What are the real pros/cons of using smaller ITBS..?

[stefz]

Hi,

This is my first post, and would like to say how excellent I find this forum. -Top notch.

My first question is regarding ITBs. What are the pros/cons of using a smaller ITB.. There are various rules of thumb regarding how much power you can generate with a throttle size X or Y.. but are there any benefits.?

I am using 4x 34mm BING throttle bodies (originally from a BMW K1100RS) on a 1600cc Zetec-S engine, the exhaust manifold is an un-equal length primary arrangement leading to a 4-1 merge collector.

Is it still true, in the fuel injection era, that too large a throttle plate looses low and midrange torque.? I understand that the mechanics of airflow have not changed with the introduction of digital injection. But I have now heard from more than one person, the phrase "it doesn't matter with fuel injection.." reffering to fuel drop out associated with large carburetors at low air speeds..and loss of low-mid range torque.

Would it not be true to say, with smaller throttle plates, air speed is much greater at part-throttle openings, and so the emphasis can be put on fuel economy and a more efficient fuel distribution..?

Applogies for the long winded post.

Regards,

Stef.

[smckeown]

In my research and experience it is the case that larger diameter TBs do indeed reduce mid range gas speed and therefore torque.

There seems to be divided opinion on this, but I have back to back tests for 45mm TBs with and without 40mm chokes, and the mid range was very different indeed between them both.

There seems to be plenty of resources on the web about the optimum diameter TBs to use for certain power targets


Sean

[stefz]

Thanks for the reply Sean,

I was starting to think this topic would have divided opinion. Personally, I'm going to stick it out with the 34mm. I have just finished my exhaust manifold, and I am extremely pleased with how its come out, I'm hoping the merge collector will help me claw back a few hp from reduced pumping loss.

My initial thoughts were, 34mm were feeding a 1100cc bike engine that revs to 9000rpm compared to the 1600cc engine I'm using (reving say to a max of 6500rpm) is very close to the original disp * max rpm.

Thanks again for your reply. It is most appreciated.

Regards,

stef.

[hjulen]

I am assuming we like slightly smaller diameters to increase flow velocity into the cylinder?

In the case of my IDF Throttle bodies (which are 45mm) vs. the inlet manifold they will sit on (which is approx 38 mm inlet bores) going to a smaller throttle body shouldn't matter too much in this logic because the biggest restrictions are the intake runners.

Right?

I am more concerned about transient engine performance (say from idle to part throttle to full throttle) being too...jumpy maybe is the word?...since changes in throttle area for such large throttle area would me large changes in airflow into the engine. A 'streetability' issue here.

[Guy Croft]

If the throttle body outflows the head there is no point whatever going bigger. So if your ITB (individual throttle body) is same-as, or a bigger bore than the port, it's going to outflow the head by a country mile and going bigger will not yield any benefit (although it will might the engine feel different).

Remember that whilst the port might be, dunno, 34mm outer diameter, say, it will not flow anywhere near as much air as a throttle body of same 34mm diameter (even allowing for the loss around any throttle plate itself), because the compounded flow losses in the port are huge.


GC

[Rich Ellingham]

So 42mm for my 16v Fiat unit, Guy?

Rich

[Abarthnorway - Remi L]

Hi Guy!


Have been watching this thread - and I find it very interesting!

The question I am about to ask might fit better in a new thread, but I think it is somehow related to this thread.

If the throttle body outflows the head there is no point whatever going bigger. So if your ITB (individual throttle body) is same-as, or a bigger bore than the port, it's going to outflow the head by a country mile and going bigger will not yield any benefit (although it will might the engine feel different).

Remember that whilst the port might be, dunno, 34mm outer diameter, say, it will not flow anywhere near as much air as a throttle body of same 34mm diameter (even allowing for the loss around any throttle plate itself), because the compounded flow losses in the port are huge.

My guesstimate would be that a 40mm carb with 34 chokes will flow approximately the same as 34mm TBs..(?)

What are the reasons for using bigger carburettor chokes in perfomance applications?


Best regards

Remi Lovhoiden

[trickymex]

As far as i understand you should only go as large a throttle body as you need for the desired peak bhp, any larger than that would only be detrimental to torque lower down in the rev range and part throttle openings to gain no extra peak bhp????

as Guy has pointed out, there is no need to go for throttle boddies that out flow the head as there will be no gain.

[trickymex]

Hi Guy!


Have been watching this thread - and I find it very interesting!

The question I am about to ask might fit better in a new thread, but I think it is somehow related to this thread.

If the throttle body outflows the head there is no point whatever going bigger. So if your ITB (individual throttle body) is same-as, or a bigger bore than the port, it's going to outflow the head by a country mile and going bigger will not yield any benefit (although it will might the engine feel different).

Remember that whilst the port might be, dunno, 34mm outer diameter, say, it will not flow anywhere near as much air as a throttle body of same 34mm diameter (even allowing for the loss around any throttle plate itself), because the compounded flow losses in the port are huge.

My guesstimate would be that a 40mm carb with 34 chokes will flow approximately the same as 34mm TBs..(?)

What are the reasons for using bigger carburettor chokes in perfomance applications?


Best regards

Remi Lovhoiden

Remi,

The role of chokes in carbs is to make the internal diamiter of the throttle body smaller to suit dofferent applications

This just means that they can be used on differing capacity's and bhp figures

EG: a 1000cc ford cross flow or a 2000cc vauxhall xe 16v engine,

both could run on a set of 45 webbers but you would be on the maximum choke size for the 16v xe engine and the minimum choke size on the 1000cc crossflow.


but you should not go for a larger choke size than you need as it will be detrimental to torque lower down the rev range.

Ricky

[Abarthnorway - Remi L]

Hi again!


Sorry for not beeing clear enough....
I understand the purpose of the choke in the carb:-)

To clearify the question in an example:

Why will a 45 DHLA with 38 choke produce bigger power than a 40 DHLA with 34 mm chokes lets say on a 131 with GC3A cams and 34 mm intake port venturi?

Both carbs should outflow the head!.....(?)

Realy curious - have given it a lot of thought and cannot find an answer.

Best regards


Remi Lovhoiden