Yamaha TTR125 CDI

ACS

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#1
I talked to the development guys at Wolf ignitions which are sold in USA as FMF (orange box ones) about developing a new cdi so that the bikes can rev out higher before the cdi shuts it down and would have a 2 programs, torque and rev in basic terms. They would probably be willing to build them but is there any interest?
 

CRGuy

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#2
I don't know but they could make some $$$ if they made some stronger lighter valve's with a Rev kit. It could be like the rev kit for the xr400.
CRG
 
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#4
Great! I think you've finally found a way to take this bullet-proof little bike and make it into a hand grenade.

Higher rpm's don't make the TTR faster. Do everything else right (bore, cam, valves, springs, etc), gear it right, and you'll have a faster mini than any other out there (just ask Powroll and BBR).

The addition of more rpm on this engine would require a few things to even allow the engine to utilize it - 1. Different camshaft than what's currently available (and works very well, by the way). 2. Much heavier Valve Springs 3. Stronger valves (stainless or ti) to handle the heavier springs 4. Piston to valve clearances of much more than you have currently. If piston to valve cl is currently in the .050" range, you'd need to go to at least .070" or greater. Also the valve to valve clearance would have to be increased.

If you're hitting the "rev limiter" right now, it's not in the ignition, it's in your head! Or I should say, your bikes head, you're floating the valves.

High rpm needs to be worked in with other modifications. Just becuase an engine revs doesn't mean it makes power. Conversely, an engine that only revs to 8,500 rpm can make considerable power.

Higher rpm in an engine not designed to take it is the fastest way to break a four stroke -- well, other than running it without oil

;)
 

ACS

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#5
I'm still thinking it is ignition as the standard setup is for a low reving engine. Flywheel size and weight gives that away. Nobody I can find does heavier valve springs. Anybody do them?
 
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#6
I think you'll also find that the cam chain setup is going to prove troublesome at high rpm with more agressive profiles and greater valve spring pressures.

It would be fairly difficult to justify the cost of a programmable ignition on a cost/benefit ratio. Although it's clear there are gains to be made there the costs to realize them would be high.
 

CRGuy

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#7
Ok say that someone build's a high performance ignition system. Ok it rev's further and add's power, But ( yes there's a catch ) if you get up in the high RPM's your engine better be timed very well becuase at High RPM's the timing is not as precise as it would need to be Is that statement true or false?
CRG
 
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#8
Originally posted by CRGuy
if you get up in the high RPM's your engine better be timed very well becuase at High RPM's the timing is not as precise as it would need to be Is that statement true or false?
CRG
I think you'll need to clarify that to get an appropriate answer.
 

CRGuy

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#9
Ok I was alittle brief.
If the TTR got in the Upper RPM's say 9,500 with stronger valve's and spring's. As the engine get's up in the 9,500 range would the engine malfunction and blow due to the valve timing not bieng precise enough to control the intake and exuast?
Let's say the rev limiter is 10,000 on this new TTR.
CRG
I think I make sense now:think
 
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#10
CRGuy,

There's lots of stuff going on in a four stroke at high rpm that will cause problems. Think of everything that has to happen for a complete four stroke cycle compared to a two stroke:

two stroke:

crank rotates
piston goes up
spark occurs
piston goes down
repeat

four stroke:

crank rotates
timing chain rotates
cam is moved by timing chain
cam pushes on rockers
rockers push on valves
valves open
valves close
crank moves, piston goes back down

All this happening at quick rpm's in a TTR will cause the following to happen:

- Cam chain will "fling" against adjuster with more force. This can cause a few things to happen. 1. Like Rich sez, the chain can break 2. chain pushes out tensioner and slows down the cam rotation (like drive chain slap when you dump the clutch). Thus allowing the piston to come up faster than the valves can move out of it's way. BAM! Piston hits valve.

- Rockers will "jump" on camshaft. This is your basic valve float... The faster the cam rotates, the harder it is for the springs to hold the rocker tight to the cam. This is especially true of a camshaft with an aggressive ramp angle on the closing side of the cam. This will cause the intake valve to stay open too long, again causing piston to valve interference. It will also create big divots in the camshaft where the rocker slams back down on it (after a jump, you've got to land somewhere!).

Okay, so we install a heavier set of springs - yes they are available. No they don't completely solve the problem. If you were to build a set that were heavy enough to handle really high rpm, would that work? Not necissarily. Now the rockers are pressing harder against the camshaft, but all the time. We've created more pressure, and pressure = heat. It also equals cam wear. Oh yea, and it can yank the head off the valve too.

Let's say we solve all these problems. Okay. Now, would a TTR 125 with 2,000 more rpm actually make more power? Not unless the cam you're running was designed to work at that rpm. Otherwise, you'd be making more noise but not accomplishing anything.

Okay, so we redesign the camshaft. Hmmmm - a little more lift, and lots more duration (so the valves still stay open long enough at that faster rpm). Crud! Now the valves are hitting each other! So, we sink the valves into the head, and remove material from their edges. Okay, they're not hitting each other anymore. But we've just reduced the flow by 20%. Now it won't run well on top end because the head doesn't flow well enough, and we've lost that quick hit off the bottom because of the change in cam profile. RATS!

I hope I've shed a little light on what it's like to really design stuff like this to work correctly.

Making major modifications to a four stroke means knowing how the specific engine works, what it's weaknesses are and where it's strengths lie.

By knowing an engine's strengths, you can build towards increasing those characteristics without making the weaknesses worse.

The TTR is not a revver. It wasn't designed as such, and doesn't perform well under these conditions. It is much faster, cheaper, and reliable to build the TTR for more torque and midrange horsepower and GEAR IT RIGHT.

Powroll has tried a different ignition on the TTR, it's the same one they're using on the CRF project bike (like the Wolff system, two stage, programmable). It didn't give any increase in horsepower over the stock ignition.

I'm not saying don't try new stuff, I'm just saying that increasing rpm on these small bikes without understanding the implications is not necissarily the smartest way to spend your hard-earned cash.
 

ACS

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#11
Ill agree with all of that but I like trying stuff.:)

I cannot see why the TTR engine cannot be made to rev harder. The XR80/100 has much lighter cam chain, valve system (in lighter I mean size of components) even the cam on the XR runs plain metal bearinds whilst the TTR has ball bearing suported cam. I know the ports are poor but with them well done it should go harder and faster. Im not talking about a 13,500 rpm screamer but more to take advantage of a bigger carb/cam/exhaust system.



Okay, so we install a heavier set of springs - yes they are available. No they don't completely solve the problem. If you were to build a set that were heavy enough to handle really high rpm, would that work?
Who does the springs? I have the VISA ready and waiting.:)