What type of exhaust for power.

storm42

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I'm usually an advocate of longer inlet tracts. 10-1/2" from the end of the velo to the head (not the valve). Were yours longer than that?
Got any specs, photos, more info of those NRP pipes?

You have a PM.
 

storm42

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Hi Ralph, Ive just checked mine ,it has about 3" movement measured at the tyre with the gearbox in 3rd. I found having the tickover set to 2000rpm helps at hairpins.

I had a high tickover by mistake once and was held a long time in the holding area, I thought the engine was going to melt.

I am going to look at my standard box and measure the movement in that.
 

Fast Eddie

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Nigel, Steve and Chris, you all mentioned that the problem might not be with the exhaust and I did have the feeling that the bike was dropping onto one off throttle, I checked the carb balance at the track and found it to be spot on, however, I did have a duff plug, I changed the plugs and the bike would hold steady at low revs and I thought at the time I had found the problem. A small sideways moment at the hairpin proved otherwise and it was just as much of a pain elsewhere. I think I may have just been underestimating just how bad the condition were, they did abandon the meeting after all.

Anyway, the bike hasn't been touched since that day and a couple of days ago I got this off the bench, which is the yellow bike I used to use on track.

View attachment 14234

and put the Seeley back on the bench ready to strip it to get it ready for the coming season. Whilst I was messing about with it I noticed there is, what I consider to be a lot of slop in the drive through the gears, here is a short vid starting with first gear going through to 5th.

I am thinking the thump the engine would give the rear tire going from off to on throttle with this kind off slop would be the cause of my problem. I will strip the box to have a look but I am guessing this is just how they are and probably why the TTI box is so slick.

I hasn't been a problem before, so like I said possibly just an overly slippy track.

Opinions please !!!


Ralph, that doesn’t look any different to mine (TTI XHD 5sp).
 

SteveA

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My 500 does that (rear wheel), it's an AMC box with Manx outer cover and Hemmings internals. Will check my TTI box on the other bike and see if it is the same. I suspect it is.

So do I, looks just like my TTi for backlash.....
 
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So what would be the best exhaust for midrange on a road bike with 920 that is using stock cam, stock rh10 head, stock carbs and maybe 9.5 to one CR?
RGM sells 1.5" twin pipes in a standard upswept shape, that is what I had in mind until now, but I'm not so sure after reading thru this thread.

Glen
 
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Moto55UK

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I had a high tickover by mistake once and was held a long time in the holding area, I thought the engine was going to melt.

I am going to look at my standard box and measure the movement in that.


Regarding the high tickover , I have a cunning device on my bike that sticks out beneath the seat so I can adjust the tickover if were held for a long period then turn it up before going on the grid; only downside is that some scrutineers insist i put a catch tank on it ,they think its a breather!
 

storm42

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Regarding the high tickover , I have a cunning device on my bike that sticks out beneath the seat so I can adjust the tickover if were held for a long period then turn it up before going on the grid; only downside is that some scrutineers insist i put a catch tank on it ,they think its a breather!

Sneaky.:rolleyes:
 

Fast Eddie

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So what would be the best exhaust for midrange on a road bike with 920 that is using stock cam, stock rh10 head, stock carbs and maybe 9.5 to one CR?
RGM sells 1.5" twin pipes in a standard upswept shape, that is what I had in mind until now, but I'm not so sure after reading thru this thread.

Glen

I tried the 1.5” pipes on my 850 on the Dyno and they did not work well at all. Dunno if the extra cc’s would change that or not?

If you intending to run with stock head and stock cam I kinda think the stock downpipes might be best.
 

storm42

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Yes and try and get the ones with the balance pipe up near the head.
 

Fast Eddie

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Dya reckon the balance pipe helps power? I’d kinda concluded it was more about noise reduction...
 
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Yes, it seems Norton twins like small diameter pipes.
Funny thing is I have the stock 1.5 " pipes on the 650ss and it works great.
This 920 would have smaller pipes than the 650.
But that is what Dunstall came up with as well.
I believe he also put small diameter pipes on the 650 and found more speed.
Here we are in Vincent world putting full 2" pipes on a prewar 500.
The dyno says it needs it.
The Black art of exhaust design!

Glen
 

storm42

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Dya reckon the balance pipe helps power? I’d kinda concluded it was more about noise reduction...

That black bike of mine came with the balance pipe and it never pulled as well when I put the 2 separate pipes on it. I always figured it was like those bulbs they used to fit close to the heads on early race pipes back in the day, the extra volume helped with torque but when the engine was revving, the gasses just went straight past and let the exhaust do what it did.
 
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Dya reckon the balance pipe helps power? I’d kinda concluded it was more about noise reduction...

That's what Doug Hele said it was useful for.
In effect it did add power as it allowed a free flowing exhaust to meet new noise regulations that otherwise would have stifled power.
For most of us roadbikers, we don't have to worry much about noise regs.
We can thank the Harley straight pipe crowd for that.
So if one ignores noise regs and uses straight thru silencers, does the cross over pipe do anything other than tone down noise a bit?
I have the stock crossover pipe on the MK3. It's nice in that the bike is reasonably quiet at low rpm even with open peashooters. From midrange on it has a great bark.
So the balance pipe is useful in that I can choose to keep noise levels low if needed.

Glen
 
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The best exhaust system for power is two separate skinny pipes with megaphones. The move the power band upwards and make it much stronger. However the point at which the power comes on is much stronger, so if you are racing - the slowest corner on the circuit must be fast enough that you don't fall through the bottom of the power band and have to slip the clutch mid-corner. If you are forced to do that, you will never be smooth enough to win races. I also think the balance pipe was about noise. It doesn't really matter what you do, the best power always seems to bring noise problems. Exup was probably a modern answer. It is supposed to improve mid range power. But if you fit dramatic cams, they increase the overall power - Exup then cuts the noise. The net gain is an increase in torque. You could use Exup with a 2 into 1 pipe. I have noticed that with the modern superbikes, a lot of the guys remove the Exup and live with the noise. It depends on whether the noise meter is commonly used at race meetings. Like fuel checks, if you want to use methanol in race classes which do not permit it.
 
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My Seeley 850 has a 2 into 1 exhaust system which has skinny header pipes and a tail pipe of the same length, but twice the cross-sectional area of one of the header pipes. So there is no restriction at the collector. I use an absorption muffler. The noise is horrendous. The motor delivers massive torque from bottom right up and through the top of the usable rev range. If I fitted Exup, I could kill the noise and probably go faster. But it is just that bit more complication. So I'm staying away from it until the race officials start reacting. The other thing I have done is advance the cam so the exhaust works harder. You probably need to be using methanol to do that.
 

Fast Eddie

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Wouldn't be too difficult to fit separate pipes to that to see if you think the balance pipe gives more torque, you have the perfect testbed.

Indeed, test both on your Dyno Hill Glen and let’s answer this once and for all !
 
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