Velocity stacks question

[MartijnElanS4]

Hi everyone,

I have a question: another Elan owner advised me to change the original Weber velocity stacks to shorter ones so it would pick up better from low rpm’s.
All of course with the intake cover mounted as I read a fire might occur.

Also he advised to add an oil catch can.

I am curious on your thoughts.

All the best,

Martijn

[tonyabacus]

Martjn
As you may guess from what you put in your posting, altering the velocity stacks (trumpets) can alter the characteristics of the engine performance, but not just in the pick up phase. So whilst you may see a small improvement in initial pick up of the engine, what else is happening elsewhere in the power curve, does this mean for example that you also need to change certain jets to compensate for the improvemet by fitting the trumpets.

It can become more technical than just changing the trumpets and the only way to know for certain is to put the car on a rolling road once the change is made. This will show the effects of the change throughout the rev range, but make sure you go to someone who understands Weber carburettors.

As for an oil catch tank, unless you intend to use the car in competitions then you may not need one. The other occasion you may need one is if you have a particularly oily engine that pushes out excessive amounts of oil through the breather. This can happen sometimes if there is excessive crank case pressure for example.

So the question has to be do you need to make these changes for a technical reaon or some other reason
Tony

[MartijnElanS4]

Thanks Tony!

Appreciate your time. The guy was/is very direct in his views. ‘You need to shorten the stacks and need a catch can. Also change the tires to Dunlop. It is the only way to go’.
As he does rally driving all his life I assumed he has the knowledge.
But as I understand from you the changes he suggests is with racing in mind.

That is certainly not my intention: I just want a healthy engine with good pickup from low down to up in the ref range; not only at the high end.

When finished, I will drive it a bit and then let experts fine tune it on the roller bench. If needed I will folow their advise.

Martijn

[nmauduit]

actually, all things equal otherwise, shorter "trumpets" would favor higher rpm rather while longer ones would tend to increase torque from lower rpm (hence favoring pick up)... if you're questioning your engine behavior I would eventually encourage a tuning on a rolling road with someone seasoned with LTC ... there can be various stages in tuning, you may want to start with making sure the richness is correct all around (esp. around 3000 rpm, the transition between the "idle" circuit and the "main" circuit), that the ignition circuit is in good condition and timing appropriate.
If you're suspecting this racers's advice may not apply to your car you may want to have someone else trying it out for comparing the two opinions.

the search for the last hp is a long and treacherous journey, good luck...

keep it on the road !

[MartijnElanS4]

Thanks! I will do that. I would like that the engine is set up nicely through the whole ref range increasing drivability.

actually, all things equal otherwise, shorter "trumpets" would favor higher rpm rather while longer ones would tend to increase torque from lower rpm (hence favoring pick up)... if you're questioning your engine behavior I would eventually encourage a tuning on a rolling road with someone seasoned with LTC ... there can be various stages in tuning, you may want to start with making sure the richness is correct all around (esp. around 3000 rpm, the transition between the "idle" circuit and the "main" circuit), that the ignition circuit is in good condition and timing appropriate.
If you're suspecting this racers's advice may not apply to your car you may want to have someone else trying it out for comparing the two opinions.

the search for the last hp is a long and treacherous journey, good luck...

keep it on the road !

[Baggy2]

There's a lot to be said for the moto " if it aint broke dont fix it "

[alan.barker]

A simple question i'd like to ask has this Rally Guy got a car with a LTC Engine.
Rolling Road session seems to me the way to go.
Alan

[Andy8421]

Hi everyone,

I have a question: another Elan owner advised me to change the original Weber velocity stacks to shorter ones so it would pick up better from low rpm’s.

I am curious on your thoughts.

Martijn

Martijn,

My initial thoughts are you need to go elsewhere for your advice, your Elan owner friend has it backward. The general rule of thumb is longer inlets increase low end torque, shorter inlets increase high end torque. So to increase low end pickup, you would have longer, not shorter inlets.

Its a complex subject. Most descriptions involve high and low pressure waves moving up and down the inlet tract - the further the pressure wave has to travel, the lower the RPM that the effect becomes noticeable. In reality, each subsequent wave interferes with all the previous waves and complex standing waveforms develop, so its not as simple as longer = lower, particularly for engines with labyrinth style inlet manifolds.

For a simple setup like the Elan, with one straight inlet tract per cylinder, it would be safe to say longer = lower.

Here is a good write up:

https://www.emeraldm3d.com/articles/emr-adj-length-intake/

If you don't fancy wading through the report, here is a graph comparing two very different lengths of tract (40mm vs 330mm):

[MartijnElanS4]

Hi everyone,

I enjoy the feedback given by you and introduce me in the complexity in fine tuning an engine.

I made a double cappuccino and read the article. Interesting to read the dynamics of pressure waves.

When my car is done I will bring to renown specialists to get the engine performing right.

Have a nice weekend all!

Martijn

[512BB]

The first thing that needs to be fixed before starting the engine is the lack of thackary washers on the carbs. They appear to be solidly mounted.

;Leslie

[MartijnElanS4]

Hi Leslie,

You mean between the carbs and the engine? I will look into it. Thanks!

[alleggerita]

What type of Webers are these? Do I read 48? My eyes might deceive me but I can't read '" DCOE" before the "31" although they definitely look like DCOE's.

[MACCA.GLM]

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

[gav]

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]

Hi,

Here is a better picture. They are DCOE indeed.

What type of Webers are these? Do I read 48? My eyes might deceive me but I can't read '" DCOE" before the "31" although they definitely look like DCOE's.