Keihin FCR 41 Carbs for Big Block

[gsxr1385]

Here's a question for those experienced engine heads on this forum. I am planning a 90 GSXR 1100L big engine build, either a 1342 (83mm x 62mm) or 1385 cc (Bore 83mm x  Stroke 64mm). This will have a heavily worked head as well (ported, big valves, cams etc.); everything in this engine will be heavily worked. I have all the parts ready to start, except those awaiting final engine specs.

 I think I'm settling on Keihin FCR 41 carbs. Picked up a set that has an intake spigot with an ID of 37.5mm, seems small for carbs this size. Per Keihin's charts this spigot was for a GSXR 1100 water-cooled. I found out these spigots will fit the stock 90 GSXR 1100L carb rubber boots (for a BST 36).

However a different intake spigot (33B) can be purchased with an ID of 40mm and will fit the GSXR 750 L&M carb rubbers. I know as I've done this before with a set of FCR 39 with intake spigots (21C) having the same dimensions as the 33b's.

The head will be ported to match the rubbers I use. So the question is what are the pros and cons of using the different size intake spigot / rubber combination? Smaller 37.5mm vs. larger 40mm with a head ported to match?

 

[Gixer1460]

I won't advise one way or the other except to say I have the big ones for a 1460 engine and using 1100 'M' rubbers when NA and 46mm Taper Throttle Bodies with EFI and turbo, Big motors have big lungs so shouldn't have to breathe through straws.

[gsxr1385]

Bigger venturi is the way I was leaning, but reasoned a smaller venturi could increase torque/horsepower at low and mid RPM's vs. larger venturi's (higher velocities better cylinder filling) but would starve at higher RPM's with too much pressure drop through the restriction.

Hmmm 1100 M rubbers - 46mm? May need to look into that...

That's one hell of a motor 1460 … and turbo'd whoa talk about arm stretching !

[Gixer1460]

8 hours ago, gsxr1385 said:

Hmmm 1100 M rubbers - 46mm? May need to look into that...

That's one hell of a motor 1460 … and turbo'd whoa talk about arm stretching !

Hmmm - confusion! 'M' rubbers won't take 46mm spigots - i've made tapered adapters from 46 to 40mm to suit the rubbers!

The motor was built for a turbo, lower CR, turbo cams and timing and yes it was an arm stretcher but it has no shortage of torque when used NA. - FCR's , Akro pipe & Dyna2000

[gsxr1385]

On 3/27/2020 at 5:05 AM, Gixer1460 said:

Hmmm - confusion! 'M' rubbers won't take 46mm spigots - i've made tapered adapters from 46 to 40mm to suit the rubbers!

The motor was built for a turbo, lower CR, turbo cams and timing and yes it was an arm stretcher but it has no shortage of torque when used NA. - FCR's , Akro pipe & Dyna2000

Thanks for the clarification … I read in the carb/rubber sticky that rubbers from 1100 M&N are 45mm ID (for BST 40mm?)… checked afterwards from parts suppliers  that the rubbers for the 1100 (USA) K/L,(BST 36), and  1100 M/N (BST 40?) are all the same so was confused. Guess I'll stick with 750 L/M rubbers.

Wish I could have watched your engine build!

 

[Jaydee]

22 minutes ago, gsxr1385 said:

I read in the carb/rubber sticky that rubbers from 1100 M&N are 45mm ID (for BST 40mm?)

Just after double checking a set of 1100 M rubbers, measured 45mm inner diameter. 

I have FCR 39'S to go with them. The spigots are marked 21C which correlate to Kehin code SKU 020-444. Could not be a better fit. Same sipot is used on Keihins for SV650's btw.