Trials/road engine specification

[Howard Wright]

Hi All

I’m beginning to think of the engine build for the Brookfields special.  I’ve started a new thread as it may be of use to others contemplating this kind of engine.

The car will be used for vintage trials but will also need to earn its keep as a tractable road car. Most of all I want reliability.

So far I have collected a 1930 coil crankcase, block and 1929 3 speed gate change gearbox.  I have a good flywheel clutch etc although the ring gear will need replaced. I’ve also acquired a 1 1/2” billet crank and a set of Nippy rods which seem in good condition and fit together well. I intend to have these crack tested and balanced. I have a 1 1/4” SU and appropriate manifold and a bunch of bananas exhaust manifold. 

So here are some questions

What cam should I use? Could anyone suggesting alternatives to a standard cam please explain why.

Should I have the flywheel lightened?

I’m leaning towards the standard gearbox and back axle ratios but is this sensible?

Finally I intend using a DS4 or DK4 manual advance dizzy. Again is this sensible?

I’ve built two road engines in the past but would welcome any advice on this build.

Cheers

Howard

[Bob Culver]

I guess auto advance could be useful for trials where revs may drop way down and driver very busy, but I would retain the manual override whatever. Can then assess the suitability of the  advance curve and static setting. i found with road car with manual advance could often avoid a double declutch down change for momentary steep bit.

[Alan]

I wouldn’t be over ambitious in tuning the engine. For a trials engine I’d keep standard cam but with increased radius followers. I’d use Phoenix splash fed crank and new rods, but nippy ones are ok. I’d use a Ricardo head and that’s about all I’d do to the engine. I would however be using decent manifolds and a 11/4 SU carb. Really it’s all about gearing. I’d be talking to Andrew Bird about a lower first gear ratio and fitting a 5.625 rear axle ratio. But then it’s years since I did a trial. Perhaps Mr Jones knows better.

[Zetomagneto]

Hi Howard, Dave Dye is your man for a Ricardo head, and followers, I have had both from Dave at very reasonable cost,they are in, and it is on my newly built, as yet unfitted engine.

[Henry Harris]

Trials with a vintage A7 is all about gearing.
Standard 3 speed gearbox with a 4.9 rear axle is nbg for trials 1st gear 15.9:1 oa
Standard 3 speed gearbox with a 5.6 rear axle is nearly ok. 1st gear 18.2:1 oa
Standard 4 speed gearbox with a 4.9 rear axle is better. 1st gear 21.4:1 oa
You'll need as much power as possible from low revs. and an engine that runs perfectly. I use standard valve gear and high compression.

[Howard Wright]

Hi All

Just to say thanks for the advice.  I’ve contacted Andy Bird and Dave Dye and await developments.

Cheers

Howard