shortened jackshaft

[tonyabacus]

As always the forum has generated a number of responses and for that many thanks for all the contributions.

Why fit a shortened jackshaft was asked, as my engine is apart I am looking at as many opportunities to make it better when rebuilt, so its not just about a jackshaft, that being one line of enquiry. As we all know the forum has members with many different experiences that we can all tap into which was my intention. Now having seen some of the responses I am better placed to approach one of the specialist engine builders and ask relevant questions, but with a small amount of background information gained from the forum and other avenues of investigation.

If this is a known modification then it has been done and repeated as a worthwhile modification, but what are the advantages or is this purely a gain for racing engines or does it have any value in ordinary road car builds is what I was trying to establish.

I also guess that like many of the forum members we have an opportunity to look and learn from one another, and curiosity as they say broadens the mind.

[rgh0]

For race engines where you are chasing every fraction of a HP at the back wheels reducing even the small drag from the rear cam bearing is worthwhile. Eliminating every gram of rotational inertia and imbalance even in a half engine speed shaft is also worthwhile. Not wasting oil lubricating a bearing you don't need also saves power and reserves it for the bearings you do need..... its all the details.

How important or even noticeable all this is for a road engine is debatable but it is certainly less important than in a race engine. However it is a low cost modification that removes one bearing and the small chance of failure of that bearing so it is a reliability improvement no matter how small...... Even for a road engine its all in the details even if those details are just for personal satisfaction

cheers
Rohan

[HCA]

I have not seen this written anywhere, but for me, the important things on my new engine are balance and zero oil leaks. So anything that reduces oil splshed where it is not necessary to have oil is a bonus.

No fuel pump cam lobe is my reason - a) no splash and b) jack shaft in better balance….

[2cams70]

The main question you need to ask yourself on a road engine is whether or not you’ll ever decide to go back to a mechanical pump. With a shortened jackshaft of course you can no longer easily do that. Some on this forum are in the 100% originality camp. To a large extent I agree because the more you delve deeper into things you come to realise why things were designed the way they were back then and that simple is often best.
Other’s may have comment on the reliability and any issues associated with the commonly available aftermarket electric pumps. I’m in the process of pondering this subject myself also as there’s so many to choose from - even from the same manufacturer!

[normanjsmith]

I didn't shorten the jackshaft but did machine off the unwanted lobes to improve the aesthetics.

[HCA]

The main question you need to ask yourself on a road engine is whether or not you’ll ever decide to go back to a mechanical pump.

I think I can safely say, on behalf of all electric pump converts, this will never happen

Other’s may have comment on the reliability and any issues associated with the commonly available aftermarket electric pumps. I’m in the process of pondering this subject myself also as there’s so many to choose from - even from the same manufacturer!

Older SU pumps posed the odd problem, but modern pumps are pretty foolproof especially the better ones from Facet or Holley. Of course with electric pumps - if you are worried about reliability - there is the luxury that allows a second pump. And if aethsetics is your thing, an in-tank pump - fit and forget

I didn't shorten the jackshaft but did machine off the unwanted lobes to improve the
aesthetics.

And you reduced weight

[2cams70]

As far as electric pumps are concerned facet for example offer at least three different types . Pro’s, Con’s of each? For me at least the minimum criteria are:
Fuel pressure for Webers correct out of the box. To me having a separate fuel pressure regulator should be superfluous and completely unnecessary.
Just one pump. No other lift pump - nothing else
Enough flow rate capacity for at least 150 - 180HP.
I know Huco’s have been talked about positively but Facet is the big player and they offer many types for some reason that would all seem to do the same job equally well. I don’t mind being able to see the pump by the way so no need for it to be in the tank.

I must say though that I’ve had nothing but reliability from mechanical pumps in the past. True you have to crank the engine for a bit if the car is left standing for a while and the fuel in the carburettors evaporates. With this engine though the flow rate and engine speed parameters may be more than what the mechanical pump was designed to cope with.

[HCA]

I am through and through fuel injection where pump choice is easy! I am far from knowledgeable on the detail you look for from these pumps, so cannot help on suggesting a type, but I would have thought the tecky guys at Facet or Huco would tell you exactly what to use? No?

[2cams70]

I am through and through fuel injection where pump choice is easy! I am far from knowledgeable on the detail you look for from these pumps, so cannot help on suggesting a type, but I would have thought the tecky guys at Facet or Huco would tell you exactly what to use? No?

Thank you - when it comes time to make a purchase of one I'll probably do that too. There just seem to be so many different designs with seemingly similar end result specifications!

[tonyabacus]

So from jackshafts to fuel pumps, amazing how the forum throws up so many different extrapolations from a single query.

But as far as fuel pumps are concerned people will always be split in their thinking on this sort of subject. There are pro's and cons for both, but as already pointed out the pump manufactures often work hand in hand with the engine manufacturers to find the best compromise. As for our Lotus Twincams then we have to realise that we are going back to at least 1962 and perhaps earlier when the engine was being developed. Lotus obviously found that the mechanical pump was more than adequate for the carburettors they were using. However as time moves on other options appear and so people started to look at electric pumps, which are also developed with engine and carburettor parameters in mind.

One of the main drawbacks with the mechanical pumps on our twincams is accessibility given its location, so maintenance and repairs are not quite so easy. An electric pump and if necessary a further fuel filter/pressure regulator can be sited in a more accessible position and can with a little bit of ingenuity hidden behind a piece of carpeted trim in the boot. Most electric pumps can be purchased with pressures suited to carburettors, however care should be exercised as many pumps advertised are for fuel injection systems that run far higher pressures than carburettors can withstand. So a pump with a range of 1-5psi together with a pressure regulator that can assist fine tuning are all that is required.

Thanks to all for the responses to my original enquiry regarding jackshafts, I think the different contributors have open the subject up to give us all a better understanding of the what and wherefors to their use. Just make sure that if you are selling a car or engine with one fitted that you pass that information on to the new owner, so that they are not left scratching their heads when trying to refit a mechanical pump!
Tony