ESE's works engine tuner

[F5 Dave]

Except that when you raise the peak revs 4000rpm any factory balance attempt - you've just negated.

[TZ350]

Hi Fritz. We finally make (paramotor) engines with opposite exhaust,

Very clever piece of work, I like it. Love that exhaust, brilliant idea.

[wobbly]

There are several factors at play with removing the balance shaft ( apart from the balance factor ).
First is that if the engine is re balanced correctly, there will be no discernible vibration,there are millions of single cylinder engines running around that dont make the rider loose
concentration due to bad vibes.
Then there is the issue of inertia.
The balance shaft runs at crank speed so in effect adds to the rotational inertia of the engine.
Loosing the balance shaft will change that inertia considerably.
There are two schools of thought on this.
Of course basic physics says a low inertia system will accelerate faster,but in a 2T it has been well documented that high inertia cranks make way more overev power.
Take a stock RS125 Honda and remove the flywheel, suddenly it wont rev at all.
That is why HRC and VHM offer high inertia option cranks for total loss use.
And take a look at the amount of Mallory on the periphery of an Aprilia or TM125 kart crank - they didnt add all that heavy metal,to make the engine slower at the rpm its used at most.

[lodgernz]

Here is some idea of the Aux duct layout.
Two things to note is that the main duct roof slopes down to form a hump,and the floor is horizontal ( making a ski slope hump ), to reduce the area where the Aux side ports intersect.
And the spigots floor should be parallel to the duct floor, and the roof has all the angle change to make the oval ( with ears ) to round transition.

Wobbly, I suspect that the dimensions of the exhaust duct as in your drawing are based on experience and experiment, rather than on empirical theory. In that case, I also suspect that the engines that have provided the data have mostly been "square" in bore x stroke. My Honda T-port engine is badly oversquare at 53 x 44. Do you think the same duct length dimensions still apply, based as they are, solely on the bore size?
Could the exaggerated bore/stroke ratio require remodelling the "1.5 x bore" and "2 x bore" data?

[wobbly]

Absolutely the use of " badly " oversquare dimensions will affect the duct geometry.
The end of the road effect of the big bore is that this will severely limit the achievable bmep.
You can see this going on in Robs GP-NSR110.It needs absolutely at the limit port timings to only achieve 12 Bar bmep.
When the bmep is limited like this,the end result is alot lower power capability - thus the bulk amount of cold mixture needed for sufficient plugging effect is reduced commensurately.
So using my simple guidelines ( as you say based on high efficiency square engines ) is applying a flawed rule of thumb to a flawed engine design.
Well spotted - never even remotely occurred to me.
Thus the reverse will apply to some extent with say 250cc road racing engines derived from the very undersquare MX bikes that run 66.4 X 72 stroke.
But as the best design would be that as used by Honda in its V twin 500 GP engine of close enough to 68 square,the difference in duct lengths in that case arent huge.

[lodgernz]

Absolutely the use of " badly " oversquare dimensions will affect the duct geometry.
The end of the road effect of the big bore is that this will severely limit the achievable bmep.
You can see this going on in Robs GP-NSR110.It needs absolutely at the limit port timings to only achieve 12 Bar bmep.
When the bmep is limited like this,the end result is alot lower power capability - thus the bulk amount of cold mixture needed for sufficient plugging effect is reduced commensurately.
So using my simple guidelines ( as you say based on high efficiency square engines ) is applying a flawed rule of thumb to a flawed engine design.
Well spotted - never even remotely occurred to me.
Thus the reverse will apply to some extent with say 250cc road racing engines derived from the very undersquare MX bikes that run 66.4 X 72 stroke.
But as the best design would be that as used by Honda in its V twin 500 GP engine of close enough to 68 square,the difference in duct lengths in that case arent huge.

Thanks Wobbly. I think you are saying that my duct needs to be shorter than your guidelines suggest for a nice square engine. Please correct me if I'm wrong.

[peewee]

Thanks Wobbly. I think you are saying that my duct needs to be shorter than your guidelines suggest for a nice square engine. Please correct me if I'm wrong.

maybe theres some way to test this in engmod. otherwise a real world test is the only way. my engines are mostly oversqaure like yours

[husaberg]

Except that when you raise the peak revs 4000rpm any factory balance attempt - you've just negated.

I don't agree, I say this as the counter balance shafts as I believe a general rule are just as effective at any revs.
What I think is occurring is that when the engine is modified for higher revs as you say.
When these modifications occur the primary balance is also changed ie different rod, rod length, crankpin and piston weights etc
If these changes were accounted for and the crankshaft was rebalanced back to its original factory balance factor (generally 50%) as it was originally designed at there should be no real change. Despite the higher revs.
I base this assumption on nearly all counterbalanced singles regardless of the revs they operate at have both their primary balance and secondary balance shafts factors set at 50%.
Removing the balance shaft in itself generally requires rebalancing the crankshafts primary balance factor anyway.
Yes of course you can do this and it will if done correctly to suit the engine speed and chassis as Wobbly says above,
But while this no doubt works it only works over a narrow rev range because it just moves the out of balance forces to another rev range rather than lessening them over the whole spread of engine revs as a balance shaft so effectively does.
Just my opinion of course I used to biff them out of course same as everyone else did.
But after reading the opinions of Frits, Jan and Ian Harrison I changed my mind.
I also think the contra rotating Balance shaft on a two stroke also has advantages with lessening the gyro force (I would like to try a CR500 with one and see if it really steers better)

[F5 Dave]

There are several factors at play with removing the balance shaft ( apart from the balance factor ).
First is that if the engine is re balanced correctly, there will be no discernible vibration,there are millions of single cylinder engines running around that dont make the rider loose
concentration due to bad vibes.
Then there is the issue of inertia.
The balance shaft runs at crank speed so in effect adds to the rotational inertia of the engine.
Loosing the balance shaft will change that inertia considerably.
There are two schools of thought on this.
Of course basic physics says a low inertia system will accelerate faster,but in a 2T it has been well documented that high inertia cranks make way more overev power.
Take a stock RS125 Honda and remove the flywheel, suddenly it wont rev at all.
That is why HRC and VHM offer high inertia option cranks for total loss use.
And take a look at the amount of Mallory on the periphery of an Aprilia or TM125 kart crank - they didnt add all that heavy metal,to make the engine slower at the rpm its used at most.

So. . . Why?
To an extent there is the stored energy which you have done work to accelerate. I can accept that a clutch start would benefit at least with roadbike gearbox for that potential energy release.
But overrev important in higher gears takes a while to expend. As the combustion efficiency fails and pipe cools surely the power collapsing with that much drag on it couldn't be compensated by the stored energy?
Or are you saying as I think you are, an engine capable of producing power after peak just can't with too light of a flywheel? (Or rather rotational mass) just can't think why that would be as I finish my piece of fudge. (Very nice, but a weakness that clearly didn't help my 50cc racing career)

Damn, this is going to bug me.

[jfn2]

wobbly.... For us poor hillbilly folk what is .. commensurately. Where do you guys pull these words from? Please explane.

[husaberg]

wobbly.... For us poor hillbilly folk what is .. commensurately. Where do you guys pull these words from? Please explane.

The poor hillbilly types that's auriferous.....

So. . . Why?
To an extent there is the stored energy which you have done work to accelerate. I can accept that a clutch start would benefit at least with roadbike gearbox for that potential energy release.
But overrev important in higher gears takes a while to expend. As the combustion efficiency fails and pipe cools surely the power collapsing with that much drag on it couldn't be compensated by the stored energy?
Or are you saying as I think you are, an engine capable of producing power after peak just can't with too light of a flywheel? (Or rather rotational mass) just can't think why that would be as I finish my piece of fudge. (Very nice, but a weakness that clearly didn't help my 50cc racing career)

Damn, this is going to bug me.

Wob and Frits can correct me if I am wrong but I believe the higher inertia keeps the crank turning at a more consistent speed ie less stop start at TDC and BDC.
I have a pic of a Aprilia crank with a heap of tungsten inserts added for increased inertia somewhere. they chose to do this rather than simply making the other areas lighter with alloy plugs.
Of course I could be said some of the reason for this was likely to avoid have cut outs in the crank wheels, but they sure put a heck of a lot of weight into that crank.

[wobbly]

As I scored 98% in Scholarship English I rarely suffer from anomia,so expanding commensurate ( yea that's in google ) by adding ly makes it a " doing " word for you hillbilly folk ie
the act of making something commensurate.

But getting back to important shit - Frits has always criticized my blatant uselessness in applying rules of thumb to the duct geometry based on the bore size and the port effective area.
He is of course annoyingly correct,the whole process should be tied to the Blowdown STA.
Thus we would have an exit geometry that is related to the power achievable , of a certain swept volume , at a certain rpm.

Once some clever fuck with more time and intellect than I , looks seriously into this , then the theory could easily be extended to a new relationship that ties the Blowdown capability to the volume of available
cool plugging charge needed, to support the bmep being produced.

That would be a cool Masters or PhD project , if 2T technology was even remotely interesting or relevant ,to engineering students focused on the future electric shitter everything - no " engines " involved.

[tdc211]

My duct outet on 104cc engine is a 32. But not far back It's a actually a 30.2mm. So I use 30.2mm in sim. And the roof is pretty close to flat. I literally copied the 13 up ktm85 roof.
As the older (pre 13) 105 cylinder was completly different here .

[Larry Wiechman]

Frits has always criticized my blatant uselessness in applying rules of thumb to the duct geometry based on the bore size and the port effective area.
He is of course annoyingly correct,the whole process should be tied to the Blowdown STA.
Thus we would have an exit geometry that is related to the power achievable , of a certain swept volume , at a certain rpm.

These rules of thumb for exhaust duct and pipe geometry work fine for well developed 125cc engines. Are they scalable to use on a 86 x 86 500cc engine?
The insight into the use of EngMod was fantastic, Thank You!

[philou]

Wob and Frits can correct me if I am wrong but I believe the higher inertia keeps the crank turning at a more consistent speed ie less stop start at TDC and BDC.

less speed variation on a rotation cycle leads to a less disturbed flow

difficult for me to explain in English