Is it worth it to use a bigger throttle body on a Fit?
 

Hey guys.
I have been messing with Hondas for a looooong time but none of the new drive by wire cars. I bored a 50mm Fit DBW throttle body all the way to 58mm. Has anyone used a larger TB in their fit? Any worthwhile results?
http://www.maxbore.com/fittb1.jpg
Factory butterfly over the 58mm butterfly.
http://www.maxbore.com/fittb3.jpg

Have you ever seen a child struggle to draw liquid through a large straw. You know how they don't really have the lung power to make it happen easily and when the beverage finally makes it to the top of the straw its volume is more than said child can drink.

LOL I like the above explanation.

I am going to give a sort of simplistic explanation because I have not concentrated much on learning about modifying the Fit for power gains. And the reason I haven't concentrated on learning in that area is that it's .. difficult.

Draw a parallel to operations theory. Your assembly line works only as fast as the slowest robot on the line, which is the limiting factor. If you swap out that robot for a faster one, say twice as fast, you don't get 2x the speed - the line speed goes up to the speed of the next most limiting factor. The same principle applies to any system you want to make better - you identify the current limiting factor and improve it, then the next one and so on.

On many cars there is a very distinct first limiting factor of the air intake, which is often routed through a complicated series of boxes and filters (as it is in our car). And so people replace the stock intake on the Civic Si and gain 2lb-ft and 13 peak hp or something. After that something else is the limiting factor, maybe the cat on the exhaust. But on the Fit there are a LOT of limiting factors close together, whether it's the restriction of the valve openings, or the intake runners, or the same on the exhaust side, or the cat. So this means, if you relieve one limiting factor, you may not see ANY gain immediately because even if you chose the most limiting factor, there is another one limiting the airflow to approximately the same amount. Compared with an engine with limiting factors spread out more, you have to relieve many of them before the flow can really speed up.

It's for this reason why NA builds are considered so hopeless on this car. They aren't "hopeless", they're just lots of work, and money, because any one change doesn't help much. The ironic thing is this is actually the hallmark of a good design - the engineers have already identified and improved most of the limiting factors to the point where further improvements are cost-prohibitive.

But as I said I'm not an expert. Someone else can and likely will identify WHICH factors are most limiting for the production of power :) .. thanks.

I am simply trying to acertain if there is a market for larger TBs for the Fit.
It doesn't look that way judging by the responses so far.

have not tried it, but very interested if there was options of a bigger dbw throttle body!

you're going to get your car tuned, right?

On the original Civic we substituted a Datsun Aisan carb on the Civic and got measureable improments because it was a rejetable carb. On an FI sytem you'd have to recalibrate the FI program to provide the fuel for the increased air which now won't calibrate the flow with the MDI. And going from 50 to 58 might help at max rpm, but dawling about town might be a severe handicap. Only a trial would tell you; who knows the temp change might be just fine with needed fuel. Course, perhaps the cams won't help much either. But it might be worth a few more hp. Only trying will decide.

Tagged for interest.

i put a bigger throttle body and intake in my old integra backin the day.....bad idea lol

Back from the Dead!

Sorry Maxbore got the responses he did. Thanks for joining this forum (Back when you did). I called Maxbore today and they stated they could bore out the stock 2015 Honda Fit throttle body. I'm currently in the market to find a used one if possible. A new one costs $460+. I may even consider sending in my stock unit but am not looking forward to any down time.

In the days of carburetors you could gain a little better air flow at the expense of fuel efficiency by getting a larger venturi carburetor. Since the throttle body is no longer the fuel metering device for a fuel injected engine, except as part of the position map along with incoming air temperature and pressure on these generation fits (there's no MAF sensor), I would imagine the engineers designed the size of it meet the needs of the intake system. In other words, they could have made it bigger but it would be of no benefit to an air pump (an engine) that can only suck in so much air.

The design of carburator size and fuel injector systems are the same. Engineers choose a venturi (which is the throatbore) of a throttle body based on the maximum air flow that will result in a pressure drop that will range over the minimum to maximum air flow that will result in good fuel-air mixture in thecombustion chamber. While carbs absolutrely depend on the pressure drop to suck fuel thru a jet, or jets, FI depends on pressure to inject the fuel by time from a computer that measured air flow. Still the air flow must produce a negative pressure drop to be sure the fuel is injected at the right place to atomize in the flow.(which it does far better than carbs, hence carb's demise).
The only reason for increasing throttle body bore is to permit greater air flow, which will increase higher rpm. Of course that means less control at low rpm, just as it does with carbs when venturi size is increased.
Increasing throttle body bore does nothing for your car unless top speed is the issue.
There are some studies to show better mpg by specially shaping the throttle bore to generate better atomiztion of fuel and specific portions of air flow into competition engies in NASCAR but results are not public. The shape of fuel discharged from injectors is part of those studies. Probably auto manufacturerers as well.
So if you experiment with your throttle body look at its shape and the injectors outleets.
RC, long better supplier of Hobda engines and now injectors, just may be the better way to go now. Just be sure you let RC know whether performance or mpg is your primary target.
That is an interesting project. Have fun.

Ready
 

**Bizzump**

Maxbore,

I'm ready to get this done. Sent you a PM & an email to your email address. Let's make this happen. Ready to send you my throttle body tomorrow. I'm looking to have it back for testing out next week. Thanks!

**Yes, I know he hasn't been active on here since 2012 but I'm trying all avenues of communication**.

His site is still running and i'm sure he still has business. The prices are fantastic for what he does. I am very curious to see if this works well on the Fit.

The problem as mentioned (at least for the GD3) would be the intake then the head. It would be interesting to see if it would gain or lose power. The 2015 i'm sure is a different flow all together.

To help on this a little, I did send him an email regarding GD3 and he replied very quickly and answered my questions professionally.

I'd like to see the results from the GD guys that tried or are going to try this.

Yep....
 

I just got back from racing my RAV4 at the Fall Nationals at Englishtown Raceway in my RAV4. It's late but I plan disconnecting the Fit throttle body tonight (After dinner) and possibly sending it out tomorrow (overnight). Will see how simple this is since I'm no mechanic. I should have taken a day off to make this happen more smoothly. Would love to have the throttle body back by Wednesday night.

He replied to me quickly twice already. I'm not concerned for gains nor losses. It's all good fun and exciting. This little Fit is getting quicker by the day. Will see!

Just what do you think enlarging the TB bore will help performance? you need to understand its only purpose would be to allow more air to flow but makes measurement at low speeds faulty and thus performance. But go ahead so we will be entetained by your results - if you tell us.
BTW have replacementOEM TB handy.

I am looking forward to entertaining the FitFreak Fit community. ;)

It was interesting the measurements you took on the TB especially to see 3 different tapers occurs. All and all going to 58 may help things since its not such a huge incase from the plenum.