Highest power exhaust for GSX-R750 L1 2011-2023?

[ptuomov]

I’ve got a lot of points in the areas that matter and interpolation for the flanks is hard to screw up.

The real-life engine power curve is pretty sensitive to the valve clearance, as is simulation.

In general, the fit between the simulation and chassis dyno results is pretty close, better than I expected.

[ptuomov]

With primaries shorter than 45cm, the torque curve dips in the middle of the power band. 38cm vs. 48cm primary simulated below…

Thank you for taking the time to show me your simulation results. They are very different than imagined. Obviously more going on than can be addressed with a simple equation.

I can rationalize the dip in the following way. If the exhaust does nothing, the current short equal-length intake runner length tubes (from GSX-R600 or Venturi Systems) tune a high pressure wave to overlap 11-12k and 13.5-15k. However, the intake is off tune 12-13.5k at overlap causing a torque curve dip. The best results come from exhaust primaries that tune a high vacuum at overlap to that 12-13.5k rpm range. This rationalization is consistent with simulated pressure traces.

[nitro2]

With primaries shorter than 45cm, the torque curve dips in the middle of the power band. 38cm vs. 48cm primary simulated below…

Thank you for taking the time to show me your simulation results. They are very different than imagined. Obviously more going on than can be addressed with a simple equation.

I can rationalize the dip in the following way. If the exhaust does nothing, the current short equal-length intake runner length tubes (from GSX-R600 or Venturi Systems) tune a high pressure wave to overlap 11-12k and 13.5-15k. However, the intake is off tune 12-13.5k at overlap causing a torque curve dip. The best results come from exhaust primaries that tune a high vacuum at overlap to that 12-13.5k rpm range. This rationalization is consistent with simulated pressure traces.

The best exhaust tunes a high vacuum at overlap from south of 9k all the way to north of 15k

[ptuomov]

Thank you for taking the time to show me your simulation results. They are very different than imagined. Obviously more going on than can be addressed with a simple equation.

I can rationalize the dip in the following way. If the exhaust does nothing, the current short equal-length intake runner length tubes (from GSX-R600 or Venturi Systems) tune a high pressure wave to overlap 11-12k and 13.5-15k. However, the intake is off tune 12-13.5k at overlap causing a torque curve dip. The best results come from exhaust primaries that tune a high vacuum at overlap to that 12-13.5k rpm range. This rationalization is consistent with simulated pressure traces.

The best exhaust tunes a high vacuum at overlap from south of 9k all the way to north of 15k

What it does below 11k rpm is irrelevant in this application. And 11-12k matters just for the start.

For the last thousand rpms of the power band, 14-15k, the mandated stock exhaust cam is clearly too short. In my opinion, this means that one wants to (1) have a lot of flow area and (2) start pulling a big vacuum earlier in the exhaust stroke than would otherwise be ideal. In any case, that’s my theory.

[nitro2]

What is limiting you to 15k, is there an rpm limit imposed by some rule, or is 15k simply where the powerband wraps up as the combo currently stands ?

[ptuomov]

What is limiting you to 15k, is there an rpm limit imposed by some rule, or is 15k simply where the powerband wraps up as the combo currently stands ?

Rules mandate stock camshafts and 15,000rpm rev limiter.

Also, given those two rules and the throttle body dimensions, the intake is also effectively fixed (hard to imagine the shortest trumpets not be optimal).

[gruntguru]

On setups using two short and two long intake runners it is unlikely the optimum exhaust primary length will be the same for all cylinders.
Adding the appropriate longer primaries to the two cylinders with longer intakes might shift the power peak too far, but the solution would be to reduce the stagger of both the intake and exhaust primary lengths.

[ptuomov]

On setups using two short and two long intake runners it is unlikely the optimum exhaust primary length will be the same for all cylinders.
Adding the appropriate longer primaries to the two cylinders with longer intakes might shift the power peak too far, but the solution would be to reduce the stagger of both the intake and exhaust primary lengths.

The first thing that came off the engine when that engine was installed to this race car by the previous owner was those unequal intake trumpets. It’s a race car.

[ptuomov]

I've been playing with the simulator some more. It appears that, at least with these stock cams, the primary pipes are the only thing that really matter to power at the relevant power band of 11-15k rpm. 4-1, sequential 4-2-1, 360 4-2-1, etc. all give very similar results if the primary is the same until the first collector and only thing that seems to make any difference at the collector extension / tailpipe is sealing it with a cork (power goes down if the seal is good!). Under 11k rpm, huge differences in the torque curve depending on how the collectors are set up, so for a street motorcycle this exhaust and intake trumpet design is a really interesting problem.

[pcnsd]

I've been playing with the simulator some more. It appears that, at least with these stock cams, the primary pipes are the only thing that really matter to power at the relevant power band of 11-15k rpm. 4-1, sequential 4-2-1, 360 4-2-1, etc. all give very similar results if the primary is the same until the first collector and only thing that seems to make any difference at the collector extension / tailpipe is sealing it with a cork (power goes down if the seal is good!). Under 11k rpm, huge differences in the torque curve depending on how the collectors are set up, so for a street motorcycle this exhaust and intake trumpet design is a really interesting problem.

I was recently going through the design and simulation of four stroke engines looking for useful bits and in chapter 6, (Empirical assistance for the Designer) Prof. Blair offers some interesting design guidance for simulation as well as some very simple equations for rendering staring inputs for both intake and exhaust lengths. The basic suggestion is to set the intake lengths so that the ramming pressures are maximized around your target peak HP and then use the exhaust to fill in the troughs in between ramming peaks. The intake formula uses 2 possible constants (ramming factors), to account for the difference between 2V and 4v designs (more or less... done with a bit more finesse) and claims a high degree of accuracy in the outputs therefrom. On the exhaust side the equation includes a calibration coefficient that must be refined through simulation and no claim of high fidelity without.
For your engine it suggest you are looking at about 212mm.

Capture1.JPG

[ptuomov]

I've been playing with the simulator some more. It appears that, at least with these stock cams, the primary pipes are the only thing that really matter to power at the relevant power band of 11-15k rpm. 4-1, sequential 4-2-1, 360 4-2-1, etc. all give very similar results if the primary is the same until the first collector and only thing that seems to make any difference at the collector extension / tailpipe is sealing it with a cork (power goes down if the seal is good!). Under 11k rpm, huge differences in the torque curve depending on how the collectors are set up, so for a street motorcycle this exhaust and intake trumpet design is a really interesting problem.

I was recently going through the design and simulation of four stroke engines looking for useful bits and in chapter 6, (Empirical assistance for the Designer) Prof. Blair offers some interesting design guidance for simulation as well as some very simple equations for rendering staring inputs for both intake and exhaust lengths. The basic suggestion is to set the intake lengths so that the ramming pressures are maximized around your target peak HP and then use the exhaust to fill in the troughs in between ramming peaks. The intake formula uses 2 possible constants (ramming factors), to account for the difference between 2V and 4v designs (more or less... done with a bit more finesse) and claims a high degree of accuracy in the outputs therefrom. On the exhaust side the equation includes a calibration coefficient that must be refined through simulation and no claim of high fidelity without.
For your engine it suggest you are looking at about 212mm.
Capture1.JPG

With 15k redline and throttle body rules, the intake is at a corner solution in terms of length and diameter: the optimum is the shortest possible trumpet on top of the throttle body element.

[nitro2]

Maybe I missed it but what was the application ? Mini/micro sprint, or motorcycle, or other ?

[ptuomov]

Maybe I missed it but what was the application ? Mini/micro sprint, or motorcycle, or other ?

https://www.instagram.com/p/CysLPd9NVe7/?img_index=1

Crosscart Xtreme. 420kg/926lbs minimum weight with the driver after the race. Solid rear axle and no differential so the inner rear tire needs to lose traction in the corners like in a gocart. Tracks are usually 1km stadium tracks with about 50% dirt/gravel and 50% paved/asphalt. Very popular and highly competitive class, especially in Northern Europe.

[nitro2]

Maybe I missed it but what was the application ? Mini/micro sprint, or motorcycle, or other ?

https://www.instagram.com/p/CysLPd9NVe7/?img_index=1

Crosscart Xtreme. 420kg/926lbs minimum weight with the driver after the race. Solid rear axle and no differential so the inner rear tire needs to lose traction in the corners like in a gocart. Tracks are usually 1km stadium tracks with about 50% dirt/gravel and 50% paved/asphalt. Very popular and highly competitive class, especially in Northern Europe.

Well those look like a real handful. Probably spend a significant % of their WOT time approaching or on the rev limiter, at least in the dirt sections.

[ptuomov]

Well those look like a real handful. Probably spend a significant % of their WOT time approaching or on the rev limiter, at least in the dirt sections.

Less than you’d think, because they have surprisingly good traction even on the dirt sections. You can hear in this video when the lead cars shift and hit the limiters at top speed:

https://m.youtube.com/watch?time_contin ... e=emb_logo

The cars have enough power that just a couple of horses don’t make a car a winner and driver is the most important part of the equation.