Cam clearance Fiat vs aftermarket
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Piero
Cam clearance Fiat vs aftermarket
Why are the Fiat cam clearances quoted greater than aftermarket competition cams?
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Guy Croft
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It was always considered - in the mainstream engine industry - that very low valve lift can be ignored.
This assumption is valid to a point, saying that the air and exhaust flow at lifts below 0.5mm (or so) is so modest that, in terms of the camshaft duration, they can effectively be ignored. This method of presentation of 'effective' duration with wide clearance makes the characteristic (on paper) look shorter than it actually is at running clearance.
However, more lately, software analysis is tending to show that, due the complex behaviour of pressure waves, even very low lifts have a place in the scheme of things.
Aftermarket cams - well, it all depends how the manufacturer decides to present the information. The quoted duration of course depends on the running clearance at which is presented. Some quote it at running clearance, some at zero. For my own part I'm not remotely interested in duration as an arbiter of how well a cam works. I'm only interested in the lift integral (area under the lift vs degree curve), lift at TDC, dwell at TDC.
This assumption is valid to a point, saying that the air and exhaust flow at lifts below 0.5mm (or so) is so modest that, in terms of the camshaft duration, they can effectively be ignored. This method of presentation of 'effective' duration with wide clearance makes the characteristic (on paper) look shorter than it actually is at running clearance.
However, more lately, software analysis is tending to show that, due the complex behaviour of pressure waves, even very low lifts have a place in the scheme of things.
Aftermarket cams - well, it all depends how the manufacturer decides to present the information. The quoted duration of course depends on the running clearance at which is presented. Some quote it at running clearance, some at zero. For my own part I'm not remotely interested in duration as an arbiter of how well a cam works. I'm only interested in the lift integral (area under the lift vs degree curve), lift at TDC, dwell at TDC.
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Piero
Thanks once again Guy.
The reason I ask, is that I have found some one willing to make me a set of cam's with distributor drive already on ex cam.
There lift is, (I feel a bit to much) @ 11.4. with a clearance of 00.4
If I was run the cam at Fiat 00.8 clearances, I would end up with a true lift of 10.6 (stop on to what I want)
I cannot quite work out how it will affect my duration (in terms of actual degrees lost) or any other knock off effects for that matter.
The reason I ask, is that I have found some one willing to make me a set of cam's with distributor drive already on ex cam.
There lift is, (I feel a bit to much) @ 11.4. with a clearance of 00.4
If I was run the cam at Fiat 00.8 clearances, I would end up with a true lift of 10.6 (stop on to what I want)
I cannot quite work out how it will affect my duration (in terms of actual degrees lost) or any other knock off effects for that matter.
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Guy Croft
- Site Admin
- Posts: 5039
- Joined: June 18th, 2006, 9:31 am
- Location: Bedford, UK
- Contact:
You might need another cam, you should not open up the running clearance to reduce the lift, if the maker says 0.4mm (16 thou") that's almost certainly what it needs (usually +/-) 0.05mm (2 thou").
If you run a wider clearance it can wear out the cam quite quickly, because the bucket misses the low rate opening and closing ramps and skips straight onto -and off - the cam flank.
You'll have to walk a long way to get much more than bare functional info about anyone's cams by the way, it's all rather secretive.
GC
If you run a wider clearance it can wear out the cam quite quickly, because the bucket misses the low rate opening and closing ramps and skips straight onto -and off - the cam flank.
You'll have to walk a long way to get much more than bare functional info about anyone's cams by the way, it's all rather secretive.
GC
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