Flow test lift increments

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Flow test lift increments

Post by MichaelMoore » July 10th, 2011, 3:59 am

Guy, here in the States lifts on a flow test generally start at .050" and then use that number as the increment for higher lifts. On my small vintage motorcycle heads (roughly 100cc/cylinder 2v) I've been starting at .050" and then incrementing with .025" lift, as max lift on the available cams is .250-.280" and .050" didn't give many data points in that range.

In the book and video it looks like you instead start at something like 5% of the valve diameter and increment with that percentage until you get to the maximum cam lift (or slightly more) that you might have available.

This had me puzzled for a bit wondering "why's he starting with that odd number and using those "not a round number" increments?" until it dawned on me that you might be using percentages.

I'm curious how you decided to use that standard and what you see the advantges are vs set numbers (whether .050" or mm's). It seems like you might be doing percentages so that you can do ratios between different engines/different valve sizes, looking at lift percentage/valve size/flow/displacement and getting something like a relative discharge coefficient so you can judge if you are getting reasonable increases out of any particular valve/engine.

But if you started with a valve of one size in the head and test with increments based on it I'd think you'd want to keep using those increments even if you bumped the valve diameter up significantly, so that you've got numbers you can compare to the original spec. I suppose that if you wanted the "comparable across all engines" data once you'd gotten percentage improvements you were happy with from the big valve you might then do a final test with percentages based on the new valve diameter.

Since I'm just starting out with flow testing seeing your different numbers made me stop and think, as I'd not seen anything but .050" increments and had presumed that there must be a good reason everyone seemed to go with that. With small valves/small lifts it makes sense to use smaller increments and your percentage method (if indeed that is what you are doing) automatically adjusts for those changes in the size of the parts.

thank you,
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Re: Flow test lift increments

Post by Daveyboy » July 11th, 2011, 7:55 am

I would be interested to know how lift increments were dedcided upon too as I also noticed that the lift increments were not the same for say head X and head Y. Although they often tended to end up at max lift around 12mm the lower lift heights seemed to vary.
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Guy Croft
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Re: Flow test lift increments

Post by Guy Croft » July 11th, 2011, 8:24 am

The Superflow manual suggested using lift/valve diameter (L/D) to determine the lift points so I just started using it. It doesn't matter at all what increments you use.

And when you get close to peak valve-in flow (or know you've gone past it because the flow doesn't go up anymore) it's worth deviating from the basic L/D structure and putting your own settings in to find the exact peak flow point.


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