The Speed Trap : How fast is too fast ?

If you look at the A series port you could ( maybe should) come to the conclusion that the whole ”keep the airspeed up” idea by using a small port is actually quite odd.


This a port mould from an unmodified 202 head. It is very clear that the airspeed will drop like a stone after it gets through the first bit of port ( the nr 1 port) as the CSA increases quite dramatically, afterwards it will need to speed up to pass though the ” nr 2 port”  which is basically the short bit before the valve. From an efficiency point of view this is probably less that ideal, the expansion will need a bit of energy and same goes for the conversion from pressure to velocity when it speeds back up through the valve aperture.

How much speed do you need ? I would recommend downloading the RET acticle ” back to basics” from Prof Blair.

After a getting some very nice correlations of my humble model with actual dyno data without a huge lot of trouble, I had a very hard time of getting a similar engine but bigger engine to do the same. Try as I might it just refused to play along. It also refused to respond to cam changes, exhaust changes and produced a identical power curve with a 649 cam and a F11 cam. Ergo something was amiss.

After a LOT of simulations going nowhere, I fiddled a bit with Pipemax . I noticed that the near optimal airspeed at 28 inch depression is equivalent of about Mean mach index of Mach 0.25 or about 80 m/s  (260 ft/s) mean port speed. The maximum speed is about double at Mach 0.5 which is fast but not so fast as to cause choke.


At around peak torque the airspeed is around 80 m/s. In other data ( say a 911 engine but allso my small bore models) this hold true for near peak power as well.

Choke speed is the speed where the pressure differential is so great that it stalls, viz it does not matter how much lower you make the pressure downstream, the speed will not increase.

This is the reality for a non turbo charged engine. As the up-steam pressure is fixed at atmospheric and the speed is now fixed at choke speed you are stuck with the mass flow you have. If you have a well wave tuned intake and a good exhaust you will probably not hit a brick wall. First of all it does not hit this very high speed all of the time and with the proper wave tuning you will alter the density of the air transiently.

An engine is not an air pump. It is a chemical reaction vessel with wobbly bits attached. So in the end it is not CFM but mass that is important.

What happens is that, while you can’t make the air go any faster, it will move a bit more mass at the same fixed speed.

If you have the turbocharger you can increase the density upstream all the time, so even if the flow is choked you can still increase the mass at a certain power cost to drive the Turbo.  That is why you can ”solve problems” with excess boost I guess.

Back to the speed trap.

How fast was the port at 8000 rpm on my simulated engine ?

Mean Mach index 0.323 (so about Mach 0.65 at max speed well into choke hazard territory). Almost 110m/s or 360 Ft/sec mean speed on EngMod4T STA.

At 9500 it is up to 130m/s or 460 Ft/sec .. well you get the idea.

If you stick a 12G940 head on a small bore engine you will probably be on the other end of the spectrum especially when you stick great big valves with a huge throat on. It will get a nice speed at 7000+ rpm but probably be a bit low on speed lower down when you use a bit cam as well. If you use a short cam if will be less of a problem as you have very little overlap .

Where is the rub then ?

Find out where the fastest part is and make that bigger. Voila problem solved.

Problem is that to get good flow you can’t really make the throat any size you like. Accepted choke size for an A series is 0.86 to .88 maybe 0.89 times the valve diameter.  So that is 31.82 mm diameter giving the numbers above, or 33m which is a tad over 0.89.  Still a  MMI of 0.3  and 102 m/s or 335Ft/sec  which is still way fast.

Even if you got to 91% (which is probably not good for flow) it is still fast (97 m/s).


Is it a bit of a speedy pickle?

When I asked GR about the port speed he came up with yet a different number ( lower this time) . Hmmm. one of the values should be true then.. now which one is it. 🙂


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