MRP ExhaustThe last pipe I have around to test is the MRP 50-70cc exhaust. I had no plans of testing this exhaust, but I was asked to try it. I'm not a big fan of using tuned pipes for 50-70cc engines on the 90-100cc engines.
My first 90cc engine had trouble with overheating and soft seizing when I used this pipe. Initially the compression was a bit high on that setup, but even after lowering it I had issues with soft seizing over time. I added a 19mm carb as well, and that didn't stop it. It seemed to be a gradual heat buildup. I could take short rides and it was fine, but on a longer run of maybe 15 miles or more when I was cruising fast it would eventually start showing signs of overheating and ultimately soft seize. I got a 90-100cc pipe and the problem went away.
I have always thought the problem is that the smaller stingers on some 50-70cc tuned pipes are too restrictive for the 90-100cc engines. Here is a quote from A. Graham Bell's book,
Two-Stroke Performance Tuning.
"
You may find that a minor reduction in pipe diameter will raise the power
output, but do be careful. A stinger pipe smaller in diameter or longer than specified could easily result in engine overheating and seizure."
Bell goes on to specify that 50-80cc engines should have a stinger inside diameter of 17-19mm and 100cc engines should have a stinger with a 19-21mm inside diameter. The MRP exhaust's stinger has an 18mm
outside diameter. So there's my reasoning for preferring not to use this pipe with 90-100cc engines, on to the test.
One thing that I like about the MRP exhaust is the 2 piece design. The short header section makes it easy to bolt the exhaust to the cylinder without holding up the whole exhaust. It also allows the header and pipe to be at different positions relative to each other, which can help with mounting it on some setups.
Everything fit well without issue. This pipe has been through hell and back, being the first performance pipe I ever owned. I've crashed with it on one of my scooters, had the bolts back out of the mount and dragged it home, and who knows what else I might be forgetting. It's not nearly this ugly stock and comes with a clear coat and blue silencer.
Here's a sound clip of the exhaust. This pipe has been repacked since it was stock. In stock form, this pipe was quite loud. After a couple of months it got really loud. I remember riding around and I could startle some people by revving it when I was riding in town. Right now, it's packed with FMF's 2 stroke muffler packing. It seems to work much better. The pipe is still loud, but nowhere near as bad.
Main jet size did not change from the baseline 90 main jet. The needle clip was set in next to the leanest position. I reduced slider weight from 6g to 5.5g. The MRP pipe is not revving any higher than other setups that used the 6g sliders, it just seems to lack the power to get the RPM up without the lighter weights. I stuck with the 2000RPM clutch springs installed for previous pipe tests.
I went on a 43 mile test ride and observed 41.37MPG, being pretty liberal with the throttle. WOT cruising speeds ranged from 48-52MPH. Max speed was 58MPH. Power felt pretty similar to the baseline test with the stock pipe. The cruising speeds and max speed are the same as the baseline. Max RPM was around 8,800, which should be a bit below where this pipe is meant to perform. Upper 8,000 to 9,000RPM seems to be the rev limit for this phase of the project with any pipe though. As I have mentioned in past tests, I think the 14mm stock carburetor might be the restrictor here. Based on just the performance data here, I see no real reason to use this exhaust on any tuned 90-100cc. I know the pipe made some power on some of my 70cc setups, but it's not impressing me with the "big" engine and small carb for sure.
Unfortunately, the lack of a performance increase isn't the only reason I wouldn't run out and buy this pipe for your big minarelli build. After the lack of performance for the last couple of exhausts, I've been checking some things out to make sure it really is a fair test and nothing else is wrong. I found a very small leak at the base gasket, but nothing that I feel would be noticeably detrimental to performance.
When I pulled the cylinder off to reseal the base of the cylinder, I found another problem. I was hearing detonation at times on the test of the MRP exhaust and, considering my past experience with this pipe, I was expecting a seize at some point. I never had any soft seizes though, so I figured everything would probably be fine. Not exactly. As you can see in the image below, part of the piston crown has been melted away. This is aligned right in the center of the exhaust port and toward that side of the piston.
Here's the underside of the crown.
Here's another quote from A. Graham Bell's
Two-Stroke Performance Tuning. This is from the same paragraph as the first quote I gave you, explaining what to expect when the stinger causes engine overheating.
"
Engine overheating, in its early stages, is indicated by the presence of oil burnt dark brown under the piston crown. On the next stage the burnt oil turns black, until finally 'death ash' appears. After this, the piston can be holed at any time."
Normally, I don't remove the head often. I leave everything alone as much as possible. Luckily, at least for the sake of this information, I started a spark plug a little off angle when I was tuning this exhaust. The threads weren't bad, but I opted to remove the head to run a tap through it to clean the threads up and hopefully avoid any future issues. Since I had the head off, I took a look at the piston and cylinder to see if everything looked alright, and the damage to the piston was not there at that point. So the piston managed to make it through multiple exhaust tests with no damage, but when I use the exhaust with the small stinger it begins to melt.
I feel as if this supports what I've been saying for a long time... don't use a 50-70cc exhaust on a 90-100cc minarelli or at least make sure you know a bit about the pipe first. Some exhausts will have larger stingers, so they aren't all bad. The milder street pipes that are not designed as a true expansion chambers are probably alright. I've had no issues with the stock 49cc exhaust or the 50-70cc Leo Vince SP3.
