frEEk
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posted March 17, 2002 01:42 PM
compression and octane
here's a question/topic that has long bugged me. i'm hopin some of our more knowledgeable members will know something about it (i'm sure a couple will).
how do you design an engine to handle more compression without having to resort to very high octane levels? used to be cars were running like 8.5 or so stock compression. these days alot do more than that & are still fine on 87 pump gas. the highest i ever heard of in a highly modded Z car (i'm a datsun Z car buff in my non-bike moments) is like 10.5, and that required higher octane gas (like 91/2 or better). how come our bikes can run over 12:1 and be ok on 91 pump gas? i understand that it is mostly in combustion chamber design, but exactly WHAT is it about a combustion chamber that allows u to run higher compression on the same octane gas? i'v heard about squelch [sp] pads being good, but dont know what that means.
so, any thoughts? anyone care to share the full physics behind this? i'v always relished the idea of one day modifying a head on one of my Z cars so it can run like 11:1 on 91 or less octane. should improve mileage and power a bunch.
   
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BA
Pro
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posted March 17, 2002 02:43 PM
my uneducated guess
part of what allows todays cars to do more compression is the aluminum heads and better cooling system design.
Aluminum disapates heat better than old iron heads, but you probably knew that.....
Also, some older cooling systems used to flow from the bottem to the top. Now they make the coolant hit the Head area first, when the coolant is at it's coolest right out of the radiator. (cars anyway) This has helped to a degree also.
Then their's the kidney shaped combustion chamber, and I'm sure you've heard about "swirl". Seems like I read something about getting the incoming gas/air to flow by the exhaust valve was a good idea because it would help bleed heat away from the valve. Just like sodium filled exhaust valves! They run cooler than solid valves and I noticed recently that they are being used again in new engines! (Ford used them back on mid-60's 427's but they didn't end up being strong enough for endurance racing I think so that idea's been dead for a long time, until recently)
Oh yeah, there's also the importance of a "hemi" type design. That helps too. The Kaw 7R (or so I heard) has a Hemi type design, and back in 2000 or so, Kawasaki took that head design and applied it to the 6R's, and I think the 9R's. (probably the 12 too, but I haven't read that specifically). There are more cars these days using the Hemi head design again, and I believe that ALSO allows for the ability to run slightly more compression with all other things being equal.
ALL THOSE THINGS ADD UP!
I don't recall anything about "squelch",.......do you perhaps mean "quench-area"?? Maybe it's the same thing?
I think in some slightly older designs, that the quench area was best described in a piston design that had the valve relief's on one side, on top, and large hunk of extra metal on the other side, on top. (maybe like a "domed" piston)
Well, that dome area would obviously be compressing things sooner than the 'other half' of the piston and as such, it would cause the air/fuel mixture to 'move' or 'swirl' (as it were) in the cylinder during the pistons' compression stroke.
I don't spend a lot of time these days looking at piston design's, but I don't recall having seen anything like I'm thinking about for a long, long time. I think we've moved beyond that design. It's all about an efficient combustion chamber these days.
that's my .06
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krexken
Zone Head
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posted March 17, 2002 02:54 PM
A lot of it has to do with cam timing. Long duration camshafts will generally let you get away with more compression. The long duration tends to bleed off cyclinder pressure where detonation is most likely to occur. The squish you refered to is the area usually around the outside of the chamber or the flat surface of the head that is part of the chamber. By reducing this area, or optimizing the gap between the piston and head there, more of the mixture can be better ignited and burned in the middle of the combustion chamber. I guess you're trying the squeeze most of the mixture into the center of the chamber where it can work more efficiently. The size of the bore has an affect on chamber effeciency. On a big bore motor like a big block Chevy, the flame has to travel much further than it does on a bike motor. Flame travel takes time and gives it more of a chance for detonating if not burned properly. You might want to go look at the Tech Article section on the Suzukihayabusa.org board. There's some serious gearheads hanging out there talking bout stuff like this.
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silver01bullit
Expert Class
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posted March 17, 2002 03:31 PM
The more dome you've got on a piston the more octane you're going to need. Like you said, combustion chamber shape and DOME SHAPE have a lot to do with it. The shape and size of the dome is going to ignite the mixture earlier, needing the higher octane. Digital fuel injection along with smaller domes and smaller combustion chambers allow US to control when the mixture detonates, not the piston dome. There's a million other things that play into this as well....
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frEEk
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posted March 17, 2002 05:38 PM
cool, i knew there'd be some people here with serious knowhow.
the improved cooling point makes alot of sense; simply keeps the intake heat down to help stay away from combustion temperature.
i think "quench" is right. but i dont get, is if u are actually trying to slow down the burn to avoid pinging, then wouldnt a big bore/spread out a/f mixture be a good thing? that way the flame actually has to travel. just as water injection helps an engine keep fomr exploding in crazy boost conditions. i understand buring faster, (or more to the point, buring while under higher pressure) increases the efficiency of the burn, but seems to me it would work against getting more compression out of a given fuel. course, i'f ur trying to add compression, then redusing compression by slowing down the burn... kinda defeats the purpose ne?
i suppose the most obvious issue in combustion chamber design would be to avoid hotspots, ie. keep the pressure and temperature within the chamber as even as possible, avoiding pinging/pre-detonation/whatever. but at the same time, its not that simple i'm sure, otherwise comming up with an optimum chamber design would be fairly simple, a straight mathimatical exercise. so what's the catch?
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Wildman
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posted March 17, 2002 07:36 PM
Well, in this area nothing is simple.
Definately, smaller bore size allows higher compression and/or lower octane without detonation.
You are *Not* trying to "slow down the burn". In fact, the faster you can combust the whole mix *without problems* the later you can spark it and the more power you can make.
The whole tumbling mixture thing is to get the flame front moving through the mixture faster.
So, (as I ramble along trying to make sense) If you have a slower burn. Then you have to fire the plug sooner. While the mix is burning, the piston is traveling up in the bore and the pressure is rapidly rising. This creates possibility of the mix igniting spontaneously in other places. Something about this, and I'm not sure if it's the meeting of the two fronts or some other thing, but anyway something can cause local explosions. (stuff moving faster than the speed of sound) These can scour the boundry layers of gas away from the metal surfaces. This is Very Bad. The metal cannot take the heat. Without the insulation of the boundry layer of gasses, it will rapidly fail. This is what causes the cutting torch like damage in engines.
Ummm, quench. It's kind of a trick that does two things. It's an area where the head and piston come very close together, basically as close as the designer can be sure they won't hit. It will push the gasses out of that area as the piston rises adding dramatically to the turbulence in the gasses (air/fuel mixture). And it effectivly makes a smaller combustion area rather like having a smaller piston for just a moment. I don't know if they are using it on modern bike engines, perhaps not. The chambers are not all that large anyway, and they can get a lot of turbulence with the intake design. Having high intake velocities can really help with turbulence in the chamber.
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frEEk
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posted March 17, 2002 10:06 PM
if this area were simple, we'd all be designing our own engines instead of just bolting on cans 
ah turbulence. that term is definitely batted around alot. i remember a year or 2 ago reading/hearing that some tuners were suggesting NOT polishing intakes so the rough surfaces could help create more turbulence. guess in direct injection engines turbulence is largely a mute point. not entirely mute, but close to it i suspect. which makes me think, why is direct injection not used in bikes yet? sportbikes seem to be pretty advanced compared to car engines most of the time, and cas are startin to use direct injection gas engines (in VERY limited models mind u, nto even sure if its available in NA yet). so why not bikes? that, and the double sparkplug thing. obviously going to help with faster burn there. again, i have heard of a model of car or 2 which use this, so why not popular w bikes (outside of racing perhaps)? i can imagine the VERY limited room in a bike's combustion chamber may be one of the reasons. what with all the values we're tryin to stuff in there, where the hell ya gonna sqeeze a second plug? i'm actually a bit surprised no-one has come out with a new standard plug size, one with like half the diameter. i would think it is possible.
anyway, i'm startin to degrade to the point of blabbering here. definitely past rambling already.
but one thought...if slowin down the burn with water injection (or natural gas/propane injection? thought i heard of this once) in a high boost turbo engine works to add power via allowing more boost, would adding water injection allow an otherwise unchanged 12r engine to run, say, 15:1 compression? and would that even add any power? wouldnt the power loss of a slower burn cancel out the gains from higher compression? hmmm....
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kcadby
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posted March 18, 2002 03:30 PM
Good thread frEEK...
The top of ZX12 pistons are pretty flat (thanks to small combustion chamber in head)...this helps in preventing hot spots on the pistons that create detonation BIG TIME...
The spark plug being in the center of the combustion chamber (unlike...off-set in a 2-valve motor) makes flame travel more even in the cylinder making it possible to use less timing (less timing...less chance of detonation)...
Cam duration makes a big difference in how much (compression) the motor makes at low RPMs (helps prevent detonation also). The ZX12 cams have WAY MORE duration than most (ALL???) cams in "normal production" applications!!!
That's also one of the reasons that newer (than 70-80s) CARS have more compression now...they ALSO have longer duration cams...
ZRXDean and I recently installed cams in his ZRX that had a lot more duration than what was already in it...it turned over SO much easier (faster) with the starter that...he thought he left the spark plugs out!!! (not realy but you get the idea???)... MUCH less "cranking compression"...
Once the engine is at higher rpms the "super charging" effect of the longer duration cams takes effect...air still rushes in even after the piston is on it's way back up on the compression stroke (IF the VELOCITY is high enough for the cams duration)...
All of these things (and many more) make it possible to use lower octane fuels)...
Y2K can (and has) told us about how the "newer fuels...resist detonation also"!!!
As for "polishing" ports...the air right up against the SURFACE...ISN'T moving like the air above the "boundry layer" so...polishing the SURFACE is a WAIST OF TIME...
A rougher surface does create some turbulence that helps "keep the gas suspended" in the air also...
If anything...polishing the exhaust port will slow the process of carbon building up some...and it DEFFINITELY makes it easier to "clean" (remove carbon) on a motor that is taken appart regularly (race aplications)...
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frEEk
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posted March 18, 2002 07:41 PM
thx kcadby, but its the people who respond who make it a good thread, not the one startin it
u bring up a couple VERY interesting points. but i dont think i see the complete picture of a couple of them.
cam duration: i certainly understand that having longer exhaust duration would help lower cranking effort & prolly help keep things cool by getting rid of more hot exhaust gases. i am however unsure about the intake duration. i do understand that benefit of allowing more time to pull air in (and that intake-stroke-plus thing is very cool!), but i dont understand how it helps with the octane situation, or lowering cranking effort for that matter. i can see that any vacuum created at the bottom of the intake stroke because of valves closing early would be a problem, but that effort would be cancelled out by the vaccuum being reversed in the beginning of the compression stroke.
polishing: i'm confused as to if u feel this is a good or bad thing. obviouslt good for carbon buildup (tho personally i have never seen carbon in the intake tract of any reasonably maintained car), but does it actually help power? i assume the purpose of polishing is to reduce air friction & thus help get more air into the cylinder, but does that really work? significantly in any case? i'm particularly confused by ur statement where u emphasise [sp] polishing of the SURFACE as being a waste of time. what else can u polish? (i am definitely a greenhorn in terms of engine tuning etc).
btw, don't wanna be pulling big long answers from ya if ya dont got time, so no worries if thats the case.
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Wildman
Expert Class
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posted March 19, 2002 05:33 AM
Freek, you said with direct injection turbulence might be a waste. No, it isn't. Picture a flame front traveling smoothly through the cylinder. It takes a certain finite (and not that fast) time for the flame to move. Ever seen a long trail of gasoline ignite?
Now picture that flat flame front is a piece of paper. If we crumple the paper up into a ball (turbulance) the flame front becomes effectively much larger in area and takes much less time to move though the whole volume of the cylinder.
more later....
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kcadby
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posted March 20, 2002 01:06 AM
 Edited By: kcadby on 20 Mar 2002 01:41
The intake valves stay open on MORE of the COMPRESSION Stroke (up-stroke AFTER the intake-down-stroke) when the intake cam has longer duration (the ZX12s intake cam closes the valves at 74* ABDC! that's 16-deg from HALF WAY UP!!!)...thus you will have "less CRANKING compression"...this is why you ALSO loose some low and mid-range power with longer duration cams...and once again...less compression = less chance of detonation
The inertia of the air rushing in (at higher revs) is what KEEPS filling the cylinder MORE...even as piston is comming back up on compression stroke with longer duration intake cams...
Another words it keeps filling the cylinder AFTER the "vacumm" has stopped...
If the intake ports are VERY rough then it will help to polish them I.E. polishing meaning...with 120-grit...NOT with super-fine sanding rolls...
The boundry layer will be there no matter how good (fine) you polish the surface so...that's why I say "fine" polishing is a waist of time
I was talking about "fine" polishing the exhaust ONLY for easier carbon removal...and...I don't even bother doing that anymore anyway...because my motors live too long for it to do any good 
And believe it or not...if you take ANY motor appart (or can see in the intake ports all the way to the valves) you WILL find "some carbon" around the back side of the intake valves and the intake port area close to the intake valves...this is because the intake valve starts to open BEFORE the piston is all the way up on the EXHAUST stroke (ZX12s intake valves open at 46* BTDC) longer duration means it open even sooner...
The ZX12s intake ports get "brown" pretty fast because of the super-long duration cams
If you take those covers on your airbox off and shine a flash light into the intake (open throttle) I am SURE you will see what I'm talking about
If you have a manual??? it even tells (and shows) you to check AND clean the inside of the throttle bodies every so often...Page 2-12!!!
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Wildman
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posted March 20, 2002 06:43 AM
More random thoughts
Ummm, you mentioned spark plugs. Specifically smaller plugs.
It's been done. the plugs being used these days are noticeably smaller than the ones in old school stuff. Smaller plugs=bigger valves.
Still, I think it might be difficult to make the plugs too small. The closer together the charged surfaces are (positive and negative, whatever you want to call them), the easier it is to foul the plug. I suppose that if you could assure a clean enviroment you might be able to make a functioning plug smaller, but you can't assure that in an engine.... Perhaps in a race only engine where it is dissasembled, cleaned, and checked after every race.? Does anyone know what they use for plugs in F1?
quote:
cam duration: i certainly understand that having longer exhaust duration would help lower cranking effort & prolly help keep things cool by getting rid of more hot exhaust gases. i am however unsure about the intake duration. i do understand that benefit of allowing more time to pull air in (and that intake-stroke-plus thing is very cool!), but i dont understand how it helps with the octane situation, or lowering cranking effort for that matter. i can see that any vacuum created at the bottom of the intake stroke because of valves closing early would be a problem, but that effort would be cancelled out by the vaccuum being reversed in the beginning of the compression stroke
Ummm, I'll try to explain some more of this.
When I learned the 4 cycle engine in school, we talked about the valves opening and closing at the ends of the various strokes. But they don't.
Cam timing is (like most things in engineering) a compromise. You can set cams up to make a lot of torque at low rpms with very little overlap. (Think Harley) This will keep intake gasses from escaping out the exhaust valve and from being pushed back out the intake valve. You will get higher cylinder pressure at low rpm's, and therefore more "effective compression". (not to be confused with the "compression ratio" which is just the volume at bdc divided by the volume at tdc. However, with this kind of cam arrangment, as rpms rise the engine becomes less able to fill the cylinder with enough fuel-air mix. Power drops off as engine speed rises. This is because of a couple of complicated things, but using the pistion to pump exhaust out and intake charge in becomes less effective at higher engine speeds.
Now, If we make a cam setup where the intake valve opens a while before the piston finishes the exhaust stroke and closes a while after the piston starts the compression stroke, at the same time, the exhaust valve opens a while before the piston finishes the power stroke and closes a while after the piston starts the intake stroke, ie lots of "overlap". Then we have an engine which makes little torque at low rpms, but more torque at high rpms and therefore more horsepower. This is a typical sportbike engine.
At low rpms, when the piston starts the compression stroke, some mix can be pushed out both the valves reducing effective compression and lowering the risk of detonation at the lower speeds where it is typically more problematic. (lugging the engine with high throttle and low rpms)
As the engine speed rises, this cam arrangment starts working a little like a 2 stroke engine. (more power!)
What I mean is this; When the exhaust valve opens, the piston is not all the way done with the power stroke, but most of the available work has been gotten out of that charge. The benefit of leaving the charge in for a little longer is outweighed by benefits of getting the valve open early. When they teach you the 4 stroke cycle in school, they tell you the piston pushes the exhaust out. That's not true. The exhaust is at high pressure and when the valve opens, it's own pressure pushes it out. This gets the exhaust moving fast and it develops inertia. Then, before the exhaust valve closes, the intake valve opens. It is not the piston moving down in the cylinder which causes the intake charge to come into the cylinder either. The exhaust's inertia creates a low pressure area (vacuum) in the cylinder which the intake charge rushes in to fill. This is why I say it's acting a little like a two stroke, it is not the piston's movement driving these gasses.
The inertia of the intake charge is used to have kind of a "supercharge" effect at higher rpms. The fuel air mix actually will fill the cylinder at a higher pressure than the outside pressure just from rushing in before the door (valve) is closed.
There's more of course. Anyway, to address the point. From the standpoint of filling the cylinder this arrangment is less effective at low rpms, yet the benefit is to allow higher compression ratio while having lower effective compression at low engine speeds where detonation is more of a problem.
Truly variable valve timing offers the promise of an engine with low end like a cruiser and high end like a built ZX12 in a smaller package. But the compression issue will have to be addressed. There is too much benefit to having a high compression for the engineers to be able to ignore it.
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frEEk
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posted March 20, 2002 09:26 AM
thx fer the expanded explanation guys. i actually already understood most of it, but a couple new points were rather interesting, particularly the pseudo 2-stroke operation.
as to polishing, i always assumed that it was just the main channel that was polished, where i didnt see much use. certainly polishing the backside of the valve & the area nearest the chamber would be useful, but i didnt think u could actually get in there to do that, other that using an extrude-hone machine in any case. polishing the exhaust side of things definitely makes sens. for some reason i always thought only intake was polished. leave it to me to only hear half the story told to me 
i like the reference to variable timing too. i have always been unsure of how much that would help, seeing as how u can just setup the cams for max top end power and leave it at that. admittedly, there is a very real use for adding lowend drivability while keeping good topend performance, but i never saw beyond that. but thinking about it now, since u can vary the valve timing, you don't have to sacrifice top end power in the interest of decent bottom end performance. so u actually end up with considerably higher top end power, as i have seen with a number of car engines, like the new tiburon (don't hit me!)
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Y2KZX12R
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posted March 20, 2002 12:22 PM
Freek! great topic!
What about using the second and third order pressure waves to boost compression on a wider rpm band?
And effective canceling out of out of phase waves?
Then theres artificial velocity enhancement using flow seperation at lower port velocity.
And volume density enhancements.
All these things enhance dynamic compression.
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Y2KZX12R
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Wildman
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posted March 20, 2002 05:07 PM
Well, now you'd be getting me in over my head.... Like I said, there's more.
As I understand it, various waves and vibrations are exploited by the engineers to help tweak things. Without a doubt, 2 stroke exhaust pipes are designed to use the reflected wave to help stuff charge back in before the piston closes off the port(s). I think some of that may be going on in 4 strokes also.
Airboxes are another big area where there is much more than meets the eye going on. Something about tuning the vibrations can help fill in flat spots in the torque curve IIRC.
This is beyond my understanding..... perhaps someone else can step up.
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frEEk
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posted March 20, 2002 07:17 PM
would you happen to have a visual representation of the pressure waves u'r talkin about y2k? i can imagine how they look actually, but i am somewhat curious where they come from? interaction between the individual intake tracts perhaps? i would think that with the rather short intake runner length on a bike (as compared to a car), pressure waves should be simpler. AS to density improvement, i was gonna say making the intake out of plastic (as is starting to happen) rather than aluminum to help keep that part of the system cool, but of course thats already the case on our bikes. and its so short anyway, as already mentioned. on my car, that big aluminum intake sites right above the headers. it gets hot as hell. no idea about flow inhancement. kinda like using narrower headers to increase exhaust gas velocity? i can see closin dow one of the intake valves at low rpm to keep the velocity up might help. this way u may get some of that supercharging effect even at lower rpms.
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kcadby
Pro
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posted March 21, 2002 12:47 AM
Put THIS in yer pipe and smoke it!!!
I have HEARD that the Formula 1 cars are working on using...SPEAKERS IN THE INTAKE to CREATE the PRESSURE WAVES when they WANT TO!!!  EEEEEEEEEEEEEEEEEEEEEEEEK!!!
Brings NEW meaning to...MY boom-box is BADDER than YERS!!!
UMMMmmm how many AMPS you got there bub??? LOL
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Wildman
Expert Class
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posted March 21, 2002 04:53 AM
Hehehe, That's the kind of shit you get when you have a money is no object, few rules kind of racing series...... I like those things.
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If you build something Idiot Proof, The world will build a better Idiot.
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Y2KZX12R
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posted March 21, 2002 05:55 AM
Kcadby, yea sound cancellation is getting alot of attention lately. I dont know very much about the application to engines. Its been tried in mufflers for several years strictly for noise control.
Get some software if you guys want to explore hard core engine tuning. The good stuff is expensive($300-$3000) but there is some cheeper stuff out there. Dyno 2000 is one for example. Its fairly acurate at 90% of its capability. It lacks some of the real tuning features in the big dollar software, but its fun. Dyno 2000 wont go down to the short stroke of the kawis but will work for Busa and gsxr1000's.
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Y2KZX12R
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frEEk
Administrator
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posted March 21, 2002 09:47 AM
oooooo....speakers to control pressure waves. man, thats why i love F1 over cart. just so much more tech. i know it makes the sport stupid expensive, but damn, do i like learning about all the cool stuff they stick in those engines. pretty amazing stuff. what i wouldnt do to stick an F1 engine in my car i'v actually heard of one or 2 custom cars somewhere that basically ddid that, granted it had to be detuned slightly so u didnt have to rebuild every time u went to the grocery store
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Y2KZX12R
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posted March 21, 2002 06:23 PM
yea its too bad about the CART and IRL deal. Really ruined a great class of racing.
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