Velocity/mach index

[superbike]

Hi guys

I would like to advance my porting to the next level.
Alot of the american guys are porting heads to a certain velocity level wich i think is 320 fps.

Where do i start with this and what is the mach index all about ? Is it a calculation or spreadsheet that is used for guidence.

I normally only gas flow one particular model and over time i have been able to guestimate if the ports are too big for the cam its running.

some of the slow opening long duration cams need a smaller port than the shorter duration ,high jerk ,high lift type of cams.

Im just hoping to get a better understanding and better my future dyno results...

All comments welcome.

[Guy Croft]

Chris hi

I'm not fully clear on all aspects of this phenomenon yet but I'll do my best.

Maximum Mach index occurs at maximum power, from what I know so far the most powerful units run very close to trans-sonic - even though it is believed that the higher the Mach index the more likely the intake is to choke, ie: stop flowing. From what I can tell, engines like Vauxhall XE units of 270bhp or more are achieving instantaneous Mach numbers around 0.61, other top engines 16v and 8v are seeing 0.5-0.55 and these simple but very laborious asessments done by me on my own computational tables seem to be borne out by the very advanced Virtual 4T software simulations. Far higher than I thought previously.

So, what about your Mach index? The controlling section in a head like yours with 2 inlet valves per cylinder may be the splitter regions or the curtain area under the valve (throat diameter x Pi x lift). A detailed study of both areas especially the curtain area change as lift increases determines the controlling section at any given time in the intake event. Your maximum instantaneous piston velocity is at around 80 deg after tdc and using my method of ratio bore to minimum intake area (in your case the combined splitter area) to simulate pressure ratio effect between bore and port and taking:

bore 100mm
stroke 61.5mm
rod length 124mm
Mach = 354.8m/s at 40 deg C

Thru 33mm dia splitter - peak air velocity is 0.449 Mach = 521ft/sec
Thru 35mm dia splitter - peak air velocity is lower, 0.400 Mach = 462ft/sec

The combined splitter area of the 33mm dia setup for example (std head) is 1720sq mm. If we take the valve throat diameter as 37mm on a 40mm valve the curtain area for two valves is 37 x Pi x Lift x 2. Let's assume 11mm valve lift and we get 2582 sq mm, so from that we can say that the controlling section at full lift is definitely the splitter areas. However at lower valve lifts this changes, eg: at 6mm lift the combined curtain area is only 1395mm. I hesitate to draw the conclusion that the valve region will choke at that lift (although it definitely will at very low lifts), because it depends what piston speed the engine is doing at that lift point.

You will realise readily a full analysis would be pretty time consuming, and even if you did it it would neglect the effect of pressure waves in and out of the port etc etc. That kind of in-depth study needs software, but the comparison between curtain and port are is still useful.

You will note straightaway that the achieveable on-engine velocities thru the intake are way in excess of anything you could simulate on a flow bench, this is because the engine operates at pressure ratios say, 10x higher (or more) than a flowbench can generate.
This is proof, if you needed any, that the tests are only useful for comparative purposes, and the tuners you mention are probably testing at quite high velocity rather than 'developing the ports to cope with high velocity', so to speak. You cannot tell from their test velocities what the Mach numbers on-engine are going to be, 320ft/s is only 0.28 Mach, which is nothing really.

Provided the port is big enough not to choke - and looking at the Mach numbers I have worked out for you it certainly appears that both the 33m and 35mm port are suited to the achievement of high rpm peak power speed (say the 10,500rpm you gave me). I know you have increased your head flows by careful testing and development, my feeling might have been to not actually enlarge them, but dyno test only will prove the benefit of what you've done. Big flows per-se don't prove that much, unless you have dyno tests behind them. What they will be like as medium for lower-end and mid-range power I wouldn't care to say - could well be some cylinder filling issues down there at the lower bore-port pressure ratios you get with part throttle and lower rpm, but with big ports like that I would certainly be looking for an inlet cam with high lift, quick lift and early achievement of peak lift and plenty of dwell.

Hope this helps a bit, and thanks for posting rather than pm,

GC