DON'T UNDERSTAND COMPRESSION
- Thread starter 99'busa
- Start date
There's more to the setup than just the static compression numbers. The actual compression ratio can be changed by cam shaft timing and profiles. I think you'll need to provide more details about what you are trying to achieve.
you can change "effective" compression by these but the compression ratio is a "mechanical" figure..
Total physical volume of combustion chamber with piston at TDC vs volume with piston at BDC..
The compression on the other hand can be changed with cam profiles, cam timing or anything else that can physically alter air volume (water injection for instance)
What kind of racing are you intending? IME if you are going to get serious about any racing, the machine needs to be dedicated to such.. Riding your racer on the street just uses up machine and tends to get you in trouble.
For the weekend warrior or saturday night drags, leave the thing stock for the most part.. Get your pipes, PC3, wheels and other "street friendly" mods and leave the motor alone. Reliable is a great thing for a toy IMO
EDIT: Turbo pistons lower the mechanical compression drastically so you can pack more air in the motor.. without a turbo, the bike performance will be AWFUL....
thanks mr bogus
im not doing any serious racing but a little 1/4 mi. every now and then. i have to get some engine work done anyway so im going with 385/385 cams carillo a beams je pistons 84mm bore. so what your saying is it doesnt matter much which compression ratio pistons i go with. my mechanic i know can help me with this i just wanted to have a little more understanding about it before i asked him which ones he's actually getting me. i'd rather get some base knowledge about it first
im not doing any serious racing but a little 1/4 mi. every now and then. i have to get some engine work done anyway so im going with 385/385 cams carillo a beams je pistons 84mm bore. so what your saying is it doesnt matter much which compression ratio pistons i go with. my mechanic i know can help me with this i just wanted to have a little more understanding about it before i asked him which ones he's actually getting me. i'd rather get some base knowledge about it first
No actually compression ratio is very important..
However, there are a number of things that happen when you start going up on bore, compression and power in general..
Engine life and reliability typically go down.. more power = more wear and tear..
You need to decide a couple things before you ever touch (or have someone else) the motor..
Just how fast do you want to go.. most any reliable builder is going to be able to quote combinations and hp specs before touching the thing.. Unless you have unlimited resources, do not even bother trying to come up with these combo's.
Cam/piston/head/pipe combos are like a puzzle.. they typically only work correctly when properly matched. This takes time, money and experience..
Budget is the other big issue.. there is the "one time" expense initially to build the thing, but it costs money to keep it running reliably. There are no shortcuts... That is why if you build a full on race motor, you do not want it just picking up miles on the street going point A to point B.. just wears it out faster..
Talk to some of the board sponsors about your ideas.. (keep in mind that they are not going to just "give" you combinations to engines etc, it takes time and money to get these setups dialed in and so you are saving money buy buying their knowledge)
They can give you approximate power levels and engine longevity numbers.. Personally? I would keep it basic to start out with.. you may find out you do not like racing (yea right) or find out the bike is fast enough to keep you happy now (and it will be ultra reliable in stock trim for the most part)
Professor could also give you some good ideas where to start on your program as well... (there is a lot more to racing than the bike.. trailer, tools, support equipment, tow rig, and all the hundreds of little details that seem to cost $100 each)
However, there are a number of things that happen when you start going up on bore, compression and power in general..
Engine life and reliability typically go down.. more power = more wear and tear..
You need to decide a couple things before you ever touch (or have someone else) the motor..
Just how fast do you want to go.. most any reliable builder is going to be able to quote combinations and hp specs before touching the thing.. Unless you have unlimited resources, do not even bother trying to come up with these combo's.
Cam/piston/head/pipe combos are like a puzzle.. they typically only work correctly when properly matched. This takes time, money and experience..
Budget is the other big issue.. there is the "one time" expense initially to build the thing, but it costs money to keep it running reliably. There are no shortcuts... That is why if you build a full on race motor, you do not want it just picking up miles on the street going point A to point B.. just wears it out faster..
Talk to some of the board sponsors about your ideas.. (keep in mind that they are not going to just "give" you combinations to engines etc, it takes time and money to get these setups dialed in and so you are saving money buy buying their knowledge)
They can give you approximate power levels and engine longevity numbers.. Personally? I would keep it basic to start out with.. you may find out you do not like racing (yea right) or find out the bike is fast enough to keep you happy now (and it will be ultra reliable in stock trim for the most part)
Professor could also give you some good ideas where to start on your program as well... (there is a lot more to racing than the bike.. trailer, tools, support equipment, tow rig, and all the hundreds of little details that seem to cost $100 each)
Good info.
Hey make sure you update us on what you decide and how it turns out... a lot of guys have the same question and it is always interesting how they end up making their decision..
My shop in Scottsdale did a lot of performance work and we were always trying to cut a compromise between "Go Fast" and "Go Reliable".. not to mention these were typically street machines that had to meet IM240 emission regs... One thing about Indiana, no smog checks or emissions tests.. (eat your heart out boys)
My shop in Scottsdale did a lot of performance work and we were always trying to cut a compromise between "Go Fast" and "Go Reliable".. not to mention these were typically street machines that had to meet IM240 emission regs... One thing about Indiana, no smog checks or emissions tests.. (eat your heart out boys)
mr bogus this question probably goes on another topic but you now know my mods im doing and at the same time im switching my front sprocket. right now its im running 15/43 (what was already on bike) was thinking of either 16 or 17 but i got use to the torque of the 15 and liked it and am afraid of going with the 17 and being disappointed even after the extra 30hp mods. or in your apinion am i worried about nothing and even going up 2 teeth in the front i will still feel the extra hp.
that would be a bit out of the realm of my experience on these bikes.. Professor would certainly know that better than most here.
I am sort of old school racer, I prefer to make one change at a time...IE engine/gearing/fuel/tires etc.. maybe my mind is getting tired but trying to track improvements and where exactly they came from is pretty important IMO.
The one thing that I find odd is that you can only get through 3rd gear in stock configuration. It would seem that the gearing issues are deeper than just sprocket/countersprocket.. It would seem that you could accelerate quicker if you could keep the rpm's in a narrower band.
We have gone to a lot of trouble on drag racers to get an extra gear shift (4 or 5 verses 3) I would myself be curious if any are working with actual gear ratios to keep these motors in the powerband a bit better or they are able to do this simply with final drive ratios (sprocket ratio)
So Rick, what do you say?
I am sort of old school racer, I prefer to make one change at a time...IE engine/gearing/fuel/tires etc.. maybe my mind is getting tired but trying to track improvements and where exactly they came from is pretty important IMO.
The one thing that I find odd is that you can only get through 3rd gear in stock configuration. It would seem that the gearing issues are deeper than just sprocket/countersprocket.. It would seem that you could accelerate quicker if you could keep the rpm's in a narrower band.
We have gone to a lot of trouble on drag racers to get an extra gear shift (4 or 5 verses 3) I would myself be curious if any are working with actual gear ratios to keep these motors in the powerband a bit better or they are able to do this simply with final drive ratios (sprocket ratio)
So Rick, what do you say?
It is all about a combination. I don't think you are going to like drag racing with a 17 front sprocket unless you go to a 45 or so on the rear.
I personally would stay away from a 15 front sprocket because of the extra stress on the chain. Some say the same about the 16.
I think you once were trying to avoid shifting into 4th. IMO you need to at least be in 4th. With your mods you may need to be in 5th in the finish"¦. This is based on ¼ mile.
This might help explain static compression ratio to you..
http://www.rbracing-rsr.com/compstaticcalc.html
Picture how much liquid this will hold with the piston at the bottom. Then picture how much it will hold at the top. If it hold 10oz of water at the bottom and 1oz of water at the top then it is a 10:1 ratio. Now when you are talking about the air/fuel mixture in the same place, you are squeezing 10oz to 1oz"¦. so 10:1 compression ratio.
Valve opening and closing can come into play with dynamic compression/effective compression but it will not affect what the static compression ratio is.
I personally would stay away from a 15 front sprocket because of the extra stress on the chain. Some say the same about the 16.
I think you once were trying to avoid shifting into 4th. IMO you need to at least be in 4th. With your mods you may need to be in 5th in the finish"¦. This is based on ¼ mile.
This might help explain static compression ratio to you..
http://www.rbracing-rsr.com/compstaticcalc.html
Picture how much liquid this will hold with the piston at the bottom. Then picture how much it will hold at the top. If it hold 10oz of water at the bottom and 1oz of water at the top then it is a 10:1 ratio. Now when you are talking about the air/fuel mixture in the same place, you are squeezing 10oz to 1oz"¦. so 10:1 compression ratio.
Valve opening and closing can come into play with dynamic compression/effective compression but it will not affect what the static compression ratio is.
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