Honda head covers
21 - 28 of 28 Posts
Ok now what I get it is with a street grind cam @ 48abc 10.4:1. If this is the case then is the trade off for bigger valve openings a more efficient way to make hp then higher compression? What the fuck? I guess this is where I should just do what you did cause it worked for you, but not knowing I think is sucky. The more math I do the more a stock cam makes sense at loudon.
Aaron
Aaron
Aaron,
Usually I only consider static compression, it's the defacto standard for determining engine compression. Too many variables and unknowns to get a good fix on dynamic compression.
So many considerations of cam timing, gas velocities, intake and exhaust dimensions and behaviour.
As for valve size...the bottom line is an engine is an air pump, the more air you can get in and out (which carries more fuel) the more power you make. In my case the larger valves are the result of computer flow bench testing that showed the valves were the main restriction point for air flow.
I'm sure there are theoritical sweet spots for intake track diameter (gas velocity), valve size and cam timing...but for now I stick with the known routes to improved performance. Sorting out optimum gas velocities and such will provide improvements I'm sure, but not until the fundemental weaknesses of the engine are addressed, such as air flow and compression ratio. As engine development goes we are only a fraction of the way to the end game. Tweaking gas velocity for greater than 100% volumeteric efficiency with intake tract size and exhaust pipe size are just that...tweaks...first we address the major factors that are limiting our hp. Primarily inlet air flow, compression and displacement.
I'll review the formulas for dynamic compression and see what I come up with, but that's a guessing game I think at this point. Real figures would requre a lot of data on intake charge behaviour, combustion chamber dynamics, cam timing, and exhaust system simulation. etc.
JohnnyB
Usually I only consider static compression, it's the defacto standard for determining engine compression. Too many variables and unknowns to get a good fix on dynamic compression.
So many considerations of cam timing, gas velocities, intake and exhaust dimensions and behaviour.
As for valve size...the bottom line is an engine is an air pump, the more air you can get in and out (which carries more fuel) the more power you make. In my case the larger valves are the result of computer flow bench testing that showed the valves were the main restriction point for air flow.
I'm sure there are theoritical sweet spots for intake track diameter (gas velocity), valve size and cam timing...but for now I stick with the known routes to improved performance. Sorting out optimum gas velocities and such will provide improvements I'm sure, but not until the fundemental weaknesses of the engine are addressed, such as air flow and compression ratio. As engine development goes we are only a fraction of the way to the end game. Tweaking gas velocity for greater than 100% volumeteric efficiency with intake tract size and exhaust pipe size are just that...tweaks...first we address the major factors that are limiting our hp. Primarily inlet air flow, compression and displacement.
I'll review the formulas for dynamic compression and see what I come up with, but that's a guessing game I think at this point. Real figures would requre a lot of data on intake charge behaviour, combustion chamber dynamics, cam timing, and exhaust system simulation. etc.
JohnnyB
Joined
·
331 Posts
man do I think not knowing is sucky too !
http://www.kennedysdynotune.com/Dynamic Compression Tech.htm
I feel like I'm reading russian sometimes...but thanks for forcing me to learn.
J "not that" D
http://www.kennedysdynotune.com/Dynamic Compression Tech.htm
I feel like I'm reading russian sometimes...but thanks for forcing me to learn.
J "not that" D
Sorry when I said bigger valve openings I meant the duration the valve was open, so longer durations in relation to the degrees the valves open and close. The way I'm reading this is that real compression is at intake valve close @ abdc. If this is the case then less degrees would be btter. Obviously something isn't coming out right but I must get to the bottom of this. I have a feeling it will be more like you'll get to the bottom and then I'll go, "Ohhh.... right... thanks".
Aaron
Aaron
Also if that formula is correct then essentially you can up your compression by making so much valve clearance that your intake never opens...
Aaron
Aaron
Well now you've kind of hit on the rub of DCR.
I'm sure you've heard that compression doesn't make hp it make torque. Ever notice that stock cams make more torque? Now notice that stock cam intake timing closes the valve sooner....increasing DCR...which increases torque....you end up with a torquier cam.
Ok...now, problem is we need to get air inside the engine too...which means high valve lift, long overlap...intake valve stays open longer....reduced DCR. So...how to we keep DCR up and still keep the intake valve open long enough to get the air we want inside the cylinder? Well....we increase Static compression ratio (SCR). Static compression ratio is a factor in calculating DCR so if we can get SCR higher...then we can run longer valve timing and keep DCR high.
The problem with "torque" cams that close the intake valve quickly is that at high rpm, where you want to produce HP as opposed to torque the cam timing is preventing enough air from getting into the cylinder. As with most things it's a compromise. Yes you can run the stock cam for greater torque...but it will limit rpm...because of reduced intake flow...thereby limiting max hp.
In the case of our engines the ideal would be to change cams (or maybe cam timing) for individual tracks. A stock cam might perform well at Loudon, a 122-20 at Mosport, a 122x4 at Bonneville Salt Flats.
If we can increase cam timing and close the intake sooner we might get more torque....decrease cam timing and keep open later maybe more max hp...tough to tell when you are moving the the exhaust valve timing and intake open timing at the same time.
This is why turbo and surpercharger engines can get away with shorter cam timing and still make hp....they can close the intake valve sooner for torque...and still stuff in the air they need with boost.
JohnnyB
PS. I'm pretty sure DCR above about 8.5:1 starts getting dangerous on naturally aspirated vintage engines. As DCR goes up combustion chamber temp goes way up.
Edited by - jbranson on May 04 2005 11:45:04 PM
I'm sure you've heard that compression doesn't make hp it make torque. Ever notice that stock cams make more torque? Now notice that stock cam intake timing closes the valve sooner....increasing DCR...which increases torque....you end up with a torquier cam.
Ok...now, problem is we need to get air inside the engine too...which means high valve lift, long overlap...intake valve stays open longer....reduced DCR. So...how to we keep DCR up and still keep the intake valve open long enough to get the air we want inside the cylinder? Well....we increase Static compression ratio (SCR). Static compression ratio is a factor in calculating DCR so if we can get SCR higher...then we can run longer valve timing and keep DCR high.
The problem with "torque" cams that close the intake valve quickly is that at high rpm, where you want to produce HP as opposed to torque the cam timing is preventing enough air from getting into the cylinder. As with most things it's a compromise. Yes you can run the stock cam for greater torque...but it will limit rpm...because of reduced intake flow...thereby limiting max hp.
In the case of our engines the ideal would be to change cams (or maybe cam timing) for individual tracks. A stock cam might perform well at Loudon, a 122-20 at Mosport, a 122x4 at Bonneville Salt Flats.
If we can increase cam timing and close the intake sooner we might get more torque....decrease cam timing and keep open later maybe more max hp...tough to tell when you are moving the the exhaust valve timing and intake open timing at the same time.
This is why turbo and surpercharger engines can get away with shorter cam timing and still make hp....they can close the intake valve sooner for torque...and still stuff in the air they need with boost.
JohnnyB
PS. I'm pretty sure DCR above about 8.5:1 starts getting dangerous on naturally aspirated vintage engines. As DCR goes up combustion chamber temp goes way up.
Edited by - jbranson on May 04 2005 11:45:04 PM
ok, that does help.
That was just enough, and at a level I can get. Especially with previous conversations about making and making torque for the 175.
Aaron
That was just enough, and at a level I can get. Especially with previous conversations about making and making torque for the 175.
Aaron
21 - 28 of 28 Posts
- This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Cafe Racer Forum
A forum community dedicated to Cafe Racer style race bikes owners and enthusiasts. Come join the discussion about restoration, performance, racing, classifieds, troubleshooting, maintenance, and more!
Full Forum Listing
Explore Our Forums
Top Contributors this Month
View All
TrialsRider
32 Replies
Youtube_Zombie
30 Replies
dirkchecken
14 Replies
Recommended Communities
Join now to ask and comment!
