Thanks for that Chris... wait till the crossflow fans see that, they WILL think they've just found an unopened Christmas prezzy! Thank you Leftcoast.... ... you're all very welcome. I'm fairly sure the exhaust flow will be very similar to the old style head as both the port size & shape is similar in std form, so when modded along the same lines to suit the same valve size they both use, the results will be similar... watch this space.
Nice 1 Mr H, thats some good work as always!! As Smudge says, Do you have any figures for a counterflow BVH?
With recent discussions on flow rates both in this thread and the 16v section. It springs to mind that basing hypothetical outputs of engines wholly on flow figures does not work. There are significant other factors involved. A recent engine built by a Pro engine builder in this post, http://www.clubgti.com/forum/showpost.php?p=2008565&postcount=224 had an output that seems to way exceeds the theoretical output based on the 1 CFM = 0.43 HP formula. If the figure of 90CFM for an 8v head is deemed as close to the max obtainable (possibly more work has been done to the head on the engine in question, but 30CFM more? ), it falls something like 30CFM short of the theoretical 123CFM required to achieve the 212BHP output of the engine.
Surely its not just the CFM of the head that determines the BHP figure? the manifold and induction system come into play too right? And how about the ability of the engine to get rid of waste gasses with the design of the exhaust manifold? Does that 0.43 figure stand for all CR's? Going with the 0.43 figure a std ABF should be giving 189bhp, or am i missing something some where?
The 0.43hp per cfm is a figure from Superflow, the flowbench people. Dan is right in what he say's, sometimes the numbers just dont add up. I actually flowtested the head off the engine Danster has mentioned... the head flowed a peak of 103cfm... I know for a fact it made 200hp, not sure where the other 12 came from tho. Either way, what I am finding is the .43 figure is a little high... in fact, when I half the 0.43 number and use it as a potential power gain over a known flow figure, the actual power gains come out very close. If I can win the battle with photobucket I'll post up the graphs I've just scanned in.
Have a look in the 16v section for a thread discussing standard head max torque outputs, relates to theoretical BHP and what has been acheived so far and how. Infact if you dont want to wade through the 16v forum heres the link.
It is not a simple calculation on a running engine. Throwing another spanner in the mix. When the head is actually on a running engine the air flow column per runner is pulsed backwards and forwards, unlike a flow bench. What I found useful is taking the actual flow numbers you published and build a "head file" in a dyno software package I have to simulate the engine output with all the other factors, including cams timing, CR, intake system, exhaust etc. So along with other "files" previously built, I can switch from say std head file to modified head file and see the delta in torque and power. Some simulation packages actually show an animation of the air/fuel mix pulsing up the inlet tract at certain rpms with the engine running.
Very true. I have a similar package on the flowbench software... punch in "all" the known data... flow, cam spec, c.r etc and it works out a potential power figure... never used it tho. I did a ported std valve head for a 1400 Saxo a couple of weeks ago... working the flow gains into potential power gains gave a possible increase of around 8-9hp (from memory)... it gained 7hp peak extra at the wheels, with gains from 2k up. There were no other changes... just the head, even the c.r increase was minimal... all 5 tho off would have given anyway. I've said it many times before... sometimes the numbers add up... sometimes they dont. I think we could drive ourselves mad with data at times... the longer we sit analising it, the less time we spend out there doing it... I certainly struggle to do both at times, along with eating & sleeping!
I love data...If you understand it and process it correctly, with the right tools, then you spend less development time to turn out good products. Less time in development = lower cost . The various software I have works out a curve for power and torque and can take a while to populate. The more infomation the software has the better the model is in generating a curve. It does take time to gather confidence in these tools though as more engines with known specs are validated and put back into the software. What ever the model says is the max power and torque or generated profile for that simulated engine hardware is taken as directional advice rather than absolute. What ever the dyno says post baselining is a pecentage change, which can be fed back into the model to determine simulated change from the baseline hardware. The dyno is not absolute either, however the telemetry from the vehicle on a track can be fed back to another software package to estimate its performance, which should be comparative to the data from the other two sources. Without data, we would still be riding 1 horsepower or even walking .
True... My question for another discussion is how did Superflow determine the 0.43 constant. There must be logic behind that constant.
Maybe its the general figure of all the stuff a flow bench does not include.....intake pulse rebound, velocity, the piston, etc....
Suspect as much. Usually generalisations like that have limitations where the result becomes unstable.
