Bike carbs vs bike itbs

I guess this would be a learn me of sorts. Im trying to decide on running 6 bike carbs or 6 bike itbs on my datsun L motor.

I feel like the carbs would be easier to setup in the short term but the itbs would offer more in the long term. Am I looking at things correctly or will they be similar?

If you think ITBs are hard to set up, you really won't like multiple carbs. ITBs for sure.

Getting 6 carburetors synced, sounds like a short trip to the funny farm. Try Russian Roulette, it will be much less painful.

They are essentially the same thing.

Both require careful balancing and tuning, whether that tuning is performed via jets, etc. or via adjustments to your chosen EFI ECU.

Throttle bodies tend to flow a bit more since they don't require chokes, etc. like some bike carbs. So you can potentially run a slightly smaller ITB than you would a sidedraft or Bike carb.

On my car with ITBs I found that they were highly sensitive to underhood heat, so a plenum and fresh, cold air is a requirement for anything other than a dyno queen.

If you can change the length of the intake trumpets, you can help reduce the loss of low end power many run into with multiple carbs or ITBs.

If you chose to go with ITBs, I'd get the car running and driving well on a single TB before converting to the ITBs, I wish I had since I had a lot of trouble sorting out which issues were due to the EFI conversion, the ITBs, etc. I highly recommend using at least a MegaSquirt 2 with the Extra code, since that provides an ITB-mode option in the code that will help with low speed driveability, which can be a common issue with ITB-equipped cars.

Good luck and I hope this helps.

In reply to Stefan (Not Bruce):

Thanks for the hints. As of now Im leaning towards the itbs because Im more comfortable around efi than carbs.

Why would you want to install 6 carbs on something that can be fuel injected with OEM parts and a MegaSquirt?

A single throttle body (no balancing) and a long runner intake tract....

Reliability, performance, ease of adjustment, simplicity, the deck is stacked against carbs. And the 6 carb cool factor is replaced by "Yes, I built my own EFI"

Similar CFM on bike carb to stock carbs?

I did ITBs on my challenge car, I was surprised it wasnt that hard actually. Im using MegaSquirt 2 running Alpha-N per the advice given to me on this board. Im using ITBs from a triumph TT600 in my opinion they are the best for this sort of thing since they are just four separate thottle bodies bolted together no ports or injector holes, its crazy easy to add/bolt two extra thottle bodies in the middle.

I converted my 4AGE to GSX-R ITB's and Megasquirt II, and it fired right up and was driveable in short order.

You want to tune Alpha-N. Don't even kid yourself to try and make Speed-Density work. I thought I could, and I am very stubborn.

I have since switched to Megasquirt I (MSII will be used on another project that needs the extra features), still Alpha-N, and it still works dandy.

I noticed no loss in low-end power at all. I felt the mid-range was stronger. Top-end seemed about the same, but I could be wrong. Throttle response took some getting used to.

I have no fast-idle circuitry, and it still idles just fine when cold (run a bin below idle speed with more spark advance, so as it starts to stall the timing advances - works great).

I'd do bike ITB's again in a heartbeat - and I have another engine waiting in line (but I need bigger ITB's for the 2.0L).

Bike carbs are missing the low speed jets i believe. Maybe wrong there but i know there is a circuit they are missing compaired to car carbs that makes them less street friendly

Looks like ITBs are the ticket for what Im wanting to do.

Ive seen mention of using R1 throttle bodies in addition to those already mentioned in thos thread. Anybody have anything to say about those?

Echo, Skinny - please tell us more. In your opinion, is any of this a feasible alternative for a street car?

In reply to nderwater:

the car drives great low end is just like stock, above 3500rpm it screams! the only thing is that the throttle is twitchy because I still have the small throttle wheel thats for a motorcycle,(small amounts of foot travel equals large throttle opening) so in stop and go traffic its annoying but I drive it almost every other day. its an easy fix that Im currently working on

For throttle response, you need to linearize the opening of the valve to the flowrate.

If you want the linear line and the throttle bodies give you the 'quick opening' line, you need the input from the 'equal percentage' line. If you were doing drive by wire, you could just program it, but if your using a cable you need a progressive throttle quadrant.

In reply to echoechoecho:

Any chance you can furnish me with the bore spacing on those TT600 throttle bodies? I googled really quick, but didn't come up with anything.

In reply to echoechoecho:

How did you deal with adding injectors since these lack bungs for them?

In reply to Fitzauto:

They are likely on the opposite side in that picture, since he used the bottom of the stock manifold.

Unless the bungs are in the head.

The injectors are on the cylinder head.

Burrito wrote: In reply to echoechoecho: Any chance you can furnish me with the bore spacing on those TT600 throttle bodies? I googled really quick, but didn't come up with anything.

The smallest part of the bore is 34mm if I recall, but if you mean the distance from center to center of each bore, Ill have to get back to you on that.

I am running a 44mm CV carb on my samurai and it is overkill on a stock 1.3 since the heads flow so badly anyway. And yes the throttle is usually really touchy with a bike carb as iirc most bike throttles are sprung. I am likely going to add a spring to mine just to make it less bouncy if you go over a bump. It will start bucking because of how stiff the suspension is.

nderwater wrote: Echo, Skinny - please tell us more. In your opinion, is any of this a feasible alternative for a street car?

My ITB's are on my street-driven Locost Super 7. Touchy throttle, but I got used to it, and while a better cable wheel would make it that much easier, it's fine. I also have a lightened flywheel, which adds to the fun.

I did a (not-detailed-enough) write up about the conversion here:

http://www.gwellwood.com/project-cars/the-lethal-locost/lethal-megasquirt/

Things I learned:

Place the IAT sensor carefully. My first location sucked. Currently it's much better (heat shielded from the engine heat).

Alpha N (I cannot stress this enough) - it's SO MUCH more driveable than SD.

I would do this again in a heartbeat. It was SO worth it. And yes, I drive this on the street, in traffic, on the highway, everywhere, and I am ~very~ fussy about tuning and making it just plain WORK. It works!

I feel like if I tried using ITBs on a car engine I would build a big plenum/airbox and use a MAF based tuning.

In reply to echoechoecho:

Feel like that would be a bit small for a 2.4-2.8 liter engine

In reply to Fitzauto:

thats .4 liter per cylinder, my car is .375 liter per cylinder.

idk if Im right but I think having small bore ITBs is a good thing because it keeps air velocity high even at low rpm, yet if you add the total cross section area of all the throttle bodys its still way more than a reasonable sized single throttle body thus flowing more cfm

My main knowledge about MC carbs on a car is via Spitfires and a fellow named Chris Cancelli who has offered Keihin carb kits for 4 cylinder and 6 cylinder Triumph engines for years. He has apparently done quite a bit of work over the years coming up with a base tune that will allow the carbs to work with the generally lower RPM range of a car engine. What he actually does is somewhat guarded for obvious reasons (once you see the price of his kits vs. sourcing the carbs yourself). So don't bother asking him. While I have no personal experience, his reputation in the Spitfire world is somewhat sketchy with angry tales of money sent and parts not received... That said, I actually have seen some of the kits in person and installed on cars, so I know they can work and work well.

From what I've gathered over the years, the rough rule of thumb is to get carbs for a MC engine about half the size of the car engine. So for a 1500cc Spitfire, one would get carbs off a 750cc MC. The general reason for this is MC engines generally run at about 2x the RPM of a car engine, so the total air flow is about the same.

Otherwise, I have to ask what is the goal and/or reasoning behind wanting ITB or MC carbs? Other that the fact that look really cool. I have some friends who have built EFI systems for Triumph engines and they've used off-the-shelf (junkyard sourced) throttle bodies mounted to a custom manifold with injector bungs (google: FIS6) run with MegaSquirt. Seems to work pretty well.

In reply to Ian F:

The goal is to be different while allowing the engine to breathe better. Im wanting to eventually open a business doing more off the wall type mods so I figured this would be a good start.