Significant Power Loss w/ Full Exhaust?

15118 Views 15 Replies 12 Participants Last post by MNNTHBX, 9 mo ago

Omarvelous

Discussion Starter · Jan 11, 2014

I've seen a lot of posts about improved top end power when switching to a full exhaust, however not many posts about the flip end...

How noticeable (if any) is the loss of power on the low end and/or flat spots on any parts of the power band?

Thanks in advance for your feedback.

Sent from my iPhone using HondaGrom.net

1 - 16 of 16 Posts

SnafuPossum

· #2 · Jan 11, 2014

any motor with less back pressure in the exhaust is going to sacrafice low end grunt(torque) for top end horsepower and the grom is no exemption to that. but considering the only time your in a low rpm is when you first take off that extra top end is a no brainer to me. its not like say a car where your always cruising around at the very gutter bottom of the powerband where you need that torque to speed up. these motors like most any kind of honda motor is made to rev so why not give it power where it likes it?

mattallac

· #3 · Jan 12, 2014

full exhaust ( on this bike ) kills the torque , and torque is what you need with this little gutless wonder . On the race track I could see not needing a lot of torque but on the street torque is all I want . My bike was a dog with a tyga moto magot full exhaust . It seems like most of the people on this forum are noobs or scooter folk , I'm a motorcycle guy who believes in function before fashion . If you want the torque from a full exhaust you need a pre muffler for the back pressure to get your torque back . It's funny ,the tyga guy tried to tell me about dyno numbers , I told him the bike is a dog with the full system and add a pre muffler . He never chimed in again ........... be careful with full exhaust and a bike that is not modified .

Reactions: 1

FXCLM5

· #4 · Jan 12, 2014

LOL i have a full yoshi, bike runs faster on the top end, i also have a knn, about low end you need to rev the clutch higher and dump the clutch problem solved.

The_Outrider

· #5 · Jan 12, 2014

Few really understand the basic physics of exhaust tuning.

At lower revs, you want efficient scavenging; at peak rpm, minimal restriction. Backpressure reduces efficiency; it's also virtually unavoidable without making ridiculous compromises. IOW, there are worse things than a little backpressure. Big & loud doesn't necessarily mean more efficient; what looks good in photos and assaults eardrums very well may not deliver the goods.

Most aftermarket exhausts are louder than they need to be, while delivering disappointing seat-of-the-pants results. That's because they have oversized headpipes, sometimes grossly oversized. That kills scavenging, taking efficiency - throttle response and power - right down the dumper with it. Exhaust gas velocity has to be maintained or the motor will fall on its face. Then, there's the effect on A:F ratio; if that's not maintained (via remapping the ecu) the engine is hamstrung no matter how well the pipe might be. Reduced breathing at the lower end and less-than-optimal A:F right through the high end of the revband can reduce average power, bigtime...what mattallac was talking about. Yes, such exhausts can deliver peak numbers...not very useful on the road, when the engine delivers less usable power below peak - where you spend most of your time.

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mattallac

· #6 · Jan 13, 2014

Thanks outrider , not a lot of people understand how a dyno works ether . The dyno has a roller that delivers a load to the rear wheel , in no way does this simulate real world riding ( up hill , down hill and flat land ,no variable of body weight on the bike ). At the dyno shop I work at there is usually no rider on the bike , the same as most of the testing done by these after market companies .The dyno is just a tool for tuning not the end all .FXCLM stated "dump the clutch ",witch can cause excessive wear on the clutch frictions and steels i.e. early clutch replacement . I guess the focus for an exhaust is don't settle with an exhaust that kills the bottom end , has poor to no r&d and has an exhaust note that a race track would not let you ride because of excessive DB levels . This bike is so new its a money grab for all these aftermarket companies to create anything to sell , it makes no matter if it makes less or neutral power as long as it looks good and it sells . "A fool and his money are soon parted "

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mr_e

· #7 · Jan 13, 2014

Finally some intelligent discussion on the matter!

My butt dyno says I get to top speed faster with my 600$ fiddy exhaust, no really, it does!

Sent from my HTC6990LVW using Tapatalk

RaZed1

· #8 · Jan 13, 2014 (Edited)

Seat of the pants I noticed no loss of low end with the M4, a slight boost in midrange acceleration, and I picked up 2-3mph on the top speed. It's not a dramatic change but definitely noticeable.

You're running the grom pretty much to the max just to keep pace with traffic, so any top end advantage is welcome.

Think of torque as "how much work a motor can do" (rotational force) and horsepower as "how quickly that work can be done" (RPMs). Power is a function of work over time. The classic definition of 1 horsepower is 33,000 pound-feet/minute. Or in other words, a 1hp engine powering a winch could lift 330lbs, 100 feet, in one minute. Torque can be multiplied with gearing, power cannot.

Say you got a sportbike and a cruiser that happened to weigh the same. They both have the same torque, but the sportbike has double the power. In theory, all else constant, they would reach the same top speed, but the cruiser would take twice as long to reach it. It can do the same job, just not as quickly.

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BKnight

· #9 · Jan 17, 2014

Great thread! I've been on and off the dyno for the last 2 months working on that right combination! Great peak power combined with mid range gains and no low end loss. It's very tough to get the whole curve to be better.

Omarvelous

Discussion Starter · #10 · Jan 18, 2014

mattallac said:
full exhaust ( on this bike ) kills the torque , and torque is what you need with this little gutless wonder . On the race track I could see not needing a lot of torque but on the street torque is all I want .

This is the main reason why I created this thread. I was looking to gather your opinions on if having the full exhaust (non-tuned) will take away that much of the torque, pep, and fun-factor of the Grom's characteristics.

With my primary purpose of my Grom, I'd love to have the extra power up top, but not if it makes the bike feel boring while getting up there.

Loving the feedback btw. Thanks guys.

"Anybody can wack a fist full of throttle & ride in a straight line... You'll see who the REAL riders are when the road bends. LIFE is graded on a turn."

IG: Omarvelous619

Daffy

· #11 · 10 mo ago

RaZed1 said:
Seat of the pants I noticed no loss of low end with the M4, a slight boost in midrange acceleration, and I picked up 2-3mph on the top speed. It's not a dramatic change but definitely noticeable.

You're running the grom pretty much to the max just to keep pace with traffic, so any top end advantage is welcome.

Think of torque as "how much work a motor can do" (rotational force) and horsepower as "how quickly that work can be done" (RPMs). Power is a function of work over time. The classic definition of 1 horsepower is 33,000 pound-feet/minute. Or in other words, a 1hp engine powering a winch could lift 330lbs, 100 feet, in one minute. Torque can be multiplied with gearing, power cannot.

Say you got a sportbike and a cruiser that happened to weigh the same. They both have the same torque, but the sportbike has double the power. In theory, all else constant, they would reach the same top speed, but the cruiser would take twice as long to reach it. It can do the same job, just not as quickly.

I had a inlet fitted to 2022 from and it made a small improvement, then I put a Yoshi full pipe on it the noise was unbearable and the low and midrange loss made me feel L had been ripped off,here in Australia it cost $1100 I wish I had just settled for the inlet.

Daffy

· #12 · 10 mo ago

Daffy said:
I had a inlet fitted to 2022 from and it made a small improvement, then I put a Yoshi full pipe on it the noise was unbearable and the low and midrange loss made me feel L had been ripped off,here in Australia it cost $1100 I wish I had just settled for the inlet.

Since I made this post on the Yoshi exhaust I made my own baffle that came to a point down inside the muffler and drilled holes until I got a nice sound and surprisingly I got my midrange back but also picked up 2 kms an hour in top speed, I'm disappointed that the Yoshi didn't come with a baffle for that price, but I.m glad I made my own because the baffle that is sold for this pipe just being a straight bit of pipe that goes up inside the muffler wouldn't sound as nice as mine so it's a win.

roadster

· #13 · 9 mo ago

Unfortunately there is often a simplistic interpretation applied to inlet and exhaust pipework which fails to understand that the flow of gases through the engine is not steady state but a series of pulses dependent on engine rpm and valve timing. Usually any increase in diameter of exhaust pipes or reduction in their overall length will ruin Honda's careful design because the resonance of the compressible gases is no longer sympathetic to the exhaust valve timing and duration. Then follows the second myth which is that the engine can be restored to full power or better by injecting extra fuel on the false assumption that air flow has been increased. Much the same problem comes from the assumption that the intake air box is "restrictive" when in fact it too is designed to resonate in a way which helps cylinder filling over the engines operating range.

Having said all that a loud exhaust sounds faster and may save weight. It will also look far better to the person who paid for it.

Now for the confession - I have bought new silencers and Dynojet electronics or jet kits for dozens of bikes then spent countless days tweaking baffles and pipework while measuring the actual performance on the road. Mostly this eventually gave a power delivery which, though not greater in ultimate output, addressed some aspects which I didn't like. For example those minor flat spots which interfered with part throttle pick up or the characteristics which make some Honda engines rather bland and unresponsive. I won't be tuning a 9 HP engine attempting to find 10 HP but for those who enjoy doing that I certainly wouldn't discourage it.

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Daffy

· #14 · 9 mo ago

roadster said:
Unfortunately there is often a simplistic interpretation applied to inlet and exhaust pipework which fails to understand that the flow of gases through the engine is not steady state but a series of pulses dependent on engine rpm and valve timing. Usually any increase in diameter of exhaust pipes or reduction in their overall length will ruin Honda's careful design because the resonance of the compressible gases is no longer sympathetic to the exhaust valve timing and duration. Then follows the second myth which is that the engine can be restored to full power or better by injecting extra fuel on the false assumption that air flow has been increased. Much the same problem comes from the assumption that the intake air box is "restrictive" when in fact it too is designed to resonate in a way which helps cylinder filling over the engines operating range.

Having said all that a loud exhaust sounds faster and may save weight. It will also look far better to the person who paid for it.

Now for the confession - I have bought new silencers and Dynojet electronics or jet kits for dozens of bikes then spent countless days tweaking baffles and pipework while measuring the actual performance on the road. Mostly this eventually gave a power delivery which, though not greater in ultimate output, addressed some aspects which I didn't like. For example those minor flat spots which interfered with part throttle pick up or the characteristics which make some Honda engines rather bland and unresponsive. I won't be tuning a 9 HP engine attempting to find 10 HP but for those who enjoy doing that I certainly wouldn't discourage it.

I wish I had read more posts before buying inlet and full pipe for my Grom, I would have saved myself $1200 and still had the same performance, I hope I have saved some other suckers from making the same mistake, just because it's making a lot of noise doesn't mean it's going faster.

jake'snewride

· #15 · 9 mo ago (Edited)

🔝 "faster" The grom is a great bike for what is is-just shoulda have been louder than a sewing machine when new, and maybe a 150cc version. My personal choice for a louder exhaust is for the safety aspect and has proven that a few times this year.
Remember "Loud pipe's save lives" I don't care if it was voted by a bunch of leftist "green" people that say otherwise, and if that bothers you then i know where u stand.😁
I didn't quite understand that until owning a bike but now i do.
Yes i do agree that there is a line to cross at being too loud but......it comes down too personal choice.

Have a wonderful day.
Oh, and don't mind me my youngest is leaving for an out of state college tomorrow and i may be a bit moody.......

MNNTHBX

· #16 · 9 mo ago

You won't lose any bottom end with a QUALITY exhaust AND proper fueling. Cut either corner, and all bets are off....

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