Velocity stacks question

[MartijnElanS4]

Thank you!!! I will change them.

Martin

You need to fit the Thackeray washers (a type of double spring washer) between the locknut and the carb flange and there needs to be a gap in the washers so the carbs can move up and down.Not solid.

Regards
G

[elans3]

The quality of thackeray washers over the last 5 years has gone down IMHO. I now use the "cosworth" type mountings, exclusively on both of my cars, ie. the rubber bobbin & two cupped washers.
Much more reliable, and easier to see that you're getting the right gap between the two cup washers to keep a "flexible" mounting, and not induce fuel frothing.
https://www.ebay.co.uk/itm/264963991708 ... R5qvn9qKZQ

[MartijnElanS4]

Hi Gav,

Thanks. I agree with you: I am not going to second guess Chapman and also prefer torque over outright horsepower.

Martijn

I am running throttle bodies so a different set up but following the same concept. I am using adjustable trumpets in conjunction with an Emerald ECU and when we were setting them up, we found that the longer the trumpet length, the better the torque and low end power which aligns with Andy8421’s points.

In my set up, because the power is modest, we found that the gains in shorter stack lengths were nowhere near as good as the gains in low end torque and the major limitation became the proximity of the flare of the trumpets to the inner face of the air box. I am using Tony Thompson;s air box which has more depth than standard but even then it didn’t allow too much additional scope.

I suspect that at the end of the day, this might become a more theoretical exercise depending on how standard you want to remain but there is a lot to be said for Chapman’s engineering skills and as is often the case, manufacturers standard is often the best compromise.

Hope this helps to some degree.
Gavin

[MartijnElanS4]

That is interesting!

Thanks for the tip.

The quality of thackeray washers over the last 5 years has gone down IMHO. I now use the "cosworth" type mountings, exclusively on both of my cars, ie. the rubber bobbin & two cupped washers.
Much more reliable, and easier to see that you're getting the right gap between the two cup washers to keep a "flexible" mounting, and not induce fuel frothing.
https://www.ebay.co.uk/itm/264963991708 ... R5qvn9qKZQ

[wdb]

My Webers have the O-ring style mounts up top and the spring-type (Thackery) mounts on the bottom. Seems to work just fine. I'm told it is done that way because the O-rings can deteriorate from exposure to fuel, which is more likely to occur on the lower mounts.

My focus would be on ensuring that the large mounting cushions between the carburetor and intake manifold are in top shape. No amount of fussing with the washers is going to correct a hardened, cracked, or damaged soft mount.

[prezoom]

The biggest improvement to air flow to the engine, is to enlarge the air box cover in the area of the rear carb. Hours of dyno work when I was racing cars equipped with Weber style carbs, showed a ratio of distance between the end of the air horn/trumpet/velocity stack and the outer edge of the air box. We found the minimum distance to be one and one half times the diameter of the air horn/throttle plate. So, with a 40mm carb, 60mm is the bare minimum. Seems to work with SU type carbs as well. In standard form, the air box cover for the twin cam is much too close to the outer end of the air horn, and even gets closer as it gets to the
rear end. The front carbs distance is much better. What's need is to make a new mold that affords an equal distance for both carbs. Keep in mind, that one cover does does not fit all, as there are several different types of covers.

[MartijnElanS4]

Hi Prezoom,

Didn’t Tony Tompson make a bigger airboxcover where the backend is as wide as the front?

Martijn

[prezoom]

I do not know, never contacted Tony Thompson. A one time mold was made from my existing front cover, that increased the rear clearance on my Elan. On my Plus2 Zetec, an early back plate was obtained, but had to be modified as the Weber intake manifold for the Zetec has a different spacing for the carbs than the twincam. Another one time mold was made for that setup. I purchased a totally fiber glass, front and back, from Famous Frank on this list, that is enlarged, that I will be using on the TC going in my Elva Coupe. I hope it fits that application without mods.

[Andy8421]

Hi Prezoom,

Didn’t Tony Tompson make a bigger airboxcover where the backend is as wide as the front?

Martijn

There is a guy on eBay, 'davenmandy0', who sells full width airboxes. I bought one a couple of years ago - if I recall it was about £90. I think he makes batches, then offers them for sale on eBay.

You could try contacting him.