exhaust calculation, 4 cylinder

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Rocksalt
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exhaust calculation, 4 cylinder

Post by Rocksalt »

Hello, I have an inline 180 deg crank, motorcycle engine and am looking to make a custom exhaust. Can I get some input on 2 different configurations? This is used for drag racing 1/8 mile in a heavy chassis (700lbs total rider and chassis)

4-2-1
4-1
This is 250cc per cylinder, running 35HP per cylinder nitrous. Peak power is 11,500 and RPM limit is 12,500. EST HP is 300HP.
The exhaust cam is 230° @ .050", centerline 109° w/109.5° lobe separation.
I currently have a 4-2-1 and believe the primary diameter is too small. (1.4@ 4" then 1.46@14" to the first tri-y).

How can I calculate primary length, tri-y exit diameters, and length between them to the final tri-y? Should I consider 4-2-1 or stick with 4-1? Would a zoomie be practical for the purpose?

Thx
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John Wallace
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Re: exhaust calculation, 4 cylinder

Post by John Wallace »

I'd get PipeMax and see what it says?

Link to Larry on this site:
viewtopic.php?f=48&t=58440

:)
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Re: exhaust calculation, 4 cylinder

Post by modok »

The primary size and all the sizes downstream,
are tied mainly the size of the exhaust valve and port, as this actually controls flow into the header.

There is not just one kind of tri-y, but actually several, and they don't all follow the same formula.

Sporty bikes tend to use an odd firing order, rather than the even firing order more traditionally used. Hard to say why, but I believe it has to do with several reasons, one being high peak power while maintaining low speed tractability, and usually with four valve heads. If you don't care if it's drivable at low speeds, then, maybe that isn't what you need.

A odd-pared tri-y tunes significantly higher rpm for the same sizes and lengths compared to even, and tends to be better at working into a relatively small final collector choke or size, meaning, the size relation from the secondaries to the final collector size or choke can be close in size... might even be the same size,

An even piared tri-y... it falls off as the rpms go past where the primary+secondary length's tries to go past the second harmonic, and odd pairing can be a solution.

I guess my main point is if you change the TYPE of header, all the tuning basically rules change too, and that's a hack of a thing to try to guess. I don't know. hopefully you can look at others running similar kinds of engines, who have already tried everything, to see which way you should go.
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Re: exhaust calculation, 4 cylinder

Post by hoffman900 »

modok wrote: Thu Sep 19, 2019 10:57 pm The primary size and all the sizes downstream,
are tied mainly the size of the exhaust valve and port, as this actually controls flow into the header.

There is not just one kind of tri-y, but actually several, and they don't all follow the same formula.

Sporty bikes tend to use an odd firing order, rather than the even firing order more traditionally used. Hard to say why, but I believe it has to do with several reasons, one being high peak power while maintaining low speed tractability, and usually with four valve heads. If you don't care if it's drivable at low speeds, then, maybe that isn't what you need.

A odd-pared tri-y tunes significantly higher rpm for the same sizes and lengths compared to even, and tends to be better at working into a relatively small final collector choke or size, meaning, the size relation from the secondaries to the final collector size or choke can be close in size... might even be the same size,

An even piared tri-y... it falls off as the rpms go past where the primary+secondary length's tries to go past the second harmonic, and odd pairing can be a solution.

I guess my main point is if you change the TYPE of header, all the tuning basically rules change too, and that's a hack of a thing to try to guess. I don't know. hopefully you can look at others running similar kinds of engines, who have already tried everything, to see which way you should go.
If you look at a modern Superbike racing exhaust system, they’re all tri-y designs and feature primary and secondary sizes pretty much equal in diameter. They then make a slow taper from the final collector to the exit.

Firing orders are all over the place.
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Re: exhaust calculation, 4 cylinder

Post by Rocksalt »

Pipemax would be a choice, but I wanted to get more details, if I could, to understand what is going on.

I see some engines paired 1-4 and 2-3, most others 1-2 and 3-4. Mine is currently paired 1-2 3-4. Firing order is 1-2-4-3.

There is great detail in the OEM exhaust. Primaries are equal length and damn near bent the same configurations. When I model the engine in software I get different results with different configurations (of course). Problem is, I don't have options for firing order, exhaust pairing, multi collector configurations.

I am assuming the current exhaust I have, has too small diameters, mainly due to the HP output(nitrous), is this a correct assumption?

Currently, I have a 4-2-1, 1.4" primary 6" long, 1.46 12", try-y exit at 2.06", 2- 2.06@ 4" long, try-y @ 2.5", 15" megaphone @ 6° w/ reverse cone @ 2.5".
This exhausts primaries are shorter by 4" than stock. Power falls off close to 11,800.

The exhaust ports are very small and have a raised section on the bottom of the port that is quite high. The total area cannot be much more than 1.5sq"

What I have is working N/A, but I think I need to try something different with the increase in nitrous flow.
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Re: exhaust calculation, 4 cylinder

Post by jacksoni »

Curious what you are using to model the engine? Pipemax will make suggestions but (at least in the earlier versions, I don't have the latest) not show you results of changes. Vannik's (member here) EngMod4T will do that and more.
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Re: exhaust calculation, 4 cylinder

Post by John Wallace »

Here's a thread on Larry's forum about Tri-y's

Tri-Y Specs

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Re: exhaust calculation, 4 cylinder

Post by Rocksalt »

I am using EAP for the modeling.

The best zoomie type wants a 1.88 I.D. @ 15" long. For what I am doing, this makes a good 20HP more over the try-y (on paper). Not sure if the program can't simulate properly and I need to trial and error. Looking for some general rules. I think I will just make a zoomie and try it. Easy to make and can see results at the track. Ideally, I would like to make a 4-1 and spray 150HP on top of 185HP.

This is a chart N/A, 70HP nitrous w/ 4-2-1, then 4-2-1 70, then 1.88 15" zoomie. 1.88 highest
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