Playing With Trumpet / Air Horn Length

[smckeown 1]

Has anyone performed (or know of any results) with playing with trumpet lenghts and pug engines ?

 

Was looking around and found these, rather nifty injector location

 

[huxley309 0]

where did you find those sean i quite fancy those myself

 

typically the more you shorten the trumpet the more you increase peak hp but at a cost of torque iirc

[Sandy 189]

350-370mm is a good starting point.

[Rik 0]

What engines that for sandy?

 

I believed or read that 300mm was more than sufficient.

 

Rik

[smckeown 1]

350-370mm is a good starting point.

 

is that a piss take ? I've never seen anything between 30mm and 40mm.

 

Also wouldnt the complete length of the inlet pipes (ie manifold, TBs and Trumpets) be a consideration here ?

[Ahl 4]

I think he maybe means the distance from the mouth of the trumpet to the valve head, the total induction length.

[Sam 4]

Yep, you don't really mention trumpet length, rather the complete induction length from valve seat to bellmouth.

[engine killer 5]

typically the more you shorten the trumpet the more you increase peak hp but at a cost of torque iirc

 

 

may not be too correct. my friend is racing 3 307s and own a dyno, experience shows any shorter (compare to optimum) the trumpets the whole power curve does shift to higher rpm but LOWER on power. the shape of the curve is quite similar when using short and long trumpets but too short (as we used to think will benefit higher rpm) the trumpet just hurt the power badly. tried both 35mm and 75mm long trumpets on dyno, max power can be acheived earlier and higher when using 75mm. unfortunately there is not any more room for any longer trumpet, otherwise i will try 90mm.

[Sandy 189]

To clarify , indeed i meant 350-370mm from inlet valve to trumpet mouth, which is the appropriate way to describe inlet length, because it takes complete account of the whole tract. From what i've seen/done/absorbed, for an engine which will probably peak between 7-8000rpm, 350-370mm tends to preoduce the best spread of power. As engine killer mentioned above, it's not like a see-saw between peak and mid range, more a fattening or reducing of the are under the curve. Some lengths will produce odd torque peaks, which can be undesirable, the smoothness and spread of strong torque delivery is the aim.

 

Also, i've seen alot of people using length of flexible hose to vary the length. One of the engine builders I work with says he's found that flexible hose results don't often translate to the same geometry fixed pipe. Difficult to say exactly why, but possibly due to a nominal damping effect on the pressure waves. He uses a variety of nylon spacers between the trumpets and bodies to ascertain the desired length, which is 100% repeatable. It may sound like splitting hairs, but the slightest variations can have a profound effect on the results, which is also why I don't believe you can model inlet length mathematically as some people claim, estimate perhaps, but not optimise.

[engine killer 5]

Also, i've seen alot of people using length of flexible hose to vary the length. One of the engine builders I work with says he's found that flexible hose results don't often translate to the same geometry fixed pipe. Difficult to say exactly why, but possibly due to a nominal damping effect on the pressure waves. He uses a variety of nylon spacers between the trumpets and bodies to ascertain the desired length, which is 100% repeatable. It may sound like splitting hairs, but the slightest variations can have a profound effect on the results, which is also why I don't believe you can model inlet length mathematically as some people claim, estimate perhaps, but not optimise.

 

 

i was thinking to use some stainless steel pipe to make a hub (like a lip) and weld a flange to it and the middle use silicon hose to vary the length to act as trumpet trial unit. but your word "nominal damping effect on the pressure waves" just give me a pause and have to think about it again. so, jenvey's spacer will probably the best try when doing dyno tuning.

[Jonmurgie 2]

I'm hoping to also do some back-to-back tests on the dyno one day with some shorter trumpets... I'm using the 90mm ones that Jenvey recommend and in a conversation with Richard Jenvey he informed me that they ended up with the 90mm length after extensive research/testing with Longmans in the late 80's early 90's with the Mi16 touring cars.

 

He basically said that if you use shorter trumpets the power shifts far higher in the rev range (too high for most Mi16's really) and you will loose a stack of torque. Hope to be able to confirm this one day!

[smckeown 1]

ok thanks guys, i'll investigate my current induction length

[welshpug 1,627]

heres some interesting facts for you so consider - the ACAV system on the 2.0 Mi16 has the following dimensions - long tract for torque - 650mm long 36mm diameter, and short tract for power - 370mm by 45mm.

[smckeown 1]

cheers

 

I have estimated that my inlet track is currently about 300mm, broken down into:

 

TBs - 118mm

Inlet - 90mm (guess)

Trumpets - 90mm

 

So does that mean it's already far too short ?

 

As the car is at QEP and i'm up in Blackpool i'm not able to measure things

 

Here's a pic of the inlet side:

 

Edited August 31, 2006 by smckeown

[Sandy 189]

Don't forget the port length. Interesting about the S16.

 

With reference to what Jon said, you can't say one size fits all regarding inlet length, there are so many variables at work, most profoundly cam profile and timing; but also valve size and port geometry, compression ratio, rod length to stroke ratio, exhaust cam timng and scavenge properties, air supply, throttle butterfly position (within the tract) and diameter and i've probably missed something! You can generalise though and in general, 350-370mm on this sort of engine is usually in the ball park.

[Jonmurgie 2]

What I am saying is that Jenvey supply the 90mm trumpets with the Mi16 kiet becauce along with Longmans they have come to the conclusion that 90mm is the best of both worlds for the Mi16 application when dealing with a failry standard engine...

 

As for Sean's engine, it's 8v and therefore I can't say with any authority what Jenvey say about that as I only spoke to them about the Mi16.

 

I am, therefore, NOT saying that one size fits all...

[smckeown 1]

when you change trumpet lengths, do you need the fueling or ignition changed ? or can you change the trumpets yourself and use a performance meter to gauge the difference ?

[welshpug 1,627]

do throttle bodies use a map sensor?

 

If so changing the trumpet length would alter the vacuum characteristics slightly but the ecu would pick up on the vacuum adjusting fuelling to suit would it not?

[Henry Yorke 269 3 Cars]

The trumpets in the top picture with the injector mounts would cause a very different "length" effect to standard trumpets as there are great big holes in the side of them for injectors and accompanying air turbulance. Also you would further be altering the density of "air" doing down the inlet tract as there will be fuel mixed in. Running 8 injectors sounds like you may as well buy your own rolling road with the time you will spend on one!!

[Sandy 189]

Jon, calm down!- here's what you said: "Richard Jenvey he informed me that they ended up with the 90mm length after extensive research/testing with Longmans in the late 80's early 90's with the Mi16 touring cars" and my point is that than information (if precise) is of no real relevance to other engines, wether you intended to suggest it was or not, that's how it read and I was merely advising that, becuase it could be misread by people that don't know different.

 

Sean- The fuelling usually gives away where the torque has picked up by the AFR changing, you should then optimise the fuelling and re-evaluate the ignition to suit, but it's not usually dangerously different if you're already running 12.5-13.5 AFR at full throttle.

 

Welshpug- TB's don't tend to work well with MAP load, sespecially with alot of overlap. Virtually every TB set up runs TPS.

[Jonmurgie 2]

Well thanks for pointing that out there Sandy... at least we won't be confusing anyone on this thread now

[Sandy 189]

I don't need to be patronised (again). You told me to calm down yesterday on another thread if you remember.

 

Grow up FFS

[Jonmurgie 2]

Oh come on... I was being genuinly thankful there for putting me and the thread straight You have PM anyhow...

Edited September 1, 2006 by Jonmurgie