Hello, Can someone please tell me at what cam lift does the stock port flow stop increasing? Also the same question with the larger 194/155 valves. Sincerely, David
May I ask why the question is phrased as it is? I sense some misunderstanding behind the question. If your answer sought is as literal as it's written, some easy Google and forum searching will answer that for you.
one set that was tested it stop at .520 lift. with only a few cfm from .500 to.520. not sure on the bigger valves but i would think very little if there was no port work done to go along with the bigger valves.
Flow at max lift isn't the important factor. A higher lift also means the valve is open LONGER at lower lifts where there is good flow. Commonly referred to as 'area under the curve'.
I haven't seen flow increase by only putting larger valves in without port work, but I refrain from time wasting and unproductive practices. The inverse is almost always true IME, with a smaller valve set onto a VJ not fully out to the diameter of the larger valve, due to the valve making an excellent door. The VJ's seat angle being 'almost' all the way out to the OD gives a great ratio of throat diameter and maximizes throat ID, which is often the limiter to velocity. I guess there could be circumstances otherwise observed and my flowbench time doesn't earn money like that per se... Engines not only make more power with increased degrees of rotation at or past peak flow, but are more responsive due to the notion that a slightly undersized port works much better than one sized bigger for high rpm hp.
So you are saying the throat should be opened up to the valve job cut? Since valve jobs vary, is there an ideal percentage of valve size to throat size for a mild Nailhead? I've also read the throat shouldn't be opened up too far.... as that reduces the venturi effect. Would be nice to have general guidelines for us DIY porters..... at least until Bob starts providing the service on a regular basis! If I may, I believe David's goal is to choose a cam for a 'very' good street build to run on premium gas with 3.08 to 3.42 gears. Joe has probably found some of the best combinations using cams with fast ramps and durations in the 220's.
Ooh, thanks for that catch! I was not stating that the throat should be opened up to the max, or obliterating any angles. Generally speaking, bringing the bowl out to the bottom of the 60* angle is tough to hurt anything. I was stating that whatever the valve job is...and picturing here a 3 angle insert commonly used...that there would be no benefit to having the valve's OD overhang the sealing angle's OD by much, just a few thousandths is enough. The whole she-bang being moved outward is what helps the relationship of valve OD/ port throat. An example I see a lot of is the valve's OD being greater than the seat's OD by .075-.100" or more does nothing to help, such as setting a 1.940" valve on a 1.875" or 1.900" seat would probably offer zero benefit. I see it enough to warrant a comment and I doubt is many ever check that. Ink left on a valve is enough to see what you need to see. I was not commenting to whatever gets decided between the machinist and the customer. I'm assuming whatever is going on there has already been decided upon. There are cases where going aggressive with throat size to the point of hurting low lift flow is beneficial, just like using a 50* or greater valve job would be. There are limitations to the cam lobe/rocker profile and if really pushing power output is the goal, it would be a good trade off to add degrees of rotation at max flow (near max lift) when the piston is at peak demand than to have the greater duration begin to hurt things at the other ends of the cycle. Basically, the air isn't moving yet until the valve is further off the seat. This is well beyond what most nailhead users are ever going to try to do, but I thought I'd comment on it as it was brought up.
