exhaust dimensions?

[Swiss]

Curious since you say you are interested in the weight, do you know what your engine weighs?

Swiss

[numbhands]

Engine weighs in at 49 kgs, so a bit of a tank really... Would have been better using a lc4 engine which is around 10 kgs lighter, but too late now to do much about it!

[EnglertRacing]

Well how does it run?

[numbhands]

It hasen't been fitted yet, I'm having real problems getting hold of some exhaust gaskets for some reason.... everything I order turns up the wrong size. Apparently there are no 35mm gaskets in the whole of the uk! think I'll get christmas out of the way now, but I'll post to let you know how I get on.

[grayracer513]

If the gasket is a round ring type that inserts into a stepped exhaust port, make a couple out of a piece of bare copper 8 gauge ground wire for building wiring. Fit them so the ends butt together in the port and it will seal quite nicely when clamped down.

[numbhands]

I'll give that a go, see how I get on. I take it you mean single core wire?

[grayracer513]

I'll give that a go, see how I get on. I take it you mean single core wire?

Yes, 8 gauge solid, not stranded.

[ingross]

i've read only now this topic which is very interesting.

I have just build supermono and racing engines so I can help you with your works.

First something about the exhaust.

For our 680cc supermono we built a special exhaust. we was looking for peak power at 9000 rpm and the header diameter is 70mm, straight section, 95cm long overall. 70mm is the diameter for the maximum power at max piston speed (about 26 m/s). straight section just to reach the max power even in a narrow rpm range. The lenght is even much important. 95 cm is intended between the ex valve and a resonating chamber. the silencer is built only after that chamber. this chamber is important for wave reflecions, without that the wave will die in the silencer without doing any work, but leaving the only gas inertia force to do all the work (which is pretty insufficient).

Your exhaust, with 60mm diameter would work very well in the 7000 rpm range, while you short lenght design system would allow the engine to rev further over the peak power without any problem.

About the lenght of the intake duct. The right way to adjust the lenght of the intake duct is to build a spacer and install it between the carbs ant the heads, with the inner diameter as big as the carbs. If you create a big trumpet before the carburettor you'll achieve noting. First because the waves reflects on the first secion improvement, right were you trumpet begins, let you trumpet totally useless, and the second reason is that, with a similar external trumpet you create a high air speed zone thus a low pressure zone where your engine waves will reflect with weaker force, making this kinda trumpet more than useless, i say... harmful for the performance.

[Swiss]

95cm/37.5" is kind of long for tuning at 9,000rpms. Have you run and tested your system on a dyno? 70mm/2.75" diameter is about right for a 680cc engine running at those rpms.

Also quite a number of people have found good success with bellmouth intake tuning. Catch is that you need to run the "Trumpet" Bellmouth all the way to the region of the Venturi, not to the large flange/filter mount that is standard on many carbs like the older Mikuni round and flat slides. As you said, the intake will tune to the first large change in diameter, which would often be the carb bellmouth in front of the slide/venturi and not a second bellmouth mounted out in front of that.

Swiss

[72degrees]

I guess these effects are a lot better understood than when Phil Irving was writing in 1948!

[numbhands]

Thats a seriously large bore Pipe you're using there!

You've confirmed what I thought about the bellmouth and intake lengths though, looks like I need to re-engineer my carb location to get the extra length i need unfortunately.... the air filter would hit the back of the fuel tank unless I can angle the carbs down a few degrees.

On the plus side, the exhaust is on and the bike has been run up, but not had a chance to test ride it yet. The uk seems to be permanently wet at the moment, maybe I should have put the engine in a boat!

And no, it isn't any quieter than it was.. Allthough it does have a nice deep note to it now, which is nice.

Plans are to get it on the dyno in jan to get the fuelling sorted, and see if all this work has been worth it! Not expecting huge gains, but if I got it right a good boost in torque of 4-5 flb between 4000 and 5500 rpm.

Something like this:-

This is a simulation of everything I've made, as close as I can get it. I'm quite interested as to how it matches up with real life...

And this is the pipe on, just to prove it's real!

Post results as soon as I get them.

[numbhands]

Finally got to have a proper go on it this weekend, and it feels pretty good!

No doubt the fuelling is all to cock, but no flat spots anywhere, and the engine feels stronger at the top end, its worth reving out to 7000 RPM now, and picks up the front wheel on the throttle now in first, which it never used to do.

Cant wait to get it on the dyno now, just to make sure its not wishfull thinking.... hopefully next week if PDQ can fit me in.

Let you know how I get on!

[numbhands]

Woke at 7:30 this morning for my ride to PDQ in -4 c conditions, and had the bike spun up on the dyno... fuelling was good, with a minor tweak need just off idle, so I'll need to fiddle with idle screw i think. Apart from that some good results!

I'll let you judge for yourselves...

Peak power is now up to 47.92 hp, and peak torque is up to 40.69, which is only a increase of about 1 peak but an increase of 6 flb where it matters at 6500 rpm ! no losses anywhere, which surprises me a bit but not complaining , ? The torque curve is wide and flat, so I'm really pleased with, and the dyno operator was quite impressed as he thought the 2.5 " pipe might have been a little too big.

Last of all, I'm quite impressed with the lotus tuning software which is a cracking tool to predict what effect changing pipe lengths will have. The resuls match up really well with the real world results, so now I'm more confident with what bits telling me about my intake length.... I'll stop fiddling with this bike one day I'm sure, but not while I think theres an easy 3-4 flb of torque waiting for very little effort!

Cheers for everybodies help and encouragement whilst I've been messing around with this, you've all been a great help!

I'l post a video of the dyno run in a bit, once I remember my youtube account password....

Edited January 16, 2013 by numbhands

[numbhands]

Here's a quick clip of the power run,bit noisy...just have to wear earplugs when I'm riding it I guess!

For those of you interested, the final dimensions of the pipe are exhaust flanges/collets 29mm id machined in a expanding cone section to 35mm id 45 mm long. Headers 1.5" Od by 467 mm long in 16swg pipe, going into a merge collector 100 mm long with a 2" outlet. This goes into a 2" to 2.5" expanding cone 70 mm long, which leads into a 2.5" pipe 705 mm long. This empties into a 2.5" bore perforated tube silencer 350 mm long. All pipework after the headers is 18 swg pipe.

Hope someone finds this usefull one day!

[EnglertRacing]

Haha Awsome!! I'm glad all your work paid off. Looks like it will pull all the way through.

Now for the longer intake manifold or velocity stacks right?

Edited January 16, 2013 by englertracing

[numbhands]

Think so, going to start making them tomorrow! I think I might need to refine the simulation a bit to make sure I get the length right, the graphs dont quite match up between real life and what I get on the PC. Dont want to waste a lot of time making something that I'm not sure will work... Am pretty happy with what I have so far in any case, dyno operator said most pegasos hes done get around 44 at the back wheel, so nearly 48 aint bad! would be nice to get the magic 50 though....

[grayracer513]

Regarding the intake tuning, Swiss makes a good point here:

Also quite a number of people have found good success with bellmouth intake tuning. Catch is that you need to run the "Trumpet" Bellmouth all the way to the region of the Venturi, not to the large flange/filter mount that is standard on many carbs like the older Mikuni round and flat slides. As you said, the intake will tune to the first large change in diameter, which would often be the carb bellmouth in front of the slide/venturi and not a second bellmouth mounted out in front of that.

An examination of the air boot of any late YZF will show this being applied, as the boot has an intake stack built in that runs all the way to the face of the carb, but is aligned with, and includes only the intake air tract. The main and pilot air circuits are fed through a separated conduit below the intake stack, and do not interfere with intake tuning.

[EnglertRacing]

What's the rough length your thinking? And which harmonic?

It's too bad you can't get your hands on 2x 31mm keihij fcrs... Might be to big..... Or even run one keihin fcr 41mm might sound small put people have ran them up to 60hp

They tend to flow a lot for their size.

[numbhands]

I currently have 2 x 33mm bst 33 mikunis, but I'm not sure how these compare in flow to an 31 mm keihin FCR? I do know slipstream tuning do a set for the SZR 660 that are 39mm , which seems huge to me! I presume they're for a full race tune 80-90 hp...

Am thinking of a total induction length of 300 mm, up from 265 mm which i have now, and this would come from spacers between the carb and head since I cant fit extra length as a bell mouth because of the existing entry on the carb. If I'm honest I'm not sure what you mean by which harmonic I'm tuning to? My understanding is the intake resonates (at the correct RPM) at a fundamental frequency dependent on the length, setting up a standing wave in the intake with constructive and destructive interference occuring at various points, the aim being to generate max amplitude of the wave at the intake valve as it starts to open. From what you say are there multiple resonant frequencies for a given intake length? Still learning this stuff so bear with me!

Have found out why the simulation was wrong which is helpfull. I inputted the induction area based on my carb diameter rather than the actual size of the intake ports, which oddly are larger. The extra port diameter holds up peak torque around the 5000-6000 mark instead of 4000-5000 letting the engine rev out better, but at the cost of torque further down. I think that explains why the bulge in torque never materialised as I expected, and it also makes me thing the port is a little on the large size. Strange for aprilia to do that on an engine designed for bottom end and midrange, but it helps me so not complaining!

It does however mean gas speeds stay a bit low until the engine really gets going, so I dont think the extra length of intake will make as much of a difference as I first thought.

Edited January 18, 2013 by numbhands