Printable Version of Topic

Click here to view this topic in its original format

914World.com _ 914World Garage _ throttle body sizing

Posted by: ottox914 Aug 29 2004, 05:18 PM

Another question for the masses. After tearing things up this season on the auto x course, I find myself about 1-2 seconds behind the CSP winners at each event. And this on Falcon Azenis tires, with the fast guys bringing their hoosiers in on a trailer, while I drive to and from each event... With the 914 possibly going to DSP for next season, I'm thinking headers, Kuhmo 700's and dual throttle bodies with KitCarlson FI. All legal for CSP or DSP. I think the tires will make up most or all of the 1-2 seconds I'm back, but like most of you, I like to tinker with the teener.

So, given that for either class, the long block needs to be stock, (cam, pistons, valves, head, compression) what would be a good way to decide what size throttle bodies to get? In a quick search of TWM, Jenvey, redlineweber, there are sizes from 40 to 50+. While for the stock long block, anything over 50 would be huge, how do you figure between 40,42,44,45,48... I searched on carbs, and it seems carb guys w/basically stock motors run 40-44's. Keep in mind, the application is auto cross, so torque is more valuable than top end HP, and sharp throttle responce a must.

thanks again.

David Parsons
ottox914

Posted by: lapuwali Aug 29 2004, 06:05 PM

I'd not run anything bigger than 40, but also remember with EFI, this is a far less critical parameter than it is on a carb'd engine. EFI doesn't depend on the air velocity through the throat generating the "signal" for how much fuel to use. Also, because there won't be a venturi in there, you're going to see a lot more flow through a 40mm throttle body than you would through a 40mm carb. Most carb'd cars running 40mm carbs have 30-32mm venturies in there.

Since you're forced to stick with stock heads and the stock cam, I'd doubt you'd see any benefit on using anything larger.

Posted by: J P Stein Aug 29 2004, 06:18 PM

I'd leave the motor alone....unless it's in pain.....and work on the suspension. More bang for the buck there, me thinks.

Vracers are a good tire. They like a 6 or 7(better) inch wheel and around 2 deg negative camber. I *think* the V710s are available in 205 X 50 X 15.....they are atleast a second quicker then the Vracers on a 50 sec course...cost more too .

It's pretty easy to spend 2-3K( or more) on a 914's suspension/tires/wheels....depending on where (car mods wise) you are now.

Posted by: Brad Roberts Aug 29 2004, 08:25 PM

What JP said Those long STOCK intake runners and LARGE intake plenum promote mid range torque. Move to a short intake manifold and 40mm throttle blades and everything will get screwed up. I totally agree on the Kit Carlsen setup... work on the suspension...work on seat time.

B

Posted by: ottox914 Aug 30 2004, 07:33 AM

Lets just say the suspension and seat time are covered. I am one of our club instructors, and after 3 seasons of tweeking, the suspension is where I want it to be. On small lots we limit entries to 90, 150 for our larger lots, and we nearly always fill. That said, on sloppy 2 season old azenis skins, I have yet to finish below 30th overall, or top 5 in street tire classes. I think with fresh kuhmos there will be some suprizes in store for next season...

As far as the motor goes, to bad Jake is out this week, I'd like his input on "the combo". Just not sure if the kerry hunter header, KC managenent is enough to take advantage of the individual throttle bodies on a stock long block. Anyone got some engine building programs that could run a sim or 2 to compair systems?

Another thought, if the stock plenum and runners is the way to go, would a larger throttle body on the plenum help? Our stock one looks pretty small. Since I'd like to break out and do some regional, and maybe national events, I'd rather not leave anything on the table if the rules allow the mod.

thanks again

David Parsons

Posted by: DNHunt Aug 30 2004, 08:00 AM

I dynoed my 2 liter last year. I changed to Megasquirt, MSD 6A and a 50mm throttle body through stainless steel heat exchangers. HP at the rear wheels was 80.5. I made too many changes at once to know if any one thing would help. My feeling was the car had better throttle response but that was seat of the pants. Also I noticed better response at lower RPM's

Blyseng and rhodyguy were at the dyno session and we talked it over at lunch. We felt that the internals were the limiting factor and the factory did a pretty good job with D-jet when it was new. I'm planning on using the same intake on my 2270.

I know that some folks have changed to headers and picked up horse power. I also suspect that tunable ignition in the Kit Carson engine management might make a difference.

Dave

Posted by: ottox914 Aug 30 2004, 08:40 AM

DNhunt- thanks for the reply. I did a G-tech on mine, and found changing from the factory air cleaner to a short piece of hose, and a round K/N stuck right under the engine lid, (w/no rain tray) added 4hp! What model of 50mm throttle body did you use, and was the mounting a project or not so bad?...

Posted by: DNHunt Aug 30 2004, 10:52 AM

I bored the stock one to as large as possible then fit a Subi butterfly to the shaft and bore.

Dave

Posted by: ottox914 Aug 31 2004, 10:42 AM

Sounds like what I need to do... How did you do it, was it hard, and would you do it again, and if so, what might you charge?

Posted by: DNHunt Aug 31 2004, 02:16 PM

Actually, I didn't do it. I had it done. $50 at Jack's Machine Shop. I can't say it made any difference. All the changes I made added up to a car that runs really well. But, I just did too much at once to be able to say that any one thing was magic

Dave

Posted by: ottox914 Aug 31 2004, 02:40 PM

thanks-

Posted by: Mueller Aug 31 2004, 04:17 PM

a larger throttle body is not needed until you have a 1" or more differance in vacuum at WOT.....I've never measured the values of a stock engine...maybe Jake has???? I'll know in a few weeks when I get my car running again.

Posted by: ottox914 Sep 1 2004, 08:13 PM

Please explain about this 1" difference at WOT. 1" difference between what and what? Let me know what to measure and I'll get some real world info to play with.

Posted by: lapuwali Sep 1 2004, 08:20 PM

I think what Mike means is 1" of mercury, or the manifold vacuum. Ideally, with the throttle wide open on a normally aspirated car, the vacuum in the manifold should be 0 (that is, exactly equal to 1 atmosphere). It never is, of course, because no throttle flows perfectly. However, anything below 1" of vacuum (or about 0.95 bar, if you'd rather think in absolute metric values, 1.0 bar being ideal) means the throttle body is already flowing so close to ideal that adding a bigger one won't make any difference. If, at WOT, you're showing more than 1"/0.95bar, then increasing the size of the throttle will help.

Posted by: ChrisReale Sep 1 2004, 08:21 PM

I say throw some money into a set of stickier tires.