downunder

This is actually an answer to Daniel from PNG in another thread (the poll on nitro %) but I figured I'd make it a thread on it's own as a "how to" guide.

First up, I've never found an engine yet that won't work perfectly well with 80/20 straight out of the box. All engines come ready to use nitro so their compressions are fairly low (around 9:1) so what most do is find a fuel with enough nitro to suit. I worked the other way round and found a compression that suited no nitro.

Basically, raising the compression is just a matter of making the entire combustion chamber volume smaller. The thing here is that this volume includes the volume underneath the cylinder head and the piston at TDC. That's the squish volume and can be around 20% of the total combustion chamber volume. Taking out a shim reduces this squish volume so it raises the compression a little but not by much so more drastic measures are needed. By drastic I mean machining the mating surface of the head so it goes further down the bore and closer to the piston.

But before you do anything at all you need to do some quite careful measuring. First you have to find the exact swept volume of the engine by measuring the bore and stroke. A ".46" won't necessarily be an exact .46 for instance.

Then you have to measure from the top of the liner down to the piston crown at exact TDC and measure the combustion chamber protrusion (with or without any shims but measure the shims too). The difference between these figures tells you what the squish clearance is (the gap between the head and the piston). Usually this is quite large, maybe .020" to .030" or more, and I have a self imposed minimum clearance of about .010" to be on the safe side. So if you found a figure of say .034" then you know you can safely machine off .024" if necessary.

Now comes the really hard part . You have to find the exact volume of the combustion chamber in the cylinder head. For a .46 size engine this will be somewhere around .8cc and the only way I've found to measure accurately enough is to invert the head (with a plug fitted of course) and slowly fill it with turps (kero) until it's dead level with the outer edge of the squish band. I can measure the volume down to about .01cc but it needs a lot of care and a steady hand . But that's the kind of accuracy needed.

Once you've done everything above you can now work out the compression of the standard engine. You have the swept volume of the engine so call it A. Add together the volume of the combustion chamber to the volume of the squish clearance to get the total volume (call it B). It's compression is worked out by (A+B)/B.

From here on everything is just maths and I do it by making up a spreadsheet so I can just enter numbers such as what happens if I machine off a bit from the head or take out a shim and the final compression just pops up. You could do it the other way round and make a spreadsheet where you put in the compression you want and the result will be how much to machine off the head.

Finally, by experimenting I found the best compression for zero nitro was 13.5:1 but anything around 12:1 is worthwhile. Coincidentally, another guy in the club had been experimenting with 4 strokes and he found 13.5 was ideal for them too.

dalkhaw

Hello Downunder
Many thanks for the detailed information and sharing your experience in finding the ideal copression ratio [through hard work!] in the forum. This gives us a good starting point and I will look for an opportunity to try it out.
Wishing you a meaningful Christmas and a Prosperous New Year.
Warm rgds
Daniel

dalkhaw

Downunder
Did you experience problems with idling using zero nitro?
Rgds
Daniel

downunder

I've never had a problem with idle on any engine so far. With my test engine (an Enya 60X) I found I had to lean out the idle mix considerably and then close the barrel further to get the idle speed way back down because the higher compression made it much more efficient at idle speeds. It then idled much more smoothly and lower than it did as standard. I had a 20% increase in HP on the same prop, fuel and plug calculated from the rev increase. The other thing I noticed was that the engine unloaded far more in flight.

Actually, when I did the experiment I wasn't sure what compression I'd end up with or if I'd be able to skim enough from the standard head to go high enough without getting too close to the piston so I made a new head with a slightly smaller combustion chamber. But before doing that I figured out how to make it a variable compression as well so I could then alter the compression while it was running to get an immediate change in revs. I couldn't alter the compression at full throttle because there's too much pressure inside the engine so I had to back it down to idle, make a change, then go back to full throttle. It worked so well I just left that head on the engine to go flying .

dalkhaw

Hi Downunder
Now you led me to the next question: how does your "invention" look like that enables you to alter the compression while the engine is running? Did you put in a compression bar just as in diesel engines?
Now that you have been succesful in flying with non-nitro fuels, why then is nitro fuel marketed so strongly? Increase sales?

Rgds
Daniel

Sport_Pilot

Fine point here, taking out the shim reduces volume over the entire diameter of the cylinder, that includes squish and main combustion chamber areas. Still a small amount though.

downunder

Daniel
I think nitro fuels are pushed in America for much the same reasons HP of engines is always stressed in the advertising. If it's got more HP than the opposition it has to be a better engine. So if more nitro gives more power then you just gotta have it. The fuel manufacturers must love this because what they don't say is that then you'll be back sooner to buy some more because the engine will use it up faster .

And no, I don't use a compression bar like a diesel because that'd have to go right where the glow plug is. Which would make it kind of awkward putting on the glow ignitor .

Sport_Pilot

Nitro doesn't really cost that much more, you get more power from the same weight engine, and it makes the engine easier to tune. Some low nitro engines are difficult to geat a good transition on hot days, for those 5% nitro usually helps. Course if you live in a country where nitro is the equivelent of $100 per gallon VS $40 in the US then this may not be such a good solution.

XJet

Spoken like a true American (no offense intended).

Come down here and buy some nitro, you might change your mind :-)

True -- I think it's the "ease of tuning" that might be the biggest benefit of using nitro.

A couple of guys around here have been trying to run their ASP 4-strokes on 0% nitro fuel and wondered why their runs were always so inconsistent -- even the change in tank head was causing major problems and the needles were *really* sensitive.

I gave them a jug of 10% and the problems just went away -- plus they got a bit of a performance boost as well. They both promptly rushed off and bought some nitro (albeit wincing at the hit on their wallets).

That's the truth brother -- I consider 10% nitro a "high nitro" fuel and have never run anything over 20% (even that was only on my 1/2A engines so they didn't suck too much).

BHogue

Hello Fellas i took downunders advise and xjet took a os 46 la shaved the head mixed my fuel 80% alcohol 20%caster tuned the motor ran real good.Way to much oilfor 46 la went to 16% oil running a hot os a3 plug and let me tell ya it came to life!!!!! Has more power about 400 rpms more transition from low to high was better and more responsive than the 15% nitro i was running.ill tel ya i really love the price to!!! Yall take care fly well.

gkamysz

My Feedback: (19)

That's very interesting. 13.5:1 is much higher than I would have expected to be suitbale for FAI fuel. That approaches what I've been using for my diesel converted four strokes. The adjustable compression glow head was something I was going to make for some experiments. It's a good solution for this experiment.

P-40 DRIVER

You could do the same thing using a little algebra if you knew the comp ratio of the engine in question. I suspect using 9 to 1 as stated above would be close enough to start with.

area of bore x stroke + Y = comp ratio x Y where Y is the volume of the head + volume of the bore above the piston at tdc.

gkamysz

My Feedback: (19)

You could use math to find head volume, but I have found that few engine manufacturers publish the real compression ratio, and effective compression ratio is actually much more important.

downunder

Seeing effective compression ratio has been mentioned I'd better say that any figures I've given have been geometric compression ratios...obviously, seeing I used the engine's stroke as part of the formula. Virtually all RC sport engines have an exhaust timing within a degree or so of opening 75 degrees BBDC so using geometric will give the same results for nearly all engines.

Effective compression (defined as starting when the exhaust port closes, which seems logical ) actually varies with revs reaching a peak at the max torque rev. Either method would work but if you used effective compression then the max would likely be more like 9:1 instead of the 13.5 I used.

I thought I'd better point out the difference in case someone tried to modify an engine to get a 13.5:1 effective ratio. Then you could really run it on diesel fuel .

The only manufacturer I know of who's always (or at least, used to) publish compressions is Enya.

P-40 DRIVER

Having said that, what would be a good volumetric compression ratio for a four stroke. In other words do 4 strokes start out at lets say 8 to 1 and I need to raise it to 10 to1 for 0% nitro. I suspect there must be a ratio or formula for deriving best compression ratio for ideal timing and how nitro changes that compression ratio depending on how much it advances the timing. A rule of thumb so to speak. The only 4 stroke I have tried to run on 0% is an old Magnun 120. It ran fine and certainly does not have alot of compression. An old truck of an engine to be sure. Interesting topic.

gkamysz

My Feedback: (19)

In my research four stroke compression ratios vary widley. From roughly 7.5:1 to almost 15:1 geometric, which is the only figure used for four strokes. Saitos have the highest compression ratios.

http://www.i4cproducts.com/compression_readings.htm

downunder

There's a guy in our club who's been independently experimenting with his 4 strokes to find the optimum compression for them to run on zero nitro and he's also found that 13.5:1 is ideal. We were both surprised when we discovered we'd both been playing around with compressions for the same purpose.

Early Saitos originally had very high compressions so they could be run with zero nitro but Americans started complaining that they didn't run very well or overheated etc because they automatically used fuels with high nitro. Saito then had to reduce the compressions to suit the Americans because it was the biggest market.

I suspect that the 4 stroke with the lowest compression is most likely the YS because apparently it needs about 30% to run properly even with their supercharging system. This is a kind of win/lose situation because while high nitro definitely adds power a low compression makes an engine less efficient even when using a high powered fuel. The edge goes to nitro but at the expense of very high fuel consumption.

I guess a rule of thumb could be made by saying that if 13.5 is ideal for zero nitro and 7.5 is ideal for 30% then a compression half way between these two at 10.5:1 should be ideal for 15% nitro. In fact this is probably quite close.

gkamysz

My Feedback: (19)

That sounds like some fun testing and engineering. I did the same when converting anything from FS-26 to FS-91 to diesel.

P-40 DRIVER

At $8.50 a gallon FAI fuel is mighty appealling compared to 20/20 which cost around $20. I think I will stick to 10% castor/syn for now because it will run about everything I own and is not to expensive.

RVM

Has anyone done this to a Saito? I've been toying with the idea of doing this to my Saito .82 by shaving off some of the flange used to mount the head to the crankcase. I don't have any machining experience, however, nor the proper tools, so I have not tried it yet.


I'd like to increase the compression ratio on my Saito .82 to optimize its performance with the fuel I am currently using (25% Omega). I've seen people put 40% nitro through this engine so I figure there is a lot of room to really up the compression.


I'm glad to see you did this downunder. It is something I have had in the back of my mind for quite awhile. If you have any information or advice for modifying a Saito to have more compression please do post it!

gkamysz

My Feedback: (19)

You should check clearnaces. I don't think there will be enough room for this in the Saito. They tend to have high compression to begin with. Many Saito engines are already above 13:1.

My FA-80 looks like the piston was hitting part of the combustion chamber out of the factory, so things are tight in there.

downunder

With a 4 stroke there are two ways to raise the compression. You can either skim the crankcase to lower the barrel or you can make a longer conrod. Personally I'd rather skim the crankcase because if you overdo it then you can add shims . The one thing you really have to know accurately is how much clearance you've got inside to play with. This could be the piston to head clearance and the closest approach of the valves to the piston around TDC. Although I've never owned or played with a 4 stroke I can see where the problems are, especially with a Saito which I believe has an integral head/barrel.

The guy I mentioned in post #17 did his experimenting with Saitos (I think mainly an 80) and I seem to recall him saying he machined about 1mm off the crankcase but that was to use zero nitro.

gkamysz

My Feedback: (19)

I'll be measuring my FA-80 in the near future as I'll be converting it to diesel. I understand the earliest versions of the FA-80 were above 15:1. Valve clearance is certainly an issue in smaller engines or any engine with canted valves like the Saito.

blw

My Feedback: (3)

There was a post and pics of someone doing this, and I think it was with a Saito. I think it was someone in Germany, but I could be wrong. Maybe a search attempt would get the thread. I do remember that it was well written with a lot of thought behind the process.

I haven't heard that Saito dropped compression to suit the American market, but that is okay if this is true I suppose. Other markets are shifted to suit sales, and not always to the liking of others here. Unless someone has seen this actually stated it may be just be lore.

A lot of people think that Saitos run really, really great with more nitro, but they really don't get much better in most cases. Saitos run well on 10%, and some people run 5%. I've heard a lot of people say they run some nitro just to help the idle characteristics.

Sorry to hear that nitro is so expensive in other places.

RVM

For those who are interested, here is a short thread about raising the compression on Saitos. Clicky.