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Underbody braces, turbos and more!

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PostPosted: Sat Jan 05, 2013 11:53 am 
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Not sure if it will help me "improve" my Calmini, but yes, please do post those pictures.

I'd love to also see the calculations you used to choose primary length and so on as well, as I would enjoy the continued discussion on header design theory--particularly I'd like to know how your 4-1 differs from a Cultus part and see the change in the powerband you got by changing the parameters.

I don't think the Calmini is really so bad, but the non-traditional design (unequal length primaries, sequential pairing) while effective enough, makes it hard to guess what making changes to the stock down pipe it is paired with might do to the torque curve characteristics--I don't think the usual calculations will apply. In the long run perhaps I will simply have to build my own as you have done, or simply buy a Cultus extractor as I had intended to do in the first place.

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PostPosted: Sat Jan 05, 2013 3:39 pm 
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I do remember having this conversation back in the late 90's with some Honda guys on how to improve the low end a mid torque range with his custom down pipe on his header. For his CRX . And they talked about Nology had come up with a some exhaust tip. But any ways i found it maybe this will help youhttp://www.nology.com/trexdetail.html


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PostPosted: Sat Jan 05, 2013 9:04 pm 
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Oh, that's an interesting solution to bring up 8)

As an owner of one of these:

Image

I know that the way to build the best exhaust is to use one's engine parameters and calculate the correct dimensions of all of the exhaust components needed to achieve the best scavenging at the rpm you want using either certain rules of thumb (if you keep it simple) or modeling software if you want it more complicated.

Two strokes actually benefit from reversion as well as scavenging so it's fairly "easy" to build an expansion chamber that is tuned acoustically to the RPM range you are looking for the powerband to "hit" on each cylinder. The problem is that they are peaky as crap because the pipe is out of phase with the motor at all other rpms. So a theoretical solution is a pipe that can change dimensions with rpms so that it works throughout the powerband. In practice, I know of no design that has ever been raced or entered into production in full-scale motorcycles that works like this to change the shape of the expansion chamber, though modern two strokes do use "power valves" which change the the diameter of the exhaust primary for a similar effect and effective spring-loaded expansion chambers have been made that do this for gas powered RC vehicles.

The same effects are relevant (along with some other parameters) for four strokes, except we are only looking for scavenging and trying to prevent reversion, so we only want to design something that acts more or less as a simple megaphone, but will also harness the power of the additional pulses to keep up a constant rate of flow in the pipe.

Unfortunately the math at this point, as far as I know, starts to get close to hopeless in complexity so that even with the best designed header, what you really get with respect to the torque curve from a whole exhaust system is a bit of a crap shoot unless you take every dimension of every piece of the system into account when you design it and have the ability to calculate what is needed based on these parameters. Thankfully there's hope that with some experimenting with exhaust dimensions you can get closer to what is best for a given application if at first you don't succeed, even if you don't have a modeling lab. The Nology product referenced is presumably an aid to doing just that, allowing you to tune backpressure across an rpm range by adjusting the spring pressure, similar to a supertrapp system, but allowing for a for a broader torque curve (in theory).

In practice I searched and information on that Nology system is virtually non-existent as if no one has actually ever used one :alien:. Supertrapp, on the other hand is pretty widely used and is an easy way to tune an exhaust system to bring back some bottom end at the expense of HP.

Unfortunately, both of those systems only tune the needed restriction all the way at the end of the system, which I believe is also less efficient than creating the restriction closer to the collector, and they can't possibly flow as well as a simple smooth restriction further upstream. I assume the best approach to tuning a given exhaust if you can't easily change the primary size or length would be to experiment with different restriction sizes on a megaphone after the merge collector, and, as I attempt to visualize this in my head :roll: this would work better because it would "suck" more effectively after pulses travel through it than a restriction all the way at the end :huh: That's why I was looking for some information on what happens to the torque curve when you play with the length of the secondary pipes of a sequential 4-2-1. I know that short primaries in general (which I can't do anything about) should make for an early peak torque and make more torque above peak, but how do the secondaries affect this setup?

Anyway, a Supertrapp (and perhaps the Nology thing, if I could actually find one) can be very easily tuned experimentally without welding in different sections of pipe, presumably at the expense of a small amount of efficiency--I really wonder how much, because...

Given that I've already got an exhaust that isn't quite ideal, I may be convinced that if I fool with the stock down pipe that I should keep the dimensions roughly as they are even if I do replace the merge collector to get improved flow. So far the powerband is not bad at all anyway and if I don't like it I can try to tune it with a Supertrapp. The whole business is so much lighter and flows so much better than stock that I am still far ahead of where I started.


Thoughts? =)

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PostPosted: Sun Jan 06, 2013 1:19 am 
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the schnurle principal 2 stroke does, indeed, rely on a reflected wave to help stuff fresh charge back into the cylinder just before the piston closes off the exhaust port. to do that the exhaust system uses the expansion chamber that has both forward and reverse cones, the reverse cone reflecting a percentage of the exhaust pulse back up the pipe.

tuning for that reverse wave is what develops the power band of the engine, the range of rpm where the engine is in it's best position to develop power.

when i used to make all the calculations for exhaust design i converted rpm into frequency and used the electrical engineering equation for tuning tank circuits, considering that the harmonic point was analogous to an engine's power band. i built some wicked 2 strokes that way.

the end of the pipe can be used to dial in an exhaust which is what a tailpipe restrictor does. you can bump up low end torque nicely with a restrictor on the end but you pay a price on the top end.

it would be cool to build a variable exhaust restrictor and actively control it using electronics, close it down for low end torque and open it up for high rpm operation.

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PostPosted: Sun Jan 06, 2013 11:17 am 
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Here's one you can actually buy:

http://apexi-usa.com/shop/index.php/ecv-pipe-type-exhaust-control-valve-5.html

You'd just need to get a motor to pull the cable and figure out how to build and program the controller.


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PostPosted: Sun Jan 06, 2013 12:18 pm 
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Caaarlo wrote:
I think it's time to post the construction of my 4-1 header soon. It was custom designed for 11:1 CR, 212/340 cams and an 8600 RPM limit. The peak performance of this header falls within 7000-7500 RPM. I'll scan the drawings and post them here. I'm sure you'll find it useful.


I'm also interested to see these. 8)

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1991-GTi: JE 75mm 11:1 pistons,Ported head, Single UD pulley (OCC),Sandros chip,Aluminium flywheel,3tech 222/365 cams, Cultus headers,Cultus IM,50mm tb,Crane cams adjustable cam gears,Apex suspension, 4.39fd GB.
1989-GTi: 3Tech 210/340 Cams,Cultus IM, B&G springs, TD04L turbo, Apexi SAFC, Suzukird UD pulley, Circuitse7en dual boost controller, AEM wideband, AEM water / meth injection kit, HKS bov.
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