9 replies to this topic

[nicklouse]

https://youtu.be/jba...mIm-i3NzIlwLn4w

well worth a watch.

 

will be interesting when he gets onto the 1275s 

[whistler]

Watched it yesterday. Very informative but would be interested in a wider study of cone v foam filters as well. Is it worthwhile going back to the Hawthorn or drilled air boxes for increased torque or power figures?

[coopertaz]

good video and agree will be interesting to see results for 1275. only one issue all testing was done with bonnet open ( i know the point was flow testing) but how will having it closed affect these results as holes underneath and towards the rear will be drawing hot air from rear of engine compartment. Holes at the front face getting cooler air, especially if a distributor guard is fitted as this will direct air towards the filter, would be the best!

Edited by coopertaz, Yesterday, 10:05 AM.

[coopertaz]

Watched it yesterday. Very informative but would be interested in a wider study of cone v foam filters as well. Is it worthwhile going back to the Hawthorn or drilled air boxes for increased torque or power figures?

i remember the keith Calver article in miniworld that ken refers to and he did test the k&n straight onto the carb and found that removing the elbow on the carb inlet led to a reduction in low end torque, ram length change as referanced in the video, and only a couple of bhp over a drilled airbox at the top end. the reason i began drilling the front of mine in the first place.

[Ethel]

I don't think he's quite right about how filter flow restriction effects AFR, the carb venturi works on velocity "variable venturi - constant depression". It's the air's density that's dropped (fewer oxygen molecules by volume). Though a HS carb (at least) has atmospheric pressure, outside of any pressure drop caused by the filter, pushing fuel towards the venturi.

 

Would've been interesting to compare some of the factory improved cases & a bell mouth or ram pipe added to the original snorkel - that's what that case came with originally.

[ACDodd]

I have a part answer for you on the open vs closed bonnet effect. As part of the training I was delivering we did
That test, on this car peak power was reduced by 0.5HP with the bonnet closed vs open.

In an ideal world many things can be tested, but it takes time. Happy to do any tests that people want, however it needs to be funded, and that is where most people go quiet!
And trust me, if you can only get down to 4inhg of vacuum in the carb, fuel is sucked out of the jet. This is physics fact. This extra fuel is in addition to the normal Venturi effect.

Ac

Edited by ACDodd, Yesterday, 04:31 PM.

[Ethel]

It's a moot point, I'm not doubting the effect. Everything else being equal, you can't have lower pressure without also having less mass. Lower pressure in the venturi will also act on the piston, which has a much bigger area than the jet, with the effect of inserting more needle in to the jet. 

 

There's 2 constants attempting to close the venturi, the piston's weight & the spring. To open it there's the pressure differential above & below the piston, both will be subject to any effect a filter has. You're left with the Bernoulli effect that lifts the piston proportional to the air velocity to meter the fuel by the area of the jet (according to the needle profile).

[ACDodd]

You forgetting the basic fact that there is
Still a vacuum over the jet. This pulls more fuel for the same needle postion. This vacuum is in addition to
The Venturi effect. The piston is in the same position regardless of the vacuum. The same as it would be if
The carb was presurised as in a blowthrough turbo application. I know this because the needle station positions do not change in use regardless of filter flow potential.

Ac

[Ethel]

If there's additional vacuum over the jet, then there's also additional vacuum under the piston.

 

What are you measuring your flow potential in? If it's CFM, or similar, then you're saying the same as me - it's volume, not mass (which is lower because pressure is lower because of the filter's restriction).

 

You make an SU boost sensitive by feeding boost pressure to the float chamber, it's the same as as a normally aspirated setup using atmospheric pressure to raise the fuel level in the jet as the venturi effect acts on the opposite end to the float chamber.

 

Carbs have to operate in a world where weather & altitude constantly change air pressure. An SU is effectively a U tube barometer with a venturi on one end. That venturi is regulated by a spring & 2 chambers that use the relative pressure difference from before & just after the venturi to compress the spring. 

[ACDodd]

Yes you are correct the operation of
The piston is irrespective of the vacuum
Or pressure over the jet. This is precisely why the piston position is unchanged when there is a vacuum in the nowt tract due to a restrictive airfilter. This is in addition to to the vacuum
Generated by the Venturi affect.

It's these basic facts you begin to understand when you actually measure what happens when these ingenious devices are in operation. The float bowl in this case has no bearing on the operation of the jet.

Ac