I had occasion to open up our Great planes GP-42 engine and noted that some to the fuel/air passageways between the liner and the case did not line up. I did ensure that the slot in the top of the liner is aligned with the pin in the case, so I know the liner is in correctly. In one area, there was well over 1/16th of an inch where the overlap in the case reduced the opening in the liner.

The engine will be replaced soon with a PRO-46, so the GP-42 is available for play/modification. I was looking at trying to increase the airflow inside the engine by porting the case to match the liner. There are also some sharp casting lines and corners that look like they should be cleaned up too.

My feeling, from working on automotive heads, is that if I can clean up the flow, without touching the port sizes, shapes, etc., the engine should be able to breathe better and therefore make more power.

Has anyone ever ported these little engines? Is it worth the effort? I just see so much that is hindering good flow; it just seems like a shame not to clean it up.

Jack

You need to run some numbers with it stock at first then do your modifications and run it again.

However a Pro 46 would be more responsive to modification.

Sure it's worth it... 500-1000 RPM gains aren't unreasonable. BTW, your approach sounds well thought out, so I doubt you'll have any trouble...

Now, if the engine is truly an experiment... you of course can go a bit wilder with the porting and see larger gains (obviously). [8D]

As w8ye says, get some "before" numbers and do let us know how you make out!

-Joe

Hey Joe. This thread sounds familiar , you goof.
Why YES the GP42 responds quite well to internal porting! Been there done that, Loved It!!
Joe wants the before numbers cause mine wasn't running right before I modded it.

Here's a link to a thread in the R/C car engines forum...
Some of the early pics seem to be missing, but there's 20 pages for you to check out...[8D]

http://www.rcuniverse.com/forum/m_52...porting/tm.htm

I was told by Dave Shadel of Performance Specialties that exhaust port matching (i.e. matching the muffler entrance to cylinder exit), would have no measurable effect...

...Unless, that is, the number of 20,000 RPM is significantly exceeded...


This is true even if the port in the cylinder casing is much larger than the muffler entrance (picture yourself the open exhaust stack of an OS.40-.55FX/AX/SX and the entrance to the UltraThrust UT-4 tuned muffler, or its spacer... It seems to offer a significant obstruction to flow, but it hardly has any effect).


As to the intake ports in the sleeve; in most engines, they are rarely matched to the bypasses in the cylinder casing, but as long as the sleeve port is larger than the bypass (no physical flow obstruction), I guess power gains from port matching at normal RPM, would not be large either.

Just be gentle , like a street port job on a set of small block heads . Think smooth not glassy. Think if I was airflow would I want to have to jump over that? But it sounds like you are a fellow gearhead so you already know what I am talkin. Mine ran fantastic !! And I didn't do the cutting of the couterweight mod cause I didn't want to change the balance. However the inside of the crankshaft is always in need of "improvement"

I do the port mod to all my glow twostrokes and the least increase I got was 200 rpm and the most increase was 600 rpm (by the port mod only). The 600 increase on the Tower 75 was very noticeable as it went from 14400 rpm on the APC 12.25-3.75 to 15K.
This mod along with some crank and muffler work and some playing with compression can sum up to very respectable increases in power. And it is quite fun to experiment a little too.

I second that motion. It is fun. Just remember to be moderate and you will stay out of trouble.

Hi Jib, I have a friend that made this work on a FP 40,and he told me that the mod worked nice. He noted a slight increase in the power, but also the fuel consuption increased too.

Buy two engines, cut on the first one until its junk (doesnt take much) and then enjoy the second one as it came. It takes many hours and dollars to learn enough to effectively redesign a modern engine. It is easy to optimize an engine engine for nitro content, rpm range, and tuned pipe use. This still takes a lot of effort to find the perfect combination.

If the factory spent enough time and effort to optimize the flow characteristics by eliminating flash = hand grinding, then the engines we enjoy so much would cost considerably more. Smoothing out the inside of the crankshaft intake port for example. As they come from the factory they are usually very roughly cut, but a little polishing and you will see power increase. I have done a GP42 and the results were wonderful.

Instead of following Kweasel`s advice you should start on a junk engine for practise and determining the right tools, that`s what I did many years ago. I started on a worn out TRX .15 engine from an rc-car and then some more or less junk aero engines.
If you have a steady hand, the right tools and feel up to the task, then just do it (be moderate and think before you cut) and you will enjoy more power from your engine than your buddy get from his
If you know this is over your head THEN follow Kweasel`s advice and just enjoy them as they come from the factory, but then you will not learn much about engines.

Do some research on the web before you go, there`s alot of info out there, especially in the different car /engine forums.

Typo:-)

Pretend it's just a pair of 1976 318 Plymouth heads.

Jack,

You've had a lot of excellent suggestions so far. May I add one or two?

Depends on what you want to use the engine for, for one thing... Extreme power is usually pretty harsh on an engine, and durability will likely suffer. If you're going to use the engine for something flat-out, where the increased wear and fuel consumption are normal factors to live with, "flowing" the port passages might help.

If you want to have a reliable, powerful engine that will stay with you a while, perhaps the most extreme cutting isn't where you should go. Maximum-flow capable engines can get very twitchy and unpredictable to operate. If you want to fuel up, hit the prop with the starter and fly, a pretty near plain vanilla engine will do you a fine job.

Also, excess flow capacity IS possible. For certain uses, keeping the flow velocity high - to keep the fuel and air well mixed and turbulent for an optimal 'burn' - can be more important than the theoretical possibility of getting maximum volume up to the chamber. ...which doesn't help if it isn't swirling hard enough to burn really well. To keep the charge velocity high, smaller passages and some surface roughness inside them help.

As one responder mentioned, if you do carve on the passages, settle for smooth, not highly polished.

You mentioned that the case passages were smaller than the cylinder ports? the 'edge' where the flow opens out through the port might maintain some part of the desired turbulence...

If you have the owners' manual, check out the rated power @ RPM info. The cylinder ports are not the only elements in an engine's volumetric efficiency. Changing only one part of the system may or may not make a positive, beneficial change. From what I've seen, few recent engines are port-limited - schneurle porting allows MUCH more charge transfer than the old crossflow baffled piston layout. Almost 3/4 of the sleeve diameter is used for charge transfer, and exhaust uses much of the remainder. Flushing that much of the cylinder with cool fresh charge is also a great aid to cooling... Used to be only about 90° (each) of cylinder circumference used for bypass and exhaust ports...

Great news gang. I just picked up a very well broken in GP42. It is dirty and covered in castor but the only problem I see is the wrist pin is stuck probably from all the dried castor (bless them) So I shall establish a baseline , optimize the stock configuration with proper fuel plug and compression changes then start in on some minor porting. I just live for this kinda stuff.

The carb barrel/venturi opening of the TT .42 GP is only .220" IIRC and I think if you were to open that up a little...say to .250" perhaps, that would be a good start. (that and a better muffler, maybe the GMS .40 muffler would work? )

The carb is a bolt on type, and that makes it virtually impossable to swap carbs...unless I suppose you wanted to machine some sort of adapter.

When I modify a sleeve, all I typically do is to radius the lower lip of the port openings on the outside of the sleeve...and I have seen 200 to 500 r.p.m. increase depending on the engine. It also depends on how efficient or (relatively ) crude the engine is in the first place.

I got 400 r.p.m. more from my O.S. .61FX for example with the sleeve mods and a little polishing of the crank.

the sleeve. Cylinder liner? Yes I have to agree. I try not to touch the liner if I can get away with it, but when I see a sharp edge I just have to knock it down. To maintain the throttle response it is advisable to keep the carb as small as possible. I have installed smaller carbs on some of my engines and found the throttle response improved greatly. I do however recomend polishing the "stack" and radiusing the intake to the carb somewhat. [8D]

The throttle response will definitely be better with a smaller venturi...
I think though that a .030" increase won't hurt that (not much anyway? ) and the top end will be stronger. (.220 is pretty small )
You could however "bell mouth" the openings in the carb barrel...and leave the central portion the stock dia. That will also give you a true "Venturi" instead of a straight through barrel opening, and the fuel draw might actually be improved, as well as the top end.
I agree Jeff...the area above and below the carb barrel can be re-worked and you can see an improvment.

IMO, with airplane engines, the top r.p.m. is generally not high enough to warrant/need extensive sleeve mods like we see being done to some of the car sleeves and cranks. Just a mild clean-up will suffice.

As mentioned previously...when it comes to material removal, it's better to go at it conservatively...at least at first.

All cleaned up and ready to go. Just gave it a run. Sig FAI fuel, Merlin red plug, APC 10X4 prop, OS 40 unbaffled muffler. Turned 13,600 without a hitch.

I didn't do anything to the engine but clean it and put it back together, still has the head shim installed. Even after 2 tanks of fuel and too hot to touch is has a lot of pop to it.
Any suggestions from the crowd?

Excellent help/feedback! I really appreciate it.

To answer a few questions:

W8ye – Of course I’ll baseline the thrust before, so I’ll be able to compare the final results. If you don’t hear back from me, it means I screwed it up big time! <Joking>

ProBroJoe - No – It won’t be a wild porting job. I’m pretty cautious, because I’ve seen epoxy patches where the porting went through the wall. <ouch!>

Jeffie8696 – thanks for the boost of confidence that the GP-42 responds well to porting.

The comparison to heads is how I think about it. I did mention porting, not port and polish. I’ve never polished anything except piston tops and one set of exhaust ports. I’d love to knife edge the crank, balance the entire rotating assembly, etc., etc., etc., but I think that’s out of my home workbench capability. I probably won’t do much to the crank or rotating parts for the reason you mention – balance, except that smoothing the flow into/through the crank and bell mouthing the air/fuel exit from the crank sure looks appealing and since it’s very close to the centerline of the crank, the weight is not so much of an issue.

Proptop – Wow, thanks for the link. There is a lot of great stuff in that thread.

DarZeelon – Wow, some of what you write is surprising, especially the part about a step caused by a smaller passage at the muffler opening not being an issue. That is almost hard to believe – sorry. Fortunately, my muffler is not a concern, as the ports match perfectly. My primary concern is where the case shrouds some of the sleeve opening, and sharp edges/flashing, etc.

Lots of great stuff here. It’s nice to know there are some like minded folks on the board. I’m a long time car guy, relatively new to RC. I’ve rebuilt car engines, trannys even fuel injected one carbed car (megasquirt EFI – www.msefi.com) My 13 year old was the driving force into the hobby, but once I opened up my first nitro engine, I knew I’d be doing work inside. I guess I just don’t know when to leave well enough alone. <Blush/Grin>

Over the weekend I finally fired up the used PRO-46 I was planning to upgrade to. It was a used engine from an estate sale that I disassemble and cleaned up. It vibrates badly at higher RPM’s, so I may not be willing to start cutting on our primary engine, the GP-42 until after I figure out what is going on with the bigger one. In either case, it’ll be a mild porting job. This is for a fun plane and just like my cars, I want a reliable engine with a nice fat power band.

Thank again for all the pointers and input,

Jack

Rock On Brother. I will get us started. Don't expect too much from me too soon, I have other duties. Beeing a Dad.

If you were close I would bring you my AL60E , I lost 2nd gear. [:@]

is there a website that show areas of concern, such as as what to look for... i have some engnes i could experiment on

I think there is a link at the top of this thread.

Jeffie8696,

You don't need no "steenking" second gear. Until it was totalled, I routinely skipped 2 and 4 when accelerating my Subie on the highway.

Here's that link - It's a great thread. http://www.rcuniverse.com/forum/m_52...porting/tm.htm

Jack