Are bigger carbs better?

**nzspokes** · 24th May 2012, 20:17

Just mucking about looking at the spec of my GPZ400 and the same year 600. The 400 has 30mm carbs and the 600 has 32mm. Jet sizes etc are pretty much the same, even use the same needles.

So would running 32mm carbs on my 400 give me more power? Or will it kill an torque I have?

**pete376403** · 24th May 2012, 20:27

If the ports dont match up to the bigger carb then its probably going to be a wasted effort. Maybe if the ports and valve sizes are increased it might do some good way up at the red line but low and midrange power might suffer accordingly.
I'm of the opinion that the engine designers usually know what they are doing.

**nzspokes** · 24th May 2012, 20:30

Interestingly the rubber spacers between the carbs and head are the same part number.

**bogan** · 24th May 2012, 20:38

No, bigger carbs will have a lower air velocity, therefore less vacuum through the venturi, which will prevent them from getting fully on the main jet. Which will then make em pretty hard to tune! And you would need to re-tune.

The only way (assuming both are tuned properly) carbs will give you more power, is if the offer significantly less flow restriction on the intake side, allowing more gas to enter the cylinders, as far as I'm aware, the intake length plays a bigger of a role in this than carb diameter.

**98tls** · 24th May 2012, 20:42

If you do nothing else then theres little point,bit like those that spend big $ on an exhaust without doing anything else then more on rocking up to a dyno expecting a revelation.

To stupid to be called idiots.

**nzspokes** · 24th May 2012, 20:49

Originally Posted by 98tls

If you do nothing else then theres little point,bit like those that spend big $ on an exhaust without doing anything else then more on rocking up to a dyno expecting a revelation.

To stupid to be called idiots.

My exhaust is far to big for the motor.

**bogan** · 24th May 2012, 21:01

Originally Posted by nzspokes

My exhaust is far to big for the motor.

So you'll be losing a bit of power there anyway. Stay with the current carbs, and if you want more power, do a proper tuned exhaust, then tune the carbs to suit, and maybe go to pods if your airbox is restrictive.

**nzspokes** · 24th May 2012, 21:17

Originally Posted by bogan

So you'll be losing a bit of power there anyway. Stay with the current carbs, and if you want more power, do a proper tuned exhaust, then tune the carbs to suit, and maybe go to pods if your airbox is restrictive.

I think my air box is fine apart from the inlet size. Ive removed the snorkel and thats given her more low end. I would like to try making the inlet bigger but would want to measure holes as its no point making the inlet bigger than the carb sizes. The exhaust is the exactly the same as the 600. Its got 4 into 2s with big cans but with a small outlet. If I was to change it I would go down to 1 good muffler.

I have been offered a full stainless Motad system out of a ZZR600 i think it is. He thinks it will fit but im not sure.

**FJRider** · 24th May 2012, 21:44

Regardless of the size of the exaust or carb ... the fuel/air has to get past the valves .... and the trick to more power is getting more fuel/air past the valves when they are open (to get a bigger bang). Either by forced induction, enlarging the valves, or extending the length of time the valves are open.

**bogan** · 24th May 2012, 21:45

Originally Posted by nzspokes

I think my air box is fine apart from the inlet size. Ive removed the snorkel and thats given her more low end. I would like to try making the inlet bigger but would want to measure holes as its no point making the inlet bigger than the carb sizes. The exhaust is the exactly the same as the 600. Its got 4 into 2s with big cans but with a small outlet. If I was to change it I would go down to 1 good muffler.

I have been offered a full stainless Motad system out of a ZZR600 i think it is. He thinks it will fit but im not sure.

You might want the inlet a bit bigger than your carbs to account for more turbulence in the airbox, but position probably plays a part too, getting/removing resonances at different RPM.

Header diameter is not a bigger is better deal, you want the correct size to achieve the best air velocity at max RPM.

Rule of thumb calculation I've found online is: Area of Primary Pipe (sq in) = RPM × Cylinder Size (ci) ÷ 88,200
so 12000x6.1/88,200 gives 0.83 sq in, or .51in radius, or 26.1mm

Having 600 pipes will mean the gas travels at 2/3rds it optimal speed at max rpm, so your scavenging isn't going to work as it should; less flow velocity, is less flow inertia, is less new air pulled through, not to mention the effects from having the air column out of time for the header length.

This is why I laugh at the boyracers who put 3in exhausts on their mazda starlets and end up being passed by scooters and old men with zimmer frames. It's probably also the reason why there is the myth that backpressure is required for a motor to produce max power.

**nzspokes** · 24th May 2012, 21:52

Originally Posted by bogan

You might want the inlet a bit bigger than your carbs to account for more turbulence in the airbox, but position probably plays a part too, getting/removing resonances at different RPM.

Header diameter is not a bigger is better deal, you want the correct size to achieve the best air velocity at max RPM.

Rule of thumb calculation I've found online is: Area of Primary Pipe (sq in) = RPM × Cylinder Size (ci) ÷ 88,200
so 12000x6.1/88,200 gives 0.83 sq in, or .51in radius, or 26.1mm

Having 600 pipes will mean the gas travels at 2/3rds it optimal speed at max rpm, so your scavenging isn't going to work as it should; less flow velocity, is less flow inertia, is less new air pulled through, not to mention the effects from having the air column out of time for the header length.

This is why I laugh at the boyracers who put 3in exhausts on their mazda starlets and end up being passed by scooters and old men with zimmer frames. It's probably also the reason why there is the myth that backpressure is required for a motor to produce max power.

Problem is that they only made one system for the GPZ. 400/600 has the same. I could just run my system into one muffler, that wouldnt be hard to do. I may try lagging the pipes. It worked well on my honda.

**FJRider** · 24th May 2012, 21:53

Originally Posted by bogan

This is why I laugh at the boyracers who put 3in exhausts on their mazda starlets and end up being passed by scooters and old men with zimmer frames.

But ... the sound you hear from them is a cool echo ... that's gotta be worth another 1.5 Hp ... and doubles the value in parts of the car.

**bogan** · 24th May 2012, 22:08

Originally Posted by nzspokes

Problem is that they only made one system for the GPZ. 400/600 has the same. I could just run my system into one muffler, that wouldnt be hard to do. I may try lagging the pipes. It worked well on my honda.

yeh that should help, keeps the exhaust temp up, which raises the velocity, and opening up the muffler should do the same.

Originally Posted by FJRider

But ... the sound you hear from them is a cool echo ... that's gotta be worth another 1.5 Hp ... and doubles the value in parts of the car.

and if they use stainless pipe, it quadruples the value! have you seen how much the pay for stainless scrap metal now

**nzspokes** · 24th May 2012, 22:23

Only problem with lagging is the pipes start behind the radiator and the wrap makes my hands itch like a bastard even with gloves on.

**carburator** · 25th May 2012, 02:50

the 30 in theroy will flow 82CFM the 32 97CFM

from experience I have a pair of 39FCR's on the bike
Ive had 41's FCR's on there ( tuned ) the 39's gave a better
range of power also better seat of the pants feel too.

better to have a broader range than try and live in a small powerband.