The issue with running a overly tight head on the car engines is they wont idle down correctly when you let off throttle.... With a tight head they always want to "ring-a-ping ping ping ping" when you come off a high speed pass.......... And the only way to get rid of it is to drown it out by running a overly rich mixture, which substantially reduces the power of the engine............ Some engines these days come with as low as 0.014" clearance and will ring-a-ping ping horribly if you try to tune then to the edge.,...They run like scalded dogs tho , but only if your willing to put up with the ring-a-ping ping when you idle down after a hard pass....

Personally I can't stand engines that do the ring-a-ping ping on idle down... I have spent years playing around with head buttons trying to find the perfect balance that allows my engines to reach warp speed RPM"s, carry crisp throttle yet still idle down smooth and silky after a hard run...... I see far too many engines at the races that ring-a-ping ping piing, to the pint where I believe some people assume its perfectly normal and acceptable....

In the end its a very fine line finding the perfect balance in the head button that allows the engine to make full power and RPM, yet still retain the ability to idle down quick and smooth after a hard pass........

Very true Maximo, pushing it to far will give a nervous engine that´s more of an headache and not something that defines a well tuned engine.
The trick is to get everything so right that you get most of the benefits without any downsides and to get that it´s more then just removing a shim and call it a day.

The "ring-a-ping ping" sounds like the engine may be "4-stroking."

(1) Have you ever tried restricting the amount of air that is supplied to your air filter? It seems counter-intuitive,
but it allows you more control over the engines rpm range, and you can still make more power despite the
restriction, and increase reliability.

(2) I was also wondering if a 3 needle carb, helps reduce the "ring-a-ping ping."

(3) Ever try using crankcase pressure instead? the constant 4 psi it produces might help your peak rpm.

(4) I suppose running a glow plug 1 heat range colder when using tight squish defeats the purpose of your glow
plug choice?


Your post initially made think about how many people recommend to adjust the carburetors high and low needles.
The process is completely backwards (To Me) from tuning any other engine, nitro or gas? I can't get my head around it.

When I adjust a carb, I adjust the low speed first, because it is always flowing through out the entire rpm range.
The high speed circuit is dependent on the amount of fuel supplied by the low speed circuit.

If you adjust your carb using the method recommended in the R/C Tech Sticky, and adjusting the low speed screw
last, you are inadvertently readjusting the high speed screw richer or leaner after you just got through setting it.



Maximo, Can you or someone else briefly explain why the "R/C Tech Sticky" method, is the recommended tuning
for Nitro RC Cars?

Thanks


Bill Mazz

I'll bite...

The high speed needle is the first metering device that the fuel encounters when it enters the carb via. the little banjo fitting that the needle and seat mechanism attaches to. Then the mid speed (which is actually the seat for the low speed needle) and its adjustment only aligns the bleed holes up with the high speed needle seat or doesn't depending on how you have it set (however, big changes can be had by adjusting this correctly) then the last thing is the low speed screw which by adjusting you control how much of the needle seat that is covered or blocked by the low speed screw (kinda like a slide needle on a dirt bike)

Therefor the HSN controls the entire fuel setting of the engine at all times. If you have your low speed set right and you richen the HSN then you must readjust the LSN to compensate.

And it has been my experience that the ring-ding-ding people are complaining about is actually a false idle that is induce by too rich of a low speed mixture setting masked by a higher than normal idle speed to allow the engine to idle without stalling due to the rich condition at idle. This can be eliminated if the low speed is properly set. This can in many case cause an engine to be undriveable however as the throttle response can be too aggressive for most to deal with. (If you are working with a strong engine in the first place)

BK

Wmazz, I will reply to your request via pm when I get home

A few points.

1. As mxwrench was saying, our needles are in series with each other, not separate circuits. With a short needle carb, the moment you open the throttle just a bit, the low speed needle is doing ABSOLUTELY NOTHING. That's why I prefer to call it an idle mixture instead of low speed. So while the idle needle has no effect on open throttle mixture, the high speed needle does effect it because the fuel has to go through both at idle. If you tune the idle mixture first, then start leaning the top, you are also further leaning the bottom. Long needle carbs are a bit different but everything still mostly applies.

2. A lot of things are different in car motors compared to plane and boats due to the need for a broader power band and smooth, crisp throttle response. Performance boat and plane motors usually need to make their power at just a specific rpm. An offroad motor needs to make power from clutch engagement (I would guess between 10 or 15k) up to about 35k or so. If all you want is more peak power, that's easy. Just drop a good onroad motor in it. They make power from around 20k to over 40k with most of it between 30-37k. Let me know how that works out for you.

3. Peak power is just not that important. While I have no way to prove it, if there was I would be willing to bet everything I own that no offroad worlds has ever been won by the post powerful motor. Driveability, reliability and mileage are MUCH more important.

4. I have played around with tighter squish/more volume in the dome in onroad motors. I have found that yes it can work. I also find that you get a super tight tuning window. Get it right, you're fast. Even slightly wrong, you either go slow and run out of fuel or go fast but blow it up (unless the classic lean bog forces you to richen it before that happens).

5. Piston cooling doesn't seem to be an issue for our motors. I have seen all kinds of crazy piston failures but never a melted one. I know this is a major issue in larger 2 strokes but it doesn't seem to effect us.

You should start a class of some sort, online, and charge a small fee, and inform the nitro community the basics. You my friend, really know how to explain things. I was trying to think of a way to explain the whole HSN vs. Lsn thing, and gave up, cause I'm not literate in that way. Thanks for your great input! Kudos to you for a great explanation!!!

Immediately after reading mxwrench's post, I pulled my carb out of my desk drawer and had a Huge Duhhh! Moment.


Sounds like you are on the edge where the engine is so lean that combustion temps are lower
than normal, and so the head temps are lower than normal. If so, this might be a good reason
for restricting the airflow to the air filter.


I have noticed that about onroad nitro cars. Do they come on their pipe twice? The second time
harder than the first? I have used this as a tuning technique that adds another 15 -20% more HP
over the normal peak HP, on some types of race engines. I am really curious if they use it in nitro
R\C.


Some parts of these engines drive me crazy, the idea of running an engine wide open through an
rpm range where the engine is barely accelerating drives me crazy. I understand the clutch gives
the cars a "sling shot" effect, but there are so many engine parts that seemed to be designed for a
"max rpm" number, rather than maximizing acceleration after peak horsepower. It drives me crazy.

Thanks for the responses!


Bill Mazz

Great explanation Wingracer!

The engine-mfg has in 9/10 times done the their best to suit as many owners as possible. So in 9/10 times there is little to be gained without the lose of something else.
Today with good CNC-machines the engines are also closer in tolerances, so they don´t add as much of a safety margin to them.

But if you have an interest in trying and to learn, go for it!
But as always, you should begin with knowing what you have and make plans to were you want to take your engine.

A couple of tips, if you ask me... If you do want a broad power curve, run large squish area and try to gain as much in the geometry of the chamber as possible without changing compression or squish velocity to much.

And I do a apologize that I´m not as good as Wingracer to explain things!

About HSN/LSN, pics do say so much more....

Do some engine tuners avoid the break-in process by hand lapping their pistons?

Is it possible to buy replacement pistons, instead of matched sets, or pinching a cylinder.

I would love to see Dykes rings to be used as a replacement part.

There are reasonable priced 37cc single cylinder 4-strokes available, that could be fitted for
1/5th scale. Especially if the rule committees would adopt the 2:1 displacement rules for 4
strokes like the AMA did.


wingracer, or anyone:

As I understand it, the basic strategy for tuning a nitro rc is to set the clutch up to take advantage
of the acceleration on the front side of the curve, in order to slingshot you through the backside of
the curve. Use 30% nitro and relatively hot glow plugs to maximize that acceleration, and through
good tuning you strive for the most controllable, drivable combination possible?

Why can't I just make the most power possible and gear it right? Tune it to be the most controllable,
drivable combination possible, without detuning the engine.

I know I am not the first Dumb mule to want to this but can you tell me why this didn't work?

I have done it before, In motorcycle speedway racing. I took an overpowered European long track
engine, and found a solution that made it so drivable and trackable on the short tracks in the USA.
The bike got great traction, and even with worn out tires.

My brother was a top 10 speedway pro, and 4 of the top 5 riders had the same engine, but they
used it as a back-up bike because they couldn't get it to hook up. I know this may seem to be
apples and oranges, but the principals for making an engine trackable is the same.



Bill Mazz

Hand lapping? I'm sure there may be some tuners/Mfg.'s that do that to a point, but a brake in is still needed. Maybe more so for everything else in the engine. I think most of the high end engines run a set for replacement pistons ans sleeves. Again there's a reason for that. Having rings to replace instead of the P&S as a set would help, but at what cost or gain. I'm sure rings have been tried in car/truck engines at one point, but I do not know how well that worked or didn't work. If I had to guess, there may be a point where you simply couldn't make the needed rings small enough and with the consistant tight tolorances needed. Say a .12 engine or smaller.

On the 4 stroke side, a 2:1 displacement could help, but at what cost? That's seems to be one of the big catches. Bigger the displacement, the more weight that gets added to the chassis. At some point, it's going to be too much. The bigger catch is that they don't have the high rev RPM's like the current engines do, and with only a single gear tranny being used, there's only so far you could go before something has to give.

Answers in red.

Its all a balance.... area under curve, peak output, idle characteristics, throttle response etc etc...I have thousands of hours testing on the dyno to find the sweet spots of many of these engines...