from my experience, 2 inch is good but for a hi po engine, 2.25 non mandrel with one muffler and a hot dog would be good, no droning and good flow (as good as 2 inch mandrel and about half the cost if the shop does it for you).

hmmm thanks everyone. no real consensus on what's best though! I had thought 2.25" exhaust. Kelford recommended 45s.
We'll see how it goes. A datsun book I have for the L-series recommends 3" exhaust for 4 cyl L series race cars. That made me think bigger might be a better bet. We'll see. It'll probably end up with whatever is easiest to get the car running again. I'm sick of having it parked doing nothing.

2" is too small for anything other than a standard A15. 2 1/4" is fine for good midrange power but for a full out 8000rpm A15 2 1/2 is best.
We actually used 2 1/2 on our best 1300 Rally motor because it was working between 5500 - 8500rpm.
Regarding the carbs, I would tend to stick with 40mm, unless again it is a full house 8000rpm monster. It will definately help the torque with the smaller carbs.
In regard to Dynos in the NI I can't actually recommend any in particular, Stirlingmac may be able to help there. My only comment would be to try to find someone that uses a "sniffer" type exhaust gas analyser to check fuel mixture rather than O2 sensors.
Hope this helps.

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

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'?)

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 .

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.

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...

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

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 ........