I just ran some tests on a mod I did to my ZDZ50NG.
This can apply to other engines.
The mod was to slightly increase venturi size with a slightly larger carb.
I was looking for possible more revs on tuned pipe/smaller prop setup.
I already had some background on performance with the supplied carb.
The new carb didn't add much at all.
In that the engine was not really smooth at full throttle - I wondered if it may be preigniting a bit -
So I started backing off the recommended 6-7mm setting.
The starting (idle) rpm dropped --and the full power rpm went up.
So it appeared that the increase in cylinder pressure was causing the problem- just as it did on my supercharged Olds-way back when - I had to retard the mag as I added more blower boost -or I got detonation and power loss.
Anyway -
I settled on 4mm advance ( I use a mm measure as its per the ZDZignition folder setup)
The engine responded very well.
I ran some repeated tests on the engine on 22x8 and 22x10 Mejzlic -stock props and a 32-1 Mobil1 MX2T fuel mix.
The tachs had fresh batteries and readings were steady and repeatable
the pipe used -a muffled sample I am testing - .
The tuning length is same as KS- I am set at 75-cm -to 70 cm
At 70 cm -the 22x8 ran easily over 7700 rpm.
on the 75cm setting - the 22x10 ran 7400 rpm.
Both smooth setups.
The apparant problem I was having was preignition and or detonation due to the higher cylinder pressure-from a tuned setup.
Maybe -- higher octane gas would have cured this - don't know .
This is the first time I have actually been able to see where increased cyl pressures were a bit much for stock timing.
If you are into tuning - you might try running the timing back a bit on full pipe setups- it may/maynot help.
Anybody else note the same thing on piped engines?

I have one customer who does boat engines, they have to retard the timing when the engine is on the pipe. But thats with a G-62 based engine turning 18,000 rpm and with a hard tuned pipe.

That must be one of those overweight underpowered Zenoah engines that "they" say are not modern enough

Dick, what grade of gas were you using? In Germany, ZDZ recommends 96 octane, but the best we can do over here with car gas is 94 octane so I could see you having to back off the timing especially if you were using 87 octane.
Charlie

Well - I buy what is called 91 - but honestly -i don't know what it is.
One other factor - the engine was loaded with the 22x10 prop--which is what I intend to use for the aerobat it is going into .
I wasn't trying to brag up any numbers - just asking if others had noted the same thing on setups where the LOAD is heavy..
I was interested in moving usable torque band up - not just looking for rpm.
When I did same thing on my blown OLDS- as a kid - I got far more torque-further up the power band and NOT any top rpm increase. BIG difference.
This is a different exercise than setting an engine up to run at very high rpms .
When I ran my twin Wankels tied together on a 2-1 Gilmer belt setup - I ran em at 17500-
different exercise.
So --tho I had only hoped for better breathing allowing the 22x8 to really unload in the air-- I also got a real boost in torque!
Thanks for the feedback!

Dick

I always enjoyd reading your many inputs, hence the following question for assistance.

Where I live in Australia, we can get 98 octane fuel. I have a ZDZ 80 that I will soon be running. Is it better to run the ZDZ on the 98 or should I go back to 91?

If I run the 98 should I change the listed igntion setting?

Many thanks

Cheers

Phil

Dick, I would think that if you went to AV gas and advanced the timing back to 6-7 you would gain some additional power. But of course then your pipe may need adjusted and in the end – no more power.
The slower burn of alcohol gives you the ability to greatly advance the timing allowing you to burn more fuel and is the primary reason that our glow engines and alcohol drag cars run faster that the petro equivalent and AV gas will certainly slow down your burn.
It nice to see that someone is experimenting with these engines.

Yes - there is a point where one must COMPROMISE for a task.
If I were after just rpm - I would go for speed props, and let er rip -
been there -
If money were no object - I would run 40% nitro and alky
been there --
But I needed a light engine- on pump gasoline (?) that was tractable and quiet .
The only point I now see as a real concern, is possibly, extending power available , past good long term reliability of the engine.
Been there
with my first effort being a 274 cu in flathead Ford. A runner but a bit delicate.
That one proved to me you can exceed the design limits if you don't watch out!
These little two stroke buggers are a bit the same - so I think I will run this setup for a while and make certain I don't beat up the bearings etc..

Hey, AV gas is pump gas, you just have to get it out of the pump at the airport

Phil, if you can get 98 octane I'd use it in your ZDZ. Going to a lower octane wouldn't gain you anything. If the engine will run fine on 91 it'll run fine on 98 too, but if you have to retard your timing on the 91 octane to avoid detonation then the 98 would actually help you make more power.

Dick, I agree with Wings that you'd get more power if you ran on 100LL and advanced the timing to 7mm.
Charlie

Tango

Thanks for the information.

Cheers

Phil

Dick I have done a lot of experimenting with different fuels on my ZDZ 80 single and my 160. I went so far a to buy the racing stuff you know $80.00 Canadian for 5 gallons. Now you have gone the other direction and retarded the timing,which is the same as adding octane,because of the increased cylinder pressure. Anyway what I found on a full length tuned pipe the racing gas was good for 100 rpm over aviation fuel. Idle and mid range were slighty better, but no enough to justify the cost. Our 91 premium fuel works very much like the aviation fuel but smells a lot more. I did not try 87 octane. I spent weeks fooling around with this I tried different header lengths with differens fuels,it still comes down to what you want to accomplish I was looking for a quiet set up that would give me the most power on a particular prop that being a bolly 32x12 on my 160. I did not tune for top rpm as that was simply to loud 5900 rpm.

Well - once I got to thinking about it - I remembered our supercharger experience -where simply backing off a few degrees - fixed the problem .
Power went up and the heat went down.
So how to get more power with less heat?
Cold power?
Noooooo
we simply tried start stop setups and kept reducing timing till engine ran smoothest with largest power increase.
eliminating the pre ignition lowers the heat.
This is definately cut and try -but today -which was cold outside showed more power yet -simply because it is cold .
On the 160 - on 10/80 tuned pipes -KS an overall 11-12" of header to pipe seems very good here -
We don't use Bolly -but the other glas props such as Mejzlic and Airwild(?) 32x11 seem well suited to this wetup . so far the timing has not been fussed with to optomise -the planes are very strong as it is .
But we will get a chance to see where max setups are to be found.
I have a new ZDZ100 on two tuned cans -sent by "XYZ mfgr" for testing - these seem to boost pretty well - they are much like my Vtech stuff I did for the 40 cc ZDZ.
If present senses of direction holds true - the more boost used - the slower the timing may need to be for best result - simply not sure as the load of the prop is another factor
a very high revving prop say 15000 rpm may need more advance .
It is a cut and try.
When I did my first Chev V8 - I ran a total of 50 degrees advance! to get best top power. the next cam setup dropped this a bunch to low 40's.

Dick

I was at a Pattern comp in Canberra, Australia this last weekend. Your information and theories were being deeply discussed and appreciated by quite a sizeable groups.

You just never know where you information finishes up.

Cheers

Phil