gas speed vs flow rate with regards to bhp/torque

Competition engines and ancillaries - general discussion
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smckeown
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gas speed vs flow rate with regards to bhp/torque

Post by smckeown »

Hi Guy,

I have read lots of your posts on here, especially in the area of flow development. I have not however been able to get your opinions or results with regards to analysing gas speed wth regards to attaining specific torque targets.

Can you spend some time explaining your thoughts or findings with regards to gas speed vs flow rate with regards to bhp/torque.

My brief experience with heads i've purchased in the past is that it takes significantly good gas speed to attain high torque levels in the mid range, whereas high flow rates support a higher peak power figure.

thank you
sean
205 8v Track car
Guy Croft
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Post by Guy Croft »

sorry for not picking up this interesting question sooner but I could not give much of a defintive answer until today as it's actually something i have been working on quite intensively in recent months.

I have read lots of your posts on here, especially in the area of flow development. I have not however been able to get your opinions or results with regards to analysing gas speed wth regards to attaining specific torque targets. Can you spend some time explaining your thoughts or findings with regards to gas speed vs flow rate with regards to bhp/torque. My brief experience with heads I've purchased in the past is that it takes significantly good gas speed to attain high torque levels in the mid range, whereas high flow rates support a higher peak power figure.

Flowbench flowrate is not related directly to power in any shape or form! I am sure of this now. All the flowbench tells you is that you have generated a % gain in flow (or more correctly minimised the losses) compared with how the head was before and that will most likely show up as a gain in power, although not by the same percentage. To match flowbench data to bhp you have to have accurate dyno feedback and then you have a modest degree of certainty regarding the flowbench target.

Attempts to analyse airspeed on flowtest and compare to engine airspeeds are, in my view, pretty pointless. The governing thing is the pressure ratio between the port (with a valve in the way) and the cylinder (with a piston moving down at very high speed). That is the phenomenon that governs whether the air travels from (more or less) atmospheric pressure in the port to the cylinder at depression. The higher the pressure ratio the faster the air enters the cylinder thru the inlet valve. On-engine Rp values are 10 times higher than you'll get on a flowbench ie: impossible to simulate that way.

There is (like with everything) a limit to how much flow you can get for a given piston speed and bore area and it's set by trans-sonic speeds causing choking in the port at the smallest cross-sectional area. We're talking Mach number.

Mach for air at 40 deg C is about 385m/s and at Mach 0.6-0.65 we know that flow is going to start 'choking' ie: the port won't flow any more. Simulations all seem to say the same thing. Best peak power results (based on dyno tests of several engines compared with calcualtions and simulations) seem to fall in the range 0.45 to borderline 0.60 Mach. If the Mach numbers there are not exceeded at peak rpm (in other words the rpm where we want peak power) then that port is going to be plenty big enough to achieve high volumetric efficiency at peak torque rpm. I'll be dwelling on the maths relating to port size much more closely in the future to see if this proves a viable way to optimise heads.

Mach number too high can be from valves too small, cylinder capacity too big, piston speed too high, etc etc. I'm fairly certain now that this is why you can put a head flowing 85cfm on a 1600cc engine and get 178bhp and put that same head on a 2 liter and only get 155bhp.

Having found out that rather useful piece of information about optimum Mach, it now tells me, I hope, how big the ports/splitter regions/seats etc need to be for a particular size of engine and peak rpm, all I have to do then is modify the heads to that size, minimise the losses ( as outline above).

I hope this is interesting and if it doesn't answer your question I'll be pleased to talk more.

GC
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