70cc class Dyno day

[drf256]

Longer stroke saws can make the time x area much greater than a short stroke saw. Has a lot to do with valve timing (even though there aren’t any per se) vs a 4 stroke.

Just another variable.

 

[mdavlee]

Torque is useless in a high revving 2 stroke except on a dyno to calculate horsepower. Horsepower att a usable range is the winner every time.

 

[00wyk]

OK, this is a bit of a rant, but stick with me. Butt stick? Anyways...

The point I was making earlier is to facilitate more torque from a two stroke - for the same rpm - you need to be able to feed it, and you need to be able to balance it.
This means a crank and an intake/exhaust system that will not fit on a saw. This is the main reason you won't see significant torque gains simply with stroke on a saw.

How a four stroke makes more torque for a given RPM is the intake is made longer, the valve timing has less over lap, more fuel is made available at lower rpm, and of course, the stroke is lengthened with the necessary balancing added. This means that, for a given engine speed the velocity of the intake is now higher. The more intake velocity, the more fuel you can throw in that mix. An intake and crank designed for low end torque in a petrol 4t is difficult to make high end hp with as it has more mass to spin around and balance(requiring expensive light weight parts for higher rpm performance), and the valve timing becomes an issue pretty quickly. If you keep the valve timing the same in something like a 351windsor, but increase the rpm, now with the smaller stock valves necessary to make high velocity at low rpms you are simply going to choke the intake and add all sorts of harmonics not conducive to high rpm flow, and we'll ignore the much higher forces on the crank and rod/bearings for now. It can make high rpm hp and torque, and certainly is often done(just look at all the 351 Windsor and 350 chevy strokers and LS engines), but it is costly and you must plan the entire engine around the set up from top to bottom to gain the benefits of stroke for torque and hp. Variable valve timing using larger valves, variable intake runners, tuned intakes, tuned exhausts, huge carbs, etc. But, in general, if you start to give a 4t the over lapping valve timing it needs to feed and clear the combustion chamber for high rpm hp, you start to lose the velocity and the ability to completely fill it at lower rpms. It's just physics. You can bypass it a good bit with variable valve timing(VVT/VTECH/VANOS/etc), but it's hard to add valves to a 2t chainsaw engine...

Configuration is also why larger displacement 4t engines make more torque at lower rpms almost regardless of their design - they have more reciprocating mass, more room for intake and exhaust, more room for stroke.

The same thing happens on a 2t, but in a different way. To make a 2t breathe better at a particular rpm you need to mess with the timing. There are no valves(usually), and there are no cams. So you have to adjust the intake, transfers(sort of like the 2t version of valves) and exhaust timing permanently, and rather invasively. There is no VXFERT(variable transfer timing). This means you have to shoot for a max hp at a certain rpm. If you want it to fill the combustion chamber at mid rpm's, you have to make the intake longer, the xfers longer, the carb larger(not just the venturi width but also the length), and the stroke has to be much longer. Even if you go back to the 70's, 2t strokes are way over square(short stroked).
But wait - there IS variable xfer timing. Only it is on motocross engines. A combination of reeds and runners and xfer design gives a larger 2t engine something akin to VVT. How large are the xfers? On some 2t MX engines, you can cut the piston into four pieces and shove them into the lower xfers and they'd fit. The xfer runners are also much, much longer than on a chainsaw engine. So for a certain stroke you will get more velocity at a given rpm vs a chainsaw engine. You simply can't find that sort of room in a chainsaw chassis.

Which brings us back to the dyno thread. What I found interesting is the Echo performed very poorly VS the other 70cc saws. Yet, in a video that Mitch recently did, it performed rather well stuck in to a huge piece of wood for a long cut vs a 565 and a 572xp. So what's goin on? I think Mitch did say the Echo was broken in, and it needed to be...

 

[Sawrain]

If we are talking 4t, which engines make the most torque per litre? It’s not those described at built for torque, but High performance, high revving ones, and (as you mentioned) variable valve timing Lets them widen their power bands.

Below is a list of highest normal and available factory naturally aspirated torque/L engines I found online, best engines at the bottom.



Some of These free flowing headed built for power beauties have 100%+ VE, when exhaust pulse vacuum, overlap, and intake resonance lines up for outstanding specific torque outputs per litre, I don’t know (but there might be?) any examples of naturally aspirated engines built for torque that make more torque per litre than these built for response/performance/power engines.

Here is the dyno plot from a 20 year old bmw e46 M3, #3 on the list for torque, pretty good torque for a 3.2l naturally aspirated motor, and note most torque available at only 2000rpm.


Why post all this? Just a discussion of the following two common ideas that are not always true.

High revs horsepower = low torque?

And built for torque actually equals getting torque.

I must note I do not consider forced induction engines a fair comparison here.

 

[srcarr52]

I am very interested in this stroke thing.

Only 1 of 4 long stroke saws out performed the short stroke counterparts...

That was the 661 stroker.

The rest of the "long" stroke saws have little to no more tq than the short stroke.

Dolmar 37mm
572 39mm
Echo 620 37.6mm


Just wonder if it has something to do with the operating range? Maybe if one saw used a big heavy long stroke and a lower range? It may show greater tq?

That big echo had 7 ftlb and a 44mm stroke.

But all the other shorter stroke saws made even more tq than the big echo? Granted that was ported vs stock.

More questions than answers.

You're going to have to test my 395 stroker than.

 

[Sawrain]

I would expect a stroker saw to outperform it’s counterpart, if the stroking caused a gain in capacity.

 

[Red97]

You're going to have to test my 395 stroker than.

When I return...

 

[Red97]

You're going to have to test my 395 stroker than.

Would almost have to do tq per cc...

See if the stroker out does the regular stroke.

 

[ranchdadmike]

Thanks guys for getting this stuck in my head all day:

 

[Piston Skirt]

Which brings us back to the dyno thread. What I found interesting is the Echo performed very poorly VS the other 70cc saws. Yet, in a video that Mitch recently did, it performed rather well stuck in to a huge piece of wood for a long cut vs a 565 and a 572xp. So what's goin on? I think Mitch did say the Echo was broken in, and it needed to be...

In his first video he cut with a box stock, not broken in saw.
In the second one he already had transfer caps ripped off & grinded

 

[dustinwilt68]

Torque is useless in a high revving 2 stroke except on a dyno to calculate horsepower. Horsepower att a usable range is the winner every time.

This statement really makes sense when you put a longer bar on the newer 70cc saws, some really shine and some just feel like they loose it in the cut ie: usable power range.

 

[00wyk]

I'll just leave this here(with the caveat I am a Harley owner, but love the Indians too):

 

[dustinwilt68]

Great video!

 

[Red97]

So since we are at realitivly the same rpm range the larger bore will typically out perform the longer stroke.

Makes sense, as in a 2 stroke it needs the bore space clear exhaust to fill with charge. That is probably why "normal" numbers don't tend to perform as well in the longer stroke saws.

I'm still trying to figure a correlation between dyno, and feel myself.

 

[00wyk]

So since we are at realitivly the same rpm range the larger bore will typically out perform the longer stroke.

Makes sense, as in a 2 stroke it needs the bore space clear exhaust to fill with charge. That is probably why "normal" numbers don't tend to perform as well in the longer stroke saws.

I'm still trying to figure a correlation between dyno, and feel myself.

'Variables' again. After all, there is a reason a motocross motor is square. It produces good torque at mid RPM and good HP at high RPM.
A square engine will almost always have more usable range than over square, but you try and fit it in a chainsaw chassis.
Chainsaws have to be small and light and have no transmission or water cooling. The only gearing they have is a sprocket. As the video shows, if you extend the stroke, you start to see all sorts of issues when trying to run it at higher rpm. Vibration, cooling, weight, size - all become factors. You also need high compression to make it all work, which means now you have a saw that is a huge pain to start, and the ignition needs to be spot on or there's issues. Chainsaws don't sell for much unless it's a 500i, so that's an issue as well. Whereas a Yamaha YZ125 goes for nearly 7g's out the door and weighs more than I do.

Most racing 2t GP engines were between 56X50mm and 54X54mm range for the 500cc class. They usually red lined at 13-14,000 rpm.
If you take a 2t engine, add water cooling, a good stroke, and insane compression, insaner ignition and carburetion, you get this:

https://global.yamaha-motor.com/showroom/cp/collection/racing_yzr500_0wl9/

Many V4 500cc 2t GP bikes were detuned from their max HP(which was often ~200+bhp), to make them more ridable and controllable in a frame that weighed in the high 200lb range(some of the 2 cylinder ones were under 250lbs and still produced 170hp - compare this to a YZ125 that weighs 210lbs and has 35hp and scoots right along). To this day, folks who remember, know the 500cc 2t GP bikes were the most brutal vehicles ever made.

I know what you're thinking. 'That exhaust doesn't look so impressive for a 2t bike'. And in that pic, it doesn't. How bout this though?

 

[Red97]

Yeh, no comparison.

These saws don't have enough real estate to even come close to a performance 2t engine

 

[Sawrain]

I'll just leave this here(with the caveat I am a Harley owner, but love the Indians too):

Did not get far before some questionable statements were made, 1m 40s, large pistons have weak forces because combustion pressure is too slow and spreads out thin? Huh?

The 2005 bmw P85 f1 engine had a 98mm piston and ran to 19,800rpm, no magic “race fuel” used, seemed to work ok.

3m 30s, suggesting a big ol V twin is Running out of legs up top because the flame front can’t catch the piston, really? better reasons could be less head area on long strokes motors have less room for valves/ports causing Lower volumetric efficiency, limiting revs, or more realistically that the engine was simply designed more for bottom to mid range power, not top end, so cam profiles and porting Should be pretty high on the list of why it’s running or of legs.

And after all he said that long stroke Harley has less torque per Liter than the Indian!

Harley sportster iron 1200
Capacity 1202cc
Bore/stroke 88.9mm x 96.8mm, 0.91 ratio
Power 71hp
Torque 73 lb-ft (61 lb-ft per liter)

Indian Scout,
Capacity 1130cc
Bore/stroke 99mm × 73.6mm, 1.34 ratio
Power 100hp
Torque 72 lb-ft (64 lb-ft per liter)

Harley-Davidson CVO Road Glide
Capacity 1923cc
Bore/stroke 103.5 x 114.3 mm, 0.90 ratio
Power 105hp
Torque 125 lb-ft (65 lb-ft per liter)

Bmw s100rr
Capacity 999cc
Bore/stroke 80mm x 49.7mm, 1.61 ratio
Power 205hp
Torque 83 lb-ft (83 lb-ft per liter)

Bonus, bmw p85 race engine
Capacity 2998.5cc
Bore/stroke, 98mm x 39.75mm, 2.47 ratio
Power 950hp+
Torque 265 lb-ft (88 lb-ft per liter)

The P85 being an extreme example of a high rev (19800rpm) high volumetric efficiency engine with a very short Stroke making more torque per litre, you might even say much more, some chainsaws have more stroke!

The Bmw s100rr and road glide are not bikes you normally compare, but you might find the numbers interesting.




Chode.

 

[00wyk]

He's being very general. If we get too technical this discussion dies.

ETA, what he doesn't show you, and no one really does when they are trying to discuss Harleys, is the Harley is a cruiser. Not a sport bike. Why would it have a sport bike engine? If you want to eat up HWY miles, you don't want something buzzing in your ear for hours. What you want it something that sounds like a real engine.

My Dyna has 85hp and 95 ft lbs of torque. All it has is a street cam, a programmer(and I make my own maps), and thinner gaskets. Nothing you can't do at home yourself. This is considered a stage II bike. Stage I is exhaust.
What those numbers don't tell you is it has 95 ft lbs of torque nearly from idle up to 4.5k rpm. It revs to 6200rpm, eagerly. This means if I am not careful with my throttle, I melt the back wheel from a stop if I am in a hurry.
Harleys are not under powered. They just aren't racers. It's not a kids' bike.
And ain't no modern Indian bike gonna sound like my Dyna and shake like it:

 

[Wolverine]

So that shaking is desirable?

 

[00wyk]

So that shaking is desirable?

Nice bike. I think another thing most Harley detractors don't realize is most Harley owners have already been where they are.
When I was young, I enjoyed having a ZX7 and riding around. Hell, I enjoyed my YZ125, too.
Even raced a Yamaha in the super sport class at the time. My last race was Laguna Seca.
Good times. But do you really think you're gonna be riding on a race bike in to your 40's? On the street? I didn't. You'd just look silly.
Here I am about 25 years ago with my ZX7 and my FZR400(The Honda behind me is a bud's I am wrenching on):

Now I enjoy a nice ride up the coast of Ireland on the Harley. Here she is with a different seat.

Ride safe. And don't ever forget we're all on the same team.