# 2 Stroke Cylinder & Case Porting for more HP!

### Help Support Homebuilt Aircraft & Kit Plane Forum:

#### Armilite

##### Well-Known Member
Every 2 Stroke Engine is different in what can be done to make more HP, since this thread is about Porting will try to focus on just that aspect! For our purpose's here, 2 Strokes for Ultralights & Kitplanes were not after every last bit of HP we can get, as were not turning them any more Higher then Industry Standard for Planes today which is 6500rpms. Could you go a little Higher, Yes. The Rotax 618UL was rated at 6750rpms! Using 6750rpms vs 6500rpms equals 250rpms is like 3hp difference.

Your Bore is a Circle or let's call it a Radius and once you Cut Ports in that radius, there is a General Rule of Thumb Formula on How Wide you can make that Port before the Ring might get stuck in that Port.

Some 2 Stroke Porting enhancements you can make to make AirFlow better in your Engine with simple tools.

I'm going to show you my Single Skidoo/Rotax 335(78mm x 70mm) vs my Skidoo/Rotax 340 TNT(78mm x 70mm) Intake Ports for comparison.

335 Stock 26.5mm x 47mm

340 Stock 29mm x 47mm

With the different Port Heights 26.5mm vs 29mm, they will have different Port Timing! But can they both be Improved upon, Yes, using the General Rule of Thumb Formula, they can be Widened from 47mm to 51mm. Could the Standard 335 be Ported to TNT Spec's, Yes, and you have your Port Timing changed doing that. Were Not building a Race Engine, so I'm not getting into all the Port Timing changes you could do if Racing.

You also want to Match your Carb Intake to the Cylinder External Intake Port on the Cylinder. On some Rotax's there are different Size Carb Intakes. Always check OD & ID. A Cheap Air Die Grinder & Carbide Bit and a Porting/Polishing kit is all you really need.

You're basically going to do the same thing to the Exhaust Port. Match your Wye Pipe & Gasket and External Port taking out rough spots, sharp edges.

#### Attachments

• 18 KB Views: 20
• 10.6 KB Views: 24
• 10.9 KB Views: 22
• 13.7 KB Views: 17
• 37.3 KB Views: 22

#### pictsidhe

##### Well-Known Member
The trick to porting a two stroke is knowing what changes to make. Surprisingly often, that means making some ports smaller. Enlarging the easy to get at bits without knowing what you are doing is unlikely to yield an improvement.

#### Armilite

##### Well-Known Member
The trick to porting a two stroke is knowing what changes to make. Surprisingly often, that means making some ports smaller. Enlarging the easy to get at bits without knowing what you are doing is unlikely to yield an improvement.
==================================================

Can we for pete sake just Focus on some mild Porting of 2 Strokes commonly used on Ultralights and Kitplanes, which 85% in use are based off Skidoo/Rotax Snowmobile Engines, with you not trying to confuse People more with your Old Motorcycle Racing Tuner days. It has No Basis here in this thread. Were not Porting an Engine for Racing use. We are not changing Port Timing from Stock, we are not changing any Cylinder Port Angles. Were concentrating on things anyone can do with Simple, Cheap Tools.

#### pictsidhe

##### Well-Known Member
You change the the ports, you change the power characteristics. Changing size and shape makes as big a difference as changing timing. But you have never ported a single engine, so you wouldn't know that. I've done several non-racing engines. The principles are identical, you just don't push things so far.
Have you even used an air die grinder? How big a compressor did it need?
Go do some actual porting and see what happens to the engine, then you can tell us about things that actually work, or don't.

#### Armilite

##### Well-Known Member
You change the the ports, you change the power characteristics. Changing size and shape makes as big a difference as changing timing. But you have never ported a single engine, so you wouldn't know that. I've done several non-racing engines. The principles are identical, you just don't push things so far.
Have you even used an air die grinder? How big a compressor did it need?
Go do some actual porting and see what happens to the engine, then you can tell us about things that actually work, or don't.
=====================================================================================

Yep, it's called making more HP at the same Max Rpms! I already said were Not changing Port Timing! You apparently can't Read either. Ya I know your a Legend in your own mind! "The principles are identical, you just don't push things so far." Duh, I said we were doing Mild Porting things anybody can do with simple, cheap, Air Tools.

I suggest you finish reading that 2 Stroke Book I gave you the link too, then go buy that cheap Rotax Single and show us all your stuff Tuner Boy.

#### Attachments

• 252.8 KB Views: 5
• 19 KB Views: 7
• 41.8 KB Views: 6

#### pictsidhe

##### Well-Known Member
=====================================================================================

Yep, it's called making more HP at the same Max Rpms! I already said were Not changing Port Timing! You apparently can't Read either. Ya I know your a Legend in your own mind! "The principles are identical, you just don't push things so far." Duh, I said we were doing Mild Porting things anybody can do with simple, cheap, Air Tools.

I suggest you finish reading that 2 Stroke Book I gave you the link too, then go buy that cheap Rotax Single and show us all your stuff Tuner Boy.
Deep wisdom from someone who has never tuned a single two stroke.
Die grinders may be cheap, but the compressor to run them certainly isn't. Anyone who's actually used one would know that.

#### Turd Ferguson

##### Well-Known Member
The trick to porting a two stroke is knowing what changes to make. Surprisingly often, that means making some ports smaller.
Define "surprisingly often"

Can make substantial change in power output on a two-stroke without touching the ports.

#### pictsidhe

##### Well-Known Member
About half of the non-race tuned 2 strokes I've seen have had some ports made smaller. Less of racing ones.
Outright flow doesn't make hp on 2 strokes. They are complex interactions of waves and flows. Change the relationship between those and power can change, often substantially. Without either a clue or some trial and error, probably for the worse.
Rotax has been building engines since 1920. Pretty good engines, too. I think it's a pretty safe bet they've found all the easy hp themselves.

Page 2, first paragraph of the Jennings book:

"Another mistake commonly made, sometimes even by those who have enjoyed
some success in modifying two-stroke engines, is to believe in a kind of mechanistic
magic. Bigger carburetors, higher compression ratios, altered port timings and expansion
chambers often do bring an improvement in power output, but more and bigger is not
magically, instantly better. All must work in concert with the basic engine, directed
toward the particular application, before they constitute a genuine improvement. You
cannot treat them as a voodoo incantation, hoping that if you mutter the right phrases and
stir the chicken entrails in the prescribed manner, your mild-mannered, all-purpose
chuffer will be transformed into a hyper-horsepower fire-breather. With a lot of luck,
you might get that result; the chances heavily are that you won't."

#### Turd Ferguson

##### Well-Known Member
All two-stokes are inescapably tied to the laws of physics as well. Bore, stroke and ratio of those measurements dictate the basic power output range of the engine. All the other black magic stuff merely pecks at the fringes. I think we are in agreement that if you want a two-stroke powerplant for your homebuilt aircraft and want to use the word "reliable" in the same sentence, need to start with good genes because those engine designers/builders have done a lot of "experimenting."

#### proppastie

##### Well-Known Member
Log Member
those engine designers/builders have done a lot of "experimenting."
never designed an engine , but have thought about perhaps cobbling one up out of existing parts and maybe a billet milled case....Or cutting the transmission/gearbox off a motorcycle engine.I do wounder how much is calculation and how much is innovation/experimenting.

#### Turd Ferguson

##### Well-Known Member
never designed an engine , but have thought about perhaps cobbling one up out of existing parts and maybe a billet milled case....Or cutting the transmission/gearbox off a motorcycle engine.I do wounder how much is calculation and how much is innovation/experimenting.
I have had the same thoughts but I think if I could come up with a good PSRU, that would solve many "engine" problems.

#### pictsidhe

##### Well-Known Member
Getting a new or modified two stroke to play ball can be a lot of work. Highly rewarding when you pull it off, though.
The easiest way to build a custom two stroke is to use ready made cylinders from sonething that already works. Changing rod length will alter the timing and tune, but otherwise, it'd be mostly a mechanical issue.
Most motorcycle engines have a gear primary reduction to a clutch basket on a shaft. Seems ripe for adaption to me. You'd probably need a new shaft to hang a prop on, but you have the gears and somewhere to put a TV absorber.

#### jbiplane

##### Well-Known Member
never designed an engine , but have thought about perhaps cobbling one up out of existing parts and maybe a billet milled case....Or cutting the transmission/gearbox off a motorcycle engine. I do wounder how much is calculation and how much is innovation/experimenting.
Till the end of year going produce in my company 2 model of 2-stroke engines:
3 cylinder inline and 4-cylinder boxers.

They will use Rotax 503, 550 air cooled and water coolled Rotax 551 compatible cylinders producted now in Russia.
Some old draft pictures. Will not post detailed information before produce and test

#### Dana

##### Super Moderator
Staff member
Most motorcycle engines have a gear primary reduction to a clutch basket on a shaft. Seems ripe for adaption to me. You'd probably need a new shaft to hang a prop on, but you have the gears and somewhere to put a TV absorber.
That's what I did, for a paramotor. It was powerful and smooth running, but heavy.

#### Armilite

##### Well-Known Member
I have had the same thoughts but I think if I could come up with a good PSRU, that would solve many "engine" problems.
================================================

I would really like to hear, How a Good PSRU would Solve any 2 Stroke Engine Problems, other than the one used originally in Direct Drive Mode, which we already know isn't a good idea, if rpms are turned very high.

#### Armilite

##### Well-Known Member
never designed an engine , but have thought about perhaps cobbling one up out of existing parts and maybe a billet milled case....Or cutting the transmission/gearbox off a motorcycle engine.I do wounder how much is calculation and how much is innovation/experimenting.
=========================================

Why, go to all the trouble to reinvent a 2 Stroke Engine already built? For Part 103 Ultralight. The Rotax 277UL in Stock form is rated [email protected] With minor Upgrades it can be made even more reliable. With some other upgrades it can even make more HP. So good for 25-35hp.

The Smaller 185UL would need a few more Upgrades to be more reliable but could be built for 9.4hp-24hp if ran up to 6500rpms.

The 377UL/380F could be built to handle 35-50hp range if turned 6500rpms. The 447UL could handle the 40-60hp, the 503UL could handle 50-70hp.

There is literally many 100's of old used planes sitting around the world and 1000's of Old Snowmobiles with the same Rotax Engines. There are many Kawasaki 340 & 440 Engines sitting on old Sleds that were used on many Planes. On top of the 25 other Brand of Engines used on Sleds.

Before you go trying to build a New Concept Engine, you really need to learn how to Upgrade and Fix an existing Tried & True Rotax 2 Stroke Engine used on Aircraft with probably millions of hours on them.

Most Companies usually have 1-2 People who can really design something unique, the others follow there lead.

Motor Cycle Engines have not been a good Canidate in the past for Normal Airplane use account the PTO is to small. Could you use one, Yes, but for how long? Again, what's your life Worth? What's your Plane Worth? Do you have many Thousands of Dollars to Ground Test your New Concept Idea?

I do like the idea of some Billet Parts, but what do you really need made? The only Case's that needs updated and made better is the Singles, 185UL and 277UL. But very few Planes even used the 185UL. I would like to see a Billet 277 Type Pro8 Free Air Case made that could handle both 277F(72mm) Cylinders and 550F(76mm) Cylinders. Any Cast Head can be machined for a Higher CR. A slightly bigger Billet 277 Type Free Air Head would be nice. There is no problems with the Twins, Cases, Cylinders, or Heads. They can all be Ported for more hp and machined for higher CR's. Unless your going into the Engine manufacuring business where you need all New parts. Then you would have to make the Core parts, Case, Cylinders, Crank, Heads. You can source everything today. You can even Source the Core parts out today, there is many places in China/India that will make anything. It's not as exspensive as you may think. It is for a one off, but if your going to make a lot of parts it's not. IF, You can do the CAD work, and do the CNC work, and just Buy a Good used CNC Mill & Lathe, used can vary $2000 to$5000 each. You could make your own Parts.

Billet Sled Race Head $400, Cast 582UL Head$901.05.

$5000/$400= 12.5 Heads
$5000/$901= 5.5 Heads

These are still available New through CPS.
447UL Case $1150.58 447UL Cylinder$695.55
447UL Head $233.08 Single Plug 503UL Case$1228.76
503UL Head $540.26ea 582UL Case$1435.29
582UL Head $901.05 582UL Cylinder$1265.79
582UL Crank $1519.44 <-- A Comparable Hot Rods Brand New Twin Crank in Des Moines, Iowa$450-$650. A Single ATV Crank$250-$350.$5000 CNC Lathe, $5000/$1519= 3.3 Cranks

New 582UL Engine $5612.00 CNC Lathe & CNC Mill used say$10,000/$5,612.00= 1.8 Engines Everybody is different on what amount of Space & Tools you might have to start a Small Business and what Knowledge & Talents you might have. If you Sourced out all the Core Parts you would need made, and just Built the Engines you would be better off. A Single like the 277 or bigger CC would be the cheapest. Hirth 28hp Single Retail Prices. F-33 with Recoil Start$2857.00
F-33 with Recoil Start & 4V Belt Drive $3427.00 F-33 with recoil & Electric start & 4V Belt Drive$3778.00

Some parts off CHINA DIRECT to show how cheap you can get them. Just search what ever your interested in. Some Ads tell the Price and minium needed to order.

#### Attachments

• 31.6 KB Views: 14
• 37.9 KB Views: 12
• 33 KB Views: 11
• 33.8 KB Views: 11
• 33.6 KB Views: 13
Last edited:

#### Armilite

##### Well-Known Member
Some New Parts still available.
https://www.skidoopartshouse.com/oemparts/c/ski_doo_snowmobile_2002/parts

2002 Skidoo 277 Head $244.99 2002 Skidoo 277 Crank$241.99
2002 Skidoo 277 Case $684.99 2002 Skidoo 277 Cylinder$589.99
--------------------------------------------
$1761.96 2002 Skidoo 377/380 New Crank$356.99
2002 Skidoo 377/380 New Case $679.99 2002 Skidoo 380 Head S/P$133.99ea
2002 Skidoo 380 Cylinder $447.99ea ------------------------------------------------------ 2002 Skidoo 503 New Case$844.99
2002 Skidoo 503 New Crank $356.99 2002 Skidoo 503 Head Single Plug$255.99ea
2002 Skidoo 503 Cylinder \$624.99ea

#### pictsidhe

##### Well-Known Member
================================================

I would really like to hear, How a Good PSRU would Solve any 2 Stroke Engine Problems, other than the one used originally in Direct Drive Mode, which we already know isn't a good idea, if rpms are turned very high.
Torsional vibration, whirl and misc flex can do all sorts of weird and unpleasant things. Not all two strokes used in aircraft were designed to have a prop hung on the end of the crank. Some are fine, others are not. A lot of direct drive car engine conversions need an extra bearing to be reliable.

#### AJLiberatore

##### Well-Known Member
================================================

I would really like to hear, How a Good PSRU would Solve any 2 Stroke Engine Problems, other than the one used originally in Direct Drive Mode, which we already know isn't a good idea, if rpms are turned very high.
Armilite,

Put aside PSRU / Porting and Polishing etc for the moment. Maybe this requires a separate thread and this is not a desire to commander this one. State of the Art 2 strokes in the Sled arena ( possibly Outboard as well ) have 3 major features that seem to be the template of their high-performance and a level of reliability at those power levels. The 3 ingredients are Reed Valve, EFI / Transfer Port Injection and or DI, and a Power Valve on the exhaust side. Of the manufacturers in the Av-Space, their are those with Reed and EFI systems, but up to this point no one in that space has a Power Valve ( except on converter/vendor who is using Sled Engines as a base ) . The question is why? weight? complication? cost? all of the above? Even in flight applications the RPM's 2 stroke flight engines are operating in are not that different then sleds.

Look forward to your and all 2 stroke enthusiast responses on these questions.

My Best,
Anthony

#### proppastie

##### Well-Known Member
Log Member
=========================================

Why, go to all the trouble to reinvent a 2 Stroke Engine already built? For Part 103 Ultralight. The Rotax 277UL in Stock form is rated [email protected] With minor Upgrades it can be made even more reliable. With some other upgrades it can even make more HP. So good for 25-35hp.