From inside the engine industry...

Our research reveals that - according to Rich Bartlett, General Motors’ Assistant Chief Engineer on the new GM twin-turbo 3.6 litre V6 project, Cadillac LF3, the engine will make 420ps (414bhp) at 5750rpm and 585nm (431 lbft) at 3500rpm. So that is 115 bhp per litre; quite impressive until you remember that this is a turbocharged engine, with 4 valves per cylinder, DFI, high tumble porting (mythology!), and dual independently variable cam timing. Blimey! Our DTMPower 2 valve 3 litre Ferrari 308 on carbs did 121 bhp per litre in 2005... WITH NO TURBOS!!

The fact that OEMs have got it so very wrong is brought into sharp relief by the quote from the same chap (Rich Bartlett) who says “ of the beauties of forced induction...” is that, “You don’t rely on tuning, with long runners and large plenums to make power – you just stuff it in!” Double Blimey! This is so far from the facts of the matter... How much better do we think this engine could have been with correctly designed porting? Then to get the same figures of power and torque the boost levels could be far lower, reducing stress on all intake and exhaust components. Or; use the same boost levels and get way more torque. They went to a lot of trouble to make the intake tract length really short so that turbo lag is minimised, they say... It is widely accepted that inlet tract length has nothing to do with turbo lag. One home experimenter even put his turbo at the opposite end of the car to the engine just to see what happened, no change in lag was found. The pressure throughout any vessel is a constant, so in that context lag is a function of the speed of pressure waves from one end of a tube to the other, these waves travel at the speed of sound, so what’s the problem?

Lag is in fact controlled by a couple of significant factors; turbine/compressor wheel inertia - so smaller is better, and exhaust gas inertia (mass x velocity) getting more gas out of the port quickly, so high gas speed in the exhaust ports helps, fast build up of that speed is a function of exhaust cam profile and combustion pressure.

Don’t forget that boost pressure is only a measure of the comparison of turbo compressor performance against the inlet port and valves ability to flow gas, big turbo and rubbish ports will mean you have a lot of boost, “just stuff it in”, all you get is a big heat build up. Actually using poorly flowing heads will make lag worse as the engine will only start to work at higher boost, that boost having to build up pressure to get the ports working, and waiting for exhaust flow to spin up the turbine which builds up the boost. A self limiting circle of problems that is eventually overcome when the boost gets high enough, blamed on turbo lag, but really it is just rubbish heads…

" A unique high-tumble inlet port design “was developed with lots of computational fluid dynamics (CFD) work; we ran hundreds of simulations to develop the ports, piston crowns, and optimal injector spray geometries,” Bartlett told AEI."

This is the point where we let the world into a secret... CFD has got it wrong, air does not flow down ports in straight lines like those you see in CFD graphics.

Our DTMPower cylinder heads flow more air in the inlet port that any others, and that means more torque, then a higher BMEP, and higher exhaust port velocity so less lag; none sometimes. After inlet valve opening our inlet flow gets going way faster than in any other heads, far better cylinder filling, higher dynamic compression ratio and better volumetric efficiency leads to better torque – everywhere…

Don’t just stuff it in… flow it through, you will get more torque. Torque x rpm equals horsepower, or PferdeStarke (PS and a bigger number as German horses are smaller it seems, but better for marketing purposes!).

Source; Engine Technology International Sept 2013 and AEI online.