Dyno disection
Road Captain
Joined: Mar 2009
Posts: 654
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Highly unlikely you're going to flatten the torque curve via altering the tune. When you look at the shape of the torque curve, you're largely looking at a map of the cylinder fill. Where the torque is low, you have poor cylinder fill, and where the torque is high, you have good cylinder fill.
When the cams have overlap (and yours do), the shape of the torque curve is largely a function of the pipe. That's because overlap in the cams effectively connects the exhaust to the intake at the very front of the intake cycle, which gives the exhaust system more influence over intake flow and thus cylinder fill. Overlap in the cams is a great way to make torque (and thus power, since power is just torque times rpm), but it relies on a pipe that works well over the rpm range you care about.
Pipes tend to alternate between pushing back and pulling during the overlap window, depending on the rpm. This is due to the pressure wave travel in the pipe and how it reflects off each end. Your pipe is pushing back at 3500 and pulling hard at 5400.
What most guys like to see is a pipe that pulls broadly over a wide rpm range, resulting in a flat or broad parabolic shape to the torque curve. This is what you get when the pipe is designed for better diffusion, i.e. it generates a broader negative pressure during the overlap window that then arrives over a wider rpm range. Keep in mind that peak torque will often come down when you broaden the torque curve like that, which really highlights what a meaningless number the peak torque is. What matters is the shape of the torque curve; a curve with deep dips and high peaks will make a bigger peak number but is generally less desirable.
That said, it's a pretty decent result. I wouldn't complain too much.
Make sure you're running the tune your engine kit vendor provided. It's critical to your success.
When the cams have overlap (and yours do), the shape of the torque curve is largely a function of the pipe. That's because overlap in the cams effectively connects the exhaust to the intake at the very front of the intake cycle, which gives the exhaust system more influence over intake flow and thus cylinder fill. Overlap in the cams is a great way to make torque (and thus power, since power is just torque times rpm), but it relies on a pipe that works well over the rpm range you care about.
Pipes tend to alternate between pushing back and pulling during the overlap window, depending on the rpm. This is due to the pressure wave travel in the pipe and how it reflects off each end. Your pipe is pushing back at 3500 and pulling hard at 5400.
What most guys like to see is a pipe that pulls broadly over a wide rpm range, resulting in a flat or broad parabolic shape to the torque curve. This is what you get when the pipe is designed for better diffusion, i.e. it generates a broader negative pressure during the overlap window that then arrives over a wider rpm range. Keep in mind that peak torque will often come down when you broaden the torque curve like that, which really highlights what a meaningless number the peak torque is. What matters is the shape of the torque curve; a curve with deep dips and high peaks will make a bigger peak number but is generally less desirable.
That said, it's a pretty decent result. I wouldn't complain too much.
Make sure you're running the tune your engine kit vendor provided. It's critical to your success.
Seasoned HDF Member
Joined: Sep 2017
Posts: 12,254
Likes: 4,946
From: England
A great explanation but the setup is **** all use when trying to ride gently round town.
I guess the guy liked the look of the pipes but had no idea they were tuned and not just a muffler.
I guess the guy liked the look of the pipes but had no idea they were tuned and not just a muffler.
Last edited by Andy from Sandy; Oct 18, 2019 at 02:17 PM.
Grand HDF Member
Joined: Dec 2012
Posts: 3,782
Likes: 1,202
From: river
Excuse me, what? Ive read this 78 times and I still do not understand what you are saying. What exactly is ****?
Road Warrior
Joined: Sep 2017
Posts: 1,609
Likes: 694
From: over the mountain
Oh lighten up, after being on here a couple of years he now knows more than Hammer Performance knows about their own stuff. LOL
Last edited by apache snow; Oct 19, 2019 at 08:06 AM.
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Seasoned HDF Member
Joined: Sep 2017
Posts: 12,254
Likes: 4,946
From: England
I am sorry I shall rephrase that.
With the power band shifted to the upper rev range the ability for the bike to pull from low rpms that you might want to do whilst puttling round town is going to be difficult or lost altogether. Where you might of been able to ride in say 3rd gear you may now need to be in 2nd to keep the revs up.
This has nothing to do with a hammer performance upgrade to the engine or the fact the engine has been tuned correctly for best performance. The picture is quite clear and the explanation is very clear the low end power and torque has been shifted.
With the power band shifted to the upper rev range the ability for the bike to pull from low rpms that you might want to do whilst puttling round town is going to be difficult or lost altogether. Where you might of been able to ride in say 3rd gear you may now need to be in 2nd to keep the revs up.
This has nothing to do with a hammer performance upgrade to the engine or the fact the engine has been tuned correctly for best performance. The picture is quite clear and the explanation is very clear the low end power and torque has been shifted.
Last edited by Andy from Sandy; Oct 19, 2019 at 11:05 AM.
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Road Warrior
Joined: Sep 2017
Posts: 1,609
Likes: 694
From: over the mountain
Well you are wrong. My Sportster1250 is 110 hp, has the 560 cams with 10.5 to 1 compression. I can ride around town in 1,2,or 3 at low speeds just fine with no jerking or surging at all. Its smooth at all RPM and pulls good from 2000 RPM to 7400 RPM. You don't know what you are talking about.