Airbox-Trumpet clearance

[john.p.clegg]

Dear All

Am now cracking on with Plan C and am in the middle (nearly) of fabricating a new airbox and wondered if the group could throw some light on the preferred distance/clearance of the trumpets within the airbox so as not to obstruct airflow? I suspect three feet would be good but let's be practical here,what would be the minimum....Anyone?

Thanks

John

[GrUmPyBoDgEr]

An absolute minimum can be calculated using the diameter of the trumpet as a basis.
The intake area is 3.142 x D x D / 4
D = Inside Dia. of Trumpet
The resulting area should be considered to equal 3.142 x D x H
H = minimum distance from intake trumpet end to the inside wall of the airbox.

That's a purely geometric solution not an aerodynamic one.

Any help?
Cheers
John

[john.p.clegg]

So for a 45mm bore,3.142.45.45/4=1591,therefore H=1591/3.142.45=11.25mm

I'll go and have a looksee,doesn't seem a lot 12mm?

Thanks

John

P.S. any more opinions

P.P.S. or are we talking about the outer diameter of the trumpet?

With an outer diameter of 60mm,3.142.60.60/4=2827 therefore H=2817/3.142.60=15mm

[paddy]

Isn't it said that the tapered airbox restricts flow to #4 with the standard 39mm trumpets? I haven't measured but I'd guess that the clearance is at least 12mm.

On that basis, would a sensible minimum clearance be the standard clearance for #1 ?

Paddy

[prezoom]

We did some testing on a dyno with regards to the mimimum distance from the face of the air horn to the side/top of the air box. It was on a 40DCNF with the top of the carb trimmed off and an air horn installed. What we found was anything less than 1 2/2 inches caused a loss in horsepower. I would believe this information would transfer to any air horn equipped carb. Similar to the Elan, the air box was sealed and utilized "ram air" from a headlight opening to supply the carb. If you don't think ram air works, in back to back testing at Willow Springs Raceway, during the same on track seeeion, without the ram air, the front straight speed dropped 600 rpm.

Rob Walker
26-4889

[jimj]

Yes, that geometric nonsense is....well......nonsense. Try to fabricate your airbox to leave the maximum space possible.
Jim

[andyelan]

Hi Everyone

Further to Rob's comments, although I'm not an expert on this, I believe ram air works not because it "rams air" into the air box, but rather because it recovers some of the dynamic head lost due to the cars forward motion. A carburettor on a moving car will be operating at below atmospheric pressure because of the air flow over it. The engine therefore will not be producing the same power as it would when its stationary. Recovering this pressure loss or dynamic head, will help minimise the power loss. Ideally whats needed, if space permits, is a properly designed diffuser this I assume is actually what the very large airboxes on top of Formular 1 cars are.

Andy

[GrUmPyBoDgEr]

Yes, that geometric nonsense is....well......nonsense. Try to fabricate your airbox to leave the maximum space possible.
Jim

Please read the question before calling things nonsense

[jimj]

I wasn`t meaning to be rude, merely noting that the 2 formulae are a complicated way of saying divide the diameter by 4, which I don`t see the sense of. Obviously the minimum is anything above zero and the most efficient is the maximum possible.
Jim

[GrUmPyBoDgEr]

I wasn`t meaning to be rude, merely noting that the 2 formulae are a complicated way of saying divide the diameter by 4, which I don`t see the sense of. Obviously the minimum is anything above zero and the most efficient is the maximum possible.
Jim

Hi Jim,

I can't find the mathematical symbols on this new fangled lap top thingy but in words here goes.
Cross sectional area of a circle:- Pi x r squared or otherwise Pi x d squared divided by 4
r = radius d = diameter
Perimeter or otherwise named circumference of a circle:- 2 x Pi x r or otherwise Pi x d
& the previously mentioned:- h = height or distance from trumpet to internal surface of air box
The calculation method I described provides an opening in front of the trumpet that is the equivalent to the cross sectional area of the trumpet itself.

John's question admitted that he realised that as much room as possible was the ideal but John needed an idea of what the minimum could be.
The calculation I offered was based only on Geometry & not on the coplexities of aerodynamic flow.

[msd1107]

RE: Comments by Rob and Andy on the possible pressure recovery of the inlet air velocity.

Ferrari and others have done some studies on this. In theory, if the airbox can convert the forward air velocity energy optimally into pressure energy, you would see an increase of around 3% at 300 kph.

Pressure recovery is quadratic with respect to speed, so at 200 kph, the pressure recovery decreases to around 1.7%.

A larger issue has to do with the properties of the plenum chamber itself, and its ability to enhance the delivery ratio. In general, the larger the ratio of the plenum chamber to the cylinder capacity, the higher the delivery ratio. This is a complicated area, and I cannot put my fingers on the study I saw of this. However, due to our cramped under hood area, there probably is little we can do.

David
1968 36/7988

[tdafforn]

Hi John,
There is a really good book by vizard on tuning the A series engine..
It has LOADS on dyno runs with different ram pipes and filter combinations..
His data shows that the flared horns that we use on the twincs actually flow less than just radiusing carb inlets by adding an elliptical flare.

closest I can find is the shortest one on:
http://www.pipercross.net/competition/products_rampipes.asp
as these are much less deep than the ram tubes I have always thought about swapping over as it would have the added benefit of giving a load more space.
Cheers
Tim

[bcmc33]

I'm in the process of setting-up the FI on my Twink and decided to revert to the original style air bos ILO the ITG filters that were on the Dellorto's.

In view of what has been said over the last 30 odd years regarding #4 pot being starved of air due to the tapered box form, I have made some attempt to rectify this apparent 'design failure' by modifying the box to a more parallel shape. I shall also put a K&N filter in the nose airflow.

The face of the air horns is level with the outer limits of the backplate.

Whether this works or not I shall see when the car goes to the rolling road over the next couple of weeks.

[paddy]

Vizard also wrote a book on twincam tuning, in which he talks about the importance of the ram pipe in optimising shock-wave induction effects. He doesn't say what the optimum length is - in any case it would depend on the engine speed at which maximum boost was desired - but he does talk about the importance of a substantial (1" radius) flare to broaden the rev range over which it has an effect.

The overall length of the inlet tract from the entry to the rampipes to the inlet valve is the key thing - why it is different on the A-series I don't know.

Paddy

[msd1107]

There has been tons of research (dating from immediately post WW 1) on the optimum length of intake and exhaust pipes for tuning at specific RPMs.

Current technology uses valves to have two lengths of the intake tract for tuning at two different RPMs. The megaphone exhaust uses a varying exhaust pipe size to spread the tuning effect over a wider RPM range. Some English motorcycles used a short widely diverging pipe for tuning over a narrow RPM range. Honda and others used a longer slowly tapering pipe to spread the tuning over a wider RPM range.

F1 engines used variable length intake tracts to smooth and extend the torque curve. This system actually moved the intake trumpets from longer to shorter, then extended them out and brought them in again, for as many as 6 times in the RPM range of 6000-20000. Even though the intake length was not optimum at all times, sub or super multiples of the optimum length still gives a power boost. Even though these engines are supposed to be peaky, in actuality they had a smooth torque curve, some times from as low as 9000 up through 18000-20000.

Most of the increase in BMEP over nominal 100% volumetric efficiency comes from converting velocity energy in the intake tract to pressure energy in the cylinder.

Surprisingly, the F1 guys have not experimented with maintaining the maximum speed of airflow through the intake tract at lower RPMs. With instantaneous gas speeds approaching or exceeding Mach .5 at peak RPM, there is considerable potential to enhance low RPM torque by maintaining a high but constant gas speed and converting that into pressure energy in the cylinder. This is not a new idea, the SR-71 aircraft used an equivalent idea to maintain inlet air speed at an appropriate level into the compressor stage. And I think some FSAE guy experimented with an equivalent idea (RET Vol 2 Issue 4, pg 37).

The research on the effect of plenum volume on delivery ratio was in Race Engine Technology Vol 1, Issue 2, pg 50 and in SAE ref no 2002-01-3318. I forget where else I have seen references to this.

Sure don't know how this affects our twincs, though.

David
1968 36/7988