75 mm throttle body on stock plenum

[96blak54]

The butt dyno often lies to you because it is attached to your right foot.

The bigger throttle body really only does anything if it was the biggest restriction in the first place.

The reason so many swear by them is that the larger TB can dramatically increase throttle response. With the same throttle angle on the low end, they let more air through than a smaller TB. However, when wide-open where it counts, the smaller TB is no longer the restriction in the system and will perform the same.

If you think I'm wrong about this show me a dyno difference with only at TB change.

If i swapped a 40mm throttle body on...there most definitely will be a dyno difference....lol

 

[Silver95bird]

The stock TB is 65mm. The plenum is 72mm on every one I've ever measured.
A 75mm TB is nice, but you wont max it out. Take either a 70 or a 75 if you get one cheap, but don't worry about every last mm. It feels nice at low throttle because more air flows at low throttle angles.

That being said, the stock plenum maxes out around 630CFM from what i read (I believe it was accufab who tested them). That's more than enough for most people.

What's your end goal?



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[OLD H2S]

The 4.6 flows 500 CFM at 5000 RPM and that is with the heads removed. You should be shifting then because the power falls off after 4600, stock plenum is fine.

 

[Adam Vinnicombe]

Hi all I'm starting to get a little confused here.
So I get the impression that the stock plenum is more than capable to feed the stock engine, I also understand that the bigger throttle body allows more air in for the same pedal movement when compared to the stock throttle body so throttle response becomes more aggressive. The part I'm a little bit confused about is will I need to machine the stock plenum for the larger throttle body?
The end goal is to fit a supercharger and run 5-8psi boost. This is not going to be a race car or track car of any sort, its my fun weekend good weather car but I do enjoy putting my foot down when I can

 

[Silver95bird]

The lower port of the plenum has a weird oval shape that IIRC is larger than the TB opening. If you want to hand port the plenum, open the TB side to 75mm if you can, and want to use a 75mmTB. The other place the aftermarket plenums are larger is the long side (outer ) radius of the 90* bend. Go look at pictures and you'll see what i mean. The bottom I would gasket match if I did anything at all. Just leave the short side radius alone, it seems messing with that usually does more harm than good.

It's not necessary, but you certainly can if you want to. I think it's more about smoothing the air flow than making more air pass through.

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[Adam Vinnicombe]

OK I will give it ago and see what happens if it goes wrong I will have to just buy a new plenum

 

[Adam Vinnicombe]

Hi all had some time over the last few days to play with the 75mm throttle body. The stock plenum well the one on my car has an opening of 72 mm and the 75mm throttle body fit straight on. I forgot to take any photos but I tapered the plenum lip from 72mm to 75mm. As I through I can not tell any difference in power but the throttle response is most definitely improved and you need a lot less pedal movement to get the same level of performance when compared to the stock throttle body.
I have removed the rpm limiter and what I also noticed was above 6,000 rpm she was still pulling hard where as when using the stock throttle body you could fill the power beginning to die off.
Plus the induction nosie has also increased in volume which is nice

 

[96blak54]

Good deal!

What will the next mod be?

 

[Adam Vinnicombe]

I'm not sure what I want to do next.
I've already
Dropped the ride height but 1 & 1/2"
Fitted cobra drilled and slotted disc all round
Corba calipers
Aluminium prop shaft
ACI system
11" Corba clutch although the release bearing is on its way out so that needs sorting.
I also have a very bad rattling sound from either the engine or gearbox when at light throttle settings /cursing conditions between 1,900 & 2,500 rpm which Im hoping is not a rod bearing and is the release or pilot bearing.
It does need a new roof at some point but if I can sort the rattling sound out and can prove its not coming from the engine then I will either fit nitrous, cam it or fit a supercharger.
What mods have you done on your ride 96bkack54

 

[Adam Vinnicombe]

Just a quick question here, my stock throttle body had a hole in the butterfly but my new 75mm throttle body doesn't. What is the purpose of this hole?

 

[OLD H2S]

Idle air bleed. Keep it.