strumpets sucking

[yankeedoodoo]

Ooopps, I think I mean trumpets sucking in air, as in air horns/velocity stacks. I request enlightenment as to what length is best and why. The plethora of dual runner induction systems on modern cars to enhance performance in a wider variety of operating conditions leaves me wondering what is best for my 200HP zetec and why. Does being @ sea level make any difference or is that covered by the ECU? A link to previous thread(s) would be appreciated very much. Thank you.

[AMMO]

Yankeedoodoo

 

You can't go wrong with twelve inches. Always a good starting place for induction lengths. The induction length is calculated on the second, third or fourth harmonic. It can also be calculated for maximum horsepower or maximum torque.

 

Do you remember the old McLaren Can Am cars with two different induction lengths on their V8s? This has been used on four cylinder Superbikes where two short and two long bellmouths are run for certain circuits. Ideally you want a long induction system at low rpm and a short one for high rpm.

 

Let me know if you want specific lengths for various rpms, I will calculate them for you.

 

Forgot to say. The total length is from the valve head to the end of the trumpet.

 

AMMO

 

Edited by - Ammo on 20 Jul 2002 09:38:23

[Morls]

Doesn't anyone make an engine with a variable length intake trumpet kit?

 

Given the common use of valves to fool the exhaust waves and V-Tec type systems, continuous tuning of the intake lengths (and airbox lid distance), seems a logical next step.

 

I'm sure some, (either SoS or BotT), bikes were playing with this a few years ago...

 

 

 

Mark

[yankeedoodoo]

AMMO: I did see the Bruce & Denny Show several times and always wondered why the stacks were sticking out every which way and of differing lengths. Now I know the good reason. My zetec is equipped with 45mm jenvey twin tb's and 90mm horns. A friend described the upper RPM performance as having an afterburner as compared to his massaged zetec. Low end torque is less impressive. So if I went to shorter trumpets it seems I would further degrade torque(?), and perhaps gain top end performance that I may not really need. Anyway, what do you suspect might be the effect if I were to fit 2 shorter trumpets and to which cylinders would they be best applied? And thank you for bearing with my sometimes moronic musings.

[AMMO]

Mark

 

When I was involved with Battle of the Twins and Sound of Singles there was nobody was using variable length induction systems. Might have happened since I left. I did do some work for Crescent Suzuki with variable length trumpets. This involved testing with various fixed length bellmouths and then making a variable setup based on the findings. It only ever ran on the dyno and it worked pretty well. The different bellmouths for different circuits was adopted as I think the variable system was not allowed in the rules. I designed and manufactured the carbon bellmouths on this year's GSXR 1000 for Crescent. Can't give you any details as Paul Denning would come around and shoot me.

 

Yankeedoodoo

 

The exact induction length is further complicated by the exhaust length and cam duration. The longer the cam duration the more critical the exact primary exhaust length is. This also affects the induction length so any calculated figures are really only a starting point. If we assume your engine makes peak power at 6,500 rpm the induction lengths for the second, third and fourth harmonic would be 20.3", 14.92" and 11.38" (not far off the 12" previously suggested). If your maximum torque was 5,800 the lengths would be 22.75", 16.72" and 12.75". So 12" would be a good compromise for torque and horsepower assuming the rpm figures are correct. If you could accomodate something longer you should give it a try on the dyno. I would think you would be very restricted for space. As the rpm at which maximum horsepower and torque go up the requirement is for a shorter system. It may be worth trying some shorter bellmouths but I think you would lose in the midrange but possibly gain at the top end (perhaps). If you wanted to mix the lengths I would suggest that either the outer two cylinders or the inner two should be different but not alternated 1 and 3 or 2 and 4. On the Suzuki it made little difference if the inner two cylinders or the outer two were longer. It was usually the inner two due to the proximity of the top of the airbox to the outer two bellmouths.

 

If you do decide to play around a bit please let us know what you find.

 

AMMO

[Morls]

AMMO,

 

I think the bike I had in mind was a Dutch? home made (maybe a University project) Single or twin cylinder thing featured in Superbike? Magazine around the time that the Assen Bott and SoS races were getting a bit of coverage.

 

This narrows it down to about a two foot tall pile of magazines, near the Stella fridge in the garage, so that's my Sunday night sorted.

If I find any embarrasing 80s / early 90s style pictures of Guzzi tuners in the articles, I'll find somewhere to post them .

 

Did the Brittain never appear with Variable intakes? He seemed to try almost every other thory that gets discussed over a pint or two.

 

 

 

Mark

[AMMO]

Mark

 

The most famous single when we were racing at Assen was the UNO Rotax. I think this was ridden by Thomas Korner. A German friend told me it was dynoed at 92 bhp at the crank with around 650 cc. If you stalled it forget about trying to bump start it. After we packed in racing there was some trick Suzuki single but I never saw this in the flesh. Maybe this is the bike you are thinking of.

 

The Britten to my knowledge never ran with variable induction but it did have some very trick carbon fibre throttle bodies. An Italian friend of mine owns a Britten and became a director of the Britten Motorcycle Company after John Britten died. I was asked to continue the development of the Britten single at one point but unfortunately the project was abandoned. The engine was the Britten V-twin with the front cylinder cut off. The induction system ended up just behind the headstock. It had three or four intake valves and three throttle bodies. I seem to recall that the middle throttle body was oval and the two outers were round. Have to dig out the info from the pile of suff I have hanging around. Shame it never happened. It would have been interesting.

 

Projects like this are a bit thin on the ground. I find them very exciting.

 

AMMO

[Matt Seabrook]

I found the 13" induction length was best for my 2000cc Zetec. It produced 210bhp @ 7100rpm and 171pft @ 5100rpm any shorter than this still produced about the same peak power but the torque was not so good through the rev range

[DohNut]

 

I was thinking that if a mix of induction lengths are being used and they are working correctly.

One pair will working better (introducing more air into the cylinder) than the others.

The cylinders are all recieving the same amount of fuel.

This will mean that the air fuel mixtures will differ between the pairs of cylinders,

It would be ideal therefore to get a lambda probe one of each pair of exhaust primarys and not just rely on the averaged result in the collector or worse, just looking at one header only.

 

You could just map it a bit rich I suppose but you are trying to do things properly

Equally this may only be a problem if the headers are 6 inches different in length.

 

 

Nick

[Blatman]

Listen to Matt. He knows about inlet lengths on a Zetec.

[allegro]

Has anyone messed with their trumpet sizes on an SLR. If so what results. I notices one 'K' owner at Curborough in may with scary looking trumpets that were claimed to improve power by 10-15HP. And I believe he proved it with back to back rolling road tests.

 

 

Andy Mac

😬 Team Langoustine. Hard Core Prawn here 😬

[AMMO]

Doh!Nut

 

You are 100% right. With the four cylinder bikes the problem is not top end power but getting the power down coming out of turns. So the two different induction lengths are a compromise to make the bike more driveable. The fuelling is monitored with one lambda probe in the collector so the fuelling will not be perfect. They even add a bit of weight in the form of a flywheel on the end of the crank to slow things down a little to stop the thing spitting you off.

 

On the V-twins there is a lambda probe in each exhaust header and because of the firing order and exhaust design, one cylinder is scavenged whilst the other has interference. If you look at the lambda traces from a V-twin at certain rpm one cylinder goes lean whilst the other goes rich. The ECU is then programmed differently for each cylinder. I don't know of any four cylinder engine that has each cylinder individually set up. It would definately be an advantage to do so.

 

Matt

 

You found 13" worked well for you. That is interesting. What cams and exhaust do you have? Calculating the lengths for 7,100 and 5,100 rpm you get 18.59", 13.6" and 10.42" for max power and 25.88", 19.01" and 14.5" for max torque. Maybe if you had room to try 14" it may be even better. Did you try anything longer? I saw a Zetec the other day that had a downdraft induction system which was about 15" long. Very nicely done. Unfortunately it was going in a Ford Fiesta.

[Matt Seabrook]

Hi Ammo I have a Dunnell 195Bhp engine with cams ported head and Dunnell 4-1 exhaust manifold going into a SBD carbon silencer. I am running Jenvey tapered throttle bodies and had 90mm air horns fitted. I got an Emerald M3D engine management system so took a trip to Dave Walkers to get it mapped. After the mapping session Dave asked if there was anything that I would like to try to seem if we could improve things. So I said that I would like to try modifying the pulse tuning. We started off by fitting a 25mm spacer between the throttle body and air horn this did not seem to help. So we tried 50mm with only slight improvements we the tried 70mm but this lost us some mid range torque. We then tried 95mm spacer and this gave us the best spread over all. We stopped at this length as for practicality reasons the air horns now stick out of the bonnet by 45mm. I do not want to hit oncoming traffic with my air filter. I would have liked to see if any longer would have helped but we had run short on time.

 

I have since made up some spacers that I now have fitted instead of the quick fit ones used on the rolling road. You can see them on my web site if you would like sorry chaps it is a Westfield but it still has a Zetec fitted here and click on cars and go to my page.

 

I know Dunnell are not cheap but when you hear a lot of horror stories about some engine tuners quoting power figures that there customers never achieve then I think that a 195 spec engine that produces 210bhp is good value.

 

All the best Matt

 

 

Edited by - Matt Seabrook on 22 Jul 2002 21:20:32

[Matt Seabrook]

Ops just thought I checked from the head to the end of the air horn not to the valve to get the 13” length. I should think it would have more like 16” to the valve. This seems to throw a spanner in the works for the calculation. Ammo how do you calculate the lengths for rpm? It just goes to show though that there is no substitute for a good rolling road session

[AMMO]

Matt

 

I saw you pics. I like the throttle bodies. Lots of people have said that taper bodies don't work but I have used them on bikes with great success. I will probably go for a set of DCOE replacement bodies although I have a set of tapered 45's from a GSXR 750 I could graft on if I spend a couple of days making adaptors etc.

 

Regarding the induction length calculation there is a long winded version somewhere in Richard Stone's Introduction to Internal Combustion Engines or a quick and dirty method from the Superflow flowbench manual. You divide 132,000, 97,000 and 74,000 by rpm to obtain the 2nd, 3rd and 4th harmonic. Length is in inches. I use the quick an dirty method as it is inevitably only an approximation and the exact length has to be found on the dyno.

 

When I was messing around with the Suzuki I made a table with all the lengths at 250 rpm increments. We lengthened the bellmouths 5 mm at a time and the best figures were very close to the calculated ones.

 

Regarding engine output figures not reaching their expectations it is a sad reality the engine are sold on numbers. I had a job recently to do some work on a quad cam, 32 valve 4 litre + Ford lump for a GT racer. Did the heads and modified the plenum, ran stock cams with air intake restrictors. Power was up from 312 bhp to 397 bhp. Everyone else was quoting 440 bhp. This thing revved 1500 rpm more than the others and when the flag dropped went past all the rest. Go figure.

[Peter Carmichael]

Mixing trumpet lengths requires different fuelling/advance at different points in the rev range. You need two maps (two separately jetted carbs??) to get it right - otherwise it is a compromise.

 

The long trumpets on Dave Jackson's car at Curborough have shown some remarkable results on the rollers, but I suspect this is because there is still a lot of development work to do on that engine - the power delivery is very lumpy and I would expect cam-timing to smooth that out and give more power everywhere. Once you have got them in the ballpark, the trumpets thing can be something a of a red herring because it is a tuned effect - what you gain in one part of the rev range you lose somewhere else. We tried Dave's long trumpets on my engine at the rolling road and it made very little difference.

 

*thumbup*253 bhp, up and running

[DohNut]

AMMO

that does remind me that Dave Walker did get a tweaked Emerald ECU to vary the advance between cylindesrs on a fiesta racer.

He was restrained by the regulations to use the original single carb manifold which flowed better to the inner cylinders than the outer, he was able to run a few degrees extra advance on the leaner cylinders (cos it burns slower).He did improve by 2-3bhp throughout the reva range.

The advantage of that situation was that he had a broadly consistent *extra flow* to the same cylinders throughout the rev range, unlike that caused by pulse tuning.

 

 

Nick

 

 

 

[Matt Seabrook]

Hi Ammo I am not sure if the tapered throttle bodies do make any difference to peak power. All I know is the spread of power no is much better than when it was on carbs. As I do not have anything to compare this with I would not like to say that T/throttle bodies are any better than anything else. I did like the fact that they are all individually adjustable so tick over is very smooth and the look the mutts nuts. Sorry that was a bit sad. What I do know is that there are some very unhappy people about after a rolling road trip. I am glad to say that I am not one. By the way thanks for the info on calculating induction lengths I do not like to be told something will work without knowing how it works.

 

Many thanks Matt

 

[AMMO]

Peter and Doh!Nut

 

Thanks for those bits of information.

 

Matt

 

Hope the induction length info is of some use. I'm itching to put my car on throttle bodies. With some slightly hairier cams (Kent FZ2002) and a little extra porting I should be able to get around 190 bhp out of my 1800. That's around the same horsepower per litre as yours. That will keep me happy. It annoys me that there is potential in the engine that I'm not taking advantage of. Hopefully will be rectified soon.

 

 

 

AMMO

[Matt Seabrook]

Hi Ammo you should easily get 190 out of an 1800cc engine a friend is putting together a sprint 1800cc on carbs and has spoken to Paul Dunnell and has been told that he will get a minimum of 200bhp. If he was on t/bodies they could go even higher I think. I myself spoke to Paul the other day and he said that an injected 2000cc engine for fast road now would be a min of 220bhp on t/bodies. Good luck and release the extra power these engines are very under rated.

 

All the best Matt

 

[yankeedoodoo]

AMMO: Thanks for sharing your knowledge wisdom and experience with all of us on these subjects! More great contributions tto the chat.

[Morls]

 

 

 

 

Mark

[AMMO]

I think the title "Windbag" really suits me as even I think I go on a bit sometimes. Glad to know you find these experiences interesting.

 

Picked up a second-hand set of throttle bodies yesterday. Only need to wait for a Pectel T2 to come up at the right price and I'm away. Yippee!

 

AMMO

[Peter Carmichael]

Am I right in thinking a windbag is the flexible equivalent of an airbox or is it one of those things you see at airfields 🤔

 

Edited by - Peter Carmichael on 26 Jul 2002 09:58:15

[AMMO]

Peter

 

Interesting induction based analogy. Unfortunately the things at airfields are windsocks. Windbags are found on bagpipes and blatchat. Superchargers and turbochargers and also a type of mechanically operated windbag used on, funnily enough, induction systems. Strange how it all comes together in the end.

 

AMMO