Hi Everyone,
I have been toying with the idea of twincharging for a few years now (initially for my VL) and now for the Bedford but i have finally decided that i am going to do it.
I have done a fair amount of research but it is always good to get people's opinions as sometimes they think of things that i havent/wouldnt.
So here is what i am thinking. I have a semi-reconditioned 253 which i have fitted with 4 into 1 extractors with 3" secondaries. Basically i will keep the extractors and mount twin symmetrical turbos (one on either side) using a custom adaptor. The turbos will be relatively small as there will be two of them and i estimate that i will only require 5psi from each.
The turbos will be mounted such that the ambient inlet is forward and the exhasut outlet is rear. The pressurised outlet pipes will run beside the headers and over the steering rack to a custom intercooler that is long but narrow (allowing it to fit with the radiator etc). The way that these are run should mean that they will never be lower than the sump so there is less chance of damage etc.
Each turbo will be wastegated on the exhaust inlet prior to the turbo inlet exhaust and referenced to the pressurised air side such that if the pressure goes above 5psi then the gates open and exhaust is re-routed to after the turbo (but back into the exhasut). This will stop them from producing more pressure above the 5psi point.
On top of the engine will be a 4/71 (or possibly underdriven 6/71) roots supercharger with the pulley made to procude 5psi. This doesnt need a wastegate/BOV etc as it is direct drive so it is always producing the correct amount of air required at any given engine RPM.
On top of the blower will be an appropriate carby with the correct internals for pressurised air.
On top of the carby/s will be a custom plenum that is relatively shallow in terms of hieght (as clearance will be an issue) but about the same width and length as the blower. Two pipes will run from the intercooler up beside/in front of the radiator (but away from the fan etc) and then diagonally across to the plenum so that they join at the front.
On the back of the plenum will be another wastegate which will be referenced to the plenum pressure and set to open at 6psi in order to prevent spikes (the 6 is allowing for some leakage as the spring starts to open towards the 5psi mark).
So the pressure in the plenum will be at a maximum of 5psi based on the turbo gates and a max of 6psi in the event of an unlikely spike. That 5psi in the plenum will then go through the carby and into the supercharger which will further compress (pressurise) the air.
Theoretically if it is 5psi on entry and the supercharger is producing 5psi then it will be 10psi in the manifold however i estimate that it will onlybe around 8psi due to inefficiencies etc.
You may be asking why bother with tubo AND supercharger but there is method behind my madness. In order to explain it i need to give some simplified background info.
A supercharger can use up to 40% of the energy produced by the engine to power itself but it provides more power than it uses resulting in a net gain. However at high RPM's superchargers tend to be a lot less efficient than at low to medium RPM's.
A turbo however uses a lot less energy to power it as it is running of the waste gasses which arent directly connected to the crank. It is NOT free energy as the backpressure caused by the turbulent airflow presses on the cylinders thus making it harder for them to move (you need to look into the complexities of fluid mechanics for further info on it). But as with the supercharger it results in a net gain. One of the main weaknesses with a turbo is that they need to spool up to provide maximum boost so they are most effective at high RPM's when there is lots of exhaust gasses.
So, the supercharger provides boost instantly and is operating at full capacity by about 1800 rpm. The air being drawn through the supercharger must go via the carby thus plenum thus intercooler thus turbos which pulls the air through the compressor wheel causing them to spool up faster (as the compressor and exhaust wheels are directly connected) thus resulting in a mechanical advantage. This means that the turbos are producing maximum boost earlier than they normally would by more easily overcoming their inertia. When the engine reaches its higher RPM's and the supercharger is beginning to become inefficient the turbos are operating at maximum and forcing the air into the supercharger allowing it to still operate at maximum efficiency.
The turbos are using energy and the supercharger is using energy this there is more gross energy being used but more power is made resulting in an overall net gain of more than each of the units could have provided on their own.
The reason for using low pressure from the turbos and the supercharger is that i am aiming to put as little stress on the components as possible for a few reasons.
1. The physical mechanical stress being less is less likely for things to break.
2. Lower pressures mean lower air temperatures meaning that more oxygen is actually getting into the chambers.
3. Dont want to break the engine just yet.
So, basically can anyone think of any issues that i am going to run into? I will list the ones that i have thought of so that you can add to them, discuss them or list new ones.
Issues:
- The height that the engine+charger+carby/s+plenum will go to
- Where and how to mount the intercooler
- How to reference the carby and what carby to use (blow through or draw through).
- rego issues
Please add whatever comments you like as the more people think about the more likely i can identify potential issues.
Cheers, Nathro.
p.s Sorry for the long post but it is a little complex and hard to describe
p.p.s Has anyone seen this done on a bedford before?
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