matching pipe and silencer for tuned length?

bclapham · May 7, 2003

OK guys, i am interested to hear what the usual suspects have to say about this!:)

i have aquired a FMF Rev pipe for this rm144 i am building. So, i need to figure whicj silencer to buy? I might be able to score a stock silencer, but if not, i will buy either the FMF power core or corresponding shorty unit.

although i am not a fast rider, i am hoping this pippe will help the bike rev a bit, are there any pro's or cons to the silencer choice???? will the shorty clash with the higher rpm pipe etc., etc.? if i manage get the stocker, should i resist shortening it?

ps. i am sure Rich R will have a look in here, can you remember a ball park jetting spec for this bike? not looking for a handout rather a ball park so i know which jets to get at the shop. if its any use, we have set up a stock 02rm125 with the tmx 38 carb on pump here with 27.5P 2nd clip and 430 main. i will be using a bruce-brew similar to what you might have tried.

thanks in advance

john stu · May 7, 2003

if you are looking for extra revs i think long is usuly the way to go but i dont know if that still stands seince you have a big bore the is modified

marcusgunby · May 7, 2003

All the gp bikes run shorty's but i think they just like the sound of them;)as steve has taught me its all about testing-no 2 two strokes will react the same to a given alteration.Its all about finding that engines sweet spot and as soon as you make one mod to it , it may suddenly work better with a longer/shorter silencer.Basically you need a stack of parts and unlimited time and you will get the perfect motor:)

steve125 · May 7, 2003

Sorry to say there Brucie boy, but your gona have too put your Happy Meal down :) and do some of your own track testing! One tip I have for you is one thing that all 4 of the big bores that I have tuned in have had in common. They all had a HP increase with a timing advance over the std. setting.

MikeS · May 7, 2003

Originally posted by steve125
They all had a HP increase with a timing advance over the std. setting.

Interesting. Since I have no dyno to test (except the RM85 100 kit). The 100 liked the timing retarded some depending on the pipe.

The TM150 timing was better od retarded some to give over rev with MR2.

Cool info... I get afraid to advance timing using race fuel but most big bore do not rev as far as a stocker so I guess it would be safe to advance it some.

One thing I found with pipes on our TM150 I had the stock pipe with a tad shorter than stock silencer but smaller diameter. it opulled well everywhere. Well we went to a PC pipe and found a boost in mid range but lost the revs. We then went to a stock silencer that was 3/4" longer but diameter went almost 3/16 larger. bottom to mid smoothed out and the revs came back :)

Alot of varibles need to be considered when tuning pipe lengths. I have been told the pipe lenght can be read on the pattern left on the piston crown.

 

 

 

bclapham · May 7, 2003

Steve: i hope your not saying that i am fat? :eek:

to be honest guys, i think i have been thinking way too much lately, :silly: especially about this bike! all work and no play, havent been riding in ages and having a stack of pieces that should be bolted together doesnt help either.

the main point i was gettting at was i didnt want to go and buy a shorty and it kind of clash with the pipe, or get a stocker silencer and cut it down to find the bike ran worse. at the end of the day, i will run what is cheapest and that i can make fit!

ps steve. i take it that when i find a hole in the piston or the plug falls apart that i have advanced the timing too far????? is there any safe way of messing with this?:)

steve125 · May 8, 2003

Bruce, I like to think that the timing advance with the big bores is not an over advance setting, but a correct one considering the larger bore size. It makes sence to me that the larger piston, along with the increased mixture volume needs a bit more time to combust properly. Ive found that once past a 2mm advance the over rev can start to suffer, even though say a 3mm advance has more peak power. So then i'll try among other things for example, 4mm off the pipes tuned length along with the 3mm advance. This can bring the over rev right back and hold the peak power at the same or sometimes even better. :thumb:

bclapham · May 8, 2003

Originally posted by steve125
4mm off the pipes tuned length along with the 3mm advance. This can bring the over rev right back and hold the peak power at the same or sometimes even better. :thumb:

steve: was 4mm of the tuned length a typo? if not what in fact are we measuring here? i could see someone whoping 40mm of the end of the silencer but not 4mm!:)

bclapham · May 8, 2003

oh, also. what do you mean by 2mm advance? is this the plug firing 2mm before TDC? so i use a dial for 2mm BTDC and then set the stator accordingly?

steve125 · May 8, 2003

Bruce the 4mm is off the pipes tuned length, not the silencer. Usually at the exhaust manifold or the pipes head pipe is shortened. The timing advance is the stator movement in mm's from whatever is stock. Clockwise is advance and counterclockwise is :confused:

steve125 · May 8, 2003

Bruce one more thing, a pipes tuned length does not include the silencer. The silencer is just part of the bleed down of the system. The tuned length of an expansion chamber begins at the piston face and ends at aprox. 1/2 way in the middle(point of reflection) of the last cone before the stinger is attached and then the silencer to that. Comprendayvu? :)

bclapham · May 8, 2003

steve- i dont want to sound like an ass (well, i suppose youve already called me fat and retarded! :silly: ), but isnt measuring stator movment a bit hit and miss??? since depending on how wide the stator plate is this could be quite an ambiguous (big word of the day) measurement????

also, back on topic- it seems i have got my terms mixed up eg pipe tuned lenght and the length of the silencer. if a long silencer supports rev and the shorty supports torque/bottom, so is it a real bad idea to mix a high rev pipe with a shorty silencer and vica versa?? eg will they compliment each other or do we get the worst of both worlds? i am talking generically here btw.

steve125 · May 8, 2003

Sigh! :think: if anything I called you chubby and slow but not fat and retarded :) Yes the stator movements can be seen as a generality because the fact is, the true amount of movement is slightly different in each motor. So whether it's 2mm on one stator and 3mm on another, if it wasn't for the dyno I would not be able to find out which would be ideal. But I do know that some degree of timing advance has increased the HP on all big bores ive dynoed thus far. Just exactly how much stator movement? I would say 2mm would be a good and conservative place to start. Tuning is always "hit or miss" :moon:

steve125 · May 8, 2003

Oh and on the silencer issue. Once more we need to cut out the generalities that short silencers give bottom and long one's increase rev. Because it's never the same given the different bikes involved and engine design combinations.  So don't worry about matching a silencer length to the pipe you choose, as it's much more important to pick the silencer length that feels best to "you" on the track. :cool:

MikeS · May 8, 2003

Steve good info...

I mentioned the burn pattern on the crown. The pattern should be slightly offcenter towards the exhaust port for best power. The way I was told to adjust this is by adding reducing spacers for the headpipe. I guess this is proven by your observations. I wish timing curves were easily and affordable to adjust. Moving the static timing helps but is not a cure all.

EricGorr · May 11, 2003

I was reading the latest TMR CR145 test in MXA and apparently FMF is making a pipe for big bore 125s. In 1996 I built a batch of 144cc RM cylinders for Guy Cooper to be used in the National Reliability series which was a 125-200cc class. FMF built him a corresponding batch of pipes and silencers to test. Theoretically a pipe with a larger diameter and shorter dwell section should work better when the bore is changed, not the stroke. At the time FMF considered marketing a special big bore pipe for the 125s but in the mid-1990s the market was very small. Now there are several companies offering big bore kits for 125s. Perhaps we can expect them to market a pipe in 2004?

bclapham · May 12, 2003

Hey, Eric reads MXA.....no wonder he knows so much about bikes! :confused:

i read that article too, it seems like MXA dont like Morgans work- they should have tried Steves engine with marcus's suspension settings, now i'll bet that would be a good bike! ;) on the pipe,i dont think it was a special fatty, they have a new fatty out called the factory, seems like they dont put the shiny coating on and charge an extra $50 soo you can scrub the rust off after you ride.

ive said it before, i think two great niche markets are custom pipes and custom cranks- a complete wiseco crank costs less than $200, RPM charges $800 to do a stoker and i think he uses the funky offset pin, which i bet costs a lot to replace. you dont need to stretch the imagination too far to see a off the shelf storrker crank..will someone let wiseco know this???? that should make rick happy!

Faded · May 12, 2003

Originally posted by MikeS
Steve good info...

I mentioned the burn pattern on the crown. The pattern should be slightly offcenter towards the exhaust port for best power. The way I was told to adjust this is by adding reducing spacers for the headpipe.

Mike, any idea on the effect of the spacers in conjunction with the burn pattern, ie- more spacers moves it toward the exhaust port or away? Curious to find out so that when the time comes for a top end I can explore this avenue.

Thanks.

MikeS · May 12, 2003

In my testing with the RM85 (100 EG kit) adding spacers moved the pattern towards the exhaust port. I am not sure on the power gains as I can not tell. But when trying to figure all the variables in tuning. I think this trick helps eliminate one. I have 3 different pipes for the RM 100 EG. Stocker, Doma and DEP. These 3 are completely different from each other in Diameter, Overall length and Expansion body lenght. The DEP is closest in shape to the stocker but the expansion area is longer and alot fatter. Overall length is a tad longer overall but hard to measure. The Doma is shorter in overall lenght, alot shorter in expansion lenght and closer to stock diameter or a tad larger at one or 2 points. The doma shape is alot different.
All 3 pipes make different power as the stocker is linear off the bottom and pulls nice. The DEP makes the most killer power all over and revs way out 1000+ rpm more than stock. But I have not tamed the extra heat this pipe produces while riding offroad. 15-20 minute motos are OK. The Doma is what we race with as it seems to hit harder in the midrange and rev up alot faster. With the stock silencer (short) it grabs the midrange real fast and signs off a bit . With the doma silencer it is smooth on the bottom and revs out better on top. Junior can loft the well or pass well with the doma.

Just some more pipe info.

Faded · May 12, 2003

Originally posted by MikeS
The Doma is shorter in overall lenght, alot shorter in expansion lenght and closer to stock diameter or a tad larger at one or 2 points...The Doma is what we race with as it seems to hit harder in the midrange and rev up alot faster.

MikeS, Thanks. Seems to reinforce what Mr. Gorr said about a larger diameter and a shorter dwell section.

matching pipe and silencer for tuned length?

bclapham

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