ask a stupid question. port and ignition timing?

bclapham

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#1
OK people- i just cant fathom or visualise this thing out in my mind. :think:

if we deck the bottom of a cylinder to change the port timing (advance i assume) does this also change the ignition timing too?

i have more questions, but i need to understand this point first. thanks.! ;)
 

jmics19067

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#2
it shouldn't, as long as the barrel stays square with with crank. the ignition timing is solely based on crank position. if the barrel stays square top dead center will be the same crank postition. Now if you change the angle of the cylinder or deck the crankcase at an angle the crank will be in a different postion to reach top dead center and that will change your timing.


or are you asking if you need to change ignition timing ?
 

Rich Rohrich

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#3
Originally posted by bclapham
if we deck the bottom of a cylinder to change the port timing (advance i assume) does this also change the ignition timing too?
Bruce - The ignition advance is based on the crankshaft position relative to TDC. Cutting the barrel does not change the relative firing position of the crank in reference to TDC or the point of ignition. The relationship of the piston to the port windows and the squish band changes which will seriously influence the conditions in the combustion chamber when the spark does fire.

Back up a step and it's fairly easy.

The idea behind ignition advance is to try and time the peak cylinder pressure to occur somewhere around 15-20 degrees ATDC, to apply the pressure in the most mechanically efficient way. The overall idea is to use the minimum timing necessary to obtain the best possible torque. That's the painfully obvious Engines 101 version.

So the point of ignition will stay the same after the base of the barrel is cut but depending on the changes to charge density, scavenging and compression ratio the optimum timing requirement may change.

In the end it doesn't really matter if the timing itself were changed because the engine's requirements have changed and the ignition really should be "tuned" to match the new conditions in the combustion chamber. :thumb:
 
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Rich Rohrich

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#5
I think we were posting at the same time :)
 

jmics19067

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#6
I am wondering now if Bruce needs a different standard than degrees for measuring rotation :laugh:
 

bclapham

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#7
Originally posted by Rich Rohrich
The ignition advance is based on the crankshaft position relative to TDC. Cutting the barrel does not change the relative firing position of the crank in reference to TDC or the point of ignition.
thanks guys, for some reason, i was seeing that the head was now closer to the piston and that was bringiing TDC in quicker- wadda mistaka to maka!

i understand now- when we first built this rm engine it seems like everything was off with port timing- of course its not erics fault since we did the best we could to bring all the pieces together from many different bikes, LOL and i couldnt tell how it was supposed to run since i was coming off the 250. ive gone to essentially stock port timing now and was running a bit of advance (at the advice of steve who has a lot of experience with these 58mms) to get a bit of grunt out of corners and the bike runs well. i just wanted to know if i could now drop the cylinder 0.2mm with a thinner gasket to a bit more to get a tad more bottom and a bit more comp- and now i think i have the room to do it.

one more question for Rich- i know that running more oxy in the fuel effectively advances the ignition- can we quantify or approximate this in any way? ie if we went from 5%mtbe to say 20% could we say this would equate to 1-2mm's movement on the stator?

oh, another question, LOL- if we stroked the crank 2mm and used a 2mm shorter rod, how would this effect port timing and how would we alter it to keep the same type of power curve? would we need to port as well?

thanks! and sorry for all the questions! :thumb: :worship:
 

Rich Rohrich

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#8
Originally posted by bclapham
one more question for Rich- i know that running more oxy in the fuel effectively advances the ignition- can we quantify or approximate this in any way? ie if we went from 5%mtbe to say 20% could we say this would equate to 1-2mm's movement on the stator?
If only life were that easy. Too many other variables enter into things to be able to answer that with any confidence. That's why they call it tuning I guess. :thumb:

Originally posted by bclapham

oh, another question, LOL- if we stroked the crank 2mm and used a 2mm shorter rod, how would this effect port timing and how would we alter it to keep the same type of power curve? would we need to port as well?
Lots of variables to think about for my four-stroke centric brain. Changing the piston acceleration rate at before and after TDC and it's effects on blowdown and transfers among other things. I think this one is better left to Eric's two-stroke centric brain. :thumb:
 

bclapham

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#9
Originally posted by Rich Rohrich
Lots of variables to think about for my four-stroke centric brain. Changing the piston acceleration rate at before and after TDC and it's effects on blowdown and transfers among other things. I think this one is better left to Eric's two-stroke centric brain. :thumb:
LOL- thanks Rich- hopefully Eric will drop by at some point on the stroker issue- i may have a source for a spare crank so i am thinking of going up a notch the next time this engine needs an overhaul, but i wonder if the cylinder will need re-porting- i know Eric is going with the longer rod in his but i probably could do without the extra hassle of the spacers and head brakets etc. either way, i need to improve, on saturday, i got lapped twice by some dude wearing an orange helmet :| now that sucked.
 

Rich Rohrich

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#10
I know Eric has done a lot of thinking about the stroker possibilities and run a number of simulations as well. I'll point this thread out to him so he'll add in his thoughts.
 

steve125

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#11
Originally posted by bclapham
 

i need to improve, on saturday, i got lapped twice by some dude wearing an orange helmet :| now that sucked.
 So Jody laid the wood to ya on his YZ again Bruce?? :)
 

bclapham

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#12
no, all them dudes are in the 50+ class- this dude was about my age and was on a full works procircuit RMF test bike; i think they only had 2-3 riders that wernt in that same race, LOL.

anyway steve, stop your poking and proding, whats your take on all of this timing stuff. :worship:
 

steve125

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#13
Ok Bruce just what is it your looking for? Well first off what is your current set- up on the RM?
 

bclapham

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#14
two questions- yes the rm, right now it is setup with stock port timing and stock pip and reeds, ~1.3mm squish. i am running the advance, around 2mm. i still feel i need a bit more bottom so i am going from a 0.5mm gasket to the cometic hi comp one which is 12 thous thick (which is 0.3048mm's by my calculation), so i will be dropping the cylinder ~0.2mms and this should bring my squish back to just a shade over 1.0mm :) should i back the timing off as well- i am just running 50/50 C12 pump right now, i think i will be OK, i might just run it like that and send Rich a plug, since i still cant figure out if the ignition is too advanced from looking at the electrode.

and once ive tried this, how will a hi oxy fuel like U4 work out? ping city?

thanks steve!:thumb:
 

steve125

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#15
Bruce leave the ign timing advanced at 2mm and go for the thinner base gasket. Ive run my squish clearance as close as .8mm with no problems so your safe at 1.1. Make sure you run at least 50/50 race/pump. Go with the new U-2, the U-4 won't have enough octane for your now high compression full race motor! :thumb: :)