4 Cycle Fuel draw

What is the consensus is this true?

I run my Saito 35 with the standard throttle (but no muffler pressure) in a CL plane (Twister) and don't see a major problem (except I do us the electric starter) with the fuel draw.

So I guess your question may depend upon if one is referring to the standard throttle setup or not.

Well, my experiments so far indicate that Zambelli is full of it. Hmm, let me rephrase that. Fuel draw is maybe (big maybe) as good as a 2 stroke engine up to a point. As venturi size goes up, the engine's ability to draw goes down. The draw drops off dramatically and suddenly over a certain venturi size. Unfortunately, that drop off seems to occur right as you're getting to the power potential of the engine. I've gotten the best power from my Saito 56 right at the point that the fuel draw becomes a problem. The power is great and draw seems ok until you pull a tight corner. Then the engine burps and coughs and sputters. Go down a couple of venturi sizes and the power drops off, but the problem, the coughing in corners, also goes away.

I've been experimenting with a couple of different options including a tank design to improve head pressure and possibly a pump or pressurized system and fuel regulator. Of course, the easiest is just to go up in displacement. With a .70 or .90, you can use a very moderate sized venturi and let cubes create the power. Cubic inches = horsepower. A lot easier to get it with cubic inches than with pressurized fuel systems or trick cams or other modifications.

Randy

PS-I still want an itty-bity supercharger.

There's something to be said about going up in cubic inches, but there is a limit we can use isn't ther?

OS does have a supercharged engine in their, but if memorie serves, the engine is at least a 1.20. Perhaps it could be modified to our use though.

Yea, there's an upper limit. Right now I think it's a .90 or thereabouts. My experience has been that the Saito .56 is plenty if the plane is light. The plane I've been testing it is isn't very light. It's got almost enough juice. Kinda wish I'd gone to the .65.

I've seen the supercharger used on the 1.20. Not even.
Randy

Sorry you took that wrong. It really was meant tongue in cheek. I've actually spent quite a bit of time on Windy's web page reading the 4 Stroke stuff. I haven't had the same experience, but similar enough considering local conditions. I've been looking for something specific. Haven't found just what I want yet, but I'm fairly close.

I suppose I should use the smiley faces.

My personal experience (13 4-stroke engines purchased exclusively for testing C/L applications - I don't fly RC) has also led to the conclusion that Bob Zambelli if full of it. However in my experience, the "it" he is full of has been good information, good advice, and sound, useful suggestions. I haven't gotten to talk to Bob often, but when I have, EVERYTHING he has told me has been spot on. I have watched his postings to various forums and learned a great deal from that as well. It is my opinion, as well, that 4-strokes have a little better, more consistent fuel draw than two strokes, both with muffler pressure and with uniflow. Bob hosts a four strike forum on Windy Urtnowski's web site that you might want to check out. It's at windyurtnowski.com.

Incidentally, if you gradually increase the venturi bore on a two stroke it gets to a point of peak power and the falls off rather dramatically, just like your Saito did. It's not a failure of the 4-stroke (or the 2-stroke) engine, it's simple physics.

Regards,

I have been testing 4 strokes with limited results until trying one of "Mr Zambelli's" venturi's. I personally am glad that he is "Full of it" and only wish I could reach this status.

Thanks Bob for all of your contributions to our sport/hobby.

Zuriel

The engine, in general, seems to be very particular to the location of the centerline of the tank with respect to the engine. There also seems to be an inability to drain the tank completely or become lean at the end of the tank.

I have heard of people running without uniflow, we tried that, the engine changed drastically (leaner) through the run. Uniflow was much better. Without pressure and on uniflow and running the engine on the ground, I was unable to turn the plane on its side (outboard wing down) to empty the tank completely, as I would do with a ST 60. The engine would just shut off. Weird.

My only explanation for the weird runs without pressure is that I use a pinhole on the uniflow vent, as I have always done on 2-cycles. I plan to test this today.

I hate to say it, but my .56 just runs better and better the more nitro you put in it. It needles like a dream on 15%. Much more touchy on 5% (with a 13-6 Bolly). The RPM really does not change much when you up the nitro, which is surprising, but boy, does she smooth out and RUN on the higher nitro.

More load, more nitro? Makes sense.

Anyway, thanks everybody (Bob, Paul, Pat, Willis). This is fun.

By the way guys, there is a lot of disagreement as to what will work and won’t on 4-cycles. Be open-minded.

So far so good.

My experience is limited, but my best, most consistent engine runs have come while running muffler pressure on the uniflow line. Others, though have run the pressure to the overflow with good results and still others prefer no pressure or even standard venting. I've been pretty lucky with engine runs so far.

Tank placement can be a challenge. As you know, a 4-stroke's centerline of the venturi or carburetor is higher in the model (with the engine mounted inverted) than a 2-stroke's. That means that the tank needs to sit higher in the nose with the 4-stroke. Since all of my full-fuselage models which have gotten 4-strokes have been retro-fits, all I have beeen able to do is shim the tank up as far as possible. Even when it won't line up perfectly, I've still had good runs with the pressure set up as above.

If you haven't already, you might check out the forum I mentioned earlier.

Good luck,

Windy's forum has been very dead lately.

On the comment about verturi size. I probably should explain that I started with a .295 venturi size (sealevel, cold air, all flights with the same 12.75 x 4.75, 3 blade "paddle" prop). I starting getting uneven runs with burping and coughing in the corners at a .330. Additional nitro (up to 15%) smoothed it out somewhat, but the fuel draw was marginal. At .350 the power absolutely went away, even in level flight it coughed and burped. I wanted to find the point where it just couldn't draw the fuel. This was pretty much it. The best run I got for just sheer torque was at .330, but just couldn't get rid of the fuel draw problems in tight manouvers. At .320 (where I'm currently running it at 12% nitro), the power is quite smooth and pretty consistent. Not there yet, but close. Still doesn't have the kind of power I want in the overheads in serious wind (15-25mph), but it's not at all bad. I still think that going up in cubes is a better answer than trying to squeeze more out of a smaller displacement, but hey, that's just a very juvenile opinion on my part.

My brother-in-law is messing around with one for kicks. Considering his experience (he's a former NASCAR engine designer and race prep mechanic), I can't wait to see what comes out of his shop. Probably double the power and torque and it will last about 2 runs before it blows up. <grin>

Now that's a cool idea. Tell Chris I definately want the converted drawing.

Remember, these are figures for a .56 2-cycle. I don't know how that translates to 4-cycle, but it seems to line up pretty close to the recommendations seen above, and, also, would verify Randy's losing the fuel draw when exceeding these "limits".

Going to larger throttle, larger venturi, will produce more power up to a point. But there is a point where fuel draw falls off and the gain in power becomes meaningless.

Leonard Neumann

I'm sure you know what you meant, and so do I, but "meaningless" isn't exactly the rught word. At some point, making it bigger doesn't result in any further power increase, because the venturi ceases being the limiting factor.

I actually run the 40VF this way - any venturi larger than about .275 has no effect on the power, because the pipe limits the rpm. I run a .280. That's why it's almost completely immune to air density changes.

Question, Brett, are you using the original OS needle with its .135 diameter, or an ST or equivilent with its .155 diameter? with an ST you could take it up to a .290. With the original I would probably stop at a .270. But, as you say, the pipe may change things.

Leonard Neumann

That's with the ST spraybar. I've run it up as large as .305, and it just didn't provide any more power. It does make it mega-hard to set and real unpredictable, though.

With the pipe set to peak at 11,000 rpm, that's as fast as it wants to go no matter what you do to the intake.

>Remember, these are figures for a
>.56 2-cycle. I don't
>know how that translates to
>4-cycle, but it seems to
>line up pretty close to
>the recommendations seen above, and,
>also, would verify Randy's losing
>the fuel draw when exceeding
>these "limits".

---disclaimer---

All of the below is purely opinion based on my limited experience and not to be taken as absolute fact.

----------------

This is exactly right. I continued to increase venturi size as long as the power continued to increase and the engine was able to draw fuel. As I noted, I reached a point where there wasn't much, if any increase in power and the engine was clearly having difficulty. At that point, I backed off to the point that the power was still good and the engine was still able to draw adequately. My only real point was that at the point of greatest power generation, the engine wasn't able to consistently draw fuel, exemplified by the burps and coughs in corners and a somewhat uneven run even in level flight. Back off further in venturi size smoothed the run and cured the coughs and burps in the corners, but there was significant power loss as well.

I'm using a modified ST needle valve, so you're right on there. I really had no expectations as far as venturi size. A friend of mine is using a .290 in his Saito .56 quite successfully and so I used that as a starting point. The best compromise (in my opinion) is at about a .310 or .320. The power is good (not as much as at .330, but still good and the engine seems to draw enough fuel without headaches. Keep in mind that local conditions make a huge difference. I'm flying as sea level in heavy, wet, cool air. For those that fly in dry air or at altitude... Your milage may vary, as they say.

Some of the problems may be due to the manifold I'm using. I plan to make another that will have better flow characteristics. I may try one such as Al Resinger was describing. Another option is to use a pressurized system with a fuel regulator. Or perhaps I'll tear the thing down and clean up the head flow. It's not exactly clean in there. Also may look at valve duration, but I really don't want to. We'll see. I think there's a lot of development room here, particularly with fuel delivery systems and props.

One last point and I'll shut up. Someone said that Saito obviously knows what there doing and that (I'm severly paraphrasing) the engine is great like it is. Anyone that spends much time working with mass produced engines knows that reliability is much more highly valued than over all performance. Saito is clearly making an engine that is highly reliable and durable. But that doesn't mean that it is performing anywhere near it's maximum or even optimum power output. I would guess that it's most important to Saito that they make an engine that the everyday guy can rely on to perform consistently and durably, much as it's most important to Ford (ok, no Ford jokes) that they make cars that are reliable grocery getters and that get decent gas milage. But I wouldn't want to take a stock Ford production car down to race at NASCAR. I suspect that there is a lot of room to modify a Saito engine to increase power production without giving up too much reliability.

OK, I'm off my soap box.

The new Dixon venturi draws fuel very effectively and gave pretty steady run from beginning to end. The lap times never changed. Problem is, the .275 venturi appears too far down on the power band and the RPM too low. We were up to a 12-7 APC to get lap times down to 5 seconds.

We can no longer use the 13” prop with the small venturi.

Also, the engine appears to be richening (slowing down on consecutive maneuvers) and generally slows down when loaded.

The first square eight is OK the second is very bad.