I see very little posted here concerning minor engine mods to free up horsepower. I have been building and Hot Rodding automotive engines for years so I feel qualified to bring up the subject. I recently obtained a used Thunder Tiger GP 42 in less than stellar shape so I used it as a guinea pig of sorts to see if I could get some horsepower . The engine wasn't beat just had not been cared for properly. Compression was good and the internals are excellent. I fully disassembled the engine and noticed the bypass ports in the crankcase had a lot of flash and sharp edges. By simply removing the flash and smoothing the transition from block to port I feel I have gained a lot of flow. I do not have any before figures but I can say that this engine will now totally blow away my OS 46 LA, an engine of similar design (plain bearing, medium price) by a very significant margin(almost a half pound of thrust on an 11X6 MA). I am not bashing OS !! My LA runs great but now my TT 42 runs better! I still have a few tricks up my sleeve fot my GP. If this performance is typical for the GP 42 please let me know as I said I have no "Before" numbers because the sad shape the engine was in. Let me add, the same plug, fuel and prop were used on both engines for consistency. I plan on doing this procedure to a lot more of my engines . Like my single port Cox 049 production engine that now turns an APC 5.7X3 to 16,700. Thanks to Probrojoe.

Grind a little off the backside of the cylinder transfer ports to smooth fuel flow. It's very common for the car guys to mod their engines in this fashion. The car guys also notch the crankshaft counterweight some how. I think they call if "knifing" I am not sure exactly what it does, but I think it is supposed to allow smoother flow of fuel from the crankshaft into the crankcase.

If you can take some material off the mounting flange of the backplate, you would be stuffing the case. Anything you can do to reduce the volume of the crankcase will improve performance. I don't know how you'd go about removing a few hundred thousanths off the mounting flange. Maybe just take it off the back of the case instead? You need to keep some clearance though. Remember, the rod is flying around in there, and it needs to float in the center of the cylinder. I don't think you'll be able to take much off, but even a tiny bit would probably show an improvement.

You can increase the intake duration pretty easily. NEVER grind off the opening side of the intake. You can't make it open any sooner than it already is. Chuck Auger told me this YEARS ago when I was grinding on my first K&B .48. I don't know why--and I never questioned him. He said don't do it, so I didn't. You CAN take some off the closing side of the crank opening to increase the duration. How much? I am not real sure. I had some numbers around, but lost them on an old hard drive. The numbers are probably posted here on RCU--I'm not a fan of keeping secrets. If ya got knowledge and somebody asks--pass it on and try not to be an arrogant jerk about it.

You can also work on the exhaust port if you're brave. I never messed with the exhaust on a glow engine with a "chrome" liner. There is always a risk of causing the liner to peel after you mess with it. But, I do know that you can take the exhaust duration up to about 160* on a Ryobi 31cc weedie before the idle goes to hell. To port the exhaust--RAISE the top of the port. NEVER take material off the bottom of the port, UNLESS the piston is BELOW the bottom of the port when it's at BDC. IF the piston is at BDC and you can still have material at the bottom of the exhaust port, then you can lower the port--BUT BUT BUT!!!--- don't lower it so much that it opens when the piston is at TDC. [X(] [X(] If you lower the exhaust port and it opens up when the piston is at TDC--you just made yourself a nice paperweight. [:@] I'd recommend that you only take material from the top of the port unless you KNOW FOR DARN SURE that the bottom of the piston skirt will not clear it at TDC and open it up to the crankcase. Moving the top of the port up increases the duration just fine and there is no loss of compression. Well, okay, maybe a little--but the gains from all that intake porting and exhaust duration will MORE than make up for it.

IF you screw it up by increasing the intake OR exhaust port--you will know it. They usually won't idle worth a darn. Like minimum 4000RPM idle or it will die. It can be saved if you stop porting and grinding right there. Put a tuned pipe on it and prepare to meet the DRAGON!! You now have a fire breathing MONSTER. It may not last but a few hours of run time--but it'll sure be fun at the club pylon races. [sm=lol.gif]

There's not a whole lot that can be done with the transfer ports except taking material from the back of the cylinder to smooth flow. If you take too much out of the transfer ports--you are increasing the total volume of the case. That slows down your fuel charge. Slow fuel charge = slow engine. [] There is also the risk of punching through the side of the case when messing with the transfer ports on the actual case. Not a whole lot of material there. Grinding on the back of the cylinder = good. Grinding on the actual case = risky.

Thats all I know. Start taching it and keep track of what you did. Make a change and then put it together and run it. Tach it. Record it. Tear it apart and grind some more. Reassemble. Check the timing with the degree wheel. Record timing. Tach it. Record it. Grind on it some more.

I had a friend do two engines 8 years ago, one a very old thunder tiger from the early eighties and a TT GP42. I tached both prior and gained consideribly. The sleeve was ported as well as the timing advanced and the crank intake port polished. This was done with a dremel tool and no timing wheel. I was very happy with the performance after.

There are threads about porting the car and buggy engines, in the car forum like [link]http://www.rcuniverse.com/forum/m_3107667/anchors_3389821/mpage_1/key_/anchor/tm.htm#3389821[/link]

If you do take one to the extreme let us know how you made out

Richard

I have found that the key to reliable performance is moderation[sm=49_49.gif]. I forgot to mention that I did do some polishing inside the crankshaft, not grinding just polishing. The only mods to the case was to take away the blockages left over from the machining process and blending the transition to the ports, the ports themselves were only given a mild polish without altering their shape or volume. The port opening in the crank was left as is with just a very minor radius to the inside of the port. I did a very small amount of work and feel the results were spectacular. I have a few more racer tricks up my sleeve, speaking of sleeves I did not touch the liner At All! I was much too worried about peeling the coating and ruining a great running engine. 11,700 on a MA 11X6 prop is worth 5.72 lbs thrust on my static thrust computer, that is compared to the 5.32 the LA made. The results were more visible with the bigger prop. I also tried a 10X6 MA and got only .22 lbs higher thrust over the OS. I think these engines are both made to take advantage of the torque and therefore swinging a big prop they show a lot more power than trying to spin a smaller one at higher revs. The intake can only draw in so much air and I am sure both these engines use the same carb as a similar .40 size engine. (but there are ways around that too, I will keep you posted) Let me reaffirm! I am not Bashing OS! I think if the same mods were made to my LA the results would be similar! I just needed a top quality benchmark to compare my mods to.

I myself am a "car guy" and its not called "knifing" actually I've never heard that before, I've modded some engines before, and it sounds like plane engines tend to peel (the lining peel) but cars theres not to many that do that, and car engines are turning 40,000rpms, but what you are talking about when you cut the counter weight, there called "turbo cuts" the help mix fuel and air, and flow, but to much can cause the engine (or the crank really) to vibrate and blow your bearings inside there. but plane engines don't turn to the rpms the cars do, so vibration will not be as bad.

but from what I know modding a car engine is different then modding a plane engine. I know downunder knows some stuff about modding plane engines.

I would be concerned about removing metal from the crank due to balance issues. I have seen the cuts you refer to and they appear to be a good idea. In automotive Hot Rodding circles Knife edging a crank refers to cutting the flat of the counterweight down to a knife edge in order to sling off excess oil sticking to it which would add undesired weight. This in addition to a crankshaft scraper (hard to describe, a series of teeth that are fixed to the inside of the crankcase that are cut very close to the contour of the spinning crank and "scrapes" off the oil ) adds horsepower without stressing the engine. If you have ever seen the inside of the crankcase of an automotive engine running you would see strings of oil being flung off the crankshaft.

jeff, I've never seen the inside of a auto engine, I don't own one and I don't think my bro or parents would let me take one of theres apart

the hole balance thing has gone on and on, but the best way to say it is (this is for car engines not sure about plane engine though) you can't balance a single cylinder engine. you can balance it for a certain rpm but will not be balance in the other rpm range. so you just don't want to make the engine out of balance then it already is.

True on the balance. You can't balance it for every RPM. You can get it perfect for a specific RPM, but outside that, balance will suffer. I think that most manufacturers shoot for a sorta "happy medium" and try to balance it--not necessarily PERFECT in one spot--but more for an overall balance that works across MOST of the RPM band. Perfect at one RPM may give you uncontrolled vibration at other spots in the RPM range. Not-so-perfect might yield a "acceptable" balance across 90% of the RPM range.

As far as engine balancing goes there's not a whole lot you can do about it without resorting to extremes like drilling the counterweight and fitting tungsten plugs. The mods the car guys do to their crankshafts simply throw them more out of balance and don't have any real affect on helping get the mixture into the crankcase. They sure look pretty though . The only mods I would do to a crankshaft would be to put a slight taper near the end opening and a small bellmouth simply to avoid turbulence as the mixture spills out into the crankcase. Polish the inside passage to get rid of any machining marks and build up the end underneath the carb to give a smooth transition for the near 90 degree bend the airflow has to take. Personally I'd avoid making any changes to the crank timing because there's such a wide difference between makes of similar purpose engines it doesn't seem to make any difference. It's only when you get to car/marine engines that there's much difference in the closing times compared to aircraft engines.

One thing that can really help down that end is the underside of the carb barrel. Ideally there should be a smooth transition from the barrel diameter to blend in with the opening in the crankcase. Even more ideally the transition should be made at an included angle of no more than 14 degrees. This allows the air flow to slow down without causing turbulence after reaching it's peak velocity through the venturi (carb barrel) and regain some of the pressure it lost. The higher pressure then helps it get through to the lower pressure when the crankshaft port opens. Talking about turbulence, a bell mouth entry into the carb helps here too. Any turbulence at the entry to the carb reduces airflow.

For the rest of the engine cleaning up the passages to deburr etc to give a smooth airflow though the liner ports is just commonsense but not polishing them. Changing the timing is hit or miss (and non-reversible) but necessary if you're after all out power, especially with a proper tuned pipe. One way to change the timing but not touch any plating is to machine/file a slight taper on the piston crown to lower it's edge at the port by the same amount you'd normally raise the port. The compression sealing area on the piston is usually well down (comparatively) from this mod so shouldn't affect the seal at all.

The other area where good gains can be made (and my favourite ) is playing around with compressions. Choose the fuel you want to use (mainly nitro content) then alter compression until you find the one that gives the highest revs on that fuel. With one of my engines I had a 22% increase in HP using zero nitro fuel with just a simple bit of machining. The hard part was experimenting to find the ideal .

Another thing with cylinder heads is that some people have found a very worthwhile improvement converting to tapered seat plugs similar to what some car engines use. A standard threaded head can be converted to using the Nelson taper seat plugs because they use a much larger thread size.

Great stuff thanks for the info.[sm=49_49.gif] Just today I was working on the carb of the GP 42 to help with flow transition into and out of the throat. I did notice one thing, the barrel opening doesn't match the throat opening very well at all. I was considering getting a used barrel if I could find one and matching the opening by slightly bell mouthing the barrel opening to avoid turbulence. I do not believe this would negatively affect the signal strength. But I think it would help flow greatly.

Update. The smoothing of the carb throat didn't do much for the lower rpms as expected but I did see about 200 rpms increase on the 11" prop but on an MA 9" it was interesting . Wants to run between 16000 and 16200. Is this typical for a GP 42?

If you have a cylinder head which extends into the liner and is a loose fit, doing something to make it a tight slip fit will give you a noticable number of RPM's. GMA said 500-1000 for a CL speed engine. My experience with CL goodyear racers confirms this.

Don't mean to discourage you but those are cheap engines not worth hopping up. You'll just burn 'em up. Trust me I've ruined quite a few. The tolerances of these engines are much more critical than auto engines and they are much less forgiving of mistakes. This is truly one area where you get what you pay for. A Jett .40 costs about $250 but at 16500 rpm with flawless transition from idle to WFO it is a beautiful thing to behold (and fly). Leave it to the experts!

Max

Ruining them IS the fun.

Anybody who start grinding on a motor KNOWS there is a risk of popping it. But you accept that fact up front and forge ahead anyway.

I've popped a Ryobi conversion engine by turning it at almost 9000RPM. It lasted about 2hrs of run time before it exploded in the air. It's a disposable engine that you can buy for under $50 brand new. You can dig them out of trash cans for FREE.

Even a $60 glow engine is pretty cheap when you think about the total cost of this hobby. Grind on it until you've either burned it up or turned it into a paperweight. It's fun!!

Yes....Ruining them is fun[&:]

As well as the knowledge learned along the way...and the satisifaction of taking a cheap engine up a notch or five

Higher power output always = more maintance cost.........the higher power output is the most fun and when it pops..........next please!

---------------


Props are just like tires on a race car. You choose the tire according to the track and the type of racing to be done, you don't select just one tire and say that that tire is better than all others.

In fun-flying some years ago, when the trend was to go large on the prop, a small group of guys discovered that the plain bearing .40ish sized engines actually did better dead lifting their light models when utilizing a 10x5 prop in our type of competition fun flying. Change the events just a little bit and you were off trying to find the best compromise prop. Never did we decide than running more prop load than that of a 10x6 was better. It always led to overheating in events such as the most loops. Some clubs even adopted a rule of once you chose a prop, you had to stick with it through all of the different events.

The point of all of this is that yes, sometimes larger props are better for some events, but sometimes they are not. I see no advantage to always lugging down a PB .40, when most of the time these engines performed much better when lightly loaded.


Ed Cregger

I too have found that a 10x5 works better on the .40 size engines for lifting off. Like 4 wheel drive for airplanes, especially if a cross wind wants to flip the plane over, great for high angle of attack climbouts.

My TT GP42 just screamed with a 10x5!!!!

Richard

My use of the 9X6, 10X6 and 11X6 were to keep the standards of testing the same for all engines tested by limiting the variables. Like using the same fuel. Apples to apples you see. All used the same plug, fuel, spinner etc. My choice of propeller will be based on further testing. I could put a 9X8 on my lightest plane and get fantastic speed but the thrust would not be enough for my heavy trainer. I would like to add that polishing the throat of the carb netted A few hundred rpms on the smallest props turning the higher rpms and almost nothing with the larger ones. As I would expect. There is stil work that can be done inside the crank I believe but the flow to obtain the torque at the lower rpms is probably just about maxed out.. This engine shows no signs of stress and I do not think that operating it at the lower rpms with the bigger props will hurt it one bit. Internal stresses increase at the square to rpm. So getting a bigger prop to do the job means less stress on the reciprocating assemblies generally speaking. Like comparing an over the road diesel truck to a ferrari. They may both make 400 hp but they do it in completely different ways. The diesel only turns about 3000 rpms doing it. I also am a firm believer in the use of castor to protect my engines and thus far have no complaints. So it's a little messy. I have plenty of grease rags.

Great news. the carb mods are great on the very top end. I used a 9X6 prop with 5% nitro Omega. And Got 17,000 . My GP 42 is showing it's age, it was not well cared for when I got it. But I got it cheap enough. the compression is getting a little weak when it gets hot. But I called ACE and finally got that head gasket ordered and installed, so we are not so over compressed now.