What type of exhaust for power. (2019)

My methanol fuelled Seeley 850 feels as though it has got 80 BHP, but I would not know what 80 BHP feels like. A torquey motor in a very light bike with close gears, can be very fast especially when it out-handles most others. I was practising on Calder Raceway, at the same tine a Don Emde with the XR750TT Harley in about 1970. It was fast but not excessively. It destroyed the timing side main bearing on the crank, so did not race on the Sunday. Judging from what I was doing, he would have been beaten.

storm42 said:

Jim, do you think the dynos Harley used might have been a bit patriotic or a bit downhill ? I have seen your drawing for the ports and whilst I can appreciate the design, I am struggling to come to terms with 100bhp from a 750 twin.

Just for comparison, Ducati claimed 84bhp for Paul Smarts 750 and that had desmo valve gear, I realise that was around 1974 and you may be referencing Harleys from around 2008 but an extra 16 bhp from a pushrod twin seem a bit optimistic.

What fuel did they use?

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With respect to performance the aluminum Harley XRs could be divided into a few different groups. The aluminum XR750 started in 1972 as a round port engine, circa 1985 the factory had the oval port version of the engine and that engine was offered to the public a few years later. At times rules forced the use of 33 mm restrictor plates in the induction system. So we have to be careful about what vintage and configuration we are talking about when we want to know the HP, e.g., did the engine employ a restrictor plate or not, was it a round port or oval port, etc. The later unrestricted engines of course made the most power as they benefitted from many decades of performance research. IMHO it was accepted that the best restrictor plate engines ran on the order of 85 RWHP and the best unrestricted engines ran in the 95-100 RWHP range. If this were not true the XR could not have been competitive with the Kawasaki referenced below.

More recently the very robust Kawa 650 Ninja engine (fuel-injected, OHC, 4-valve pent roof chamber) appeared on the flat track scene, and when stroked to 750 cc with accompanying head work was reported to produce on the order of 100-105 RWHP, which contended well with the XRs. In 2017 Indian introduced their purpose built Scout FTR750 that has dominated American flat track to date. Harley also introduced a new flat track bike that year, which has performed subpar to date. The FTR750 engine design targeted an output of 110 CSHP (fuel-injected, OHC, 4-valve pent roof chamber) and although no formal HP specification has been provided by Indian, various writers/authors invited by Indian to view dyno runs have quoted 109 CSHP. Of course both of these fuel-injected engines have a significant advantage over the carbureted XR since the FI throttle bodies provide a significant increase in airflow over a carb.

https://www.cycleworld.com/indian-scout-ftr750-flat-track-racer-cycle-world-exclusive/

Along the way there have been considerably more powerful flat track bikes raced (Suzuki) without much success, because as is always the case, the rider has to get the bike’s power to the ground and still have a tire remaining after 25 laps, which is a formidable challenge.

And as these discussion always track along, there are generally more questions than answers, e.g., which dyno (Dynojet, Ssuperflow, or?), rear wheel or crankshaft, what correction factor etc. I have no answers beyond what is presented above, but I don’t see a 95 HP XR as a stretch at all.

Torque and horsepower are two different things. However the gearbox is a torque converter. If you raise the gearing high enough, you run out of torque. On dirt there is probably a balance to be maintained. A horsepower figure for a motor probably does not mean much. On dirt, there is always the balance between slide and drive to be maintained and the performance characteristics of the motor affect that.
On bitumen, the story is slightly different Handling comes more into the equation.

I have been to the dyno today. I have had Nigel at NRP make me a twin pipe exhaust and as my first race this year is on Saturday I thought it prudent to see where the fuelling was. I wanted to try a twin pipe setup as I couldn't get it out of my head that the PW3 cam which the JS2 fitted to my engine is based on, always seemed to be used with twin pipes and therefor that probably was how Mr Williams designed it to work best.

It seems that the general feeling is, that 2 into 1 pipes give more bottom end, but I figured that with a 920 motor that probably wouldn't be a problem so I went for the twin pipes.

The red line is a run with the 2 into 1 fitted in October and the bike hasn't been run until today with the twin pipes fitted which is the green line.

One advantage of having twin pipes is that the sniffer can be put in each pipe and any differences in fuelling between the cylinders identified, I ended up with a 260 main in the right hand cylinder and a 270 in the left, I don't know why there is a difference but it wasn't possible to find out if there was such a difference with the 2 into 1.

Anyway, 10 more torques at about 3650 revs and about 8bhp extra seems a bit of a surprise, what does the collective think?

storm42 said:

I have been to the dyno today. I have had Nigel at NRP make me a twin pipe exhaust and as my first race this year is on Saturday I thought it prudent to see where the fuelling was. I wanted to try a twin pipe setup as I couldn't get it out of my head that the PW3 cam which the JS2 fitted to my engine is based on, always seemed to be used with twin pipes and therefor that probably was how Mr Williams designed it to work best.

It seems that the general feeling is, that 2 into 1 pipes give more bottom end, but I figured that with a 920 motor that probably wouldn't be a problem so I went for the twin pipes.

The red line is a run with the 2 into 1 fitted in October and the bike hasn't been run until today with the twin pipes fitted which is the green line.

One advantage of having twin pipes is that the sniffer can be put in each pipe and any differences in fuelling between the cylinders identified, I ended up with a 260 main in the right hand cylinder and a 270 in the left, I don't know why there is a difference but it wasn't possible to find out if there was such a difference with the 2 into 1.

Anyway, 10 more torques at about 3650 revs and about 8bhp extra seems a bit of a surprise, what does the collective think?


View attachment 79597

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Individual tuning of the engine always produces the best results. . . .

Very interesting results.
I also found the greatest power with individual pipes vs a 2 into 1 ( Dunstall) or balanced stock.
That was with a stock 850 running up a steep hill full bore, using a GPS as speedo.
Glen

That’s very interesting Ralph. It’ll be an even bigger challenge keeping two wheels on the ground exiting the hairpins now

I would urge listeners against making a simple ‘one pipe versus two’ comparison here though as Nigel clearly knows his stuff and his 2:1 pipes probably are like yours!

As an added bonus I imagine its easier to keep a 2:2 quite isn’t it? Or doesn’t it work like that ?

And... pictures...?!?

storm42 said:

Anyway, 10 more torques at about 3650 revs and about 8bhp extra seems a bit of a surprise, what does the collective think?

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Surprise to who?

My separate pipe system was made by Nigel for a PW3 profile.

And yes Nigel, I do think it is a little easier on noise, but not much, I think it just because there is more silencer volume overall.

Oddly, back in the days of no silencing, but big volume 2 into 1 exhaust cans, the 2 into 1s were a little quieter.

Does NRP incorporate a megaphone into his 2:2 systems? I kinda assume he does...

worntorn said:

Very interesting results.
I also found the greatest power with individual pipes vs a 2 into 1 ( Dunstall) or balanced stock.
That was with a stock 850 running up a steep hill full bore, using a GPS as speedo.
Glen

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We had a chat about balance pipes and he said there was no difference to be seen on the dyno with or without, but I did point out that he didn't start his power runs until about 2500 rpm.

Fast Eddie said:

That’s very interesting Ralph. It’ll be an even bigger challenge keeping two wheels on the ground exiting the hairpins now

I would urge listeners against making a simple ‘one pipe versus two’ comparison here though as Nigel clearly knows his stuff and his 2:1 pipes probably are like yours!

As an added bonus I imagine its easier to keep a 2:2 quite isn’t it? Or doesn’t it work like that ?

And... pictures...?!?

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I think it is quieter but we will see on Saturday. There probably are not many places where I will make use of that low down increase but it may help, again we will see. He does build in a megaphone of sorts into the pipe but it is very shallow. Looks like a wheel barrow now.

SteveA said:

Surprise to who?

My separate pipe system was made by Nigel for a PW3 profile.

And yes Nigel, I do think it is a little easier on noise, but not much, I think it just because there is more silencer volume overall.

Oddly, back in the days of no silencing, but big volume 2 into 1 exhaust cans, the 2 into 1s were a little quieter.

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I will say Steve, that along with the nagging thought that Peter Williams designed his cam with twin pipes in mind, the fact that you, with all your years on this sort of bike run twin pipes, was a factor in going down this road.

Hi Ralph , impressive dyno figures, where did you go to for the dyno and what Comp ratio are you running?

It's not unusual for individual cylinders to operate best with different "jetting"

Something similar is common in some forms/classes of auto racing where it is permitted - so called "Sheet metal manifolds." These are custom made intake manifolds where each cylinder's intake runner is tailored in shape for that cylinder. IOW, the "standard" shape in an aftermarket performance manifold is designed to fit all engines of whatever type. In a sheet metal manifold, the plenum and the size and shape of the individual runners themselves are optimized for the particular engine. They can make substantial power gains over a standard "performance" manifold. They are, of course...expensive! Essentially, they are "adjusting" the air/fuel mix for each cylinder.

Moto55UK said:

Hi Ralph , impressive dyno figures, where did you go to for the dyno and what Comp ratio are you running?

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Hi Mike, hope you are well. I went to the same dyno you used, in fact you were mentioned. I think I have it around 11:1.

Have you sorted your motor yet?

Hi Ralph, I hope Russ hasnt been giving away all my secrets!

Work on "ole smokey" is progressing , but very slowly.
cylinders are re sleeved, pistons graphite coated, new rings and pushrods stuck in an US airport for 3 weeks, crank away having the output shaft ground and chromed to stop the bearing turning, valve springs on order the only thing missing is motorvation.
Went to Darley on monday to run the Manx in the thunderfest and got drowned out! Lots of wet kit and no laps, no motorvation to be found there then.

MexicoMike said:

It's not unusual for individual cylinders to operate best with different "jetting"

Something similar is common in some forms/classes of auto racing where it is permitted - so called "Sheet metal manifolds." These are custom made intake manifolds where each cylinder's intake runner is tailored in shape for that cylinder. IOW, the "standard" shape in an aftermarket performance manifold is designed to fit all engines of whatever type. In a sheet metal manifold, the plenum and the size and shape of the individual runners themselves are optimized for the particular engine. They can make substantial power gains over a standard "performance" manifold. They are, of course...expensive! Essentially, they are "adjusting" the air/fuel mix for each cylinder.

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Russ the dyno owner did say he will treat multi cylinder engines as separate engines where possible.

Moto55UK said:

Hi Ralph, I hope Russ hasnt been giving away all my secrets!

Work on "ole smokey" is progressing , but very slowly.
cylinders are re sleeved, pistons graphite coated, new rings and pushrods stuck in an US airport for 3 weeks, crank away having the output shaft ground and chromed to stop the bearing turning, valve springs on order the only thing missing is motorvation.
Went to Darley on monday to run the Manx in the thunderfest and got drowned out! Lots of wet kit and no laps, no motorvation to be found there then.

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I had an invite for the Thunderfest and forgot all about it, sound like my dodgy memory has helped for once. Ha no secrets, no point really, my engine is what it is although I think they are fairly similar. Hope the motivation returns.

storm42 said:

It had crossed my mind that two pipes designed for a Manx but made left and right might work.

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Two pipes similar to those on a 350 cc Manx might work - reverse cone megaphones. But I would not try to race with megaphones like those on a 500 cc Manx. At some stage, you must find a corner slow enough to make your motor fall out of the power band when you are on full lean. Slipping the clutch to get it back on power, can cause a disaster. I once raced a bike which had a power band from 5,500 to 10,500 RPM. If it fell out the bottom in a corner and I slipped the clutch to get it back, it would immediately go sideways. I never raced without anxiety. The slowest I could ride it in the slowest corners was about 50 MPH. My Seeley 850 is entirely different, I never have anxiety about racing it. With the 2 into 1 exhaust, it is always there when I need it.

My two pipe/ balanced pipe/Dunstall trials on a stock did not show a massive difference. Once jetted correctly the best combo managed 3 kmh more at the hilltop.
For most Roadriders that wee bit of speed doesn't matter.
It's probably more important for most to have the look and sound they like.
Racing is another thing, an extra 3 kmh is always useful!
I intend to try all three exhaust type plus a 4 th ( 1.5" separate) on the 920.
It has a stock cam, head, carbs and 9.5 to 1 comp. so the info will only be useful to a very small number of Norton owners, maybe just one!
Side note, I found an old discussion of cam types and dyno results from a few years back.
I was surprised to see that the PW3 made 5 HP less than the stock cam at 4500 rpm, same bike same dyno, I believe same day. The PW3 made 3 HP more than the stock cam at 6500.
The owner loved the feel of the new PW3 as the power falloff then the surge gave the sensation of great power.

Glen