One Zenith Versus 2 Mikuni Carbs, Results

[leshoman]

I flew P-81 about 50 hours with 1 Zenith carb, I built the engine myself, a 94x86, Always concerned regards carb. ice.  I have been flying the 94x86 engine with 2 mikuni carbs from Scott Cassler and love the performance. Withe the Wood Bar Dyno from Steve Klibinger i have taken WOT rpm, pull test with the Tenn 58x20 prop and Wood bar dyno results with both the zenith and the 2 mikuni carbs.
Zenith Carb.
WOT 3150, 170 pounds pull, and with the wood bar dyno 3370 rpm, 36.16 hp.
2 Mikuni Carbs
WOT 3220, 177 lbs pull, and with wood bar dyno 3560, 42.5 hp. 
Please don't expect this type of change with your engine but i was impressed. 
Before someone asks the question, both carbs were running EGT of about 1300 and CHT of about 375.

[Sam Buchanan]

Les, these are good data points, thanks for posting them. Good to hear you have a strong engine.

After observing the Zenith vs Mikuni situation for four summers I've come to some conclusions that may be of interest to you.

1) The "big" engine is rpm limited by the ability of the single Zenith to flow air. I suspect the long intake runners may be a contributing factor as well.

2) Based on field experience of others and my pull tests and Dyno bar runs, I think both engines develop about the same power @ 3000-3100 rpm. The Mikunis yield more gross power because they allow the engine to breath and run higher rpm.

3) When running the Zenith, a prop with a lot of pitch is a good choice. The 45 has a ton of torque at 3000 and will eagerly spin my Tenn Prop 58x24 to 2900 static and pull 190+ lbs. That is about all it can do because of the carb, but it makes a great cruise prop and gives satisfactory climb. I see ~400 fpm which is fairly steep considering the low climb speed.

The low rpm you saw with the Zenith wasn't due to it being loaded by the prop, but by the flow restrictions of the carb. The Zenith won't develop high rpm power with any prop. The Mikunis can yield more thrust from a lower-pitched prop because it can spin up faster. But if flying the Zenith, good performance can be found with a steeper pitch.

This is one of those deals where you change one variable and a whole bunch of other stuff gets moved on the chart. Great fun to play with but not always intuitive. I've considered a move to the Mikuni setup but the Zenith is simple and provides excellent performance for my mission. My engine will fly the XL 58 mph @ a pleasant 2900rpm.

[Sam Buchanan]

One more note based on mine and Les's experience with the 45 engine:

The XL will fly nicely on 37 hp. If I was building an XL and didn't need the most aggressive climb performance available, I would use the 37 hp Casler engine. It is many $$$$s less and when propped for max power at 3600 rpm it will make for a nice flying XL. It also runs smoother than the big thumper.

Both the 37 and the Zenith 45 make about the same power, the 45 delivers it at ~500 less rpm.

[Sam Buchanan]

Zenith Carb.        WOT 3150, 170 pounds pull
2 Mikuni Carbs    WOT 3220, 177 lbs pull

I disagree with Sam here, apples to apples these results show the engine is generating more torque with the dual carb setup.  It takes a 70 RPM increase to provide the additional prop loading to balance the increase in torque.

Based on my experience tinkering with engines over the years the big difference is the elimination of sharp turns in the intake/carb rather than the change in length.  Actually a longer tuned length intake on the Mikunis would probably be of benefit and increase torque further.  Performance is much more sensitive to intake flow than exhaust flow since you only have 14.5 PSI to push air into the intake.

I don't see any disagreement. I didn't say the Zenith engine produced more torque than the Mikunis, just stated that it has enough torque to spin the big 58x24. (Since different scales and tachs were used under different conditions we probably can't reach 'apples to apples' conclusions between my tests and somebody else's.)

Update: I need to correct my previous post. Not trusting my increasingly frail memory I went back and found the data from my pull tests. I saw 170 lbs at 2800 rpm with the Tenn 58x24 when hooked up to the deer scales. This is the same number (thrust???) Les measured but ~350 rpm lower due to the steeply pitched prop. Twenty minutes later it pulled 180 lbs with a Tenn 58x22 at 3050 rpm. This seems in line with Les getting 3150 with the Tenn 58x20, but the lower thrust he saw could be due to differences in scales or maybe the 58x20 wasn't as efficient at that rpm (?).


I have no way of measuring torque, just stated an opinion based on field experience. One variable not mentioned so far is the rpm where each particular prop is most efficient. I personally think the Tenn 58x24 is in a sweet spot at 2900-3000 rpm when tip speeds are considered. But I have no empirical evidence to prove that opinion.

I agree on the intake manifolds, I just didn't specify that the bends probably have a limiting effect, was speaking in terms of the overall design of the "long Y" manifold.

To complicate matters even more, we have Steve Johnson's engine that has a single Mikuni. I have no idea how it fits in this discussion!  

Either setup delivers an engine that will fly an XL very nicely. The builder should do their homework, consider their mission and make decisions accordingly.

[stevejahr]

Zenith Carb.        WOT 3150, 170 pounds pull.
2 Mikuni Carbs    WOT 3220, 177 lbs pull.  

I deleted the HP results due to the RPM delta.  Torque and HP are equal at 5250 RPM and below that HP tends to increase with RPM (until torque begins falling off).  Thus it is difficult to judge power increase with different RPMs: is the increase real or just due to the RPM change?

I disagree with Sam here, apples to apples these results show the engine is generating more torque with the dual carb setup.  It takes a 70 RPM increase to provide the additional prop loading to balance the increase in torque.

Based on my experience tinkering with engines over the years the big difference is the elimination of sharp turns in the intake/carb rather than the change in length.  Actually a longer tuned length intake on the Mikunis would probably be of benefit and increase torque further.  Performance is much more sensitive to intake flow than exhaust flow since you only have 14.5 PSI to push air into the intake.

<recreated post... >

[stevejahr]

Sam,

I presumed the stated results were same engine/prop under similar conditions.  Rough cocktail napkin type calculations put the power increase around 4% which may not be noticeable to the butt-dyno.  This may have been part of your original point   Likewise changing any part of this or comparing other's results brings "manufacturing tolerances" into the picture so we cannot judge on small deltas.

I also agree there is an optimum prop RPM and prop RPM is definitely an engineering constraint here.
We likely agree that most intakes are designed more as plumbing than air flow devices.

Comparing dual carbs, and Y intake with a single side draft carb, and Y intake with a single up/side draft carb does get interesting/complicated.  Any carb requiring a sharp immediate turn is going to suffer though.  My intuition is a good Y with single side draft carb and a plenum at the junction of carb/branches could be very close to the same performance as the dual carb.  For design suggestions I look at modern EFI intakes where air flow is the main design parameter.  Cannot entirely ignore the differences between dry EFI flow and wet carb flow though.

Yeah it does not take too much to fly a LE but it is rare for a pilot to complain about having too much power and some of us get a lot of satisfaction from tweaking.  I think it would be a kick to hook up a custom intake, EFI with computer controlled ignition, and a tuned exhaust.  Optimum self tuning of fuel.  Multi-dimensional adjustment of ignition timing.  No hand start kickback.  Might be worth the weight.

[Sam Buchanan]

The quote attributions in this thread have gotten all messed up somehow.

Les Homan compared the Zenith with the dual Mikunis spinning his Tenn 58x20 prop.

I compared the Tenn 58x24 with the Tenn 58x22 on my Zenith engine.

Mine and Les's engines have the same bore and stroke.

[Jerry Carter]

. If I was building an XL and didn't need the most aggressive climb performance available, I would use the 37 hp Casler engine. It is many $$$$s less and when propped for max power at 3600 rpm it will make for a nice flying XL.

So, would you opt for the 37 hp engine with the 2 Mikuni carbs rather than a single Zenith? And I assume the prop recommended would be the 58 X 20?  I need to order my engine soon.

[Sam Buchanan]

. If I was building an XL and didn't need the most aggressive climb performance available, I would use the 37 hp Casler engine. It is many $$$$s less and when propped for max power at 3600 rpm it will make for a nice flying XL.

So, would you opt for the 37 hp engine with the 2 Mikuni carbs rather than a single Zenith? And I assume the prop recommended would be the 58 X 20?  I need to order my engine soon.

Jerry, the 37 seems to run great with the Zenith, Joe Spencer has a lot of data on that engine. The single carb is easier to set up but you will get various opinions on which is best.

Prop would probably be a 54x20, depending on who made it.

[Jerry Carter]

Thanks, Sam. I'll call Scott in the next day or two. I've been putting it off because I couldn't decide which version I wanted. I don't see Mikuni carbs listed as an option on his website, but I remember talking to Les at Oshkosh about this, and I know he got his engine from Cassler (as noted in the first post in this thread).

[Sam Buchanan]

Thanks, Sam. I'll call Scott in the next day or two. I've been putting it off because I couldn't decide which version I wanted. I don't see Mikuni carbs listed as an option on his website, but I remember talking to Les at Oshkosh about this, and I know he got his engine from Cassler (as noted in the first post in this thread).

If you go full case you want the dual Mikunis.

[Jerry Carter]

If you go full case you want the dual Mikunis.

So, full case I could mount directly to the firewall like Les? Zenith won't fit? Cooling issues? Carb air intake? Sorry, I know we talked about this, but my aging brain has let some of it slip. :-)

Thanks

[Sam Buchanan]

Jerry, why don't you start another thread with your questions--there is serious thread creep occurring here and getting away from Les's original post. 

[leshoman]

This was the zenith install

[Sam Buchanan]

Jerry, why don't you start another thread with your questions--there is serious thread creep occurring here and getting away from Les's original post.  

New thread is here:

http://www.eaglersnest.com/forum/index.php?topic=628.0