Earlystocks 748 carb choice

[R1guy]

Going to be running 748oil boiler in my Katana.

Earlystocks rules state I can either use up to 29mm flat slides or up to 36mm cv carbs with this engine/chassis combo.

Obviously 36mm cv carbs are just under 25% larger so what will give more top end power?- for a bike that will be ridden WOT for a large amount of its life, would I be better using vm29ss or a Mikuni 36cv?

Engine is a standard 748 oil boiler and I’ve got a choice of Micron or Yoshi exhaust system .

 

[bluedog59]

If the regs say "either/or" then you can bet they have researched that there is no clear advantage either way. Why not have both and swap according to circuit/weather ?

CV's maybe bigger but that throttle butterfly is always in the airflow, taking up space and disrupting flow.

[Gixer1460]

Maybe, but if that shaft costs 10% of the area, you are still up 15% BUT airspeed will be lower so giving a less 'punchy' response which is probably why racers like slide carbs? But Suzuki didn't fit 38mm CV's to the Race 750RR's for no reason - just sayin'!

[bluedog59]

That could also be true, it's one of those things where what may appear obvious may not actually be correct. Your point about the less "punchy" response of CV's is also important, when you start to make big bhp/ltr an engine will generally become more "peaky" and you may gain a little more bhp with slides but CV's make it more rideable.  Which leads to my question of having both types and swapping according to weather/circuit, you can bet the rule makers have done their sums and concluded there is little or no clear advantage over the season as the pendulum swings between advantage and disadvantage................................ unless you play smart and track that pendulum.

Rule 1 of building a race bike. Read what the rule book says.

Rule 2 of building a race bike. Read what the rule book doesn't say.  

Perhaps, if R1guy is willing, we could rustle up a set of each so he can dyno and track test both options 

[Sandman]

21 hours ago, Gixer1460 said:

Maybe, but if that shaft costs 10% of the area, you are still up 15% BUT airspeed will be lower so giving a less 'punchy' response which is probably why racers like slide carbs? But Suzuki didn't fit 38mm CV's to the Race 750RR's for no reason - just sayin'!

GSXR 750 RR (RK) - CV 40mm.

 

[Gixer1460]

I thought the 1100 M / N was the only bike with 40's?

[Sandman]

No, the GSXR 750 RR was also delivered with CV 40mm. GSXR 750 88/89 CV 36 mm, GSXR 750 90/91 CV 38mm.

 

 

 

[nopokewithoutsmoke]

Isn't a 36mm carb about 60% bigger in cross sectional area than a 29mm carb? Or have I missed something here...?

 

[R1guy]

•29mm has 660 square mmopening

•36mm has 1017 square mm opening

 

....if my maths is correct

Edited November 29, 2018 by R1guy

[Rene EFE]

@Leblowskiand myself do trackdays together, both with pretty much stock 749's. His is on 29 (33) Flatslides, myself on 36CV's. 

Power on the dyno is roughly the same but coming out of a slower corner, I'm fucking nowhere...

[bluedog59]

I assume you're saying that his 29's are bored out to 33 which would hand a "top end" advantage to the 36's however, I think the 29's (unbored) would still retain a lot of their advantage out of slow corners. Swings and roundabouts, just like I said earlier. The only way to sort it is dyno and track test both.

[nopokewithoutsmoke]

38 minutes ago, Rene EFE said:

@Leblowskiand myself do trackdays together, both with pretty much stock 749's. His is on 29 (33) Flatslides, myself on 36CV's. 

Power on the dyno is roughly the same but coming out of a slower corner, I'm fucking nowhere...

Hope it doesn't sound cheeky...but are you both about the same weight? And do the bikes have the same gearing?

[Leblowski]

2 hours ago, Rene EFE said:

@Leblowskiand myself do trackdays together, both with pretty much stock 749's. His is on 29 (33) Flatslides, myself on 36CV's. 

Power on the dyno is roughly the same but coming out of a slower corner, I'm fucking nowhere...

I don’t see my flatslides as a plus i cant just wank the throttle i have to rolllll the throttle on high revs and the bike is unforgiving when in wrong gear 

[Leblowski]

1 hour ago, nopokewithoutsmoke said:

Hope it doesn't sound cheeky...but are you both about the same weight? And do the bikes have the same gearing?

Dunno bout weight and gearing but i do know my bike has lighter wheels better suspension bla bla the bikes are not comparible 

 

[Leblowski]

2 hours ago, bluedog59 said:

I assume you're saying that his 29's are bored out to 33 which would hand a "top end" advantage to the 36's however, I think the 29's (unbored) would still retain a lot of their advantage out of slow corners. Swings and roundabouts, just like I said earlier. The only way to sort it is dyno and track test both.

My bike has been on the dyno we did 28 runs to get the bike as i wanted.

And yes my 29’s are bored to 33

Powerdelivery probably  makes a big difference compaired to rene’s bike also

[Rene EFE]

2 hours ago, nopokewithoutsmoke said:

Hope it doesn't sound cheeky...

I'm not fat 

[nopokewithoutsmoke]

• 

[Guest YoshiJohnny]

VM29s but bore them.... it's only cheating if you get caught... I am gonna bung a really good set on  e bay shortly as I need as much for them as poss..

 

[bluedog59]

On a bit of a tangent, has anybody thought of/ made a set of face plates (the bit you bored out ) for VM29's with a 3D printer ?  You could have 33mm and still be able to put them back to stock.