High vs Low RPM motors

[Mbirt]

Originally posted by Warpspeed:
It would be rather interesting to take those two engines as tested, fit restrictors to them, retune them, and test them again.

It could well be, that the old Harley banger with it's higher Ve at lower rpm out torques and maybe even out powers the Rotax.

The Rotax probably has much larger ports and valves, and much more radical valve timing. Strangled right down with a restrictor, it is never going to be able to reach anything like 11,000 rpm.

And at the lower max airflows limited by the restrictor, it could quite likely be operating at a severe disadvantage.

Tony, I'm not so sure I agree with these points.

Things are different when you get into low cylinder counts and odd-fire engines in restricted scenarios. We run such large plenum volumes on singles because the restrictor chokes the induction event on a pulse basis, not at steady flow. We must time-average the flow through the restrictor to negate its effect. Since the restrictor is limiting the volume of a single induction event, it limits peak torque and not peak power. If the two engines were the same, but one had more peak torque, you might find that the torque peak of one was truncated while the other seemed unaffected in comparison.

But the two engines have different firing orders and the 72 degree firing order plays in favor for the 1125 in a restricted scenario. With only a 45 degree separation between induction events, the xb-12 might as well be treated as a thumper.

Finally, with the added valve periphery of 4 valves instead of 2, the Rotax undoubtedly runs less duration than the XB-12. I found these specs for the XB-12. Taking ramps for pushrods into account, duration at 1 mm is likely over 265/265. There is no way the 1125 would need this much cam--with great thanks due to 4-valve breathing.

Timing @ .053” Lift Open / Close:
INT. 25 / 44
EXT. 59 / 10

Duration @ .053” Lift:
249 / 249

Max Lift:
.551” / .551”

[Warpspeed]

Originally posted by Mbirt:

Things are different when you get into low cylinder counts and odd-fire engines in restricted scenarios.

Very true.
Being forced to run a restrictor is a real game changer, and the more uneven the pulsing of the induction airflow, the worse it all gets.

Very difficult to predict what might happen, which is why I find this particular engine comparison a particularly interesting one.

But the thought still remains. As a restrictor effectively causes power to plateau at some relatively lowish rpm, an engine with a higher Ve at, and especially below that lower rpm may be working at less of a disadvantage with a restrictor.

Getting the hottest available high rpm race engine, and then strangling it right down with a restrictor may not always be the best approach to the problem.

[Z]

Originally posted by Mbirt:
But the two engines have different firing orders and the 72 degree firing order plays in favor for the 1125 in a restricted scenario. With only a 45 degree separation between induction events, the xb-12 might as well be treated as a thumper.

Mbirt,

Because I am a nit-picking PITA I should point out that the above quote is back-to-front. Assuming the two V-twins are of conventional design the 45 degree Harley will fire every 315-405 degrees (360+/-45), and the 72 degree Rotax will fire every 288-432 degrees (360+/-72). So the induction events on the Harley are more evenly spaced than on the Rotax, so giving potentially better breathing through the restrictor.

The very old Harley design was probably a case of the designers trying to figure out the best way to get more power from their single cylinder engine.
"Maybe increase bore and stroke?"
"But then we have to change all the tooling..."
"How about we use two of our existing engines side-by-side, parallel-twin style, for twice the power?"
"Errrr..., but then it will weigh twice as much..."
"Ok, how about we just stick an extra cylinder onto the crankcase, here, just behind the existing cylinder, and use pretty much the same crank?"
"Yeah, saves the weight of that extra crank and crankcase, and keeps it all nice and narrow. And if we push the two cylinders as close together as possible we can get pretty close to 360-360 firing..."

Z

[VFR750R]

I agree with Z's possible Harley scenario. Even more plausible when you consider the forked connecting rod design where the two cylinders are directly in plane with each other. The crank pin is not wide enough for two full size rods.

[Mbirt]

Thanks, Z. I stand corrected. I had assumed the potato-potato sound and seemingly poor VE were due to a 45 degree firing order to match the 45 degree vee angle. Should've done my homework...

[Owen Thomas]

I thought the Harley v-twins weren't "conventional design", though? Ye faithful Wikipedia tells me that Harley engines have simultaneous power strokes, which means on that engine the firing interval is actually 45 degrees apart. This is what gives them the "potato-potato" noise (still chuckling about that). I can't speak for the Rotax, but I would assume it is more of a conventional (288/432 degrees) setup, yielding smoother power delivery.

Anyways no one seems to be wrong, but in this specific scenario I think Mbirt was correct with his original statement. Regardless, looking at the pressure pulses definitely adds some complexity.

Back to the topic at hand, it seems like if an engine did have an odd firing sequence like a v-twin, the variation in manifold pressure would have a smaller effect at higher RPMs.

[Warpspeed]

Originally posted by Owen Thomas:
I thought the Harley v-twins weren't "conventional design", though? Ye faithful Wikipedia tells me that Harley engines have simultaneous power strokes, which means on that engine the firing interval is actually 45 degrees apart. This is what gives them the "potato-potato" noise (still chuckling about that).

I also assumed that too, that the Harley firing interval was 45 degrees, but apparently it is not.
The Harley ignition system is very crude, with one set of points and no distributor cap, it is essentially a wasted spark system where both plugs fire simultaneously.
This obviously cannot work if it had only a 45 degree firing separation, but it does work in a fashion with the 315/405 firing interval that these monsters apparently have.

[Adambomb]

Hmm, I was also under the impression that "conventional" for a v-twin meant that both cylinders fired on the same rotation, ie 45º bank angle = 45º firing interval. I was also pretty sure that the majority of cruiser engines used the "quasi thumper" layout to get the potato® Harley sound. It gets really obvious with the large displacement ones that idle at like 300 rpm; they just sort of go "badup....badup....badup...."

As for the 1125, will have to consult the service manual for my 1125CR tonight!

[Adambomb]

OK, I had to research this more myself, and sure enough it does appear that the Harley uses the "more even" firing order of 0-315-720. There is also some interesting discussion of Harley's trademark on the potato:

"The mark consists of the exhaust sound of applicant's motorcycles, produced by V-twin, common crankpin motorcycle engines when the goods are in use"

Harley's unique sound goes back to the beginning

What gives a Harley-Davidson motorcycle its distinctive sound?

[rjwoods77]

Fun fact:

Almost all of the WW2 high performance V12's were fork and knife connecting rods in order to keep the engines as short as possible and to reduce the vibrations as a result of offset banks...

DB V12....

http://legendsintheirowntime.c...10_sk_DB601_p177.png

RR V12:

http://www.ww2aircraft.net/for...conrods-2-resize.jpg

Allison V12:

http://www.enginehistory.org/Misc/AllisonRods.jpg