How bad is your torque curve?

[Syscrush]

Here's mine from immediately after the engine was rebuilt and before I decided to go ahead with EFI conversion - it's absolutely atrocious:



When this was done, I assumed that the horrible dip centered at 5k RPM was due to carburetion issues (you can see that the AFR drops to ~10:1 right where the dip is happening) but what I've been learning recently about volumetric efficiency and ECU tuning seems to suggest that just sorting out the fueling may not be enough to resolve the flatspot.

Because the revs are continuing to climb through that dip, the power curve doesn't look nearly as awful:



But I'm still concerned that addressing this will require a change to the cams or exhaust. I have a sentimental attachment to the exhaust and would really like to keep it. Maybe retarding the stock intake and/or and exhaust cams could smooth things out a bit. I wouldn't hesitate to give up some top end power to get a more even output from 2-6k RPM.

Has anyone else here faced such a big ditch in their midrange torque? If so, were you able to address it?

[Jeff Bennetts]

I have no experience with FI but the last time I saw a graph like that it was due to the exhaust system.

[Syscrush]

BTW - this is pre EFI conversion. I'm still in the design/planning phases of that. I've updated the original post to make that more clear.

I have no experience with FI but the last time I saw a graph like that it was due to the exhaust system.

I think that's what's going on, too. It seems like almost every 6-1 system is associated with a midrange flat spot, and my system has really short primaries - which is consistent with both the crappy midrange and the excellent top end.

The local peak at ~4.2k is kind of mysterious to me, though.

[Jeff Bennetts]

Exactly, 6 into 2 into 1, Bassani is the only system I have ever dynoed that had a small dip in the midrange, all other 6 into 1 systems seem to suffer a large midrange drop!

This is 20 year old data I’m referring to, so keep that in mind!

[bobcat]

.....no comment.

[bikeymikey748]

Interesting information/graphs, Phil.
I’ve heard it said repeatedly that 6-1s suffer from midrange flat spots. Mine seems to pull cleanly through any engine speed with no issues. Perhaps it’s because I trash-canned the DG resonator for the modded Yoshimura GSXR 1000 one that I MacGyvered?
Still, would love to run mine on a dyno sometime to see what is going on.
Best of luck with your EFI conversion, following with interest.

Mike C.

[Rick Pope]

For those of us who worship at the alter of Kevin Cameron, and before him, Gordon Jennings, there has been much written about the nuances of header length and collector design. 6 in to 1 doesn't have to have torque dips, unless there are design limitations (space? appearance?) restricting them. Big header pipes, while great for high rpm, allow the gases to slow down, ruining the resonance effect. So, it's simpler to just go with a 6-2, with it's simpler collector.

[Syscrush]

Big header pipes, while great for high rpm, allow the gases to slow down, ruining the resonance effect.

I'm not an expert on this stuff, but IMO there has to be some significant resonance between my exhaust and the OEM intake that's causing bad reversion and killing VE right at 5k.

I used to think that the dip was because of the awful AFR, but now I think that maybe I had cause & effect reversed.

[Larry Zimmer]

That is typical for many/most X into 1 systems anywhere from 2 - 1 for a 2 cyl to 6-1 for a 6 cyl. All a matter of tuning and what curve you want for a given engine. There will be some 'sacrifice' somewhere along the way.

[Rick Pope]

I first learned about this when I built my CB750 into the CB836Kafe, back in 1977. I had a Hooker header, 4-1, and was going to buy a RC Engineering pipe because it had bigger header pipes. Byron Hines advised against it for street use.

[daves79x]

That's why the D&D 6-1 with fairly small header diameter made pretty good power.

Dave

[Larry Zimmer]

I first learned about this when I built my CB750 into the CB836Kafe, back in 1977. I had a Hooker header, 4-1, and was going to buy a RC Engineering pipe because it had bigger header pipes. Byron Hines advised against it for street use.

And, that's why I took the RC off my Suzuki. Dead in the middle.

[Rick Pope]

It boils down to bragging rights about peak hp v. rideability. I have a stock, 137 rwhp bike sitting in the garage, yet I mostly ride an old tech air cooled v-twin with perhaps 98 rwhp, and loads of bottom end and grins.

[oroepke]

I realize it's an older post, but I have the exact same problem! I just put my new to me '80 back on the road. It came with K&N pods, 6-1 header and Supertrapp muffler. I haven't dynoed it, and don't plan to, but I bet my torque curve looks similar! I already have an original airbox I am planning to put on, but from what others say, the 6-1 could be another problem! I don't care about top end hp, I want a rideable bike.

[Larry Zimmer]

That's why i put the stockers back onto my Suz to replace the RC 4-1. Typical X into 1 systems loose noticeable mid-range. If one stays above 6ooo, the X into 1 is great. Otherwise, ...........