A Funny stack story

06-07 yzf600 R6

We mucked around (high tech term) with modifying stock stacks, epoxy, superglue, toilet paper cardboard modeling clay. All to no significant advantage over stock.

Then, as my machinist and I were just packing up and calling it a night, he says - do the 05 R6 2 piece Superflares fit? It looks like the same airbox hole diameter.

Well, "that won't work", the 06 throttle bodies are about 2mm bigger than the Superflares and removing the stock stacks and putting in the Superflares will leave a gap below where the original stacks and that will hurt airflow.

Well, at least they fit the airbox and there's a set sitting on the shelf.

So, "just because they fit", we installed the "too small, with a big gap" stacks that haven't a chance to work,
And did a loaded dyno test on the EC997,

First step at 3000 rpm? About 2 hp better.
Well, that's ok. Whatever. Maybe the smaller ID helped the very low end.

A word about the EC997 dynos.
It's a load dyno, designed to do steady state testing in preset increments.
As in, bike's idling, roll on the throttle, rpm increases to the first rpm target (it's low inertia so there's little load to get up to the rpm target). Back off, let the heat equalize and roll it on to the next, automatically targeted rpm point.

The dyno holds it up at the specified rpm and the dyno records both the average hp and the 5 gas EGA over the 3-5 seconds. It also displays a realtime stripchart of the power produced over that 3-5 seconds.

At each rpm point., it plots out a hp point on a chart that is displaying your "reference file". That way, as you are doing your current test, you can see if you are better or worse than your reference test. Makes it kinda fun to do testing.

The average hp is for .1ths hp repeatability and the hp Event Recorder stripchart tells you about tuning - as to whether power is increasing over time (lean and/or retarded ignition) or decreasing (rich jetting and/or over advanced ignition).

So, regardless - we get to 3000 rpm and it's a couple better than stock.
Then we get to 4000 rpm and it's still 2-3 better than stock.
Oh well, I internally sigh, we will reach that point where the small ID will hurt airflow and power.
5000 rpm, still better by 2-4 hp.
6000.... 7000..... 8000..... 9000.....
2 to 4 hp better "everywhere" and no loss of peak and maybe a hp lost at the stock redline. All with the too small diameter and that horrid downwind gap.

So! We "got it"! The stock stacks are too large ID!

Yippeee! Lets do smaller ID stacks and fix that horrid downwind gap!
That's the ticket! Gosh, are we are clever, or what!

A day's worth of programming and machining and out pop out a set of beautiful "smaller ID and no downwind gap" stacks. GuarAtEED to smoke all previous efforts!

3000 rpm.... same as stock.
4000 rpm, same as stock.
and so on. All essentially the same as stock.

Redo stock stacks. Redo the "too small with horrid gap" stacks.
Get the same 2-4 hp increases.
Redo GuaRenTEED smaller stacks with no gap.
Same as stock.......

Ok, so there we stand, depressed, but amused... right when we "thought" we knew almost everything (well, at least a lot) about stacks.......

All we learned was that the 06/07 "liked" too small of an ID and a horrid gap - and nobody else ever came out with anything else that worked on a stock engine....

Could we sell them like that? Not really. I could imagine the flames on the forums about selling old stacks that didn't fit for new bikes....

2nd side note to the 06 R6.

A week or two later, a well known UK privateer emailed to see if we had any stacks that would fit his 06 R6.
Wow! Serendipity!

Why, yes. We just found that if you stuff the goofy looking "too small, horrid gap" Superflares from the 03-05 in an 06, you gain a 2-3-4 hp all the way up to, but not including, the power peak.

Nah, that didn't work. I tried it.

Did you notice that the ID was too small?

Yep - I noticed that and I machined them out to the proper ID.

So - that verified it. It needed the small ID AND the gap.

Obviously it's a wave and cylinder filling issue, but "quantifying" what's going on is easy to do "qualitatively". In other words, it's easy to say, in hindsight, what happened in general terms but impossible to predict.

I didn't write down every thought in this story....... So many of the conclusions are the simple ones and secrets remain secrets........

Tim will agree - If one thinks tuning is easy, then you don't do it for a living.........