GUNSHIPGUNNER

FINDING TOP DEAD CENTER (TDC)

I was taught this method years ago by an old fellow who was a master race car engine builder. I have used it on everything from Jet Skis to Chevy big blocks. (using a plate bolted to the top of the cylinder in the case of the car engine)

This method totally eliminates any error caused by clearances in the con rod or the piston rocking in the cylinder.


The first step in bringing your engine "up to spec" is to find out where it is in relationship to the original design. Most model publications include port timing in their engine reviews. Once you find true TDC on your engine you can then measure your port specs to compare with published specs to see how close it is.

Why is port timing so important? Port timing plays a major role in the power generating ability, RPM range and tractability of your engine.

2 strokes (piston port) control this timing with the opening and closing of the cylinder ports by the piston and it's relationship to crankshaft rotation. These ports are located in the cylinder walls.
There is another port machined into the front of the crankshaft under the carb.

Note: Some 2 strokes use reed valves and some use a rear rotary valve. 4 Strokes control port timing with the cams and valves.

When you read about an engine and it's timing specs these specs are usually given in relationship to TDC!!

So lets find Top Dead Center. This is where it all begins.


You need a degree wheel. I glued together two of those plastic thingies (protractors) I stole from a grand kid to create a 360 degree wheel.

I found a screw with the same thread as a Glow Plug.

1: Fasten the degree wheel where the prop normally goes.

2: Make a simple wire pointer that fastens to the engine (use one of the backing plate bolts for this) (solder works well for this) and sticks out over the wheel.

3: Screw the screw in the glow plug hole far enough to keep the engine from turning over past TDC. (just barely).

4: Turn the engine over one direction till it hits the stop. (gently please)

5: Make a mark on the degree wheel under the pointer.

6: Turn the engine the opposite direction till it hits the stop.

7: Now make another mark.

Now half way between the two marks is Top Dead Center.

EASY!!

Now turn the crankshaft so this halfway mark is at your pointer.

FREEZE!!

Make a mark on both the crankcase and prop backing plate.

THIS IS TRUE TDC

Re-align the degree wheel (don't rotate the crankshaft) so the zero on the degree wheel lines up with your pointer and the marks you made in the step above line up.!!

From this point you can proceed with porting etc.

As I indicated some time back I would be willing to post some info on simple methods I use to blueprint and port 2 cycle glo engines if anyone is interested. All you need is a Dremel (or some similar high speed grinder) and a few hand tools.

Let me know if you are interested.

downunder-RCU

For sure this is the most accurate way to find TDC..the hard part is finding a 1/4x32 screw to fit the plug threads Try removing the coil from an old glow plug and gluing in a close fitting drill end or similar to act as a stop, but round off the end. Aim to have the piston contact at somewhere around 10 degrees either side of TDC but before you get any ABC pinch.

School supply shops usually have 360 degree protractors which are a perfect size.

ChuckN

Found a nice 360 degree protractor at my local Hobby Lobby. I imagine Michaels probably has 'em too.

Porting with a dremel or other high speed grinder is tedious work. The hardness of the cranks along with the softness of the cylinder sleeves can really throw you off when you are first getting started. I highly recommend cutting the ports to your target dimension in a small milling machine if at all possible. I realize we all can't afford to have a Bridgeport mill sitting in our basement (I know I don't!) but if you can find someone that does have a mill by all means get them to help you out. Or, if you haven't already noticed, small bench top mills are available from Grizzly Machine, Enco, Harbor Freight and other tool suppliers. If you are planning to do a lot of engines these little milling machines could be a very practical option.

GUNSHIPGUNNER

Heck I always use a hand held rotary grinder. For me the quickest and easiest part is the actual grinding.

The initial setup and measuring the port timing is what's time consuming.

If I'm familiar with a particular engine I don't bother checking anything with a degree wheel. I just remove a pre-determined amount of material according to what I'm trying to make the engine do.

Thanks to some friends and from my records I can usually find data telling me how much to remove from where.

Modifying and porting our simple little RC engines is about as easy as it gets. Try changing the lobe centers on a 1200 cc
Daytona or do a multi-faced valve grind on a 16 valve engine.

ChuckAuger

My Feedback: (12)

I base all my measurements for porting purposes from BDC.

BDC, roll up until the exhaust port closes (for example), take reading, roll back past BDC in the opposite direction until port closes, take reading, add readings together, and have exhaust duration.

I'm not sure I would want to do all the math to get this number based on TDC readings.

Traxxas_Tech

My Feedback: (22)

Sounds neat man

Just what can I expect to gain from porting a two stroke? and what size engine is this usually performed on? Thanks

ChuckN

Our 2-stroke model engines respond very well to port changes and compression changes. When I first tried it I was both amazed and impressed by the results. But you do have to be careful. It's very easy to destroy a cylinder sleeve or crankshaft when you're first starting out. You also have to be sure to clean up the port edges so you don't score your piston or your crankshaft bore.

4 stroken ron

My Feedback: (29)

How do you find TDC on the small gassers that have angled plugs?
Thanks.
Ron

ChuckN

It'll still work if your piston stop is long enough.

GUNSHIPGUNNER

Consider this regarding porting.

Most engines are designed so EVERYBODY can operate them with some measure of ease and success.

This everybody includes everyone from your basic Bonehead (according to 3 of my ex-wives I belong in that category)
to your basic genius. And all those in between.

So the engine MUST be easy to start, easy to get to idle reliably, produce reasonable power AND last more than a few flying sessions.

In order to fill the above requirements the manufacturers will always err on the conservative side when designing their engines for public consumption.

BUT!!

Specialty engine producers like Jett, Nelson and a few others don't need to be so conservative for a couple of reasons.

First is the price. Only a very small percentage of consumers are willing to pay the extra money these specialty engines can cost. This eliminates most of the Boneheads.

Secondly, these engine makers make it very clear that their engines are NOT suitable for nor are they intended for use by the average consumer. They also make clear the fact their engines require an extra dose of skill and caution to operate.

And they don't usually warranty them. A fellow who is unsure of his own abilities is less likely to buy one whereas experienced fellows don't give this lack of a warranty a second thought. The skilled engine user knows who does and who doesn't make good engines and they know what they are getting into when they buy from one of these specialty manufacturers.

Which leads me to the point of this post.

Your average RC 2 stroke Glo engine will respond very well indeed to a few simple modifications.

The extent to which you modify an engine can really be broken down into two categories.

The first category would include fairly mild mods to the engines port timing, compression ratio etc. While these mods are being performed to increase power output it is important to retain a reliable idle, fairly easy starting and end up with an engine that still lasts for a few flying seasons.

This type of engine can be an excellent choice for aerobatics flying, pattern type flying and in the especially demanding type flying that involves hovering, harriers and other 3d maneuvers.

The second category would encompass modifications based on obtaining maximum power output with little consideration for prolonged idling, ease of starting (that why God created electric starters!! or longevity. These mods can include radical porting, bigger carbs, increased compression Etc.

This type of engine is better suited for Pylon racing, Warbird racing and other applications where speed is foremost.

I have been modifying engines that fit both of the above categories for several years with some success. Along the way I have made lots of mistakes and probably wasted more money than I should have. But I sure had fun (still am!!)

One thing to keep in mind about modifying these little engines is that someone has probably done it all before. Made all of the mistakes, had all of the successes. The key is to find out if these fellows bothered to document their experiences. If they did, find this info and study - study - study it. That's how I id it. (and still do) I read and re-read everything I can get my hands on.

Just recently I modified two of my favorite engine for use in a couple of my profile "Fun Fly" 3D type aircraft. I use APC 12.25X3.75 props almost exclusively for 3D flying. These props take everything a little .46 engine has to offer.

As I indicated earlier, 3D flying is extremely demanding. Throttle response is critical. So is reliability. If the engine "bogs" or dies when you wack open the throttle and you are in a low hover, the results can be catastrophic. 3D flying requires you to operate the engine in the upper mid RPM range, producing almost maximum power while receiving limited cooling airflow. So any mods have to be done keeping these things in mind.

The 2 engines I reworked are an MDS .48 and a .46 Raptor. I left the carb size stock but increased the exhaust duration (so they would better respond to a tuned pipe ), the inlet duration and the transfer ports duration. I also kept the compression ratio stock as I like to use a higher Nitro content fuel for the better throttling it can provide.

Both engines now produce a lot more power right where I needed it the most (3/4 throttle and above) They have almost perfect throttle response and operate at just under 400 degrees temp.

I hope this rather long winded post is some help to those flyers considering tinkering with their engines.

There are lots of experienced engine builders on this forum that I'll bet would be willing to help a fellow get started down this path.

Good luck!!

GSG

Ed Smith

OOOOh,.......Gunshipgunner. I liked your post. But I now have horrible visions of all these "Boneheads" as you call them, thrusting into their engines with a dremel tool.

"YEAH!! I read it on the internet" he says as he gouges around with aluminum chips flying everywhere.

Ed S

ChuckN

Hey! I was a bonehead until I started gouging into engines making metal fly everywhere. The trick is to never make the same mistake twice

johnd

i have a rossi 53 which i would like to tune i need all the info i can get can anyone help?

ChuckN

Well, there are these two little thumbscrews on the carb that you turn in and out...

But seriously

What size airplane will it be on and how fast do you want to go? Also, what size prop are you going to run? If you start cutting ports you will lose some low end power. A lot if you really get carried away. Higher tuned engines are pickier when it comes to prop size. You can't just say "I want more power". You have to know where you want it.

johnd

the plane it is going in is a weston uk magnum and i would like it to rev its balls off, weston do a webra 50 that runs on an 8/8 apc prop at 19000 rpm on the ground and unwinds in the air can the same be done to the rossi? i have been told by a engine firm that the rossi has a too longer stroke to get lots of revs out of it

ChuckN

I suspect a stock Rossi 53 will rev right with the Webra. Especially on an 8x8 prop. That's a fairly light prop for a .53. A .45 ducted fan engine will spin an 8x8 19,000 no problem providing the pipe is set the right length.

Funny someone said the stroke was too long. I suspect it's the same or shorter than the Webra. The Rossi 53 uses the same crankcase as the Rossi 40 and 45.

The one thing I have noticed about the Rossi engines is that the exhaust timing is rather conservative. I would extend the timing to 165 degrees total and mount a tuned pipe.

johnd

i have a straight through tuned pipe on the 53 and i use a apc 9/9 prop. the main problem with using a 8/8 prop is the engine will not rev any higher when in the air unlike the webra and when the webra unwinds the revs stay there, would advancing the exhaust timing on the rossi help this? and do you measure the degrees for the exhaust from tdc until the exhaust port has closed?

GUNSHIPGUNNER

I agree completely. This is the best place to start when it comes to porting and will add a noticable amount of power.

I don't know why it is but I have always loved Rossi and Webra engines!!

It must be the .European mystique

Something akin to when I owned my 916 Ducati and then purchased a 1200 cc Triumph Daytona.

I would swear the Duck was faster (It DID corner and handle better) than the Daytona but I let a friend (and he weighed 180 lbs. to my 260) ride the 916 and I rode the Daytona. In straight line running I could walk away from him. But the 916 STILL felt faster when I rode it again!! Go figure!!

Anyway back to our RC engines. I have always treated engines like Rossi, Webra, OPS (my FAV!!) differently than my OS, TTs, etc. and I could not give a rational explanation as to why this is. I just do and always have!!

Check out the picture of one of my Lee modified Veco 61s

Robby

My Feedback: (18)

GUNSHIPGUNNER
I have a good idea.. Lets try starting out with a simple mod, and see where it leads to.. Then progress into more serious mods...
For example,using a .46 size motor, if I JUST remove .005 from head or cly sleeve,what difference would I see.. Ok, now if I do .010 what would I see...
Then, if I cut out exhaust outlet of cly sleeve to match block opening, what would I expect to see.. And if I were to open exhaust as such, would I realize anything if I were to open exhaust porting within cly sleeve...
One of the questions I had asked someone of recent is that if I have a .46LA <OS> and a .46FX <OS>, why do they both show using same or almost same size prop.. I would expect the FX to
be able to use more prop as it has bigger carb for example.. How come a Super Tiger .45 which has a even bigger carb and a touch more exhaust size and also shows using same size prop..
Thank you,
Robby (there are no long winded explanations,
only short attention spans)

ChuckAuger

My Feedback: (12)

Sounds like your pipe is just too long. Try shortening it up a bit and see if it won't "rev up in the air" a little better.

And I can see the wheels a turning...port timing coming. Every Rossi I have seen except for boat and fan engines is 148 deg exhaust duration. But the good news is Rossis are exceptionally responsive to changes in exhaust duration.

There is a thread below "Porting 2 Strokes" that has some basics in it.

ChuckN

Check the intake port timing on your crankshaft. A lot of times this is very conservative in order to obtain a reliable idle. If it closes too early it also can prevent your engine from unloading in the air. You would want to shoot for a closing point of at least 55 degrees after TDC.

johnd

how much can i advance the exhaust? and is that measured from tdc and what about the crank timing should i adjust that and if so should it open sooner or close later? couldnt find the porting 2 strokes thread not been using this for long

david93073

My Feedback: (1)

:drowning: WOW!!!! what alot great info, keep it coming!

Shortman

My Feedback: (21)

In the May 2002 MA magazine they have a little article on how to find top dead center.

ChuckN

Since 2-stroke exhaust timing is symmetrical (same before and after TDC) you really aren't "advancing" exhaust timing as much as you're extending it. Your average sport engine will see an improvement if you extend your exhaust duration to 160 degrees open timing. It will kill a little bottom end, though. Forget about running an 11 inch prop if you modify a .40 or a .46 for instance. You will need to run 9 or 10 inch props.

Crankshaft intake timing is measured a little differently. Don't worry about when it opens. When it closes is what's important. If you make it close later it will benefit top end power. One of the drawbacks, though, is an engine that likes to run backwards when you try to start it by hand flipping. Also, don't make the closing point later than 60 degrees after TDC. That would be way to radical for a side exhaust sport engine.