TW200 Forum banner

1 - 20 of 22 Posts

·
Registered
Joined
·
24 Posts
Discussion Starter #1
I decided to pull the quiet core out of my DG exhaust just to see what it sounds like. It was too loud for me, so I put it back in. So my question is just for curiosity.

When the quiet core was removed, the throttle lagged and there was low end power loss. If I wanted to run the bike without the quiet core, what would I have to change to eliminate the lag and get back low end power? Is some back pressure necessary or does the carb just need to be tuned to compensate?
 

·
Registered
Joined
·
7,046 Posts
How does it work that back pressure makes more torque? I have heard that back pressure is important but why? To me it doesn't make sense.

Maybe what I think back pressure is is all wrong.
 

·
Registered
Joined
·
24 Posts
Discussion Starter #4
To be honest I don’t understand how or why back pressure is needed. So if you get a DG without the quiet core, does it have a different insert with a wider diameter opening?
 

·
Registered
Joined
·
72 Posts
While I am no expert, I have always thought that this back pressure/torque relationship is a relic of the 2-stroke days where exhaust dynamis were incredibly important and determined where your power band was. I know on my RD400 with aftermarket exhaust, you defintely knew when the bike "came up on the pipes", and not always in a good way!

Everything I have ever read about 4-stroke performance mods (gas and diesel) has always involved lessening the exhaust back pressure. Yes, H-pipes, tri-Ys, and anti inversion cones are used but they are foe balancing exhaust pulses and/or creating better exhaust scavenging. Logically, if exhaust back pressure was required for torque, why would dragsters and aircraft ose straight pipes?

I think you were experiencing an air/fuel mixture issue, plain and simple. But like I said, I am not an expert and there are those on here that would be better to listen to.
 

·
Registered
Joined
·
275 Posts
I'm no expert either. I think you need to hold the exhaust back so that the intake air is slowed down. This would keep just a little exhaust in the cylinder to keep so much new air fuel from getting in. Same as running a smaller carb to increase low end. I know go cart racers use the disk type mufflers to tune the amount of back pressure for different tracks. More back pressure more low end for getting off the corner. Less pressure higher speed on entry.
 

·
Registered
Joined
·
40 Posts
Read this article by Kevin Cameron: https://www.cycleworld.com/everything-youve-ever-wanted-to-know-about-motorcycle-exhaust
KC has probably forgotten more than most of us will ever know. I continue to subscribe to the now 'boutique' CW magazine, solely for his articles.
Exhaust design was the black art for two strokes... and being able to ride a radically tuned one 'on the pipe', a brief but highly enjoyable experience. Turbo cars (think 70's Porsche) acted the same... kept in the zone... they were FUN. Racers were able to keep them in that zone...
Modern vehicles are using variable valve timing, intake tract length and today's turbos have continuously variable vanes built in to provide more or less boost as required. Take a modern 'EcoBoost' 4 banger Mustang for a spin. Even my 11 year old Buell has a variable exhaust! A servo motor and cable opens a shorter path out of the muffler... ahhh computers. Computers now control enough variables to make a modern engine powerful and 'usable' over a broad range... not just a sliver of the tach.
None of this will matter when we go electric... 100% torque at bottom of rpm range, 100% horsepower at the top...

Greg, did you make that 'tunable'? It looks like you could rotate the inner sleeve, aligning different holes/ sets of holes. Did it work? Or, was there just one sweet spot?
 

·
Registered
Joined
·
1,939 Posts
Mr. Kayak -
It's great to find another fan of Kevin Cameron...one smart MoFo!
 

·
Registered
Joined
·
336 Posts
The term "backpressure" is a bit of a misnomer in a 4 stroke context. You need to maintain escape velocity in the exhaust flow to scavenge as much burnt exhaust from the cylinder before the exhaust valve closes as possible before the intake charge enters the cylinder. The more fresh air and fuel you have in the cylinder, the bigger the bang, the more power. Smaller exhaust generally has higher velocity and pressure, larger exhaust has lower velocity and pressure. You need to balance the exhaust flow with the intake flow to maximize the available fresh air and fuel charge in the cylinder.

When you make a drastic exhaust change like mentioned in the first post, you lose exhaust exit velocity which leaves burnt exhaust in the cylinder when the exhaust valve closes, which makes it fall flat on its face at low RPM because your available combustible fuel/air charge has been reduced. It's fine at higher RPMs because you are pushing the flow faster due to increased piston speed. Depending on how much the flow has been altered, opening it up typically shifts your powerband up in the RPM range while restricting it shifts your powerband down in the RPM range.

Ideally you want to balance intake flow and exhaust flow to match your engines operational RPM range. If you regularly wind it up to 14k RPM, open it up. If you lug it around at 3k all day, leave it stock. If you're looking for more power over the whole operating range, then you need to address intake, intake ports, intake valve(s) size, exhaust valve(s) size, exhaust ports, and exhaust system, then balance that with a cam profile that makes use of all these changes to time the valvetrain appropriately to maximize the intake charge with as little spent fuel/air as you can manage. Small gains can often be had from small changes without having to touch valve sizes or cam profile, as manufacturers typically err on the side of emissions standards and ease of production over pure performance. On more complex engines, manufacturers have been able to maximize output over a wider RPM range by using things like variable valve timing, variable intake plenum runners, butterfly valves in the exhaust that change flow, etc...

I did notice when I had my TW exhaust off to wrap it, that the downpipe where the flange that meets the head is welded on could definitely use a bit of minor porting work to reduce turbulence, which will promote more linear flow and should make the whole system operate more efficiently without significantly altering its powerband. Small gains, but I'll hold off on that until I do a carb swap and maybe some intake tweaks. Might try to talk a local shop into letting me use their dyno.

Similarly, 2 strokes are more sensitive to this because of the more connected nature of the intake to exhaust path and require tuning of the exhaust which manipulates the resonance of the pressure waves in the exhaust flow to facilitate exhaust scavenging.

All of this pretty much goes out the window when you slap some form of forced induction on there. Superchargers/turbochargers force-feed air and fuel into the cylinder which pushes the spent exhaust out, and you don't have to rely so much on tuning your exhaust to maintain escape velocity.
 

·
Registered
Joined
·
119 Posts
While I am no expert, I have always thought that this back pressure/torque relationship is a relic of the 2-stroke days where exhaust dynamis were incredibly important and determined where your power band was. I know on my RD400 with aftermarket exhaust, you defintely knew when the bike "came up on the pipes", and not always in a good way!

Everything I have ever read about 4-stroke performance mods (gas and diesel) has always involved lessening the exhaust back pressure. Yes, H-pipes, tri-Ys, and anti inversion cones are used but they are foe balancing exhaust pulses and/or creating better exhaust scavenging. Logically, if exhaust back pressure was required for torque, why would dragsters and aircraft ose straight pipes?

I think you were experiencing an air/fuel mixture issue, plain and simple. But like I said, I am not an expert and there are those on here that would be better to listen to.
I had a 78 RD400 (bought new in 1978, loved that bike), acceleration went -> tach 0-4500 like a four stroke, on the pipe 4500-redline instantaneous. Like strapping a rocket on your butt lol.
 

·
Registered
Joined
·
2,209 Posts
Almost killed myself on an RD350 some years ago. It came up on the pipe while in a turn and the flimsy chassis was not up to the task to keep all the important bits pointing in the right direction.


I had a 78 RD400 (bought new in 1978, loved that bike), acceleration went -> tach 0-4500 like a four stroke, on the pipe 4500-redline instantaneous. Like strapping a rocket on your butt lol.
 

·
Registered
Joined
·
2,562 Posts
How does it work that back pressure makes more torque? I have heard that back pressure is important but why? To me it doesn't make sense.

Maybe what I think back pressure is is all wrong.
i don't think it makes more torque. i do know you can loose torque by eliminating back pressure
 

·
Registered
Joined
·
2,562 Posts
Read this article by Kevin Cameron: https://www.cycleworld.com/everything-youve-ever-wanted-to-know-about-motorcycle-exhaust
KC has probably forgotten more than most of us will ever know. I continue to subscribe to the now 'boutique' CW magazine, solely for his articles.
Exhaust design was the black art for two strokes... and being able to ride a radically tuned one 'on the pipe', a brief but highly enjoyable experience. Turbo cars (think 70's Porsche) acted the same... kept in the zone... they were FUN. Racers were able to keep them in that zone...
Modern vehicles are using variable valve timing, intake tract length and today's turbos have continuously variable vanes built in to provide more or less boost as required. Take a modern 'EcoBoost' 4 banger Mustang for a spin. Even my 11 year old Buell has a variable exhaust! A servo motor and cable opens a shorter path out of the muffler... ahhh computers. Computers now control enough variables to make a modern engine powerful and 'usable' over a broad range... not just a sliver of the tach.
None of this will matter when we go electric... 100% torque at bottom of rpm range, 100% horsepower at the top...

Greg, did you make that 'tunable'? It looks like you could rotate the inner sleeve, aligning different holes/ sets of holes. Did it work? Or, was there just one sweet spot?
no, not tunable so to speak. i did tune it for exhaust note , to where i wanted it to sound. i put a 3/4" pipe inside the baffle and drilled holes until it sounded the way i wanted. i didn't notice any change in performance
 

·
Registered
Joined
·
96 Posts
Yeah , I done the same on end cap but did re jet and shim needle, it’s all to do with pressure waves etc, the best information on exhausts that I have read is on 2- stroke expansion chambers, there shape and size determines where the power torque curve is, hope this helps
 

·
Registered
Joined
·
40 Posts
Nihil, funny you say that it needed work in the head pipe area. I picked up a stainless 225 pipe from eBay and polished it up nice and shiny on the outside. It looked pretty filthy inside, so a trip into the blast cabinet cleaned it out well, but I found the first inch or two filled with weld slag balls adhered to the wall. Die grinder with sand paper rolls used for porting polishing actually had to work to remove it. Of course, I smoothed out the weld and transition area. Bet I've got a 1/1000th of a horsepower more now :D
'Well enough' should not be left alone... it needs to be screwed with until it's 'just right' or horribly ruined...
 

·
Registered
Joined
·
40 Posts
no, not tunable so to speak. i did tune it for exhaust note , to where i wanted it to sound. i put a 3/4" pipe inside the baffle and drilled holes until it sounded the way i wanted. i didn't notice any change in performance
I spotted the inner tube and hoped you'd come up with a noisy /quiet exhaust mod. Turn the 90 tip out to the side for loud, point it down for stealthy quiet operation... then I could copy and overcomplicate it with some sort of remote control...
 

·
Registered
Joined
·
1,865 Posts
To be honest I don’t understand how or why back pressure is needed. So if you get a DG without the quiet core, does it have a different insert with a wider diameter opening?
To answer this question yes DG has three sizes of inserts. I think 1” is the biggest.

To answer your other question I think you can tune out the lag with different jets and washers on the needle. I removed the large insert and replaced it with the smallest one.
 
1 - 20 of 22 Posts
Top