GB or no GB? - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 15 (permalink) Old 12-01-2019, 10:29 PM Thread Starter
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GB or no GB?

I have gone two seasons with the GB on, and with NO mufflers. This past season, I also had an ear shave and a reject.

Should I remove the GB? I would be running straight pipes without a crossover.

How much louder are we talking?

What, if any, is the change to performance? I have heard lost performance, gained performance, lost low end, gained high end, no change.


'89 VN750 Bought stock 4/12/18 @ 17805.9 miles $800. Wrecked once. 27454.5+ miles

Completed modifications:
Removed mufflers, straight pipes out of GB. 110/90 Front Tire. Pick-up coil mod. Shindengen MOSFET FH020AA r/r. USB Outlet + Voltmeter. Repainted Metallic Blue w/ new tank bages. Ear Shave (wo/ coasters) 40/140 NO shims 45MPG. MCCT conversion.

Planned modifications:
Custom LED turn signals. Windshield. Highway Pegs. (Possible) ignition adv. Tuxedo Mod. Ignition coil mod.
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post #2 of 15 (permalink) Old 12-02-2019, 12:46 PM
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Wide open pipes usually lose low-end torque, just making a lot of noise and going nowhere fast.

If you have straight pipes off the GB now, it probably won't get much louder, but maybe will change the exhaust note a bit. I started mine with just the GB on it, and it sounded like a 4-stroke single cylinder ATV with an open pipe.

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Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
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Last edited by Spockster; 12-02-2019 at 12:52 PM.
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post #3 of 15 (permalink) Old 12-02-2019, 02:05 PM Thread Starter
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I realized this morning, why not do the math to see what RPM ranges are affected?
NONE of this math takes backpressure into account. Only self and cross scavenging effects.
https://www.musclecardiy.com/perform...formance-math/

The front pipe length from cylinder to center of GB (or H pipe) is 45 in.
This results in a self-scavenging effect around 4900 rpm.
The rear pipe length from cylinder to center of GB (or H pipe) is 27 in.
This results in a self-scavenging effect around 7900 rpm.
The total pipe length from front cylinder to rear cylinder through GB (or H pipe) is 72 in.
The timing separation between cylinders is 305 deg and 415 deg.
This results in a cross-scavenging effect around 2500 & 3800 rpm.
The stock system is designed for a broad torque band, with tuning at 2500,3800,4900 & 7900.

Removing the GB and putting straight pipes on increases the pipe lengths.
Assuming stock debaffled mufflers (18" long) in stock position (6" from end of stock pipes).
The front pipe length from cylinder to tip is 58 in.
This results in a self-scavenging effect around 3900 rpm.
The rear pipe length from cylinder to tip is 41 in.
This results in a self-scavenging effect around 5200 rpm.
There is no cross-scavenging effect.
Based on this, there should be a good torque peak near 3900-5200 rpm, with losses at 2500 & 7900 rpm.
I would expect the increased flow would offset the 7900 rpm tuning, resulting in a torque loss only at 2500 rpm.
Adding an H-Pipe would probably bring back the stock tuning, plus better airflow and high rpm.

With short straight pipes (stock length) and no H-pipe, the self-scavenging effects occur at 6500 & 11900 rpm.
Not ideal for a cruiser, but should make a great drag bike, if you lengthen the rear pipe out to 27" (7900 rpm).
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'89 VN750 Bought stock 4/12/18 @ 17805.9 miles $800. Wrecked once. 27454.5+ miles

Completed modifications:
Removed mufflers, straight pipes out of GB. 110/90 Front Tire. Pick-up coil mod. Shindengen MOSFET FH020AA r/r. USB Outlet + Voltmeter. Repainted Metallic Blue w/ new tank bages. Ear Shave (wo/ coasters) 40/140 NO shims 45MPG. MCCT conversion.

Planned modifications:
Custom LED turn signals. Windshield. Highway Pegs. (Possible) ignition adv. Tuxedo Mod. Ignition coil mod.
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post #4 of 15 (permalink) Old 12-02-2019, 02:26 PM
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Killer mathematical explanation. I always heard that there was a loss in low-end torque by removing the GB, but seeing the effects of the self and cross-scavenging exhaust as "sweet spot" rpm's makes a lot of sense.

I run a stock exhaust because I like quieter bikes, but that's just me. I got the most performance gains out of a carb clean, and the pickup mod helped take advantage of the rejet on the top end. It doesn't seem like the exhaust is a bottleneck on these, they already seem to flow great at the upper rev range

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post #5 of 15 (permalink) Old 12-02-2019, 03:12 PM
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Quote:
... resulting in a torque loss only at 2500 rpm. ...
2300-2500rpm is right where I like to shift, most of the time.
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post #6 of 15 (permalink) Old 12-04-2019, 03:49 PM Thread Starter
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I might be incorrect on my math.
The self-scavenging comes from the length of pipe from cylinder head, to where it expands.
This expansion causes a negative pulse to go back up the pipe.
That expansion would actually be at the point of the GB or H pipe. Not the center of the GB or H pipe.
That means the two lengths are 38 and 20, giving 5800 and 10300 instead of 4900 and 7900.

From this site:
https://outbackjoe.com/macho-diverti...bigger-better/
"A larger exhaust changes the shape of the power curve. It makes it more peaky, but the area under the curve is reduced. Average power is less but peak power is more. This will present as an extra couple of kW of peak power on a dyno run as it provides lower resistance at peak exhaust volume. However, looking at the entire rev range, the larger exhaust has deviated from the optimal size and average power over the full rpm range will be reduced. Maximum torque may be less and low rpm will produce less power. Fuel efficiency will also be worse, since most of the time your engine operates in the bottom half of its rev range."

The math on the pipe diameter itself (which affects flow AND has an optimal tuning RPM) shows that the stock pipes are larger than optimal. A pair of 2" pipes can flow about 3 times the needed airflow, and have a small boost at 11,100 RPM. Stepping DOWN to a 1.5" pipe still flows almost twice what is needed, and has a small boost at 6100 RPM instead.

Quote:
Originally Posted by Spockster View Post
Wide open pipes usually lose low-end torque, just making a lot of noise and going nowhere fast.

If you have straight pipes off the GB now, it probably won't get much louder, but maybe will change the exhaust note a bit. I started mine with just the GB on it, and it sounded like a 4-stroke single cylinder ATV with an open pipe.
I missed this post earlier! Hmm...
Ya, that seems about how I'd describe it, but have a pretty decent sound when riding.

Quote:
Originally Posted by Spockster View Post
2300-2500rpm is right where I like to shift, most of the time.
I assume that's your RPM after shifting. That seems low to me.
I usually shift at 4k, dropping to 3k after shifting. Unless I'm having fun, then it's 6K to 4.7K.
I don't like giving it WOT until I hit 3K, to avoid lugging.
I don't think I'd miss the torque at 2500, since I'm not pushing it at that speed anyways.
I would miss the help at 7900 though. The torque is already falling off at this point, so losing more wouldn't be great. However, my updated math above says that this higher tuning point is actually > 10k.


'89 VN750 Bought stock 4/12/18 @ 17805.9 miles $800. Wrecked once. 27454.5+ miles

Completed modifications:
Removed mufflers, straight pipes out of GB. 110/90 Front Tire. Pick-up coil mod. Shindengen MOSFET FH020AA r/r. USB Outlet + Voltmeter. Repainted Metallic Blue w/ new tank bages. Ear Shave (wo/ coasters) 40/140 NO shims 45MPG. MCCT conversion.

Planned modifications:
Custom LED turn signals. Windshield. Highway Pegs. (Possible) ignition adv. Tuxedo Mod. Ignition coil mod.
Jason Pittenger is online now  
post #7 of 15 (permalink) Old 12-04-2019, 07:09 PM
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When I'm shifting at 2500rpm, I'm generally taking it easy, but short-shifting and getting up to the speed limit quick. It feels like it's right in the sweet spot of torque, not lugging.
I've seen it over 10k rpm, but that's a good way to get into trouble, or a ticket. If you're first in line at the green light, you can break away from traffic without telling half the city.

Actually got to liking the 2300-2500 shift, just feeling the torque and enjoying the sounds, and it keeps me out of trouble with the law. If I come through the gears at high rpm, they're going to know from quite a distance. This is with stock pipes, but the earshaved carbs make their own noise.

2003-21k mi
Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
Repair Manual: http://www.mediafire.com/file/mj7z81..._Catalogue.pdf

Owner's Manual: http://www.mediafire.com/file/nscb5f...ers+Manual.pdf
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post #8 of 15 (permalink) Old 12-04-2019, 07:27 PM
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If you want a fast bike, buy a fast bike. Don’t care about the math, I, along with the general public don’t like loud bikes. Running straight pipes with no muffler is just giving the finger to everyone else on the road. If that’s your intent, then don’t be concerned over how much power you got on a 63hp motor.
JMHO

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post #9 of 15 (permalink) Old 12-05-2019, 11:24 AM
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My neighbor has a Kawi cruiser, not sure of the size, not a 750 but not the biggest cruiser either, parallel twin engine. It's got straight pipes cut off just behind the footpegs (which are not forward like ours).

It's the loudest, and slowest, thing on two wheels. My VN would leave it in the dust using only 2nd gear. It's just unreal how that bike struggles to move, but I can hear him crossing the nearest bridge which is at least a mile away, maybe two.
Jason Pittenger likes this.

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Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
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Owner's Manual: http://www.mediafire.com/file/nscb5f...ers+Manual.pdf
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post #10 of 15 (permalink) Old 12-06-2019, 03:31 PM Thread Starter
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Quote:
Originally Posted by Knifemaker View Post
If you want a fast bike, buy a fast bike. Don’t care about the math, I, along with the general public don’t like loud bikes. Running straight pipes with no muffler is just giving the finger to everyone else on the road. If that’s your intent, then don’t be concerned over how much power you got on a 63hp motor.
JMHO
You're absolutely right. No amount of mods (short of a super/turbo charger or Nitro) is going to make this a fast bike. Even then, might as well just buy a different bike!

I just enjoy knowing absolute numbers (2500 rpm) instead of "low end" torque. The joy (for me) is in the results, the math, and the modifications. I can't leave something alone. I have an itch.

I am louder than stock, but I don't need the loudness of straight pipes.


'89 VN750 Bought stock 4/12/18 @ 17805.9 miles $800. Wrecked once. 27454.5+ miles

Completed modifications:
Removed mufflers, straight pipes out of GB. 110/90 Front Tire. Pick-up coil mod. Shindengen MOSFET FH020AA r/r. USB Outlet + Voltmeter. Repainted Metallic Blue w/ new tank bages. Ear Shave (wo/ coasters) 40/140 NO shims 45MPG. MCCT conversion.

Planned modifications:
Custom LED turn signals. Windshield. Highway Pegs. (Possible) ignition adv. Tuxedo Mod. Ignition coil mod.
Jason Pittenger is online now  
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