on info from Jenvey site (lots of info in few words, with examples, summarizing long discussions on other sites), 35mm TB would be fine, but as I move it further away from the valve, it shall be bigger. Maybe this is why 40mm or 42mm work so good on many engines.

Now a bit on intake design: why do people find it easier to split the throttle bodies and fabricate everything around, including mounting flanges on IDF intake manifolds? It seems easier from my point of view to take original manifolds, cut the intake pipes and weld back some stainless pipe to the original distance of the ITBs. I see most of the people around have the skills to weld, but I don't think many have a CNC at home to make flanges. Also, nobody seems to consider the intake design. It is so much easier to create a proper intake by welding the needed pipes. Is it a reason not to design own manifolds in stainless steel?
I'll start with a simple picture, and comment on what I understand from Jenvey site:

intake.JPG

Red is cylinder head intake port, green is intake manifold, yellow is TB, blue is velocity stack (trumpet).
Based on Jenvey, the overall intake length shall be 350mm for a 9000rpm engine. This means 484mm for a 6500rpm engine. IDF velocity stacks (easily available) are 55mm or 38mm (D in picture). A TB similar to IDF is 84mm (C). It means the length of the intake manifold and intake port shall be 345mm (A+B). Now, I do not have an IDF manifold to measure, I would be interested if somebody can do the measurement. I can measure during the weekend the approximate distance from the valve to the cylinder head mating surface.
The minimum distance from the valve to the butterfly shall be 200mm for a 7000-9000rpm engine. This is related to the previous measurement of intake port and intake manifold length.
I would be curious if the legths match the theory from Jenvey.

[TZepeSH]

I measured a cylinder head port, it is approximately 80mm in length. Can somebody measure the lenght of an intake manifold, for example like the one below (CSP, for upright conversion of type4)?

intake_CSP_upright.jpg

[falcor]

https://www.deimosaircooled.com/media/c ... mage-1.jpg

I've got these intake manifold with my 40 mm itb's. The centre line of them is 13 cm or 5,1 inches.

[TZepeSH]

Thank you for the info Falcor. So it seems that the overall length of the intake would be ~10cm short, according to Jenvey. This means one more point for making own intake.
Another thing: what shall be the diameter of the intake manifold? Same as the ITB size?

[falcor]

Just make the connections as smooth as you can without any steps in them.....and yes the valve to butterfly length will be too short to get use of any of the harmonics in the rpm span usually used for aircooled fours.

So making your own intake would solve some of this but you will probably run into space issues depending on what your car and engine bay looks like.

[TZepeSH]

I totally missed the space issue. Thank you for pointing that out. I have a bug, so space is small... No way to get the proper intake in there unless I cut the body and hood, which I don't want to do.
Sooooo, back to the main question. What size throttle body shall I use? If I go with 36mm from Honda VFR, I can use IDF intakes. If I need to go higher, I can either fabricate my intake to fit the spacing of the ITBs, or split the ITBs to fit the IDF manifolds. What to do?

[TZepeSH]

I bought a set of VFR800 throttle bodies, complete, for the fabulous amount of 24 dollars. They are 36mm and spacing between is 92mm. I have seen several topics and I like how they were installed.

The simple question, because Google is not my friend today and does not give me this info, and neither my wife knows: how much does a 36mm throttle body flow? @28" would be better to be easier to compare with head flow.

[TZepeSH]

I just found here on STF a good piece of info, http://www.shoptalkforums.com/viewtopic.php?t=1687

Here's some numbers to play with. I have a chart of DCOE flow for different venturi and choke sizes. I can approximate many numbers from it... They should flow about the same as an IDF... check this out: (numbers per throat):

40mm throat, 28mm venturis: ~140CFM
40mm throat, 32mm venturis: ~167CFM
40mm throat, 34mm venturis: ~173CFM
44mm throat, 36mm venturis: ~207CFM
48mm throat, 40mm venturis: ~240CFM

So, my 36mm ITBs will flow at least 207CFM. If I consider 100%VE, at 6000RPM I need 217CFM. This is pretty close, and in the same direction as the advice in this topic, consistent with info from Jenvey, and with other calculations from other sites (ITB flow = 1.4x port flow, ITB size = 1.25x port size...), so I should be OK.