Q: with the exhaust, I have found that since installing a new fully functional one (ie no more holes or falling off hotdogs) the mixture is unbearably
rich, (I am goind to change my carbi as i can't take the flooding anymore). I did notice an increase in power (whoohoo 77mph.. don't laugh, this is
festy we are talking about ). Why is this so? if the previous exhaust was
full of holes yet it was idling ok etc. I now have twin cannons with pipes welded in (sound dampening :lol ) but I get backfiring, flames from time to
time (toastie d' pedestrians as dey duck avoiding the terrorist attack :P) etc.. I know its to do with the carbi... but what role does the exhaust
play with the carbi, where you have the outlet valves etc is this the 'back pressure' thing, a buildup of gasses in the heads?
if i have pissed anyone off with this sorry, direct me to a website or thread which tells me in detail nn[ Edited on 20-8-2005 by $2.63 homebrand baja ]nn[ Edited on 20-8-2005 by $2.63 homebrand baja ]
$2.63 homebrand baja
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posted on August 20th, 2005 at 01:17 PM
ok rather than just delete thread, someone out there may find my adventure into 'how workie firey bits' helpfull. so for 2nd question I suggest
google up 'what is backpreassure vw'
A.k.a.: Knob Jockey
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posted on August 20th, 2005 at 07:00 PM
The dude is a little confused as to what "backpressure" and "scavenging" actually mean and what the overall effect is.
Put very simply, the well designed zorst system must result in a minimum of back pressure (positive pressure) possible and a maximum of scanvenging
(negative pressure) effect at the correct TIME.
The issue of backpressure relates to performance 2 strokes and their "expansion chamber" exhaust design which (very loosely) utillise a diverging
cone and a converging cone to create scavengine and backpressure events in the pipe. 2 strokes rely on ports and the timing of ports to get air/fuel
in and out and the correct timing of these positive and negative pressure waves is absolutely critical in the good performance stakes.
4 stroke engines have poppet valves controlling intake and exhaust events so creating a timed backpressure, or any backpressure for that matter is
detrimental to performance.
However creating a negative pressure(scavenging) in there at the correct time asists the engine of ridding itself of spent exhaust gas and actually
assisting the piston to "draw" in fresh air/fuel. The timing of this is no less critical than the timing of the 2 stroke pressure events.
This achieved with a combination of head pipe diameters and lengths (hence "Tuned Length) and the design of the headpipe collector and the stinger
(tailpipe). Its not the exacting science as it may first appear as all of the diameters and lengths are easy to arrive at mathematically but getting
the combination of them right is totally a black art. Its easy to get a pipe to produce amazing power gains in an extremely narrow rev range at the
expense of power elsewhere in the rev rang but to get useful gains of power over a WIDE rev range and minimising power losses elswhere in the revs is
the black art and the key to a good and flexible engine package.
Its all a delicate balancing act to create the biggest scavenging effect with the minimum of backpressure, over a wide engine rev range.
The last thing you want to do is create a "potato in the tailpipe" effect by increasing backpressure.
Pharrrk!
L8tr
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Im not a complete idiot, quite a few parts are missing....
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posted on August 20th, 2005 at 07:50 PM
ahh, thats why one takes things found on the net with a grain of salt, just as the carbi info above is biased towards webbers because they are selling
them (ignored that bit), but still found it interesting rather than making numerous posts annoying peeps.
For me It gave me a bit of an idea on the principle of what back preassure is, and what it does. I was curious as to why they make all these weird
exhaust combinations, there are a whole load of different things when you search it, some make sense, then the next one is contradicts the first. I
was trying to work out why the whole tuning of my carb had changed once the exhaust was changed, the hotdogs I had before had all baffle (I assume)
things inside, whereas the cannons are pretty straight through and i noticed the extractors are slightly shorter. I was just looking in an novice
sense but the principle, ie preassure etc sounded like sense considering the state of my exhaust, it also gave me an insight as to why 4 into one
exhausts are more popular, my last baja had one and once again it ran slightly differently.
Thanx for the clarification I wasn't intending to send anyone down the wrong path. But this is what a forum is for discussion, even if I do end up
making an arse of myself I am learning so much the more i go into things,
the more there is to find out.
tassupervee
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posted on August 20th, 2005 at 09:17 PM
Dude
Not at all were you sending anybody down any wrong path and least of all making an arse of urself!!
It was the article that had some flaws in it really.
You are on the right track however with your different zorsts and tuning issues tho.
More power to you dude!
L8tr
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Im not a complete idiot, quite a few parts are missing....
posted on August 20th, 2005 at 09:24 AM
nn[ Edited on 20-8-2005 by $2.63 homebrand baja ]nn[ Edited on 20-8-2005 by $2.63 homebrand baja ]
