T U R B O’ s
In the last century I wrote the following article. This I have now expanded on in my Newsletter 72.
You might ask why is Nick nattering about blowing Hot Air? Well when you fix 2 Turbo Conversions in that many weeks- no we did not "Build" them- you know they are out there.
A short, and hopefully not to Boring, technical explanation:
When the exhaust valve opens a wave of hot gasses get expelled into the Exhaust Pipe and hit the turbine wheel, which then spins and drives a Compressor wheel and that in turn Compresses sucked in Air which it crams into the Intake. The Compressed Air in the intake then Bleeds into the Combustion Chamber. When the pressure gets to high in the Intake, it is usually controlled by bleeding of excess exhaust gasses in the Exhaust manifold before the turbine by a wastegate. This in turn lets the turbo slow down and compress less Air.
Intake size calculations become very complicated if you take Finite Wave theories into Calculations, as well as pulse tuning. There are no simple rules to take into effect. You can probably spend just as much time modeling the Intake until it is right, than calculating it.
Exhaust primary length (before the turbine) can be calculated using a standing wave theory.
I have been very successful applying a Finite wave theory to this and headers- just remember you do not have an open ended collector and your gasses are far hotter.
That’s it for the Hi-Tech part, now for reality. The Compressed Air is heated by Compression and Heat from the Turbo (Turbo’s glow Red hot in Normal operation) and we all know that the cooler the Intake Air is the more Dense it is and the more performance we can expect. That is why an Intercooler gets fitted between the Compressor and Intake valve. The problem with Turbo’s is that they get VERY hot, need more complex plumbing and positioning and; Turbo lag. This is the time taken for the compressor wheel to spool up to the speed demanded by the Engine for the full Air supply. In other word low revs, low throttle and no real apparent Boost and you snap the throttle open. What happens the engine tends to bog because it cannot suck enough Air in for it to accelerate as the Compressor is not supplying enough and only until that little Air is burnt and spins the Turbine which spins the compressor to supply more air … You get the drift?
A "Compressor" is nothing else but a mechanical Driven turbo. Some are Vane type, screw type or turbine type. The Advantages are easier Fitting and the downside is you are not using spent energy (Exhaust gasses) to drive them. The problem with all Blown engines is that your Engine Management has to take on and Off Boost scenarios into Effect. Your timing has to Advance normally under vacuum driving and Retard under Boost. Your Carburetor also has to supply the correct mixture from Max vacuum to full Boost over the complete Rev range. Since EFI is upon us in a big way, I will be marketing a Supercharger sometime in 2002. So check his Website occasionally.
Also you want a pop off valve in your Intake to blow off Pressurized Air when decelerating.
So far Diesel and Petrol are the same. On a Diesel you normally have a Fuel Enrichment device on your Regulator that increases Diesel Delivery with Boost. And that’s it. On Petrol Engines you have suck through or Blow through Carb systems. And then of course the Ultimate "Fuel Injection". Which is all I am prepared to consider when installing a Turbo- and Alpine has finally caught on too.
Suck through systems are basically a ordinary Carb hooked before the Turbo and then the mixture is blown into the Motor. The Advantage is better mixing of the Air/Fuel Mixture through the swirling within the Compressor. Though you cannot run an Intercooler (Similar to a Radiator) due to the Pyrotechnics should you experience a Backfire.
Blow through are normally the more professional but you need a special Carburetor and Fuel pressure regulator to work under Boost conditions.
You get Low Boost systems running to 10 Psi while High Boost systems can run up to 2.5 Bar or More. The more Boost you run the Lower your Compression Ratio must be and/or Timing advance you can get. It all results in different Volumetric Efficiency curves (PV diagrams). Generally speaking a Retrofitted Turbo can increase Horsepower on a Generic Motor by 40 % and Torque by 50 % which can Equate to 100 Hp for every L. While the 2L Group C Engines could push 1400 Hp on a Dyno for short periods.
Every component in a Turbo system must be sized and matched. There is no easy system i.e. 2L 200 HP = this Turbo, Carb and Advance Curve. Do it wrong and you will hate Life- ooooh but right…! On the Turbo consider the inertia of the Wheels that might spin at 25 000 Rpm at Cruising to be accelerated over 85 000 under load. The A/R ratio being the ratio of the Area of the turbine inlet throat at the Venturi to the distance from this point to the axis of the turbine shaft. There are many more factors to consider when designing a Turbo system. But at least this should give you more Ammo for the next Bench racing session.
To keep your system Reliable consider:
Lubrication (when shafts spin at over 100 000 Rpm)
Cooling (Turbo 1000 C, Outside Air 40 C, Oil 140 C, Water 90 C…)
Hose types and fittings etc.
Lastly your Experience in this mystical field.
Oh yes and before you pick up the phone to order, remember most systems cost well over 10 grand!