I'm working up a design for an intake manifold and I am pretty confident about all of the parameters (runner length, plenum volume, etc) except the throttle body size. What is the optimal TB diameter for a normally-aspirated motor that should make 400-425 hp at the crank? The last thing I want to do is to go through all of the work to build an intake, then have the TB turn out to be a restriction, but I also don't want something that ends up working like a barn door either.

Any thoughts from those of you with intake fabrication experience are very much appreciated.

 

ive always thought that the plenum/runners are the important parts of the intake, and anything before that can only be a restriction, thus go big to have less of a pressure drop.

 

Well, I don't want to go too big, or the velocity will drop of very suddenly as the air enters the plenum and that screws everything up.

Right now, I am leaning towards using a 90mm TB off a Q45 or an LS series V8.

 

I could be wrong, but I think that itd be better to have as slow/stagnant of air as possible in the manifold, so that the cylinders can have more even filling.

if you have higher velocities entering the plenum, then momentum of the fluid column will take more air to the farther cylinders.

and even if you have a smaller TB, the velocity will still plummet as soon as it enters the plenum, since thats an even larger diameter than the biggest TB you could probably put on there.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by dfoxengr &raquo;</TD></TR><TR><TD CLASS="quote">I could be wrong, but I think that itd be better to have as slow/stagnant of air as possible in the manifold, so that the cylinders can have more even filling.

if you have higher velocities entering the plenum, then momentum of the fluid column will take more air to the farther cylinders.

and even if you have a smaller TB, the velocity will still plummet as soon as it enters the plenum, since thats an even larger diameter than the biggest TB you could probably put on there.</TD></TR></TABLE>

Yeah, isn't it hard for the air to make the turn into the runner closest to the TB if the velocity is too fast?

 

If it is too big, you get sluggish air that hugs the walls and does not get distributed properly. Plus, partial throttle and transitional response gets crappy. While it is obviously going to slow down to some degree as it enters the plenum, the issue is how much. And, keep in mind the air is not moving through the plenum and out the back, it is trapped in the plenum and can only go into the runners. Lastly, all the runners are not pulling air in the same volume at the same time, so they are not 'fighting' over it as much as you would think.

Please, anyone with actual experience/knowledge? I saw in another thread the target volume through the TB was about 300cfm; if this is true, I can do some math and get an approximate answer. . .



Modified by Top Ramen at 10:43 PM 5/28/2008

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by all-mtr-teg &raquo;</TD></TR><TR><TD CLASS="quote">

Yeah, isn't it hard for the air to make the turn into the runner closest to the TB if the velocity is too fast?</TD></TR></TABLE>

very. it doesnt and separates from the walls.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Top Ramen &raquo;</TD></TR><TR><TD CLASS="quote">If it is too big, you get sluggish air that hugs the walls and does not get distributed properly. Plus, partial throttle and transitional response gets crappy. While it is obviously going to slow down to some degree as it enters the plenum, the issue is how much. And, keep in mind the air is not moving through the plenum and out the back, it is trapped in the plenum and can only go into the runners. Lastly, all the runners are not pulling air in the same volume at the same time, so they are not 'fighting' over it as much as you would think.

Please, anyone with actual experience/knowledge? I saw in another thread the target volume through the TB was about 300cfm; if this is true, I can do some math and get an approximate answer. . .

Modified by Top Ramen at 10:43 PM 5/28/2008</TD></TR></TABLE>

well like I said, thats my opinion that it would distribute more evenly due to the reasons i stated.
And you probably arent describing correctly (proper terminology) what you think is happening because "hugging the walls" is a good thing.

the fact that it is trapped there makes the momentum thing worse.
if youve ever seen CFD plots of a standard manifold you would know that the cylinder closest to the opening has it rough due to the momentum I was talking about, and it also not being able to make a turn 90 degrees.

you dont have to agree with me, I was just trying to help. If you want to argue until someone agrees with you then thats fine also, youll get your answer and youll just end up doing it your way.

I would just make it so you can try out 2-3 different ones. a stock one, a 68mm and like a 75mm or something.

good luck.

 

What are the specs for your engine... I need to know, Cubic inches (cc's), max RPM, plenum cc's, runner lengths and if your head is ported or stock?

I can tell you what the optimum TB CFM should be. But in order to find one that flows that will require flow testing. I don't know if there are any flow bench numbers floating around for the Honda TB's.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by GhostAccord &raquo;</TD></TR><TR><TD CLASS="quote">What are the specs for your engine... I need to know, Cubic inches (cc's), max RPM, plenum cc's, runner lengths and if your head is ported or stock?
</TD></TR></TABLE>

Displacement - 3317 cc

max rpm - 9,000

plenum volume - 4.9L (roughly)

runner length (back of valve to end of velocity stack) - 40cm

mildly ported head - flows roughly 300

 

Your throttle body should be able to flow a Min of 450 cfm @ 85% volumetric efficiency and a Max of 530 cfm @ 100% volumetric efficiency. With your intake manifold & head volume you are probably well into the 90-100% VE range for a 3.3L engine @ 9000rpm

That said, In doing the math I figure that a 65-70mm throttle body would flow more than 550 cfm very easily @ WOT.


Modified by GhostAccord at 2:20 PM 5/29/2008

 

Great - I know I can find a 70mm Honda-style TB, so I may not even have to hassle with converting to a Q45 part.

Thanks.

 

you're right. that will flow that amount, and a stock one probably would too.
but you're still overlooking the pumping losses that you're going to incur by having a greater pressure drop, thus reducing power.

 

go with a 60 then. All I did was crunch the numbers.