On an n/a twincam engine with top end power in mind what's the consensus on advancing and retarding the in/exh cams?
Let's take a pair of 300* cams as an example. Currently timed with a 110* full lift position to open / close at: 40/80 80/40.
Actually since I asked:
Top end?
Bottom end?
Midrange?
I know there are other factors involved and power curves will overlap but what are the theoretical rules and reasoning ?
Twin cam timing
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user-23911
Re: Twin cam timing
For a start...."full lift timing" means nothing.
They won't be symmetrical cams so to start with you'll need to take some measurements........ICL and ECL.......it's half way between the opening and closing points, not related to max lift.
Measure at zero lash
Again at 0.5 mm lift
Again at 1 mm lift.
A guess.....
115 for intake
111 deg for exhaust
But it's something that needs testing on an engine dyno to get the most out of.
They won't be symmetrical cams so to start with you'll need to take some measurements........ICL and ECL.......it's half way between the opening and closing points, not related to max lift.
Measure at zero lash
Again at 0.5 mm lift
Again at 1 mm lift.
A guess.....
115 for intake
111 deg for exhaust
But it's something that needs testing on an engine dyno to get the most out of.
Re: Twin cam timing
You will need to move each cam, in each direction, and see what works best. There's too many variables, and you didn't give enough info, for a rule of thumb.AC sports wrote: ↑Mon Aug 06, 2018 10:41 pm On an n/a twincam engine with top end power in mind what's the consensus on advancing and retarding the in/exh cams?
Let's take a pair of 300* cams as an example. Currently timed with a 110* full lift position to open / close at: 40/80 80/40.
Actually since I asked:
Top end?
Bottom end?
Midrange?
I know there are other factors involved and power curves will overlap but what are the theoretical rules and reasoning ?
Is it a 2 valve or 4 valve ?
Are the ports big for the application, or small ?
What valve sizes ?
When IndyCar was running 4L N/A V8's, we saw the most top-end power, running 98 ICL and 110 ECL.
Mike Jones
Jones Cam Designs
Denver, NC
jonescams@bellsouth.net
http://www.jonescams.com
Jones Cam Designs' HotPass Vendors Forum: viewforum.php?f=44
(704)489-2449
Jones Cam Designs
Denver, NC
jonescams@bellsouth.net
http://www.jonescams.com
Jones Cam Designs' HotPass Vendors Forum: viewforum.php?f=44
(704)489-2449
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mekilljoydammit
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Re: Twin cam timing
How long is a piece of string? Looking at some examples of relatively developed stuff, big Cosworth BDA stuff seems to be around 100 intake, 103-106 exhaust centerline, old TRD Toyota Atlantic specs I'm seeing listed as 102/102, newer stuff maybe advancing the intake to 98, some Honda B-series stuff a bit wider, some Nissan SR20VE stuff running the exhaust around 98 degrees centerline, Mike just mentioned a professional racing example where exhaust went even wider... test it.
Re: Twin cam timing
Surely "300 degrees" is advertised duration (seat-to-seat), not at 1mm lift which is a more useful number.
With DOHC-4V motorcycle engines, 105 degrees is generally a reasonable first guess. If intake and exhaust durations are significantly different, arranging for the intake and exhaust valves to be open the same amount at TDC (with the actual lobe centerlines split around 105, e.g. 106-104 or 107-103) is another reasonable first guess. Piston-to-valve clearance needs to be checked and may dictate the approach used.
That's the point where you start dyno testing to see what actually happens. It is unlikely that the optimum for whatever you are looking for will be more than a few degrees different than this.
The bike engines all use a separate throttle per cylinder. That limits exhaust reversion due to excessive overlap (and the resulting part-throttle misfiring, rough running, and driveability problems); superbike engines tickover like a (big loud) sewing machine. In roadracing, having good part-throttle driveability is extremely important. If your engine (which you haven't told us what it is!) has one common throttle to all cylinders and part-throttle driveability is of any concern then this may limit how much overlap you can have.
With DOHC-4V motorcycle engines, 105 degrees is generally a reasonable first guess. If intake and exhaust durations are significantly different, arranging for the intake and exhaust valves to be open the same amount at TDC (with the actual lobe centerlines split around 105, e.g. 106-104 or 107-103) is another reasonable first guess. Piston-to-valve clearance needs to be checked and may dictate the approach used.
That's the point where you start dyno testing to see what actually happens. It is unlikely that the optimum for whatever you are looking for will be more than a few degrees different than this.
The bike engines all use a separate throttle per cylinder. That limits exhaust reversion due to excessive overlap (and the resulting part-throttle misfiring, rough running, and driveability problems); superbike engines tickover like a (big loud) sewing machine. In roadracing, having good part-throttle driveability is extremely important. If your engine (which you haven't told us what it is!) has one common throttle to all cylinders and part-throttle driveability is of any concern then this may limit how much overlap you can have.
Re: Twin cam timing
I work on single cylinder 4v atv engines that peak around 9500-10,000 rpm. My personal engine is 490cc, 96mm x 67.6mm with 37/31mm valves. I have a 42.5mm slide carb, and the exhaust is a step header leaving the head at about 1.6" od. I've got a couple cam profiles I like.
If throttle response is important, 252 @ .050 .427" lift 112ish CL on the intake side, and 262 @ .050, .4" lift 105ish CL.
If top end hp is more important, 262 @ .050 .430" lift 112ish CL intake and the same exhaust as above.
A few things I've found while playing with cam timing:
Advancing the intake hurt low end more than anything else I did, but it could benefit in the mid, but it would also fall off earlier. Retarding make the power more linear at the cost of some peak torque. It picked up at a lower rpm, and it pulled to a higher rpm. I landed at 112-114 ICL by balancing the effects in a trial and error fashion.
The exhaust side was all together different. I could pick up a bunch of torque by retarding the exhaust cam through the entire usable rpm range, but it would start to hurt top end if I get down to 104 ECL or so. If I advanced it to a 110 or 112 ECL, it would help top end, but it gave up too much torque for my usage. I usually fall around 105 or 106ECL is where trial and error found it to be happy.
I'm about a 285 lb guy racing a bunch of 180 lb guys, so I find all the torque I can make without stunting the top end too much.
If throttle response is important, 252 @ .050 .427" lift 112ish CL on the intake side, and 262 @ .050, .4" lift 105ish CL.
If top end hp is more important, 262 @ .050 .430" lift 112ish CL intake and the same exhaust as above.
A few things I've found while playing with cam timing:
Advancing the intake hurt low end more than anything else I did, but it could benefit in the mid, but it would also fall off earlier. Retarding make the power more linear at the cost of some peak torque. It picked up at a lower rpm, and it pulled to a higher rpm. I landed at 112-114 ICL by balancing the effects in a trial and error fashion.
The exhaust side was all together different. I could pick up a bunch of torque by retarding the exhaust cam through the entire usable rpm range, but it would start to hurt top end if I get down to 104 ECL or so. If I advanced it to a 110 or 112 ECL, it would help top end, but it gave up too much torque for my usage. I usually fall around 105 or 106ECL is where trial and error found it to be happy.
I'm about a 285 lb guy racing a bunch of 180 lb guys, so I find all the torque I can make without stunting the top end too much.