I measured the internal diameter of my FMF silencer today and I noticed that it is as small as 29mm on the silencer end, even though it has 32mm of I.D. on the chamber end. I have cut about 200mm (just a guess) out of the stinger pipe length to fit my frame.
I was wondering if anyone would be able to tell me if this 29mm diameter is going to cause me any trouble, especially in terms of raised piston temperature?...I'm just not that sure if it is a good idea to start my motor with a stinger that is that small, in case I seize it.
In case they are needed, the specs of my motor (that I know) are:
-11.2:1 compression ratio
-1.72mm squish clearance on first (cold, before the first start) torqueing
-98 RON octane (New Zealand)
-196 degrees of exhaust port duration
-AAEN 500 chassis pipe (32mm stinger)
-Keihin 40mm PWK
-1987 motor
-Stock reed cage with fibre power reeds
Thanks,
Ross.
And the bore size is 88.5mm
29 mm id is ok if your engine is not going to make more than about 55 to 60 hp. If your engine is going to make more power than that, you have better get the right amount of restriction on it or your piston will suffer during the long hard pulls. If the silencer has a spark arrestor it will be severely over-restricted with a 29mm Id silencer core or outlet.
Why are you not using the silencer that was made for that pipe?
This is one of the reasons our silencers will not fit any other pipe or other silencers will not fit our pipes. We have done the testing to get the whole system right. We do not want our customers shooting themselves in the foot by allowing them put something that they got off of ebay and then blame us for their engine failure or our pipe not making the power it was designed to do.
I used the FMF silencer instead of the AAEN silencer that matches my chamber because I read in one of your old posts that the AAEN silencers were causing pistons to seize.
I'll chop up the FMF silencer and replace the perforated tube with one of a larger internal diameter. Is there any size that you would recommend? I realise that you're not going to be able to say the exact size to achieve the peak power out of the chamber but what do you think would be a safe size to use?
The AAEN silencer that was causing problems was the larger Diameter style AAEN with the spark arrestor. The standard AAEN for zillas works fine.
I used the FMF silencer instead of the AAEN silencer that matches my chamber because I read in one of your old posts that the AAEN silencers were causing pistons to seize.
I'll chop up the FMF silencer and replace the perforated tube with one of a larger internal diameter. Is there any size that you would recommend? I realise that you're not going to be able to say the exact size to achieve the peak power out of the chamber but what do you think would be a safe size to use?
It was their spark arrestor design that was torching pistons. Their spark arrestors were adding too much back pressure. A little too much back pressure will add a lot of heat to the piston crown without hurting the power. If you get way too much back pressure the engine cannot make enough power to make enough heat to hurt itself. You can increase the back pressure to a critical level by making the core too small, the stinger too small, a spark arrestor that is too restrictive, a stinger that is too long for the diameter of the stinger, there is an infinite numbers of ways to get too much back pressure.
Getting the back pressure right requires a lot of testing and usually requires some pistons to be sacrificed to find the right combination of stinger and silencer for the exhaust system that it is attached to.
Ok, thanks for the info. I might start by modifying the FMF silencer to use a 32mm core and just see what the guy who dyno tunes it thinks and just work from there....I certainly won't use that 29mm core
Probably be less of a headache just to get an AAEN silencer, make sure you get the non spark arrestor one.
Am I the only one that noticed that's not a 500 chassis in that picture!?
looks like a TRX250R Honda chassis
This is the yfz hybrid that I started a build thread on a while ago
