Intake manifolds less than 41mm

[Redog1]

Can someone wise help me understand why a 41mm FCR would be beneficial when the std intake manifolds are roughly 38+mm.
Logically the air must slow down when it enters the carby's larger throat?

OR

Are FCR devotees installing larger intake manifolds?

Appreciate your collective wisdom.

Cheers

[HansJ]

I am by no means an expert, so take it for what it is.

Consider valve size, still the same, but porting helps as well. A larger carb can produce more of correctly mixed air/fuel. You are right that the air passes faster in the narrower intake manifold, or ports, an effect that one wants, so that the cylinders can be filled in the best way. The "sucking force" from the downwards moving piston is the same, if everything else is kept constant. You want speed in filling the cylinder, but you don't want resistance.

Aside from this, a FCR has better inner aerodynamics, at least with fully open throttle.

[Kayla]

Can someone wise help me understand why a 41mm FCR would be beneficial when the std intake manifolds are roughly 38+mm.
Logically the air must slow down when it enters the carby's larger throat?

OR

Are FCR devotees installing larger intake manifolds?

Appreciate your collective wisdom.

Cheers

I think the fact you haven't got a throttle butterfly hanging in the way helps lots The difference in response between FCRs and CVs is one of the reasons, then add the extra torque, power and sound (ok, not a benefit as such, but it all adds to the ride!) I think the 39s would work better on a standard bike though.

By 'intake manifold', do you mean the rubbers between the carb and head, or the rubbers between the airbox and carb? I found no real step in diameter between the carb and head when I fitted mine, and I did check.

[eisi]

Hi,

I wrote mistake...

[eisi]

Hi,

38 or 39 are better for torque, 40 and 41 are better for the top line.
At higher speeds, the gas column becomes faster and thus the engine a higher charge. Thus, the compaction increases in the cylinder.

EISI, he has TDMR

[eisi]

Hi,

38 or 39 are better for torque, 40 and 41 are better for the top line.
At higher speeds, the gas column becomes faster and thus the engine a higher charge. Thus, the compaction increases in the cylinder.

eisi, he has TDMR

[Redog1]

The internal diameter of the aluminium intake is 38mm+.
The rubber between the carby and the aluminium intake is also about the 38+mm.
I understand the the lack of other obstructions to the air flow must help, BUT and this is my real confusion, if the carby throat is larger than the manifold, is anything really gained? Again because the air velocity must drop through the carby venturi if it is larger that the manifold.

Wouldn't a carby with the same diameter as the manifold make more sense?
ie the 38mm FCR rather than the 41mm?

OR am I missing something?

[HansJ]

I think you are completely right,or at least almost

My guess is that the 41s or 38s are almost as good for an otherwise standard engine, and the major advantage is the better internal flow of the FCR design, like Kayla says.

But, if you icrease the demand for air/fuel (volume/time), my guess is that the 41s will be better in providing it. But is complicated stuff, compare with putting on stacks with different lengths. Longer stacks, lower HP but better torque and vice versa. Smaller/bigger carbs will have the same effect.

I hope Quan will come in and enlighten us, i am just speculating, and he knows

[Kayla]

The internal diameter of the aluminium intake is 38mm+.
The rubber between the carby and the aluminium intake is also about the 38+mm.
I understand the the lack of other obstructions to the air flow must help, BUT and this is my real confusion, if the carby throat is larger than the manifold, is anything really gained? Again because the air velocity must drop through the carby venturi if it is larger that the manifold.

I think I understand what you're getting at. It's down to how accurately the FCR can meter the fuel I think, and the fact the response is so crisp (as long as you don't just whack the throttle open!) instead of CVs that always seem to be half a step behind the throttle, at least that's how it felt to me after riding a CV carbed TRX recently. I wonder how good a 35mm or 37mm carb would be on a standard bike? I'd bet loads more midrange and similar top to the 41mm carbs.

[Redog1]

So where did we end up?

FCRs are best because of their mechanism

41mm, being smaller than the intake manifold, MAY not yield any benefit over a 39mm.

Does anyone out there really really know?

Thanks for everybody's thoughts.

[Mincehead]

I opened my intake stubs to match the FCR41`s bore, a completely stock motor and swap from stock CV`s to the FCR`s gave me 11 bhp, so I was happy with that.
The dyno that my TRX was put on is well known for it`s accuracy around my neck of the woods too, as for which are best, Mikuni`s or Keihins? I didn`t care to bother trying any kind of comparison as the FCR`s came with bags of jets at a reasonable price.

[Redog1]

I opened my intake stubs to match the FCR41`s bore, a completely stock motor and swap from stock CV`s to the FCR`s gave me 11 bhp, so I was happy with that.
The dyno that my TRX was put on is well known for it`s accuracy around my neck of the woods too, as for which are best, Mikuni`s or Keihins? I didn`t care to bother trying any kind of comparison as the FCR`s came with bags of jets at a reasonable price.

When you say "opened my intake stubs" are you referring to the aluminum or the rubber or both?
If you opened up the rubber, how did you do that without tearing it?

Cheers

[HansJ]

I have the TDMRs, 40 mm, and did the same, both the aluminium inletstubs, the rubber (and the head). The rubber is fairly easy to fix, it's a small flange between the stubs and carbs that has to be cut down. The rubbers look like a T if you cut them through, and it's only the lower part of the T that is modified.

[Redog1]

I have the TDMRs, 40 mm, and did the same, both the aluminium inletstubs, the rubber (and the head). The rubber is fairly easy to fix, it's a small flange between the stubs and carbs that has to be cut down. The rubbers look like a T if you cut them through, and it's only the lower part of the T that is modified.

OK, so did you just carve the rubber away with a knife, or did you do something more sophisticated?
And with the aluminum, did you just rat tail file the inside or did you have it machined?

Cheers

[HansJ]

The stubs were done by the guy who ported the head (involved some building up of material as well), but i guess you could do it with a Dremel, smoothening down the sharp edges. I did the rubber with a small sharp knife, and a dremel.