Motorbike Carbs

[zep]

I'm trying to figure out what the maximum capacity four-cyclinder you can rock motorcycle carbs on is.

I've seen plenty of 2lt Zetecs and the like with them, but will something like R1 carbs be suitable for something around 2.5 lt, or are they going to be too small?

Cheers

[Spencer]

depends on the enigne, how big are the ports? and how much air it eats, what size are R1 carbs in the first place? 45mm?

[isnowi]

It's all down to airflow, the gsxr750 carbs i had on my 1300 polo were way too big, the gsxr made around 112hp at 11000 rpm, they were only a 36mm throat.

I have seen writeups that recommend r6 carbs over the r1 carbs for engines up to 2 litres, the increased air speed helps with low down driveability i think, obviously for a race motor things would be different.

[zep]

depends on the enigne, how big are the ports? and how much air it eats, what size are R1 carbs in the first place? 45mm?

I think the R1 carbs are 40mm. The port sizing on the engine I was thinking about is 35mm.

[Testament]

16v engine or 8v?

vauge comparision 2L twincam 8v with 10/11:1 compression typical carbs for 140-180hp (depending on cams/exhaust/porting) would be 45DCOE webers with 36mm chokes port size in the head being around 35mm with a 43.5mm intake valve.

What type of carb are the R1/R6 carbs? slide throttle or some variable restriction things kinda like SU's in principle?

[zep]

8v.

No idea on what type of carb they are, I don't know a whole lot about them. Retarded, yes.

[peteretep]

r1 carbs are constant velocity (CV) type

[Steelies]

yes yes excellent more info johnny5 need input!

[Testament]

8v.

No idea on what type of carb they are, I don't know a whole lot about them. Retarded, yes.

What motor is it exactly? the more info the better, valvtrain, bore, stroke, rod length, intake and exhaust dimensions and volumes, type and lengths/sizes of extractors, what cam specs etc. etc. how high is it going to rev, what is the intended use of the vehicle etc.

[Spencer]

^ yea what he said, need more info on the engine

[zep]

Its the Isuzu 4ZE1 2.6lt 4cyc found in Jackaroos and Rodeos.

Power: 89.5 kw @ 4600RPM (with standard EFI)

Torque: 146 ft.lbs. @ 2600 RPM

Bore x Stroke (in.): 3.65 x 3.74 (92.6mm x 95mm)

Compression Ratio: 8.3:1

Intake valve: 47mm (1.85")

Exhaust valve: 42mm (1.65")

"4ZE1 conrod is 149.8mm centre to centre"

It will have 4-1 extractors, I will measure the length later/tomorrow.

Ideally it will have a cam regrind to get peak power up a bit higher.

Perhaps a head skim to up the compression (I don't want to have to have custom lumpy pistons made).

Some porting, perhaps.

Just playing with some ideas for a street car to get going next year when I've got the cash to sort out a more interesting daily.

Cheers

[peteretep]

checked my r1 carbs, they are 40mm

[isnowi]

For a street/ road car , and an engine that doesn't appear to be overjoyed about revving, i personally would err on the smaller side of things. It's probably already got fairly small ports judging by the rpm that the torque peaks at, mind you it could be designed to be like that so that it can haul the lardy jackaroo body around. Should shift quite well in a small car.

With the information you have and an idea of further work you will do you should be able to work out the airflow you will need for the power you hope to make

ie

VR = volume displaced per revolution

VE = volumetric efficiency .9 for 4 valve heads, i've been told .85 for 2 valve heads

PR = Pressure ratio 1.0 for non turbo

thus Airflow = 158cu/in X 4600 rpm X .5 X .85 X 1

__________________________________

1728

= 178 CFM therefore for the motor in it's current configuration

R1 calculation:

Airflow = 61cu/in X 10000 X .5 X .9 X1

_____________________________

1728

= 158 CFM, so not too afr away i reckon.

[Ogre]

hmm, interesting.Ive done the same calc with my AMG engine, turns out its about 190cfm.Whats that gonna be like with the R1 carbs?Ie should the carbs be abit less that the cfm of the engine?Wheres the point at which the engine will become under carberated?

[isnowi]

hmm, interesting.Ive done the same calc with my AMG engine, turns out its about 190cfm.Whats that gonna be like with the R1 carbs?Ie should the carbs be abit less that the cfm of the engine?Wheres the point at which the engine will become under carberated?

It's well documented that the 4g63's have ginormous ports and can flow stupidly large amounts of air. That calulation only shows the cfm based on the port size, valve sizes also influence things as well, perhaps it's worthwhile looking at what the motor is allowed stock (TB size ) and working from there. Going smaller than required airflow will harm maximum potential power, but it will increase low down response in the form of an earlier torque peak. All of these calculations are based on best case for maximum power...

My recopmmendation: suck it and see, it's more fun that way..

[zep]

isnowi, this information is really intersting.

As I said before, ideally I'd like to be making peak power at a higher rpm (mostly for drivability) and hopefully I can do this with a cam grind.

I'm looking at your calculations, etc, but am still unsure of the sizing match between engine and carbs. The cfm on the R1 is lower than that of the 4ZE1, but this does not mean that the carbs will be undersized, does it? The reason I'm a little worried is that I've seen them being used on 2lt engines with good results, adding 600cc could perhaps make it undercarbed.

If I can replicate the stock power and torque figures, that would be excellent. I think that I should be able to buy and set up the carbs and manifold for cheaper than I could wire in and EFI system to power this engine.

[H05TYL]

hmm, interesting.Ive done the same calc with my AMG engine, turns out its about 190cfm.Whats that gonna be like with the R1 carbs?Ie should the carbs be abit less that the cfm of the engine?Wheres the point at which the engine will become under carberated?

It's well documented that the 4g63's have ginormous ports and can flow stupidly large amounts of air. That calulation only shows the cfm based on the port size, valve sizes also influence things as well...

The early 4G63's huge ports are designed to work with the Dual runner (cyclone) manifolds, if your not going to use the stock manifold you'd be much better off with a later small port head (evo 1-3, possibly later RVR's, etc)

[Spencer]

While Isnowi's calculations are good, the are simplified and only good for a rough guide. The volumetric efficiency of your engine compared to R1 bike is in no way going to be only 5% at peak power. The R1 will be close to 100%

I would say you may be fine with R1 carbs zep, but they are leaning towards the to small side. I say this without seeing the ports and knowing much about these engines,

For reference for a decent breathing 2l 4cly there's glenn who has the wild 3sge beams, he did a dyno comparison on a stock 3sge beams with several of his throttle bodies he makes. I think it was blacktop throttles (45mm?), 48mm's and 50's. The 48mm's made more power all over the rev range than the 45's and the 50's made more power after 7600rpm. So go too small for your engine and you will loose power everywhere, not just up top. The beams has massive intake ports though, to the point where 45's are probably smaller than the port, which would cause a velocity drop at the port. I think that info's on toymods somewhere.

Anyway CV carbs are gay get 4 larger flat slides

[zep]

Intake

Exhaust

Not that that is much help. I guess the only real way to find out if this is going to work is to try it then eh.

There is a pretty worked one in Aussie (knife-edge stroker crank, longer rods, custom forged pistons, 12:1 comp) running twin 48s that made 190rwhp. Obivously I don't want to be going down this route because it's where it starts to get expensive. Carbs, cam and extractors are basically where this is heading.

What is the smallest point in diameter of 48mm IDAs?

[isnowi]

What is the smallest point in diameter of 48mm IDAs?

The smallest point will be whatever size venturis you use.

What i posted above in regards to airflow is all purely hypothetical, as spencer pointed out. 2 of the figures i used were assumptions, volumetric efficiency, and rpm. Things like port size and shape, included angle of the valves, valve shrouding, extractor design, will all affect the volumetric efficiency. I posted just to give you an idea of what the R1 carbs are designed to support. From my own experience, CV carbs are very adjustable, and even being significantly oversized a reasonable result can be acheived, if you have the money Flatslides are even more adaptable.

My 2 cents: if it was my motor and i was doing this i would be looking for something between 40 and 45mm throats, and i would be wanting to up the compression ration significantly as well. But that is basedo n nothing more than my own personel experience with bike carbs and gut feeling in regards to what size they should be.