cstraub wrote:
It requires lesss camshaft. On the circle stuff if they are running multiple tracks I will choose lobe area that is an average and then give the engine builder 2 lash settings.
It wouldn't require less camshaft assuming both tracks were the same. With a set bore/stroke/comp, un-ported 2 plane intake, and a 4412 carb, the cylinder head flow changing a few cfm wouldn't change optimum duration. For starters an apparent 18cfm gain would turn into 6-8 cfm (maybe less) when you bolt the un-ported 2 plane intake and 2 barrel carb on it. The 2 barrel carb is a much larger restriction to power than it's apparent cfm loss on a flow bench, and that part didn't change. Just for a look, I ran engine pro with a 215 cfm head and found the duration that made the most power, then added flow (cfm) in the program until the optimum duration changed. It needed 260 cfm before the program added 2* more duration. Within the limits of a given head it would take massive amounts of grinding and much different size valves before the cam requirements change.
And, without knowing how that cfm was gained, you wouldn't know if it needed more time area. Maybe the gain was with a bigger valve and it already has more time/window area. Maybe it was gained with steep seats and has a bunch less time/window area.
cstraub wrote:
Based on flow numbers I look at the the cam as can a few others and give the customer a lash setting to tray before they have to buy anything. You see and engnine with an I/E ratio of 60% and the customer has a single pattern cam you can quickly see the engine needs split. Have the customer loosen intake and tighten exhaust and the engine will respond much differently.
Based on what flow numbers? The apparent 60% ratio from flow with no intake bolted to the head, but with a short pipe on the ex port? Is that what it flows on the running engine? I've seen an intake take 40 cfm off a 300 cfm head. This was a head with published flow of 315 cfm. So now what's the in to ex ratio?
I've seen a short pipe on the ex pick up 20 cfm, but when I bolt the actual headers on the flow was less than the open port. So now what's the in/ex ratio?
I've also seen smaller headers flow less when bolted to the ex ports (yes I've flowed ex ports with actual chassis headers bolted on) yet that worse flowing smaller pipe ran faster at the track. So now what's the in/ex ratio? How good is that smaller pipe at exhausting the engine vs it's measured ability based on flow bench cfm?
Then there is the differences in flow benches. They vary 15 cfm or more. 2 heads could flow the same, yet one guy calls the cam company with a flow sheet reading 15 cfm more. Which guy gets the right cam? AFR heads are down on cfm on my bench from published numbers, so which flow numbers do you cam it to?
Then there is the issue of efficiency? Just flow doesn't tell you how good a port is, or what size it is, or how good it is at feeding the cylinder at and after BDC, or how much power potential it has. I've seen some 23* heads make 2.5 HP per cfm, and others make 1.8 HP per cfm or less on the same cubes/comp etc. Which one fills the cylinder the best, and which one needs more/less cam???
I've gained 8-10 cfm on an engine that had several cams tested in it before the port work. Then re-tested with more cams/lash/cam positions, and the trends were the same. The additional port work didn't change what the engine wanted cause the overall geometry/cubes/comp/etc didn't change.
When someone calls a Cam guy and asks for a cam, they'd be better off asking where do you want peak TQ and peak HP, and more importantly how good is your cylinder head guy at hitting that rpm and what kinda VE/power levels have they obtained with similar combs in the past?
Me, I don't agree with flow based cam specing,
But,.. everyone can keep calling their cam guys and bragging about flow numbers if you want. LOL
Randy
