Hybrid turbo..

[Mr Whippy]

They are mappable to 5.3k without fiddling with map paramiters, but most tuners won't put the fueling in higher up the rev range for some reason.

The problem is that the power curve drops off rather rapidly over about 4500rpm, mainly due to the mechanics of the diesel combustion, but also the engine configuration which is generally the same in most diesel road cars (piston velocity wise)

So although you can inject more fuel and rev harder, you are increasing inertial loads massively by revving harder (Kinetic Energy ~ velocity.squared), and you are also fighting more engine friction (friction ~ velocity), and since the burn itself is becoming hugely inefficient for a given engine torque output, you do make power, but it's horribly inefficient.


Thus most curves I have ever seen on a high revving diesel actually don't get higher after about 3500-4000rpm, they just plateau. Thus if you achieve maximum power at 3500-4000rpm, and have anything near standard 306 Td/HDi gearing, you don't need to rev beyond 4000-4500rpm, since if you change gear you are back in the peak power again anyway.

So all revving does is achieve higher road speeds per gear at the cost of a lot more heat and inefficiency. Heat is bad because you need to get rid of it. The OEM heat dissipation requirements are for 90bhp/4000rpm, so throwing in four times more fuel to make maybe double the power at 5000rpm, is a big load!



The benefits are that you can run a high peak power output if you use less torque at more rpm, which means you can then avoid heavy duty clutches/gearboxes, general drive-train stuff.
Ie, you can run a 5000rpm engine to get 250bhp, or a 4000rpm one, but the 4000rpm one will need to run about 5/4 more torque to achieve that power at those rpm. That is only really going to be a concern for people racing diesels though, not road car users...


The last car I saw that ran 5000rpm, made it's 180bhp from 3000rpm > 5000rpm. But due to gearing, it made sense to just change at 4000rpm in each gear anyway, so running 180bhp all the time from 3000 > 4000rpm... using the 4000 > 5000rpm section just meant generating more heat, more engine wear, more risk of failure, more fuel consumption, to go no faster at all.


It is one thing having a turbo compressor wheel that will flow air for a stoich burn with a given amount of fuel that equates to X power at Y rpm... it's another thing having an engine around it that will cope!

You also need to deliver the fuel to the cylinders in good time, so you need a fast injection (pistons move bloody fast at 5000rpm), so you need lots of rail pressure, so you need lots of pump pressure. A fast pump generates heat, so the fuel is hot, and expands. You also need BIG injections to make power at increasingly high rpm, since the efficiency falls off a cliff.

The mechanics simply mean it's gets really hard to do what you think is easy.


200bhp/265lbft worth of fuel at 4000rpm might be say 200mg/injection, 1.325lbft/mg/inj

200bhp/210lbft worth of fuel at 5000rpm might be say 250mg/injection 0.84lbft/mg/inj


You need to pump in maybe 55% more fuel to make the same power, because you rev 25% higher.


But, piston speed is also 25% higher, so your window to inject in a timely manner is 20% smaller.

You do however have the benefit of 25% more pump speed, so you can expect about 25% more flow from it.

So you need to get 55% more fuel into the engine, in 80% of the time. So you could say you need 25% more rail pressure to generate the required time improvement, THEN you need to pump in 55% more fuel too, and that puts a load on the pump which is only spinning 25% faster than before. The fuel will also be hotter too.

If you have a superior fuel cooler, a bigger fuel pump running a faster pulley, uprated injectors and sufficient air flow, then you could perhaps run the fuelling you want to make big power at big rpm with the matched turbo compressor.

However, you are ignoring the cam shaft profile which is out of it's depth at 5000rpm and such massive exhaust temps and pressures at such rpm. Valve lifters. Oil temps spiking. Coolant temps.

Does your bottom end have sufficient strength/lubrication to run 200lbft at 5000rpm?


Lots of variables of course.

I see people specifying a turbo to run the flow required, but is everything else capable too?

Tuning a car like this properly is damn hard work, it's a partnership between the tuner and the owner, it takes a lot of time, patience, and basically earning very very little to do it when you count out the hours you have spent. And since each car is rather unique, very little can be taken to the next project.

Ie, peak rail pressures are a function of the pump, pulley and temps they run at, which dictate how you calibrate your advance map, and that can then impact the smoke map, and on and on it goes.



Don't see this is making excuses, it's not.

Simply wanting a high revving HDi because you think it's the easier way to higher power is incorrect. You have just as many hurdles, or more, making 200bhp at 5000rpm, as you do at 4000rpm.

Specifying a turbine and compressor for a turbo is the easiest bit of the job. Making the engine injection system actually deliver it (not just requesting it in mapping, but checking you get it) and the engine physically cope with it, are the hard bits.

Does anyone have a finished one yet? Any at wheels dyno plots off a Dyno Dynamics rolling road or similar?

I've seen Pete make big power, and other high power HDi's around, but none of them are making their peak power at 5000rpm+! More like 4500rpm tops, and plateauing earlier still... and at 4500rpm, another 500rpm is a LONG jump when in the world of diesels.


Sorry for the VERY long post

Dave