Hi guys, can someone tell me what, if any, is the advantage of running 45mm carbs over 40s, if the venturi is only 32mm? I've changed to 45s, but am wondering whether it's really better than just getting new chokes and jets for the 40s?
And where can I get a good dyno tune in the NI and how much should I expect to pay?
And on another note, what size exhaust do people run? Mine has always been 2", is this big enough? Engine is 1570cc A15, 11.75:1, ported head with std valves, kelford cam can't remember specs but works between 4&8000rpm, 45s, coby extractors.
Cheers fellas

i'm pretty sure the velocity through the 45's wont be high enough to have them running at there peak performance, On your A15. Torque performance has a good reputation for dyno tuning, as per the boyracer video "HighOctane". I think 2-2.25 inch is far better to run, If you run too big you loose back pressure which gives you mid range torque, the dyno people would be able to recommend the correct size from their readings

I have to agree with the exhaust. 2" seems to be just right. We've got similar engine builds and I run 45mm carbs (no chokes though and a different design where it's actually 45mm venturies). I thought for sure I'd need smaller but Matsuoka, THE A-series tuning shop here in Japan asked for all my specs and recommended 45s and jetted them appropriately.

A good 2" exhaust system with no crimped Bends are the GO !!


A 42mm Dcoe weber ( yes 42s are RARE) with 32 mm chokes will flow more than a 45mm with 34mm chokes .. IF 35mm chokes are used the 45 starts to out flow a 42mm DCOE...
I would be using twin 45`s wITH AT LEAST 36mm chokes if your power band is 4000rpm to 8000rpm .. you will notice more top end over 6000rpm ..if you have a good C/head and You want more top end power ........

consensus says i'm putting a 2" exhaust on my A15 with twin 40mm DHLA's then

you all say 2inch system but dont say how many mufflers..

Just for a comparison..

Mates L20b

Internal and Headwork unknown.
Crow Cam Stage 3
45mm webbers.. Not sure what chokes.
Genie Extractors
2inch exhaust with 1 muffler before diff.

Works a treat.. so i dont know if 45's would be too big for a A15 but if they are choked down too 36mm they are really only 36mm's then...

I would go with at least 2.25" exhaust, unless the 2" is mandrel bent all the way back. If you go with 2" non mandrel bent like most exhaust shops make, the kinks and bends are going to restrict to way less than 2", probably closer to 1.75" or worse. This is why I chose to stay as big as I could at the time on mine and went with 2.25". I would've gone 2.5 as this is the size of my collector on the header, but the exhaust came before the header did so I stuck with it.

45mm relates to the Butterfly size ..Not choke size .. it is not possible to get a 45mm choke in a 45mm Dcoe ... as the choke is a venturi shape to work .

My 45s:




You can see how there's no choke it's just 45mm straight through with sliding valves instead of venturies.

My 2" exhaust:







You can see how straight it is and it's got one muffler plus an empty catalytic converter. Not sure how legal this is there but if your vehicle is low enough you can get it really straight like mine by going under the diff. No clearance problems yet but mine'll be lower than it is now when it's all said and done. (who am I kidding, when are these things 'done'?)

42's are #### on a small motor

had em, hated em

can fix em but not worth the money to do it cause its not certain to fix the problem, better to use 40 or 45s