How come no one has mentioned port design? Cam profiles? Squish shape? Air velocity at the venturi? exhaust scavaging? Exhaust port shape? Shrouded valve pockets? Guides obstucting the ports?
Because those weren't the topic. Jetting was.
Actually no...the topic was buddy bought a new exhaust for his bike and found after he installed them found a hollow spot around 4500-5000rpm that he was interested in getting rid of. If you want to get rid of this flat spot, without detriment somewhere else in the rev band, you better start looking at the ports.
The original post said that he WOULD NOT BE JETTING HIS BIKE. So how the topic got to be jetting, I don't know.
"Does the stock muffler flow the volume the engine produces at any greater restriction than aftermarket?"
Yes, and yes it has been proved.
And the resulting HP was....... If you effectively
passed more air/fuel through an engine, it WILL make more HP. Emphasis on effectively
Now you want to ADD the word effectively to the question. Plain and simple; every aftermarket exhaust system I have ever seen, for a motorcycle, does offer less flow restriction than stock. Not "effectively" or "like" or "similar to" or "resembling". I said an aftermarket exhaust DOES offer less restriction than stock.
Here is where you are going wrong: If you install a less restrictive exhaust on a motor it WILL take in MORE air, the problem now is the size of the jets can't keep up. Sooooo effectively, you seem to like that word, you will get 0 HP increase until you increase the amount of fuel going in. That is the carb's job.
Tuning is all about BALANCE. You must balance the carbs to the exhaust. One without the other is USELESS.
You asked about flow restriction, not HP gains.
"Analyze that, and show me how ANY THEORY you can find changes the fact that it mixes fuel in direct relation to the volume of air coming through it,...."
Direct relation; NO. My carburetors are attached to a device called a throttle, which can change the relationship of the volume of air to the volume of fuel. It can change the volume of air by way of the butterfly valve and it changes the volume of fuel by way of jets and circuits in the carbs. At higher RPMs there is more fuel in a gallon of fuel/air mix than at idle. See, the DIRECT relationship has changed.
I'm confused. I said it delivers fuel in direct relation to the volume of air. You say NO?! Are you saying that there's more
fuel to air at wide open than fuel to air at idle? If so, you're simply wrong. If not, then you're saying the same thing I did..... because it's right... and a VERY simple concept that fuel burns best at a specified ratio to air. And it's all of those little jets, circuits, and fancy parts that are accomplishing just that
Try this, (I have): Install an O2 sensor on a carburated bike. Watch the readout as you go through the rev band. You will find that the motor is lean at idle and at full revs and rich in the middle. The richest at the instant the needle comes out of the primary. This has ALWAYS been a short fall with carbs, any carb. Quadra Jets and ThermoQuads have the best metering.
Most Exs have CV carbs which will change the fuel/air mix with no input from the throttle. It uses throttle plate vacuum to lift the needle, enlarging the jet opening.
And it has those features, striving to keep the air fuel mixture correct at every throttle position because the venturi alone is too large to handle the full range of flows accurately through one jet. And while we're at it, if you mean it uses manifold vacuum not ported vacuum, you might want to look at that further. I haven't dwelled on this detail too much beyond the basic idea of its function because I was just trying to use it, not design it, but it's NOT manifold vacuum that's utilized.
NO NO NO NO, I said throttle plate vacuum. If you used plenum (you call it "manifold vacuum") vacuum the slider would be pegged to the top of it's travel most of the time.
Actually what a CV carb does is keep the flow through the venturi at a CONSTANT VELOCITY to make it easier to tune.
(What is "ported vacuum"? I have never heard this term before.)
You CANNOT use a vacuum cleaner analogy because a vacuum cleaner delivers a constant vacuum, a motor does not. That lack of constant vacuum is a real problem in tuning.
And THAT'S where this discussion belonged all along and IS the source of potential
for needing changes. The pulse is effected slightly, IF there's a true change in flow, and the ability of the fuel circuit to respond to those pulses is the real world side of what has the potential
to create a problem that the theory doesn't need to account for. In theory it requires NO change, in practice it may
. Emphasis on two words, "IF" and "MAY". Depending on the carburetor and the effective range of the change, it may or may not require a change. Experience on these suggests that they're good as is with the exceptions as noted and posted about.
You have to figure out what throttle plate vacuum is. Your argument would be valid if we were talking about a multi cylinder motor with a shared plenum, but we're not.
You'll get into overlaps of the circuits, efficencies of the carb design, and find the real world imperfections that never fully realize theoretical perfection at every RPM. It also requires an actual change to require actual attention, also the point of the original discussion. If you've got a problem, it should be able to be tuned out. I've not encountered those problems and HAVE tried larger mains AND shimmed needles, both too fat, and it WAS subtle. I'm back at stock with the pilots out 2 1/2 turns and the 3,000 to 4,000 or so RPM flat spot GONE.
"...why did FOG use the analogy that it's like changing cam timing.... not that it DOES change it...."
He didn't say it was "like" changing the cam timing, he said it "effectively" changed the cam timing. Not that it was similar but, for all intents and purposes, does change the cam timing. It has the same effect as installing an exhaust cam with a longer overlap.
It seems that you're arguing the definition of a word and in my understanding of those two words in those two examples, they are synonymous. That's the way I read it and used it. If not, I'll let you choose the one that you prefer and just agree with that. And I'd go further to say that the more accurate description is it effectively increases the exhaust lobe duration
, but I'd understand the intent either way.
Nope not synonymous.
Has the same effect as installing an exhaust cam with a longer over lap. Not "similar", "like" or "resembling". Has the SAME END RESULT. EXACTLY.
"And I'd go further to say that the more accurate description is it effectively increases the exhaust lobe duration
Nope, longer duration might or might not get you more over lap. I am talking about "Cam Timing Overlap", the number of degrees of crankshaft rotation that both the intake and exhaust valves are open at the same time.
Now...on to Jet Kits;
I, like FOG have no use for jet kits. BUT I have a decent collection/assortment of jets and needles (as I imagine FOG does). When the bike needs jetting, I have the jets I need. I also have quite a bit of experience in carb jetting, so I have no use for a "Jet Kit" per se. (OK, one of the jet kits sold has a tapered needle that I swear by IF you are running the stock CVs, but it is available elsewhere. It overcomes the lag/flat spot the original post was talking about. Doesn't fix it, just masks the symptoms).
So there are uses for Jet Kits; If you don't have a decent assortment of jets, or when you don't have any experience (or don't want any) in jetting carbs.
Just so we all know, Carbs are adjustable. The adjustment comes from jets and needles. When designing the motor in Japan one of the specs is that it has to run acceptably in Denver (about a mile above sea level) to Richmond ,BC (Below sea level). To get this level of tune there must be compromises and one of the compromises is in carb jetting. To remove this compromise you rejet to YOUR situation.
So which bikes could benefit from a rejet...99.9% of bikes. Is it worth the effort? You decide. I re-jet when the temp changes, when the air density (barometer) changes, as the valves and rings seat in, when the sun shines or doesn't...etc etc etc.
So I will say if you want perfect, learn how to tune. If you don't want to put forth the effort and have no motivation to learn, a jet kit will get you close.
As an aside; Every race bike I have ever bought came with an assortment of jets and needles in the spares kit. (HMMM, a factory "Jet Kit"?)
As FOG says a header will get you one maybe two HP. Cost ~$600 CDN. You can pick up one or two HP by degreeing the cams. Cost ~$25... a jet kit 0 HP. Cost ~$80.
And if you're changing jets for all of the subtle scenarios you outlined, then congratulations. You're chasing fractions of fractions of a problem that's more about the quality of the air you're working with, not the quantity, and you'll never get it all back. In fact, very little unless
you're talking LARGE excursions from baseline. If the one of the key components of your air fuel mixture is faulty, you can't make it 100% with tuning and you won't get it all back. If you're racing, neither will your competitor, so relax.
Yes I rejet that often, but then so do my competitors, and YES at that point I am looking for that last HP.
I have a problem with "more about the quality of the air you're working with, not the quantity". I deal with quanity, as I have no idea on how to change air "quality". Maybe you could enlighten me.
These posts are for practical use of these bikes and once it's running nicely, not stumbling, burbling, or falling flat on its a$$, learning to ride it well will get the real results, not chasing a HP or two. And the factory WILL tell you to re-jet if you're continually at elevations like Denver (5,000' or so) and above. I've been there and 5,000 feet matters but 7,000 feet and above will REALLY screw with your performance and CAN benefit from a jet change. That's also a dramatic change in environment from the majority of dry land.
The Factory will tell you to rejet in those circumstances because using stock jets at that altitude will mean the motor is SO lean that it will burn the valves, resulting in a WARRANY CLAIM. The factory doesn't really give a flying **** how well your bike runs or doesn't. Always keep in mind the manufacturers mantra " Death before warranty"
And yes, I DO have more than a little knowledge and experience with this stuff. At the strip, being finicky has some
value. At the road track or especially on the street? VERY suspect value. "It's the rider, not the bike." I see it repeated time and again, track after track, thousands of riders, at the seventy or so track classes like I'll be teaching just this year.
"At the strip, being finicky has some
value. " Only if you put value on winning races...and that is why I enter races. I LIKE TO WIN. If I didn't I would go to track days. It is real easy to look like a hero at MOST track days...even on an EX500.