big bore motor questions

[woodsracer144]

ok when dealing with a big bore what is a good all around pipe to run? (330-370) and how much do you gain when you have the bottom end matched to a cylinder? how much will a reed spacer change? In my mind it would make the volume of the bottom end bigger so i would think that should effect something.

when your running a BB I would assume you could advance timing and things the same as a 265?

[Grande Huevos]

Originally posted by woodsracer144
ok when dealing with a big bore what is a good all around pipe to run? (330-370) and how much do you gain when you have the bottom end matched to a cylinder? how much will a reed spacer change? In my mind it would make the volume of the bottom end bigger so i would think that should effect something.

when your running a BB I would assume you could advance timing and things the same as a 265? i just got a esr 330 w there 9 porting and i was told that a 5 or a 9 pipe would b great. the 9 is more mid to top and the 5 is an all around. i had a guy owe me some money so he just handed over a brand new unused esr5 pipe to e and so far i love it! dont have alot of time on it yet but it seems very responsive from bottom to top! i asked he same question about the advanced timing and everybody said NO u cannot do this on the bb because it will make them run like crap!!!! why idk?? thats what 4-5 reputable builders told me tho

[woodsracer144]

So you don't need a bb pipe?

[Grande Huevos]

Originally posted by woodsracer144
So you don't need a bb pipe? thats the impression i am under. i had never even heard of a pipe built specifically for bb until i started messing w this forum so idk what diff they actually make?? i would think if it was that big of a deal ESR would have somthing since they have several diff pipes they sale, or at least mention somthing about it to u when purchasing a kit from them ya kno? they never said anything to me about it. I love my 5 Pipe and also have a friend w an esr 330 he runs there 9 pipe and loves it. Im not a builder by any means but iv built a couple big bores now and never ran a "bb pipe" and i always try to do extensive research on anything im buying or putting on my bikes so i kno im not gonna get the wrong thing or not b happy w it then end up replacing it again! Hope this helps

[rustyATV]

Originally posted by Grande Huevos
i asked he same question about the advanced timing and everybody said NO u cannot do this on the bb because it will make them run like crap!!!! why idk?? thats what 4-5 reputable builders told me tho

Someone double check me on all of this; it's been a while:

Larger bores = larger combustion chambers = longer distance for the combustion flame to travel = more time needed to complete combustion = more time to heat the fuel at the edge of the chamber = more detonation

They also generate more heat than the stock cylinders so, unless that is compensated for with more cooling capacity, that will also contribute to more detonation.

In theory, for larger bore motors you want more spark plugs, not more timing advance, so that you start the mixture burning over a larger area. I recall once reading an "ad-ticle" in a magazine of a 700+cc (780? I want to say?) sand drag Raptor that was modified for three spark plugs. It wasn't until they did so that they were able to get that huge motor to build enough RPM's.

And someone else may be able to speak more to this, but what I understand about pipes and displacement is that larger motors will shift the tuning of a pipe lower in the RPM range. So a mid range pipe will become more of a low end pipe as displacement increases.

Again, it's been a while since I've read all of this, so someone double check.

[woodsracer144]

hotter plug then also? what about getting the bottom end matched for the top end how much would you gain?

[rustyATV]

Originally posted by woodsracer144
hotter plug then also? what about getting the bottom end matched for the top end how much would you gain?

A hotter plug would make detonation more likely, as it would retain heat and create a hot spot. This is just one of those things about a big motor that you can't get around with some simple tuning

[woodsracer144]

Well that makes since.. hahaha

[wild250rman]

larger stinger! lets heat out better and makes jetting a lot simpler.

[red88r]

The one thing I have learned is this. We had a standard LRD pipe and silencer on our Duncan 340pv. Now the only reason reason we had this on there is because before that we had a LRD 265PV on there. It ran good...but not great.

We then ordered a pipe/silencer from Duncan and what a difference! Now I know it's prolly cuz the pipe matches the motor now. But also Duncan doesn't have a sb and a bb pipe like some of the other companies. That is probably why the LRD didn't perform as well. JMO.

[woodsracer144]

how big are the BB stinger ends 1 and a 1/8th? or a 1 and 1/4?

[Pumashine]

Originally posted by woodsracer144
how big are the BB stinger ends 1 and a 1/8th? or a 1 and 1/4? Standard TRX pipe is 1" inside at the stinger. The BB stinger is 1 1/8" inside. Now you only really need these on motors over 350cc.

[wilkin250r]

Originally posted by rustyATV
And someone else may be able to speak more to this, but what I understand about pipes and displacement is that larger motors will shift the tuning of a pipe lower in the RPM range. So a mid range pipe will become more of a low end pipe as displacement increases.


This doesn't make any sense to me.

The power range of the pipe is dependant on the timing of pressure waves, the exhaust pulse goes out, and a negative pressure (vacuum) gets created when that pressure wave hits the expanding area of the pipe. When that negative pressure wave returns to the exhaust port, it helps pull the old exhaust out of the cylinder, and pull in a new fresh fuel/air charge.

When the exhaust pulse hits the other end of the bulb, the area that starts to get smaller, it creates a positive pressure wave back towards the cylinder. It is timed to hit just after the negative pressure wave has already pulled all the exhaust out. The positive pressure wave keeps the new fresh fuel/air from escaping out the exhaust port, and helps build compression before the exhaust port closes.

That timing is based mostly on the distance those expanding and contracting areas of the pipe are in relation to the exhaust port, NOT by the amount of air coming out of the exhaust port. I can't see how a big-bore motor can change the timing of the pressure waves.

[rustyATV]

I'm going to have to try to reason this out because it's been a WHILE since I've read/heard about this. I'll try to try to back up what I say, later.


Originally posted by wilkin250r
That timing is based mostly on the distance those expanding and contracting areas of the pipe are in relation to the exhaust port, NOT by the amount of air coming out of the exhaust port. I can't see how a big-bore motor can change the timing of the pressure waves.

This assumes the flow of gasses between the two motors stays the same.

I think this is a question of fluid dynamics. One property of a fluid flowing through a pipe is, for the same pressure (don't have the book from 12 years ago right in front of me, so I could be getting this wrong) as the area of a pipe decreases, the velocity of the fluid increases. I believe (I'll check) that if the pressure or volume increased, but the area of the pipe stayed the same, then the velocity of the fluid would increase. Thus, the gasses expelled from the larger engine, flowing through a same size pipe will do so at an increased rate due to the larger engine volume and pressure the gasses are under. This will, in turn, cause the gasses to reach the other side of the pipe and be reflected back sooner than they would be for a smaller engine, and they would carry more pressure than they would for the smaller engine.

I would imagine the caveat to this is, with the higher fluid velocities, there are more efficiency losses and...I really have to go to bed, I'll pick this up later.

[wilkin250r]

Originally posted by rustyATV
This will, in turn, cause the gasses to reach the other side of the pipe and be reflected back sooner than they would be for a smaller engine,

Excellent point. I'll admit, I hadn't really considered that.

I'm really curious on the volume of the exhaust gasses after combustion. Because my instincts tell me you are correct on the initial pressure wave in the headpipe, a larger volume would be under more pressure, and thus would travel faster and reach the divergent section of the pipe sooner. But my instincts lead me to believe that the expansion chamber is such a large volume compared to the actual combustion chamber, the increased pressure isn't significant when it comes to the convergent section of the pipe.

[woodsracer144]

So how would a Lrd team b work on the sparks 330 for mx

[rustyATV]

Originally posted by woodsracer144
So how would a Lrd team b work on the sparks 330 for mx

Can't say, specifically, just make generalizations.


Originally posted by wilkin250r
Excellent point. I'll admit, I hadn't really considered that.

I'm really curious on the volume of the exhaust gasses after combustion. Because my instincts tell me you are correct on the initial pressure wave in the headpipe, a larger volume would be under more pressure, and thus would travel faster and reach the divergent section of the pipe sooner. But my instincts lead me to believe that the expansion chamber is such a large volume compared to the actual combustion chamber, the increased pressure isn't significant when it comes to the convergent section of the pipe.

While the expansion chamber is much larger than in either end, and the gasses are going to experience a velocity drop going into it, it's still a trapped volume and the gasses still adhere to the same fluid flow principles as it did in the other parts of the exhaust, so I would expect the difference in velocity in this section to be proportional to the difference in the gas flow, and not the section of the pipe.

By the way, while I should have been doing other things, I found this while searching the topic at work, today. Handy little calculator for a number of fluid flow situations:

http://www.pipeflowcalculations.com/pressuredrop/

I actually thought a lot about this while at work, today, but have run out of time to post, again. I'll try again, tomorrow.

[wilkin250r]

Originally posted by rustyATV

While the expansion chamber is much larger than in either end, and the gasses are going to experience a velocity drop going into it, it's still a trapped volume and the gasses still adhere to the same fluid flow principles as it did in the other parts of the exhaust, so I would expect the difference in velocity in this section to be proportional to the difference in the gas flow, and not the section of the pipe.



But I'm not entirely convinced were talking strictly about fluid mechanics, in fact, we might only be talking a very small (possibly even insignificant) portion of fluid mechanics.

The other aspect is the ACOUSTIC properties. Reflected pressure waves sound waves, dependent on acoustic properties, not flow characteristics.

[rustyATV]

Was able to write more tonight, despite the hour, because of dodging tornadoes.


Originally posted by wilkin250r
But I'm not entirely convinced were talking strictly about fluid mechanics, in fact, we might only be talking a very small (possibly even insignificant) portion of fluid mechanics.

The other aspect is the ACOUSTIC properties. Reflected pressure waves sound waves, dependent on acoustic properties, not flow characteristics.

You're not going to be able to do anything with acoustics until you have sufficient flow to support a pressure wave, and the gas flow is going to be determined by the dimensions of the pipe, volume of flow, etc.

Plus, on the upper end of things, increasing gas speeds leads to increasingly turbulent (read: less efficient) gas flow, which makes it harder for a larger engine to breath through an undersized pipe, where sound would not experience such limitations (I would assume)

I was trying to look for a word to describe a pneumatic system that relies of kinetic energy to work, as that's what an exhaust pipe would be (versus a positive displacement system). I found this term:

http://en.wikipedia.org/wiki/Kadenacy_effect


In simple terms, the momentum of the exhaust gas leaving the cylinder of an internal combustion engine creates a pressure-drop in the cylinder which assists the flow of a fresh charge of air, or fuel-air mixture, into the cylinder.

So the pressure wave has mass and momentum, and is doing the work of scavenging the cylinder.

Ok, here's something for you. Cracked open A. Graham Bell's book and started reading exhaust systems, to refresh my memory. In it, he gives an equation for determining the tuned length of an exhaust pipe. This is for getting into the ballpark with a design.

Length = (Exhaust Duration X 42545) / RPM

I thought this might be reworked to be used to estimate an exhaust's tuned engine speed for a certain size engine using it's exhaust duration. Doing so, I got:

RPM = (Exhaust Duration X 42545) / Length

For exhaust durations I went to a different chapter and grabbed the following numbers from Table 3-1:

125 motocross ED = 198
250 motocross ED = 184
400 motocross ED = 178

I assumed a Length of 780mm for all three, since we're talking about how the engine size changes the tuning of the pipe.

Here's what I got for engine speeds

125 = 10800 RPM
250 = 10036 RPM
400 = 9709 RPM

So, the way I'm reading this is, for shorter exhaust durations that you would expect to find on larger engines, the pipe comes into tune at lower engine speeds.

Anybody else concur with my conccusions?