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Author Topic: Why 4 Stroke  (Read 3947 times)

Offline Motorman

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Why 4 Stroke
« on: February 17, 2017, 09:03:37 PM »
I don't know anything about 4 strokes on model planes but my take on it is why use a heavy engine that puts out less power? What's the thinking on using 4 strokes.

Thanks,
MM
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Offline Dan McEntee

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Re: Why 4 Stroke
« Reply #1 on: February 17, 2017, 09:54:54 PM »
   
      You should try reading all of the sections on this forum.
   Search this section for anything by Bob Reeves, or Saito .56 or .72. If you haven't flown one, you should try one that is properly set up. If you get into four strokes, forget everything you ever  learned about running and proping two strokes and start from scratch. They require shorter nose moments due to the extra weight as suspected. They can swing more prop and the torque they put out is remarkable. I have a Top Flite Score with a Saito .56 in it that I bought to get some experience with them to see what they are like. I messed with it off and one for two seasons and could see why some guys loved them and why some guys would use them for trot line weights. It was when I read up on what Bob Reeves figured out  that I started getting really constant runs. The combination is responsible for a few trophies in my possession.  It's really a very different type of power plant, and if you can grasp the concept behind them,they are a very useful and viable power system. Paul Walker won one of his Walker Cup NATS wins with a four stroke. Like I said before, some people swear by them, some swear at them, but they are just one of the several different power choices that a guy has available to him these days.
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Offline Brett Buck

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Re: Why 4 Stroke
« Reply #2 on: February 17, 2017, 11:11:10 PM »
I don't know anything about 4 strokes on model planes but my take on it is why use a heavy engine that puts out less power? What's the thinking on using 4 strokes.

   The all-up weight isn't a lot different from a piped 40 once you add up all the parts and consider the difference in the fuel weight, and the power is more than sufficient (from a 56 on up) for any current competitive airplane. Overall power, for stunt, hasn't been an important factor since the mid-late 80's, because you can get far more than you can use.

    The pros and cons have been debated endlessly. One thing that 4-stroke definitively solve is the "excess power burst" problem that you can get with 2-strokes if you don't set them up right. The aren't going to spin up much beyond about 11,000 rpm no matter what you do, which, conveniently, is about what you need in the air for a 4" pitch prop. In fact, the low-rev systems are seemingly intended to overcome the tendency of the engine response to be too "flat" because it regulates a little too well at 4" pitch rpm.

    The cons are the tendency towards random fuel consumption, and generally inflexible response. You can't just dial in the amount of power boost/brake you want like you can with a tuned pipe engine or a feedback control electric. The Berringers used an intentionally flexible prop to help, and also invented the venturi system that Bob Reeves and others use to adjust the speed over small ranges. The random fuel consumption is mostly due to trying to control the speed with the needle, and it's a very weak effect on RPM but a big effect on fuel usage.

    At the time of the original experiments (Ted Fancher in the mid-80s), the power characteristics were a huge improvement over the ST46 and similar, with much less problems than you got with muffled schneurle engines like the 40FSR. But a few years later, along comes Bob Hunt/Dean Pappas/Rich Tower with piped 40s, which solved the schneurle engine problem, and was much more amenable to tweaking. The whole idea died until Windy/Bob Zambelli and others ran out of ST60s and discovered 4-strokes, at which point tuned pipes were a turnkey operation with far more flexibility. Some liked the heavy line tension to you tend to get, and tried it with some success, but it wasn't a breakthrough that you had to have to be competitive.

     Later, Igor Panchenko invented (as far as I know) the very-low-rev system that "freed up" the engine to be more responsive, at the cost of less speed control due to the very high pitch required. Bob Reeves applied the Berringer-style venturi, with resolved the issue with the speed adjustment, but by then, Mike Palko and others were experimenting with electric, with quite obviously more potential. So after a brief second heyday, interest sort of faded away. Some people (like those here) are still enthusiastic about it and get very good results, hence the forum for it.

     Any of these systems (tuned pipe, electric, 4-stroke, and the very advanced 4-2 break engines like the Discovery-Retro and Belko {ST60's with 20 more years of aerospace-quality development work}) is plenty capable of providing sufficient performance for any purpose, if you know what you are doing. Electric clearly has the most growth potential at this point. The others have pretty much been refined to the point that not much new is being developed. David Fitzgerald and I have made changes only in the area of fuel flow and carburetion over the past 10+years. We learned some very interesting things and have refined the run to our needs, but nothing that would be classified as a breakthrough.

      Brett

Online Dane Martin

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Re: Why 4 Stroke
« Reply #3 on: February 18, 2017, 10:36:34 AM »
I don't think they are less "power", just less RPM potential.  So prop selection for CL is tricky at first.

Mostly, it's Cuz they sound so bad @$$

Offline Target

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Re: Why 4 Stroke
« Reply #4 on: February 18, 2017, 03:27:01 PM »
Subscribed.
How does vibration compare, 2 cycle to 4 cycle? I would assume the 4C would shake less, but that is just a guess on my part.
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Chris

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Offline CL-flyer

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Re: Why 4 Stroke
« Reply #5 on: February 19, 2017, 04:53:21 AM »
Check out the RC forums for a more complete coverage of four-stroke engines in general before focusing on CL usage. I like four-strokes a lot but only use them on my RC planes. Only two-stroke and electric for CL.

In a nutshell the four-stoke uses more of its power stroke than a two to generate more torque.

Offline Brett Buck

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Re: Why 4 Stroke
« Reply #6 on: February 19, 2017, 05:57:55 PM »
Check out the RC forums for a more complete coverage of four-stroke engines in general before focusing on CL usage. I like four-strokes a lot but only use them on my RC planes. Only two-stroke and electric for CL.

In a nutshell the four-stoke uses more of its power stroke than a two to generate more torque.


   That's certainly not true in general, a PA61 puts out ~50% more torque at typical 4-stroke revs as a Saito 56, and even more difference at lower revs. The data is available here somewhere - we measured it. But that's not particularly important, since you can't really use the full capability anyway. 4-strokes (and to a lesser extent, 4-2 break engines) use large props at low revs because they have to, not because they are somehow "torquers".

    The energy from a given amount of fuel and air is generally more efficiently used for given amount of power in a 4-stroke but the power, even at low revs, is much less. As long as it is sufficient, you decide based on other issues (like the ability to regulate at usefully low revs, which the 4-strokes definitely do).

     Brett

p.s. I thought I had dug this up before, but here are the numbers. At 9000 RPM a Saito 56 puts out about 90 in-ounces (based on data from Saito/Pete Bergstrom posted here).  My PA61, set up for a launch RPM of 10100 rpm with a normal prop, dragged down to 9000 by a much draggier prop (Rev-Up 14-6) was around 140 in-ounces. We were initially surprised by this, but then it occurred to me that to run at a stable speed with an inflight RPM of 10800, 9000 is probably pretty close to the tuning peak. We run well past the peak in flight, of course. The Saito 72 peak torque is around 118 in-oz, so, still not there. Even the venerable ST60 is in between the 56 and 72.

  I also presume that the Saito data was taken at ideal mixture with the throttle wide open. The PA needle was set at the position necessary to get the launch revs with the flying prop, and then left alone as we changed to a 14-6 Rev-Up to bog it down, and had just enough load to run in a 2-stroke. A little tweaking at it could have been more, another .015 of venturi would have been dramatically more, etc.

   Note that this tells you only a little bit about how effective any of these engines might be in flying a stunt plane, but the notion that 4-strokes are "torquers" is way off. A different way of saying it is that they will *only* put out significant torque at low revs, i.e. they won't breathe at higher revs. The torque and HP curves are dropping dramatically at 11,500 rpm and look like a brick wall. Take the pipe off the PA, or shorten it by two inches, and it will happily thunder out 2-3x the power at much higher revs, meaning it keeps putting out torque far beyond the 4-stroke. Note also that this is a mere PA61(!) - and you can get an engine 25% larger!

    
« Last Edit: February 19, 2017, 06:46:51 PM by Brett Buck »

Offline Paul Smith

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Re: Why 4 Stroke
« Reply #7 on: February 20, 2017, 05:37:16 PM »
It seemed like the whole rationale for 4-stroke was a slight reduction on noise.

But with the advent of cheap and easy electrics, the need for lower dB's is satisfied and the 4-stroke is nothing but heavy, expensive, complicated junk.

If you want power and performance use 2-stroke.
If you want quiet use electric.
If you want 4-stroke use 4-stroke.

There are still a few die-hards who make Sterling engines run.
Paul Smith

Offline Robert Zambelli

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Re: Why 4 Stroke
« Reply #8 on: February 20, 2017, 05:38:57 PM »
Around 15 years ago when I was tinkering with 4S engines, I did a comparison regarding weight.
I did a weight comparison between the ST 60 and ENYA 53 4S.
Tanks included.
More info to follow.

Bob Z.
« Last Edit: February 21, 2017, 12:39:44 PM by Robert Zambelli »

Offline Chris Wilson

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Re: Why 4 Stroke
« Reply #9 on: February 20, 2017, 09:02:28 PM »
I like four-strokes a lot but only use them on my RC planes.
Probably because they throttle better when swinging scale sized props, and people can relate to them far easier.
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Offline Paul Smith

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Re: Why 4 Stroke
« Reply #10 on: February 20, 2017, 09:13:17 PM »
That was the theory.  But in actual scale contests it doesn't seem to prove out.  4-strokes seem to have MORE trouble dead-stopping the model than two-strokes.  Maybe this has to do with one-cylinder engines needing to coast through two revolutions. 

The bigger 4-stroke props might be developing too much thrust at their lowest idle speed.
Paul Smith

Offline Bob Reeves

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Re: Why 4 Stroke
« Reply #11 on: February 26, 2017, 03:07:12 AM »
My reason for going to 4 strokes was mostly financial. I wanted to be able to compete at the highest level of Advanced and knew I wouldn't be able to get there with a Fox 35 or FP-40. Having to invest four to six hundred in a piped engine set up just wasn't in the cards. On top of that if I wanted to have a back-up engine I would have to spend another four hundred or so. Discovered I could buy a really good Saito 56 used from an RC guy for a hundred bucks and for less than $300.00 have two engines ready to go.

My 4 stroke story has been told a couple times on the forums so not going to go into the whole process of how I ended up with a Saito 56 powered stunt ship that won Advanced at Brodak's twice and in my opinion could compete with the big boys in Expert under any conditions. I reached my goal which was to win Advanced at Brodak's with a $100.00 engine competing against pipes and electrics flown by some of the best Advanced flyers in the country.

I did not move to Expert simply because I have other interests and wasn't willing to invest the effort it would take to get to the top of Expert. I know myself well enough to know if I went into Expert I wouldn't be happy till I was winning. I had no doubt my 4 strokes could get me there if I was willing to invest the time it would take to get good enough. I wasn't willing to invest that time.

Offline Chuck_Smith

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Re: Why 4 Stroke
« Reply #12 on: February 26, 2017, 05:37:09 AM »
For some of us, mucking about with the engine is as much or more fun than the flying. I've been flying the same pattern since 1970, nothing new there. But engines, always something fun to try next. As noted earlier, ample and consistent power is no longer a differentiator in this hobby. Anybody can have it.

If a 4 stroke floats your boat - take up the challenge. Main thing is...have fun! One of the early drawbacks was expense. Nowadays you can pick up gently used 4 strokes pretty reasonably.

Me, I'm looking at gas now. Obscene power to weight, torque for days, reliable, super easy starting, consistent, easy cleanup and your fuel is 2 bucks a gallon. The only drawback so far is that the smaller gas engines use so little fuel that the carb passages are tiny, and ANY dirt is not-so-good, so the challenge is going to be assuring good filtration.

All IMHO and YMMV,

Chuck
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Offline Chris Wilson

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Re: Why 4 Stroke
« Reply #13 on: March 03, 2017, 08:24:41 PM »
....... I wouldn't be able to get there with a Fox 35 or FP-40.
In this country an FP 40 in a profile won many titles in a Sukhoi profile model - so it is possible.
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Offline proparc

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Re: Why 4 Stroke
« Reply #14 on: March 05, 2017, 09:29:03 AM »

Me, I'm looking at gas now. Obscene power to weight, torque for days, reliable, super easy starting, consistent, easy cleanup and your fuel is 2 bucks a gallon. The only drawback so far is that the smaller gas engines use so little fuel that the carb passages are tiny, and ANY dirt is not-so-good, so the challenge is going to be assuring good filtration.

All IMHO and YMMV,

Chuck

Gas has come on like gangbusters in the RC world. All for the reasons you mentioned. The DLE line of engines has really been responsible for a lot of that. My Saito 72 has been without question, one of the MOST reliable and consistent CL motors I have ever owned. I own 3 Saito's, and I am looking for a 4th.
Milton "Proparc" Graham

Offline Chris Wilson

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Re: Why 4 Stroke
« Reply #15 on: April 07, 2017, 09:34:39 PM »

In a nutshell the four-stoke uses more of its power stroke than a two to generate more torque.


I agree with this 'if' you are considering only the power stroke, a four stroke has far longer dwell times and will make better use of those times but sadly they only occur half as often when compared to a two stroke.

But in the power stakes what are we comparing here? State of the art model two strokes with a weird premix fuelled vintage push rod actuated 2 valve four strokes?
I know that type of  four stroke model engine is all that is commercially available but it makes you wonder about true four stroke potential if miniaturisation went crazy.
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Offline Brett Buck

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Re: Why 4 Stroke
« Reply #16 on: April 07, 2017, 10:57:08 PM »
I agree with this 'if' you are considering only the power stroke, a four stroke has far longer dwell times and will make better use of those times but sadly they only occur half as often when compared to a two stroke.

But in the power stakes what are we comparing here? State of the art model two strokes with a weird premix fuelled vintage push rod actuated 2 valve four strokes?
I know that type of  four stroke model engine is all that is commercially available but it makes you wonder about true four stroke potential if miniaturisation went crazy.

   And, as noted above, the idea that 4-strokes "put out more torque" is demonstrably incorrect, at least in their current form. It's substantially less, even at the idea RPM for the 4-stroke,  and given that, and the rpm limitations, they produce FAR less power. In stunt, this is mostly irrelevant because you can't use all the power in either case.

   But even in applications where you can use all the power, it has been proven time and again that 4-strokes cannot hold their own against 2-strokes. Even mighty Honda, at the height of their powers back in the 60s and again in the 80s, tried to produce racing motorcycles with 4-strokes and got absolutely killed over and over. They did it by cylinder multiplication and high RPM, with a 5-cylinder 125cc  engine and a 6-cylinder 250cc engine and a "4-cylinder" 500C engine (which was effectively an 8 cylinder with oval pistons, effectively dodging the rules about cylinder count), all 4 valves per cylinder, and revving as high as 25,000 rpm. The finally gave up on the idea after the failure of the NR500, broke down and built a 2-stroke, and won everything there was to win.

   You don't see any 2-stroke racing engines in full-scale racing *only* because you also have to carry fuel for the race. Doesn't matter on a model airplane or a motorcycle, but it matters a lot in a Formula 1 car, and in fact, today, they are limited to 26 ish gallons of fuel for the entire race. They manage to make it even at ~900 hp for the entire hour and a half only because they are hybrids and recover a lot of the energy.

       4-strokes are more efficient mostly because the fuel/air mixture burns very completely, because the engine uses an entire rotation to clear the exhaust gasses the previous firing. On model 2-strokes, when actually running in a 2-stroke, the new charge comes in as the exhaust goes out, and they alway mix and they always leave substantial residual exhaust behind to fire a much less clean charge. Also, some of the charge ends up coming into the cylinder and going straight out the exhaust. That's what the baffle is for - to keep the charge from just shooting across the top of the piston and out. It forces the charge up the intake side of the cylinder, and hopefully pushes most of the exhaust out in a loop, hence the phrase, loop-scavenging.

    Interesting, when a model 2-stroke is set so rich that it 4-strokes, the strokes on which it fires are MUCH more powerful than the firing strokes in a 2. Scott Bair measured it to be something like 50% more cylinder pressure. Same effect as a 4-stroke (proper) - the "dead" stroke where it doesn't fire pushes out the residual exhaust and blows an entire charge through the engine to effectively clean it out. Next time around, another new charge comes in, no exhaust hanging around, and it fires very strongly. Of course it wastes around half the fuel, but you can always make a bigger fuel tank.

     I agree that it would be very interesting to see what you could do with a small 4-stroke built to high-performance standards (DOHC 4/5 valves-per-cylinder and unlimited RPM). Not for stunt, it works fine the way it is and nobody is actually using all the power of even a Saito 72. But just as an academic exercise, what you could do if you went for it. The answer is pretty clear from motorcycle racing experience, but it would be pretty cool to build a .60 (10cc) shrunken version of the Honda 25CC cylinder (used for the 2-cylinder 50 and the 5-cylinder 125). And then maybe rev it to 40,000 rpm - likely possible, since the parts are about 40% the size they were on the motorcycle engine and it managed ~25000:







     Brett

   

Offline Chris Wilson

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Re: Why 4 Stroke
« Reply #17 on: April 08, 2017, 05:40:25 PM »
Hi Brett,
           I stand by what I say, a 4 strokes power impulse is stronger due to the time is has extract energy, the ability to use higher compressions that convert more of the fuel into heat, less resistance not having a primary compression to deal with and least of all the calorific value fuel is higher not having to deal with oil. (And probably lighter pistons as their shorten length does not have to act as valves against the windows of ports.)
Evidence of this the fact that if one engine has half the net power impulses as the other then it should make half the power but it doesn't work out that way.

And I too follow motorcycle practice having ridden for about 50 years and my current bus is a Moto Guzzi - but none of this matters here.

So why chose a four stroke?

From a purists point of view, the mechanics of operation are far more tangible although using slightly more parts vs the extreme complications of two stroke physics and interdependency of their parts.

And (since both systems are heat exchange air pumps, the inherent use of supercharging in a two stroke will always give it the edge) the fact that the 4 stroke simply uses atmospheric pressure throughout its entire intake system.

The four stroke concept is easier to understand.

I realise that most will rail against a simple non piped two stroke as being labelled as supercharged but they all are.
The primary volume must be mechanically compressed in order for the charge to scavenge and its usually about 1.6:1 compression ratio in a model engine.

The entire history of two strokes is littered with how to better compress the primary and secondary volumes and give added charge to the fuel load.
Split singles, DKW Bekamo (http://www.odd-bike.com/2014/02/dkw-supercharged-two-strokes-force-fed.html) pump engines and resonant pipes are all charging systems allowed in racing circles but when someone mentions mechanically charging for a four stroke above atmospheric it deemed as cheating and unfair - go figure!

Anyway, I could easily rave about this subject forever so I will leave it there, Cheers.

P.S. If you want to cite examples where you can use all the power you can get then look at Top Fuel Drag car engines, dominated by supercharged four strokes.

Lifted unashamedly straight from Wikipedia "  The calculated power output of these engines is most likely somewhere between 6,340 and 7,460 kW (8,500 and 10,000 hp),[8] which is about twice as powerful as the engines installed on some modern diesel locomotives, with a torque output of approximately 10,000 newton metres (7,400 lbf·ft)[9] and a brake mean effective pressure of 8.0–10.0 MPa (1,160–1,450 psi).

In late 2015, tests using sensors developed by AVL Racing showed peak power of over 11,000 hp (8,200 kW).[10]"

Can't think of a two stroke anywhere near this in horsepower per litre and the power is increasing all the time and having to be ruled against for safety reasons.
« Last Edit: April 08, 2017, 06:18:33 PM by Chris Wilson »
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Offline Brett Buck

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Re: Why 4 Stroke
« Reply #18 on: April 08, 2017, 06:15:50 PM »
Hi Brett,
           I stand by what I say, a 4 strokes power impulse is stronger due to the time is has extract energy,


     In fact, all other things being equal, and if you ignore the heroic amounts of fuel that is wasted, in typical applications like motorcycles, 2-strokes of a given displacement generally DO provide somewhere between 50% more to twice as much power. That's why Honda gave up, after compensating for the low power at low revs by trying to spin the engine at 20,000 rpm when 2-strokes were running even more power at 11,000 rpm - meaning the torque was much higher on the 2-stroke.

   The "time" part of it is just about negligible and in fact it doesn't really take consequentially longer. The rest of it is more-or-less right, a 4-stroke *does* extract more energy from a given amount of fuel , because it always fires a clean charge at a near-stoichiometric ratio, and doesn't waste a large fraction of it by letting a new charge blow out the exhaust. That's why 4-stroke specific fuel consumption is far better, not that it takes longer. It also doesn't make much difference that the 2-stroke has to compress the next charge, even when it is 4-stroking, it regains a large amount of the energy used to compress it when it rebounds. In fact, it steals some heat from the firing cycle as well, actually doing a tiny bit of work by transferring some heat into the otherwise adiabatic compression, then letting it expand again on the rebound. On a proper 4-stroke, that compression doesn't gain any work from the heat, but it *does* push out the exhaust, which is a lot more effective in making the next charge fire correctly.

   There was a long thread about this on SSW about 15 years ago, where you could see what the rpm did during a revolution.

   As noted, this doesn't matter much for model airplane engines, but people were more-or-less gutting out Saito 56s to get the same power you get with a 40VF running at 30% of capability. You only need about 1/2 a horse, that's 80% of a Saito 56, and 10% of a PA75.

The "clean charge" effect also works on model 2-strokes that are 4-stroking, which explains why you can have it change from a 4-stroke to a 2-stroke with negligible change in the power. Of course, the charge that *does* fire on a the firing stroke of a 4-stroking engine is far from ideal, it's far too rich in most cicrumstances, which is why the exhaust left over afterwards causes the next compression stroke to misfire.

   This is not a matter of opinion, it is basic engineering. Here's a plot of the cylinder pressure for both conditions:




Offline Chris Wilson

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Re: Why 4 Stroke
« Reply #19 on: April 08, 2017, 07:50:34 PM »

     In fact, all other things being equal ...........


And there is the Elephant in the room right there, both systems are not even close to being even, supercharge a four stroke and equality looks more promising.

See my postscript attached to reply before this one, if you want to argue absolutes to prove a point then look at Top Fuel Drag cars.
(1375hp per litre or in model terms 13.75hp per 10cc is absolutely staggering.)

But its easy to see here that materials will fail long before either system finds its true power limitations.

In regards to the NR 500, it was hampered by rules from being a V8 and of course supercharging was denied it but it seemed more of a learning and marketing exercise than a serious attempt to win the world title.
« Last Edit: April 08, 2017, 08:32:24 PM by Chris Wilson »
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Offline Chris Wilson

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Re: Why 4 Stroke
« Reply #20 on: April 09, 2017, 02:23:20 AM »

 You only need about 1/2 a horse, that's 80% of a Saito 56, and 10% of a PA75.


Brett,
        so my my calculations a PA 75 can make 5hp?

A CMB 80 Hydro racing engine doing 27 500rpm makes 5hp so for its smaller size its actually more powerful than a purpose built water cooled racing engine?
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Offline Brett Buck

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Re: Why 4 Stroke
« Reply #21 on: April 19, 2017, 09:55:12 AM »
Brett,
        so my my calculations a PA 75 can make 5hp?

   Conservatively. A 15 speed engine is about 2-2.5 hp (at 40-45000 rpm) , 5x as big should be able to handle a mere twice the output. Probably the only reason it wouldn't be able to be 5x is that it would burn a hole in the piston - as the engine size goes up the ability to cool it goes down.

    Of course, you couldn't just take a stunt engine setup and tweak the needle and get 5 hp. But the PA75 won a world championship running a venturi smaller than that on a stock 20FP, and a pipe that acted as a regulator to kill power above about 10,000 rpm. Put a max venturi on it, put a properly-sized pipe on it, and a small enough prop, I would expect no problem getting 5 hp out it. The engine itself is probably beefy enough to take it.

       Brett



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