48mm Itb's

[Vz_ace 0]

Would they be to big for standard mi16 engine setup? i read that they usually use 45's...

 

any pro and contras?

 

thanks for any advice!

[bales 1]

I think there is always a bit of confusion on this matter, when carbs are called 45/48's they have chokes in them that drop down the ID so they are not that bore straight through.

 

Throttle bodies however, are the same bore all the way through so I would imagine yes 48mm ITB's are pretty big for a standard mi16, I would have thought you'd have to be running around the 240bhp+ to require that big?

 

Though I could be wrong....

[welshpug 1,661]

IIRC DrSarty runs 46mm ITB's for 230 ish, DJMini runs 48mm ITB's at 280 bhp.

 

PVFCPUG runs 45 carbs IIRC but no idea of chokes, 211 ish.

Edited October 16, 2009 by welshpug

[DrSarty 90 1 Cars]

Apparently it really has no huge effect how big they are when running ITBs, as it's all down to the mapping.

 

55mm ITBs were made to run just fine - even proving Dave Walker of Emerald wrong - on a 4cyl engine at his place not that long ago. There's an article in PPC on it.

 

So I would say 'no', not too big; although you will have to have a good mapper set them up properly.

[bales 1]

Apparently it really has no huge effect how big they are when running ITBs, as it's all down to the mapping.

 

55mm ITBs were made to run just fine - even proving Dave Walker of Emerald wrong - on a 4cyl engine at his place not that long ago. There's an article in PPC on it.

 

So I would say 'no', not too big; although you will have to have a good mapper set them up properly.

 

Surely though the larger the bore the lower the air speed, therefore don't you get issues at low revs/partial throttle openings with the fuel dropping out of the mixture with overly large throttles?

 

Aren't the same issues that people encounter when using overly large single t/b's encountered when using larger ITB's such as poor throttle control at low revs?

 

Though I guess if it is proven that it works ok then I can't argue with that! But I guess there is a difference between something working and something being optimum.

 

I am suprised though purely from an air flow point of view.

[DrSarty 90 1 Cars]

I agree Balesy, and I think that's what surprised Dave Walker.

 

Somebody for clarification should dig that article out from PPC; it's at least 6 months old.

[Batfink 201]

Well Mattsav had a set on an mi16 and said it had crap midrange torque

[Sandy 191]

There's so many variables involved in this, that generalising is vague at best.

 

Generally speaking, big TBs (relatively), will cause issues with low speed running, throttle response, fuel drop out and so on. But, it's the overall tract design, injector location(s) and setting up, relative to the rest of the engine design that counts. There's a rough guide to sizing on Jenvey's website, which also suggests a base tract length and throttle/injector location, but when you get seriously involved in testing these variables, exceptions happen.

 

As base advice, if you have a standard or fast road spec Mi16, 48s ought to be slightly big,but will probably be ok if you run the injectors in the OE location, with a good inlet manifold design and it's well set up. You might find that fitting a set of chokes (eg 40mm) immediately upstream of the throttles, will help low down response.

[petert 588]

I've got 48mm itb's, with injectors in the standard position and it runs like a dream. Makes approx. 220hp.

[Cameron 16]

I think it might have something to do with the injector being close to the port, and the port being a smaller diameter that makes the difference in throttle body size have a negligible effect. The air has probably been sped up enough when it reaches the port & injector to counter any slowing caused by larger tb diameter.

[sideways danny 0]

too small will restrict, too large will just mean you have less fine throttle control. Generally 45s are spot on a for a 2.0 engine. I'm going to be using 48s on mine

[pugpete1108 62 1 Cars]

so 40mm would be way too small for a gti6 engine?

[sideways danny 0]

so 40mm would be way too small for a gti6 engine?

 

 

they would in my experience. Sure it's been done and got passable results. 40s would be what i'd expect to see on a 1.6

[DrSarty 90 1 Cars]

My 2.2 had 40mm ITBs to start with and made 196bhp.

[sideways danny 0]

My 2.2 had 40mm ITBs to start with and made 196bhp.

 

what did it do with larger ones?

[DrSarty 90 1 Cars]

what did it do with larger ones?

 

230bhp, but there was a change in inlet design too.