Epoxy?

[Anonymous]

Filling in Midrange Hole

Mark:

To fill in your midrange hole - I found increasing the exhaust canister VOLUME to be most important. I fiddled with both megaphones and canisters.....read on......

(I KNOW....theory says the tapered megaphone reduces backwards "reflected" pressure pulses.)

The problem is the VOLUME is so small on the megaphones......and if you make the megaphone volume larger....the tapered cone grows so big on the end that people think you have your pets head in there. OK....it's a stretch.

Get a large exhaust canister - steel if possible so you can braze , weld , modify brackets and adaptors on it.

I modified a steel 1993 FZR600 canister so the smaller outlet pipe within the large tip diameter "floated" within the larger diameter. Most came stock with just the small pipe flowing and a steel wall between the smaller pipe and the larger canister tip. The floating (used an internal tri-pod) allowed exhaust gases rushing out the tip to create a vacuum on the smaller pipe.

I also added additional internal pipes between the canister chambers (there are 3 chambers typically) and connected all the chambers with these internal pipes. Cut both ends off to do it then re-attached the ends.

Lastly, I grew the larger tip diameter from 1 3/4 inches to about 2 1/4 inches. This gave me a substantial boost in HP (+7 peak)

Bike had a deep sound and fairly quiet around town. When I got on it I got a little of the "ripp" that racer pipes have but MUCH less noise.

For anyone who knows 1982 GS750EZ- they come stock with 112 mains. I was running 130 mains with this setup and the primary intake box inlet opened up to about 3.6 square inches - up from stock and CO was about 3 to 4% - spot on for reliable street bike. Whoo hoo.....

I am pleased to hear my BMEP or output per liter was good. My figures were dyno'ed by the way.

Another key is that IF you open up your primary airbox inlet this much - you WILL have to run a more agressive taper on needles. THAT needle change gave me +15 HP in the midrange and finally allowed me to have lean enough crack off idle mixture and aggressive midrange acceleration. Gas mileage was about 43 to 48 MPG. You learn alot in 4 years of errors. Your setup and results may vary.... and require different changes -Dieter
PS: Compliments to the "duaneage" on velocity stacks and contorted box shapes.etc..etc.....his whole post is exactly right.

[Anonymous]

To fill in your midrange hole - I found increasing the exhaust canister VOLUME to be most important. I fiddled with both megaphones and canisters.....read on......

Ah, but did you fiddle with a true megaphone or just the V&H street meg system? Both the V&H and the Kerker megaphone systems are really just tapered core mufflers, as far as I know. They do not have a true megaphone followed by a muffler core, which is the best way to set it up. This means they behave like a typical straight tailpipe system, with a very small volume muffler, as you noted. Not really that good for performance. Mine will be a true megaphone with reverse cone on the end, followed by a small plenum chamber, then the muffler section.

On a straight tail pipe system, the muffler volume is very important to performance, as you found.

I am pleased to hear my BMEP or output per liter was good. My figures were dyno'ed by the way.

Your peak torque BMEP was excellent. Even the best current sportbikes are only making 195psi or so at their torque peaks, so you are in very good company.

Another key is that IF you open up your primary airbox inlet this much - you WILL have to run a more agressive taper on needles. THAT needle change gave me +15 HP in the midrange and finally allowed me to have lean enough crack off idle mixture and aggressive midrange acceleration.

My airbox was modified by the shop that the PO bought the bike from. The rear half is removed, with a large single K&N filter in its place. The jetting was reworked to suit this and the pipe. The bike dyno'd at 102rwhp @ 8500rpm and 69ft*lb @ 6500rpm. Not nearly as good as your effort... :? But I plan to improve on some of that. I don't know what the jetting specs are, aside from a note on the dyno chart that gives the main jet size as 132. I plan to tear the carbs down this spring to clean them and see what is going on inside them. I expect to have to increase the mains when I add my new tail pipe, as well, since I will be increasing the flow area by a factor of 6 or so... Depending on the carbs, I may look for a Dynojet kit to get the adjustable needles if the stock ones are still in there.

I never got much time on it last year (about 150km) because the fork seals are shot and the front brake pads are contaminated with fork oil, but it seemed to run pretty good, with maybe a slight lean surge at part throttle cruising. So, hopefully not too much sorting before I can start wearing it out.

Mark

[Anonymous]

Mark:

Yep - at 77 peak rear wheel this is like 112.9 for an 1100 motor.

HOWEVER - frictional losses in an 1100 motor might be higher and account for your 102 rwhp figure. Remember the 750 is a plain bearing motor as well. Do you agree? Your thoughts?

Next 60 at 5500 rpm is an estimate for the rpm, not the hp. The graph looked like it occurred at 5000 rpm but that is impossible. It was probably closer to 6000 rpm so I picked 5500 in the middle. If you see some variations in my postings-that is where that comes from. What would the BMEP be if it were 60 rwhp @ 6000rpm? List the formula too please.

Lastly, the 77 rwhp for a BMEP of 160 was a compromise. Running my lobe centers at 107 / 108 or thereabouts was not a setup for peak horsepower but rather for peak torque. If I had shifted my cam timing up around 110's I would have gotten peak horsepower. I would guestimate I could have gotten maybe 80 to 82 rear wheel horsepower. What do you think?

What fun. Incidently, I left the primary airbox on and ran a cone shaped large K & N filter - probably the same filter as you. The 750 and 1100 airboxes are identical - so I get a little "advantage" since my airbox size is larger relative to the swept volume of 1 cylinder. Heh heh.

My guess is if I opened the inlet orifice to 3.6 in sq. to match the outlet orifice of 3.6" sq. - that was the best I could do for orifice flow. The outlet size max limit prevented any additional gain by increasing the inlet side of the airbox over the 3.6 figure. To me it seemed intuitively better to keep the primary airbox for "quiet air" instead of removing completely. All of this assumes that the secondary air box (the 1 into 4 with velocity stacks) was untouched. Give me the formula and an answer on friction and so on questions. (6000 rpm vs 5500) and cam timing etc .....

[Anonymous]

HOWEVER - frictional losses in an 1100 motor might be higher and account for your 102 rwhp figure. Remember the 750 is a plain bearing motor as well. Do you agree? Your thoughts?

I wouldn't think that the losses would be much different. Maybe 1-2%, but not the 10% difference we see here. Don't forget, you have also cleaned up the head and added bigger cams and degreed them, while mine is stock aside from the intake and pipe. I want to degree the cams this spring and I hope to get another 3-4hp from that alone. If I was to assume another 5% gain from a port clean up, I would be right at the 112rwhp number. It just shows how good the basic GS four valve design was to achieve these sorts of numbers with very basic tuning.

What would the BMEP be if it were 60 rwhp @ 6000rpm? List the formula too please.

For 60hp @ 6000 rpm the BMEP is 176psi. Still pretty good for an old air-cooled beast, but not as amazing as the 192psi number...

The formula is:

HP = (Displacement x RPM x BMEP) / 793,000

Displacement = in^3 (this is total engine displacement)
RPM = RPM (obviously...)
BMEP = psi

I have also reworked it to get:

Torque = (BMEP / 151) x Displacement

Torque = ft*lb

These are both useful forms, since the twp places you are usually interested in bmep are at the torque and HP peaks. The bmep is maximum at the torque peak, since this is where your engine efficiency is maximum.

If I had shifted my cam timing up around 110's I would have gotten peak horsepower. I would guestimate I could have gotten maybe 80 to 82 rear wheel horsepower. What do you think?

I can't say. I have no experience with changing lobe centers and dyno testing the results. It certainly sounds like a reasonable estimate, given the effects I have seen elsewhere from changing lobe centers.

My guess is if I opened the inlet orifice to 3.6 in sq. to match the outlet orifice of 3.6" sq. - that was the best I could do for orifice flow. The outlet size max limit prevented any additional gain by increasing the inlet side of the airbox over the 3.6 figure. To me it seemed intuitively better to keep the primary airbox for "quiet air" instead of removing completely. All of this assumes that the secondary air box (the 1 into 4 with velocity stacks) was untouched.

I would enlarge the primary inlet more, to keep the airbox pressure as close as possible to atmospheric. I would think a ratio of 2:1 for the primary to seconday openings would be enough. You may be right about keeping the primary box and increasing the inlet area. Mine was done when I got it, so I will never know. Having a reservoir of still air is a good thing for carburation and performance. I figure I am not too badly off, as the filter is pretty well sheltered by the side panels and seat.


Mark

[Anonymous]

Thanks Mark.

When I talked about the airbox - I did not mean "sheltered air. Quiet air term meant that I have a sealed volume between the carb intake and the opening to the outside. This sealed volume acts as a damper to the inevitable shock waves that occur at any transition point. Your transition point would be at the filter.

Airboxes act like mufflers, or electrically, capacitors. I don't know all the theory, just that it is important to have quiet air unless running a tuned intake system which naturally has an RPM range for best performance.

I once had a fish tank with one of those vibrating pumps for the air in the fish tank. The tank buzzed all day long. I finally took a piece of aluminum pipe I had, cut a foot length and spliced it between the pump and the air tank. Immediately the noise stopped. The pipe acted as a reservoir or muffler and completely damped out the pumps noise. An airbox does the same thing which allows the bike to have better midrange since it quiets the intake pulses and assures intake air pressure is a constant. This is why it helps jetting too, since it is easier to find correct jetting related to a more linearly proportional intake vacuum instead of adding tuned pressure pulses to the mix.

If you can, pick up the book, "Scientific Design of Intake and Exhaust Systems". Much information in there, INCLUDING about megaphones and reverse megaphones.

Another thought....if you ran the stubby competition baffle in the Kerker systems, this allowed the start of the muffler to truly act as a megaphone AND the tip of these baffles had a small "reverse megaphone shape as well. The book I mentioned talks about muffler design AND optimum taper angle.

I believe done right, you can get as much flow AND a fatter power band with a properly setup airbox as with pods. - Dieter.

[Anonymous]

When I talked about the airbox - I did not mean "sheltered air. Quiet air term meant that I have a sealed volume between the carb intake and the opening to the outside. This sealed volume acts as a damper to the inevitable shock waves that occur at any transition point. Your transition point would be at the filter.

I understand. It does help to have the box if you can use it to tune for the resonances you want. New sport bikes are doing a great job of this.

If you can, pick up the book, "Scientific Design of Intake and Exhaust Systems". Much information in there, INCLUDING about megaphones and reverse megaphones.

I will take a look for it locally.

I believe done right, you can get as much flow AND a fatter power band with a properly setup airbox as with pods.

No question. The Jap bikes are doing really well in this regard, making tremendous power with super smooth torque curves and no noise. It is simply amazing that the current liter bikes make 150+rwhp, run on pump gas and come with a warranty. All while being as quiet as your neighbour's Taurus.

The only kink in this is that the airbox's put onto our bikes were not nearly as well refined and more power can be had simply by adding pod filters and freeing up the breathing. You may have found a very good compromise by modifying the airbox to increase flow, while retaining most of the benefits. After the many discussions here about jetting with pods, I am certainly going to keep going with my current set up until I feel there is no more to be had elsewhere. And next winter I may pull the head and have a go at cleaning the ports and trying the bead blast finish on the port walls for the finishing touches to a great motor.

This has been a great discussion, Tony.

Mark

[duaneage]

When I talked about the airbox - I did not mean "sheltered air. Quiet air term meant that I have a sealed volume between the carb intake and the opening to the outside. This sealed volume acts as a damper to the inevitable shock waves that occur at any transition point. Your transition point would be at the filter.

I understand. It does help to have the box if you can use it to tune for the resonances you want. New sport bikes are doing a great job of this.

If you can, pick up the book, "Scientific Design of Intake and Exhaust Systems". Much information in there, INCLUDING about megaphones and reverse megaphones.

I will take a look for it locally.

I believe done right, you can get as much flow AND a fatter power band with a properly setup airbox as with pods.

No question. The Jap bikes are doing really well in this regard, making tremendous power with super smooth torque curves and no noise. It is simply amazing that the current liter bikes make 150+rwhp, run on pump gas and come with a warranty. All while being as quiet as your neighbour's Taurus.

The only kink in this is that the airbox's put onto our bikes were not nearly as well refined and more power can be had simply by adding pod filters and freeing up the breathing. You may have found a very good compromise by modifying the airbox to increase flow, while retaining most of the benefits. After the many discussions here about jetting with pods, I am certainly going to keep going with my current set up until I feel there is no more to be had elsewhere. And next winter I may pull the head and have a go at cleaning the ports and trying the bead blast finish on the port walls for the finishing touches to a great motor.

This has been a great discussion, Tony.

Mark

The air in the airbox is reactive in nature and has impedance. THe air has compliance, meaning it can be compressed and it will expand at a given rate, forming a mechanical capacitor, but it also offers resistance to this effort. This gives it a reactive value.

The intake ports entering the airbox pull air from the space and the frequencies at which this occur varies greatly. At higher frequencies the airbox compliance falls and allows flow but at a price. If shockwaves develop into harmonics there will be standing waves inside the box, where an oscillation will occur. smooth corners, curves and unequal walls prevent development of standing waves.

At lower speeds the air volume becomes more reactive and that is where the damping comes in. Pods have almost no air volume so your on your own. The close proximity to the other carbs means they WILL be affected by the pulses from the adjacent carb.