M30 big bore throttle body?

[gale]

I've read a few posts that recommend not to mess with them, but you know me, can't leave well enough alone :p I bought a used throttle body to play with. I haven't taken it apart yet but my initial ballpark measurement of the butterfly diameter is approx. 64mm. What increase should I start with? A 2mm increase opens up the cross-sectional area 6.3% and 3mm takes it close to 10%. If I go too large, I'm hosed.

The engine is basically stock except for an EAT chip, 19lb Mustang injectors and a 3.5 FPR. I hope to add headers in the future. Anyone have 1st-hand experience with larger throttle bodies?

[Mobius]

From what I've read, and what's logical - there's absolutely no point unless you've ditched the incredibly restrictive AFM first.

Once you've got a nice free-flowing AFM in front of the throttle body, then you might see some benefit from enlarging it - but right now you have a big spring loaded plate in the way that's being way more restrictive than the extra 2mm or so of throttle body wall.

[Robin-535im]

I think the MAF is a good idea too. I've been thinking about building up a 3.7 or 3.8 and part of that would mean getting more air in. I'm assuming that the volumetric flow rate goes linearly with area so if you increase the displacement by 10%, you'll need at least that much more air (and fuel) going in and out.

Would you do it yourself and machine a new valve or send it out?

[shogun]

Have a look at this MAF performance upgrade
http://www.racingking.net/Includes/SearchModel.asp?Model=E34
and here
http://members.roadfly.org/louielouie/AFM_adjustment.html

[gale]

I don't have the aptitude to mess with a MAF/AFM conversion & don't care to open up a can of worms to get it to idle properly or eliminate flat spots.

I've heard all along there are negligible gains to be had with just a big bore throttle body alone & there are instances where it will actually reduce performance, probably from going too large & messing up the intake air velocity, but I've also read a few posts from some who have added only a larger throttle body & report noticable mid range & high rpm improvements. I'll machine the used donor myself. The stock bore has a straight round cylindrical bore thru the middle with a straight conical taper at the entrance and exit. There is a nominal .010-.020" step on the radius at the transition from the taper to the center bore that will clean up if bored 1mm oversize, but I'm thinking that small an increase is so trivial, there wouldn't be any perceptable difference so I'll start at 2mm over on the diameter. So far it doesn't look like the tapered entrance/exit profiles need to be re-machined.

The new oversize butterfly disc is easy to make, simply pressure pad a 1/16" brass plate on a lathe & machine it to size. I'll measure the stock disc to see if it's slightly eliptical since the butterfly closes just shy of 90 degrees. I have a method to turn an eliptical disc on the lathe as long as it has no more than say 5 degrees tilt off 90.

The slot in the throttle shaft will have to be widened, that can be accomplished by hand with a jewelers file. With the new disc machined to size, the 2 screws on the throttle shaft hole locations can be scribed and match-drilled.

I might spring for a set of headers when Ireland gets them into production, or score a set on ebay & maybe a cam if it needs the inevitible headgasket job, but other than that, it's time to quit spending on big ticket items for this car other than routine maintenance & tires. So far I'm into the throttle body for $25 & the rest is just a few hours time. They charge an arm & a leg for bored/exchange throttle bodies. I'm thinking I'll start at 2mm overbore & see what it does. If it's noticable, I'll drive it like that for awhile & then go to 3mm later on. If it messes it up, I can always go back to my stock throttle body. I just passed smog with flying colors & I'm good for 2 more years, so it's tempting to replace the cat with straight thru pipes.

[Robin-535im]

I don't have the aptitude to mess with a MAF/AFM conversion LOL - of all the people on this board... I'd bet you could calculate the proper settings on the back of an envelope and have it work perfectly the first time :)

The TB sounds like a fun project. Do you have a g-tech or something to record performance data before and after?

[gale]

LOL - Do you have a g-tech or something to record performance data ..."

No, not that sophisticated. All I go by is my "butt-o-meter". Who in their right mind is going to track a 7? (LOL). Just looking for cheap ways to make it more fun. I rescued the car from my wife back when it was still an automatic, when she was getting ready to trade it in on a mini-van (oh, the horror!).

Ever since then it's been a hobby to tinker with, nothing scientific about it, just for fun.