Throttle body advice

[r10crw]

Been thinking about what throttle body to use on my Audi engine and starting to get a little confused! Typically in the past Audi engines in 111 chassis have used the 52mm K series throttle body which Bernard has used without issue up to 370hp. I've got DBW which I'm away to convert so trying to find a suitable throttle body. Currently it has a Dia of around 60 mm so 52 is quite a drop and that would have been standard at 225 never mind 400+. So I was advised by several people to go for the original DBC throttle bodies as used on the early passats etc with the AGU engines. These were also around the 60mm. I do know that there are guys pushing 550hp through a 62mm without issue.
Trouble is bigger isn't always better and you will suffer with throttle response so started to look a little more into it. I found a page which gives a formula for calculating the TB size needed which for my engine at 8000 rpm this comes out at around 28mm. Now I guess this is the minimum size for a 1.8 with 81mm bore and 86.4 stroke before the throttle restricts the air flow. Trouble is Ill be pushing around 2 bar of boost through the same throttle body and therefore more air so does this mean I need to double the surface area of the throttle body or perhaps triple it?
The formula does not account for butterfly either so probably need to add a little to accounf for this.
Essentially I want to know if Id be fine using the 52mm but with a small sacrifice in power, maybe 20hp but with improved throttle response?? Any thoughts?

heres the page that has the formula,

http://www.hipermath.com/engines/throttle_body_size

I could always just go for a 52 for starters and measure for pressure drop across the TB?
Incidentally my manifold has 60x60 mm bolt pattern so anyone that has a throttle body which fits this and has a pot on it let me know!!

[philthy]

Foudn this which might help - http://forums.vwvortex.com/showthread.p ... 932b08ee5c

This is with a Gt3071 so smaller flow rate too. I think the 52mm would be sacrificing a lot of power and risking Detonation, but its not like you dont have enough already!

[r10crw]

Thanks mate, did see that so was kinda the reason for looking more into this, according to that more is better. I think I will settle for the 60mm TB available on early cars, realistically I cant gain much more without changing or porting the manifold.
TBH Im not looking at big power to start with, if I get 300 as a starting point Ill be happy but would like to future proof any upgrades that I do once Im comfy with 300.

[robin]

I know next to nothing about this, but ...

I assume turbo sits in front of t/b?

What diameter is the turbo egress port?

What diameter is the manifold ingress port (sounds like 60mm)?

The t/b diameter by itself is pretty irrelevant if not matched to the rest of the system.

Whenever your diameter narrows the velocity increases.

Whenever your diameter increases, the velocity decreases.

You lose some flow rate at each discontinuity (whether a bend in the pipe, a narrowing or a widening) due to the eddies, reflections, etc.

Bigger is not inherently better especially if it introduces unnecessary discontinuities.

Given that you are not changing the manifold or the turbo your best bet would be to match one or other of these, in my opinion, as this will preserve diameter.

BTW, the WRC cars make 300-odd BHP through a 32mm tube on the atmospheric side of the turbo (by definition you can only see 1 bar drop across that, at which point you would have a perfect vacuum all the way to the cylinder - not much use ); so you don't need 60mm on the compressed side where you'll have higher pressure and thus (relatively) lower losses.

Cheers,
Robin

[r10crw]

Thanks robin, still got to take the turbo side appart so not 100% on sizes, look forward to going through it on route to the pub...

[robin]

I feel we cannot tackle Kalman filters until at least 2 G&Ts, 2 beers and three big crisps ... until then we can make stuff up about turbos instead

Cheers,
Robin

[tut]

Gin in freezer, tonics, lemons, limes, glasses in fridge, cashews, pistachios, almonds, (don't do peanuts), in little blue bowls.

tut

[r10crw]

Arrived in the post yesterday, 98 passat 1.8T TB has the potentiometer, DBC and around 60mm diameter Also has the 60x60mm spacing so all sorted now just gotta do the loom
Craig