Flow bench numbers and engine simulation software

noneglibob said:
c] What actual info do you get from a flow bench and how is it presented? Do you get a cfm/minute figure.  Presumably you need to know what the head flowed before you do any modifications.

And finally, I was wondering about leaving the inlet port walls with a textured/ dimpled finish to help with laminar flow. My very limited experience with aerodynamics suggests that this might be beneficial. Any thoughts or data on this?
Click to expand...

Your assumption is quite correct, Bob.

Before altering anything within the ports, you need to know where you stand....  While working on H-D Sportster heads for the M-W Sportster Cup effort, I found that a, say, 7% increase in flow would almost exactly work out to a 7% increase in power!  and, we weren't allowed to alter the port at all, no material removed from head or manifold, not even blending.  The increases arose from valve seat configuration alone (which was allowed)!  So, the valve seats are just as important as port shape and surface.  Our (basically stock) engines eventually ended at 74hp, a huge step above the stock 45....... but the Yanks were already pushing 80!  We still had a lot to learn. ???

Later flow bench experiments with other heads showed that port surfaces finished with a 150 grain flap wheel gave excellent results, slightly better still than highly polished.

piet
 
as long as we're talking about ports...

here's an interesting except from Ian Postlethwaite, Lotus Engineering:

"The design of a modern intake port geometry is a compromise between high speed flow and low speed tumble. The tumble is used at low speed to augment combustion at a time when the flow has generated little turbulence during induction.

As the speed/flow rate increases, the tumble increases and flow coefficients decay. At the higher speed/flow rates the flow has sufficient turbulence, and high tumble is not required. Therefore, a port designed to deliver sufficient tumble at low engine speed will produce a surfeit of tumble at the higher speed/flow condition, at the expense of flow coefficient.

The port for a high performance engine has a steep angle, which endows it with a good flow coefficient. The lack of tumble is not a shortcoming since, in this type of engine, the high engine speed and flow generate the
turbulence required. This type of port would produce relatively poor low speed performance. On the other hand a port for a typical passenger car engine has a much shallower port giving good tumble at low speeds but compromised flow at high speed. The bulk flow structure in the cylinder, described as tumble, is generated by separating the flow from the floor of the port just upstream of the valve. This effectively forces the flow over the top of the valve and produces the tumble. Clearly this does not use the full flow area effectively. If the separation could be reduced at high speed then the port flow capacity would increase.

With this in mind Lotus has developed an innovative port design (figure 2) which has shown improvements in flow coefficient of about 8%. Vortices are generated that draw the flow back towards the port floor and re-energize the boundary layer to offset the flow separation which causes the tumble, the flow attaches more to the inner radius of the port with the modified geometry. The modified geometry shows a similar effect at lower speed and consequently reduces the tumble also. However, the reduction in tumble at low speeds is small and is more than compensated for by the increased flow."


this was pulled from a product brochure i'm certain some of you will find of great interest:
http://www.cd-adapco.com/products/brochures/industry_applications/autoapps.pdf

when you get down to the lotus head you can see how the designer created vortexes with a dropped floor
and while not so easy to do in a small port consider the pic of a S shaped bend in a 727 engine and look at the generators on the short side radius of the bends to keep the boundary layer intact much like they use on upper wing surfaces to avoid stall at slow speed or great angles of attack

michael, i looked at my old sim program from motion. dyno2000. a step below dynomation and engine analyzer pro both of which are full wave action simulators. i definitely need to upgrade  ;)

cheers,
scott

 

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Hi Guys
A very interesting thread.
I'd be  interested to hear Phils' take on mods to the 750.
I think you mentioned to me once that the small carb head was a good starting point is that as it has smaller ports it allows more leaway for porting or some other advantage?
I,m thinking of making a simple flow bench and having a go.

Regards

Rick
 
Hi Rick - only just spotted your post - Yes - i prefer to use the old 30mm inlet port head - it can then be bored out to suit the 36mm carbs - or larger by going in at a steeper angle - about 10 degrees more to decrease the valve angle to port angle. i've sectioned a damaged head and made sure that the new port misses the valve spring lower support face - especially if going for a large capacity motor with 40mm or so carbs and a bigger inlet tract. There is a lot of hand finishing required to blend in new port with the original, but afterwards it'snice to see how much more of the inlet valve is exposed to the incoming charge. Of course the stub face needs refacing to suit new angle and the stubs replaced with larger I.D ones. a 44mm inlet valve can just be shoehorned in if going for a larger capacity, along with a 36.5mm ex valve. I counterbored the head around the inlet valves with a 2" (51mm) cutter with a radius ground on its cutting edge and blended the sharp edges by hand to avoid shrouding the valve at all.
            Compression ratio needs careful checking afterwards, but a reasonable skim of head will rectify the extra cc's added to combustion chamber - Regards - Phil
 
Years ago, Gordon Jennings experimented with fitting some straight tubes into the intake track of a BSA (as I recall). Mounted the carbs on the exposed pipes and that allowed a straight shot down to the intake valve. Very impressive tooling and looked great.

As Gordon exclaimed after much hard work. "It didn't work worth a tinkers whatsit"

Without a taper and other flow mysteries, the engine only ran well at somewhere around WFO.

This was done well before flow bench work was available. It was also early in his tuning experience. Later he was clearly the Grand Master.

There is a book or two, out of print and probably available authored by him. Also a book "The Design and Tuning of Competition Engines" which is very helpful with some flow data, based on piston speed, camshaft and port taper. Still relevant information today.
 
Hi Phil
Thanks for that.I asked because I have access to a small carb head that I'm hoping to modify.I've just been doing a tig welding course but I'm still not that happy about welding cast ally so will have to find some other stuff to practice on.
I'm curious about taking out the steel scull welding up and reprofiling the combustion chamber.

Rick
 
Hi Rick - If i ever re-find the sectioned head - i'll try and photo it - i seem to remember that the skull has a sort of flange aroud it embedded in the aluminium - it is also very thick - in places it is nearer to the outside surface of aluminium than you may think - would make welding a new shape in  "interesting" to say the least. I saw an SFC all aluminium head re-shaped into a bathtub shape - allegedly done by Brettoni - Steve elliot owned it - but can't get in touch with him ATM - Regards - Phil
 
That looks like a really nice job, sadly Mez ports no more.....RIP.
 
I suppose other options could be to reprofile the steel scull and ceramic coat.Be nice to have some squish.

Rick
 
Does anybody use Extrusion-Honing these days? (Gritty Putty). It was all the rage in the 70/80's - in the 2 stroke off road world.  Not much use on a Lav because the ports are relatively short and accessible. Paul.
 
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