Carburettors for B18 + K Cam ?

[sleek lemur]

Hi everyone,

I'm currently rebuilding a B18A and looking to uprate it to compression ratio of 9.5 (maybe 10.1) /1 (following advice on posts in this forum), with a K cam.

What carbs would you suggest for this engine?

Any other comments welcome!

[classicswede]

keeping single carb or going twins?

[sleek lemur]

Cheers, CS.

I'd imagined that it would need twin, but really, whatever would work and whatever I can get hold of. Presume will need to source inlet manifold as well, as have standard one.

This is a fun project for me for over the winter (or however long it takes), so ideally would look for something which needs work and (obv) for which parts or available.

[Vintagewrench]

Twin SU's in excellent rebuilt condition with KD needles works very well on a 10:1CR B18, and was standard equipment on B18B's in later 1800s, and 123 GT's. Its what we are presently using on our 1800s racing and road car B18B and very pleased with the combo.

[Ron Kwas]

SL;

Indeed...as VW notes, the twin SU is a very respectable setup, including for motorsport...and it's factory original!

...a very nice pic of the SUs btw!

Cheers

[sleek lemur]

Many thanks, Vintage Watch. What size SU do you use?

[142 Guy]

I am not particularly knowledgeable about carburetors (other than the Mikunis and Keihins on motorcycles); but, once you have changed the compression ratio and the camshaft you no longer have a B18A. If you want a low risk road map to follow, you seem to be well on your way to transitioning to a B18B. If you follow that road map twin SUs would be the path.

The only concern I might have is the K cam. I have a B20E with the D cam and the K cam is the later post '72 emission control evolution of the D cam for use on the later D jet and K jet systems. It has a nudge less overlap than the D cam. From first hand experience, with my B20E idling around 875 RPM the manifold air pressure is a relatively high 60 kPa. The K cam with slightly less overlap may idle with a slightly lower MAP. The later B20F engines are reputed to have a smoother idle than the B20E.

My limited second hand knowledge of the SU is that they are a bit sensitive to manifold vacuum (more being better at idle). The SUs will probably work with the K cam. Compared to the original B18B cam, the idle may not be so smooth if that is of concern to you. If you crank the idle speed up the MAP drops (vacuum increases) and the SUs may work just fine.

[sleek lemur]

Hi 142,

I am basing my initial selection of cam on what Brookhouse recommend. Thank you for your detailed message; I'm doing this purely for the fun of it and should increase my understanding of the science behind the cams so I can understand the science behind any recommendation.

Yes, I am aiming for a B18B spec, 123GT, optimised with our 60 year knowledge of the engines. I bought a B18A because it was cheap and the seller seemed like a thoroughly decent fellow (Member "Old Fart"!). unless I find space to restore a complete car, will probably sell the completed engine.

Right now, I have given it a preliminary clean and torn it down, so I have quite a way to go before I buy a camshaft.

I intend to follow the 6 P's.....

[Burdekin]

HS6 are the standard twin SU size/type used.

[Derek UK]

123GT spec used a C cam. That was their best standard cam with carbs. D came along with the D-Jet F.I. and the K with the follow up K-Jet. K cam design slightly improved emissions, the regs for that getting tighter at the time. Very slightly less lift than the D and with less aggressive lift pattern. It's reputed to run slightly quieter and smoother than the D. Both perform better than the C cam.
If the smoother quieter running is true it may be a better choice as on the road I doubt if you'll notice much difference in normal day to day driving. Not saying that you couldn't tell the difference if you back to back tested identical cars whose only difference was the camshafts. Both have their fan clubs.