midget,
1/ The inlet ports are slightly better, ~ 1% or so. The shape of the curve is pretty much the same.
2/ The exhaust ports though are worse, ~ 8%. A significant difference.
I'm not yet sure why the exhausts vary so much. I ground these valves before testing, so the margins are thinner, but I'm not convinced that is the difference. I'm going to have to carefully measure the port's mcsa to try and get a handle on the difference. So even though these heads are the same casting number, they might be from differing "batch lots". I'm going to carefully check them over for production date codes, if any. This would be a good question for Steve, how to "decode" any production casting codes.
Flowtestboy
Flowtestboy: though the 4 valve may behave differently and this head may be different, a lot of people seem to think that exhaust ports are TOO BIG to start with and that continually hogging them out and putting in bigger valves is counter productive. Some of this is to allow bigger intake valves ( making the exhausts smaller that is) but also just that the flow dynamics of the exh port and attached header/exh pipe can benefit by the smaller port/matching pipe. The flow may increase on the bench but what does the HP do? this is of course the question. In your experience with similar configurations, which I know is extensive,what do you think the 8% difference might do on a running engine? (my comments above are primarily associated with 2 valve v8s so may not apply at all) Just asking. ( methinks question answered with dyno time..... )
Jack,
I see this every day that I run flow tests or simulations. The presumption that some modification is going to perform the same from one engine type to another, is a giant mistake. Even when the engines are quite similar, differences occur. Just because a certain porting modification works on a V8 Pontiac, doesn't mean it will work on a V8 Olds. Too many variables. The flow and performance differences between 2 valve and 4 valve engines are quite large, basically for the same reason.
On this Rover head, the ports at the manifold face are actually quite large. Where the ports are divided internally however, the minimum cross sectional area is pretty small. I believe that this is where the peak flow is being choked off. I want to finish some flow demand simulations prior to any "enlargement" of the mcsa though. "Hogging out" the ports is EXACTLY what I do not want to do, because that will turn this experiment into a 11,000 rpm "screamer".
One of the other factors that limits valve size is the valve centerline spacing, obviously something that is "fixed". And the use of a 75mm/75.5mm bore diameter also restricts the maximum diameter of valve that can be used.
The 8% less exhaust flow significantly alters the flow ratio between intake and exhaust. I have a certain flow balance % value in mind for this engine. That percentage usually works well with the same cam grind on both intake and exhaust. Flow balance % below that value typically requires different cams from intake to exhaust. I can usually make anything work, given enough development time and dollars. But knowing the data values ahead of time, allows a more "thoughtful" approach to the cam(s) selection. On any project that is funding "challenged", it just makes sense to try to make the best choices on the first shot, something that some forethought allows for. "Development" always occurs on the dyno after "initial engineering". The trick is to use the "initial engineering" to eliminate the poor or bad choices. I've also called this process the "evaluation and planning stage".
The bottom line is: Engines DO NOT CARE about numbers and percentages, flow rates, etc, etc. The end focus is to produce bhp as opposed to information. But the "information based approach" works better, in my experience.
To hit my target, I want to use a gunsight rather than throw a dart. It just works better for eliminating piles of shiny, useless parts.
Next time we meet up, I tell you some "tales of the drag shop". You're buying.
Flowtestboy