Throttle Body Size Advice

I've been kicking around the idea of throttle body size, and was interested in this groups opinions/experiences in that area.

My setup is a 306, TFS High Port heads ported/polished, GT40 Intake, b303 cam, hooker super comp headers through 2-1/2" exhaust all the way back. I believe the blower is an A trim, makes 8-10 psi right now through a 65 mm throttle body and EGR Spacer. This is in an 89 mustang coupe, with a 5 speed, 4:10 gears and all kinds of other stuff like MSD ignition components, etc.

Will a 70 mm be worth the investment, or is a change really not going to net any worthwhile HP gain? Car is street driven to/from work and errands. Hasn't seen the track since the blower was installed, but ran 13.0 N/A

Any help is appreciated,

Charlie

70mm is the largest I would recommend since your only using a A-Trim and none of your other parts need anything larger and you will not feel any negative effects from going to big.

But no positive either.:huytsao:

You will see many upgrade to larger TBs but unless you are seeing 500+ HP the 65MM is more than sufficient.

I could get into velocity of the air flow making more torque, but that gets REAL deep! In short the smaller TB gives better velocity than a larger unit for obvious reasons...more air moving through a smaller opening equals more velocity. That equates to better torque and it is torque you want on ANY street driven car.

You are wasting your money getting a larger TB IMHO.:(

BTW, I'm installing the EXACT setup you have on my Ranger 5.0 this winter and have done exhaustive research on all aspects of your setup. Have you been on a dyno yet? What kind of tune are you running?

70mm is plenty enough for sure. Unless you have a Kenne Bell it's a waste to go any bigger. They gain good from less inlet restrictions. I had a friend that thought because he had a 75mm MAF he needed a 75mm TB! LOL And it was a NA 302!!! He wondered why it was a little dead down low...

Lots of POSITIVE effects. You need a 70mm because you are Supercharged and pushing/forcing air into your throttle body. Just don't go larger, but who would because after 70mm everything gets sized down to fit anyways. Your supercharged so even if you want to listen to others that responded most what was said is not a factor.

To take advantage of the change another dyno session would be in order.

Your HP will go up slightly....VERY slightly...and your peak torque will move up to a higher RPM. Again, a proper retuning is in order.

I'd go alcohol injection instead of a larger TB. Then you can advance your timing and gain noticable HP and more importantly for the street, more torque.:teeth:

70mm TB is not about horsepower but taking out a restriction on the Supercharger at WOT. And if the torque does move up better for you since a 5.0 has to much torque under 3000 rpm anyways to get traction on street tires. I was driving a 500+ whp 5.0 daily and even when it had no blower I could rev to 3k rpm and drop the clutch in 3rd with 3.08 gears and never get traction till the top of 4th. That was with a 70mm and Cobra intake. If you need a new TB get a 70mm if not keep the stock unit just don't spend the money twice and waste money on anything under or above 70mm.

Thanks for all the input. What I'm taking from the advice is that installing a 70 mm will not make a big change in the performance of this setup. I guess that I will hold onto that money for something else.

Tominator asked about my tune, etc. I've not been on a dyno with this rig, and so my tune probably is leaving some power untapped. This forum is full of knowledge about doing things right, so my tune is embarrassing to report. But, I have 24 lb/hr injectors, run an FMU, base fuel pressure is about 32 psi, timing at 10 deg with the spout out, BTM set at 1 degree per lb of boost. I think my fuel pressure under boost goes to about 60 psig, haven't looked in a while. When I first installed the supercharger, with the fmu that was provided, my fp went 100 psig plus and it would stumble the car. After the fmu change, that problem went away. I have o ring heads, and haven't had any real trouble since installing the sc.

Any tune suggetions? I err on the safe side to hopefully prevent problems.

I know that bigger injectors and a custom tune would be in order if I ever got serious. Problem is, to get serious with this car would also require some other issues to be addressed like rear axles, different rear ratio, different carrier, etc. With two kids (9 and 5), the disposable income isn't what it used to be. Truthfully, the last time I ran the car was about two months before my oldest daughter was born and the transmission gave out to the tune of about a grand. Not cool with a 7 month pregnant wife! Some of my other interests (boating, fishing, 70 Monte Carlo) also get in the way and compete for disposable income.

Thanks again for the advice,

Charlie