how to get more speed from gas engines? - Page 2 - RC Groups
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Dec 02, 2012, 01:45 PM
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DarZeelon's Avatar

More speed!


If the OP wanted more power, than methanol based fuel could help...
As I wrote, the net gain in HP is 24%.

But in the USA a special license is required to purchase methanol.
I am not sure about the availability of 95/5 methanol/oil fuel, or about the price of methanol there.

And the aim of the OP is more speed, which would ultimately mean a sleeker model, with a thinner wing, smaller engine cooling intakes with baffled flow guides, retractable landing-gear, Etc..

Even if your could extract twice the power from the same DLE 55 engine, the increase in speed would only be 26%... The 80 mph would become 101.

I made a miscalculation in post #8, which I just corrected (luckily no one else caught it...).

The speed gain from switching to methanol would be 6 mph; not just 2. I.e. 80 will become 86 mph.
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Dec 02, 2012, 05:19 PM
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earlwb's Avatar
Actually you can buy methanol here in the USA OK. Most all of the glow fuel companies will sell you a 55 gallon drum of methanol if you want to pay truck shipping on it. The restrictions are all on nitromethane though. For nitromethane you'll have to jump through hoops and lots of red tape to buy it in quantity. But if you have a drag strip nearby, they may sell you some though. But since the Oklahoma City bombing they tend to be very careful with who they sell it to. But then unless you live in a USA locale where glow fuel is expensive, the costs for buying glow fuel isn't all that bad. Years ago a group of us used to get together a buy a 55 gallon drum of fuel and fill up our fuel cans as we needed it. But that was in a locale where for some odd reason glow fuel was more expensive though.

Now it has gone up about one US dollar since I took this pic though. This is in our Dallas Fort Worth Metroplex area. The big hobby shops here sell it for very reasonable costs. The low price is FAI fuel and 5%, 10% and 15%.
Last edited by earlwb; Dec 02, 2012 at 05:22 PM. Reason: wrong picture
Dec 03, 2012, 10:02 AM
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ChillPhatCat's Avatar
280cc engine that runs on alcohol or gas:

5Hp more if you run alcohol.
Dec 03, 2012, 10:12 AM
Registered User
DarZeelon's Avatar
It would have been nice if this company proofread its advertisement, before airing it...
It's full of 'typos'.

A gain of just 5 HP is too modest; only 14% (should be 24...).
Dec 03, 2012, 10:20 AM
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ChillPhatCat's Avatar
A gain is a gain when it comes to racing. I imagine that they hired people based on their engine building skills and not their command of the written language.
Dec 03, 2012, 10:28 AM
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gkamysz's Avatar
Supposedly this airplane does over 125MPH with a DLE55 and APC20x12 clocked with radar and GPS? This is far more than I expect so I don't know how they were doing it. Maybe with an 18x16 prop. The fuse doesn't have a tunnel for a pipe, so that would be a lot of work.

The USRA unlimited racers run methanol in Walbro carbs without issue for quite some time (a whole season) before overhaul. This first is hand from people who are directly involved. These guys run highly modified engines and if they were running gasoline it wouldn't be pump gas so the cost of fuel would probably be even greater for gasoline than methanol. When they are spending 20kUSD per airplane and have more than one or two for a race, I doubt the cost of fuel or carb rebuild kits is a concern, just the cost of doing business.

Dec 03, 2012, 11:08 AM
Registered User
Originally Posted by rjstrickjr
If you want more speed out of a gas engine and maintian the low cost of gasoline? Run 92 octane, A high quality synthetic 2 stroke oil at 45-50:1 and as someone already stated a smaller prop with more pitch. That would be the easiest and quickest way to see the improvment.

A common misconception that higher octane gives you more power. Octane is an ignition inhibitor that makes it harder to light gasoline on fire which is added to gasoline used in higher compression engines to prevent detonation (pre-ignition). Higher octane gas only helps if you have a higher compression engine (in the 13:1 range), so on a DLE-55, 92 octane is a waste unless the compression has been increased.
Dec 03, 2012, 09:35 PM
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ChillPhatCat's Avatar
Originally Posted by S3NFO
A common misconception that higher octane gives you more power. Octane is an ignition inhibitor that makes it harder to light gasoline on fire which is added to gasoline used in higher compression engines to prevent detonation (pre-ignition). Higher octane gas only helps if you have a higher compression engine (in the 13:1 range), so on a DLE-55, 92 octane is a waste unless the compression has been increased.
Good point to pick up on, with automotive engines, high octane is only good when you advance the timing and/or compression of the engine... for two strokes with un-adjustable timing, it's only needed for high compression.

It's not the octane that increases power, but the agressive engine setup that makes the power, and an increase in octane is needed to prevent detonation... to stave off a side effect.
Dec 04, 2012, 05:51 AM
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DarZeelon's Avatar
Higher octane does not mean a fuel is less flammable; it means it is less sensitive to detonation.

Detonation is a mode the fuel is consumed, but not in the normal way it is burnt in an engine.
The flame-front does not propagate from the ignition point, causing the combustion chamber pressure to rise gradually, reaching its maximum value after the piston passed TDC and is going down...

It instead ignites spontaneously from several locations and explodes; causing shock pressures, that cause extreme loads on many parts and making that 'ping' sound (that knock sensors are designed to detect); and also a different scent of the exhaust fumes.

What is known as a 'lean-run' in a glow engine is actually detonation, resulting from the leaner mixture being quicker to ignite and faster to burn.

Because of this, the needle in a glow engine is actually the user's 'ignition timing control'.

For gas engines, the C/R limit is much lower than 13:1, to seek a higher octane number fuel...
Last edited by DarZeelon; Dec 04, 2012 at 11:34 AM. Reason: Wording.
Dec 05, 2012, 12:15 PM
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Nav_Aids's Avatar
Actually all 3 of you (S3NFO, ChillPhatCat, and DarZeelon) are somewhat correct in what you are saying about higher octane fuels. All pump gas has a certain amount of natural octane rating but, a lot of fuel companies use additives to raise the octane rating of their fuel. Speaking from experience I can tell you that if you use a high octane fuel like 110 in a 9.5:1 compression engine it will fire the mixture but, it will not burn the fuel fully. It will run like it has a miss. In our racecar we have 16.5:1 compression and run VP C16 (117 octane) with 26 degrees timing. I tried to run this fuel in my car with 9.5:1 compression and could barely keep the engine running. IMHO =<9.5:1 use 85 - 87 octane, >9.5 but <10.5:1 use 92, >10.5:1 use alcohol or racing fuel starting at 100 octane.

The Reid Vapor Pressure is totally different between 87 octane and 117 octane, the C16 RVP is almost 5 times lower than 90 octane, and the evaporation temperatures are higher in the C16. Takes a lot more compression to get the C16 to burn right (Combust and Deflagration) but, to an open flame or spark the C16 is easier.

Reid Vapor Pressure Wiki:

Volatility Wiki:

Deflagration Wiki:

Last edited by Nav_Aids; Dec 05, 2012 at 12:17 PM. Reason: missed an equal sign
Dec 05, 2012, 03:12 PM
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DarZeelon's Avatar
Good post, Ray!

You got deep into subjects and details never explored here.
Jan 27, 2013, 06:46 PM
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scirocco's Avatar

Is pitch speed relevant for gas?

Hi folks, lots of electric experience but just starting in to gas with a used 77" CMPro Extra 330l and most likely a DLE-55.

I've been wondering what sort of speed to expect because it's a big factor in required servo torque. I saw the post in another thread about the 125-132mph Revolver with a DLE-55 and a 20x12 IIRC. The DLE-55 specs say max power at 7500 rpm and useable rpm up to 9000. At 9000 rpm, pitch speed is only 103mph. I understand that props can drive an acft a bit faster than pitch speed, but 25% faster?

I gather that with a 22x8 I should expect about 7500 rpm static, but even if it unloads in flight to 9000 rpm, that's less than 70mph. Now 70mph is not a problem - it's heaps fast enough, but I had the impression these 1/4 scale gassers were a bit faster than that.

What am I missing here?
Jan 27, 2013, 09:57 PM
Registered User
kerwin50's Avatar
a higher pitch will give you the speed you need. you may need to drop in prop size to get the rpms up.
A G62 on a pipe flys an AT 6 texan well over 150 mph.
Jan 28, 2013, 08:54 AM
Registered User
Regarding the original post, we're talking about somebody new to gas - and suggesting he start right into custom blended fuels and modified compression ratios?

Maybe I'm just getting old, but I think the DLE 55 plan is an excellent one with plenty of (factory) support nearby if (when?) needed. Start there, and when he gets his knees to stop knocking as it comes right out of the box, then see about doing something to make it go faster.... Walk before running?
Jan 28, 2013, 11:39 AM
Hope to get out of life alive
kenh3497's Avatar
I have a fairly high performance motorcycle that many have said need to run 92 octane fuel for maximum performance. I decided to do a test of sorts. I checked mileage with my normal 87 octane fuel and 92 octane fuel. I normally run 89 octane 10% Ethanol blend. I started with a tank of 87 to "clean out" the 89. I ride back and forth to work so I used that as my test track. The weather was about the same for the entire test. I checked my mileage for two tanks of 87 octane and two tanks of 92 octane with an extra tank of 92 to "clean out" the 87. At the end of the test I had a reduction of mileage with the 92 octane fuel. It wasn't much and maybe with in the error margin of the test

IMO the 92 had a delayed burn which equates to less cylinder pressure / less performance. Which required a larger throttle opening to make the same power. My "butt dyno" or seat of the pants could not detect a difference in performance.

Is / was this a valid test? I don't know. I do know it cost a lot more money to fill the tank on my bike using the 92 octane fuel with little if any gain. If I had been able to advance the timing of the engine I may have seen a tangible gain.

Take the above for what it's worth.

I don't believe our small engines warrant using anything but 87 or 89 octane fuel. Using the same FRESH fuel from the same supplier every time would be a better bet IMO.


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