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Can someone explain how retarding the exhaust timing can give you better performance?

Thanks,

Lars
 

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An engine gets its energy from combustion. The amount of compression and combustion determin the power. When the exhaust timing is advanced the exhaust valves are opening to soon which in turn is a loss in compression which is a loss in power. When you retard the timing on this quad you are actually correcting the timing by putting it where it is needs to be. So pretty much you are keeping your power in the cylinder longer. If you are adding fuel and increasing air why let blow out the pipe the whole point of increasing these is to burn it for energy to cycle the motor. This is the best I can explain it. I'm sure that someone out there can do a better job.
 

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Originally posted by Heinriken@Nov 10 2003, 11:25 PM
This is the best I can explain it. I'm sure that someone out there can do a better job.
Uhhh... I thought that was very simple and understandable. Good work.
 

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And now for the ultimate dumb question. WHy the heck would Yamaha do this? WHy not re-use the YZ cam and save some money instead of goign out and manufactur a different cam?????
 

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because some people cant handle 50 ponys, they would simply kill them selfs, i have been riding for 15years an i still come on to her to much in the wrong spots, if u know what i mean..lol :rock :yfz :yfz
 

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They didn't do it because of emissions. I've been thinking about this tonight. By advancing the exhaust cam they widened the LSA or lobe separation angle and reduced valve overlap. Stock, the exhaust lobe is further from the intake lobe, which does broaden the powerband but reduces peak power and emissions.
Remember the 4 stroke model: Exhaust, intake, compression, power. Some guys might think the exhaust event comes right after the intake event. Contrare, it's the opposite. The exhaust stroke expells gases on the up stroke, the intake stroke draws in air on the down stroke. The compression stroke occurs on the up stroke and the power stroke is the result of the spark going off, igniting the mixture.

By retarding the exhaust cam you now get more overlap and exhaust scavaging when the intake event occurs. This is due to the intake valve is beginning to open when the exhaust is not fully closed. It takes a bit longer to build cylinder pressures like this and you get more emissions by forcing out more exhaust. The scavaging is because you've opened the intake valve, letting in more fresh air, forcing out nasty exhaust fumes. That is the main reason you get more power. Better cylinder filling once that exhaust valve gets closed. Duration and lift remain the same, the power is made from valve timing and scavaging created from overlap.


Note:
I have not drawn this out yet since I do not have these cam specs, and it's very late, so bear with me if I have made any errors.
 

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Barely Legal, if you get timing specs and can draw it out on a grid, I would really like to see it. This sounds like a great explanation for the benfits of retarding the cam. Can you be sure there is valve overlap with the exhaust cam in the retarded position? I'm assuming there's none when the cam is in the original position.

I be intersted in knowing if the cam duration can be increased by the ~11 degrees if someone designed a cam that opened the exhaust the same time as stock, but closed the same time as in the retarded position.

If I were to guess, Yamaha probably made the exhaust duration shorter just so they had the option of easily changing the cam timing. I remember this modification was being done as far back as the WR400F. I don't think a mechanic discovered it by mistake, I suspect the factory designed the exhaust cam to work this way.
 

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Overlap would make the emissions worse because in all reality some freash raw fuel and air get drawn out the exhaust.
 
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