Science warning 

Your thoughts please, I'd like to tidy it up and submit this to CAMS.

To start with the 1.36 diesel forced induction factor has no basis in reality, 170% power increase on a diesel with the addition of forced induction is pretty easy. I propose a 1.7 forced induction factor the same as spark ignition engines.

some abbreviations..

IDI - InDirect Injection Diesel
DI - Direct Injected diesel.
BFSC - Brake Specific Fuel Consumption
VE - VOlumetric Efficiency
gms = grams
ccs = cubic centimetres

NOTES:
# Diesel burns slower, optimum rpm of light vehicle sized engines is 4000 rpm, past that the fuel is only partially burned when the exhaust valve opens and power begins to drop rapidly no matter what you do. We have assumed the optimum engine speed for spark ignition port injected typical high performance engine as 7000rpm.

# For E85 BFSC note the increased fuel consumption as a result of lower AFR, the volumetric efficiency increase as a result of increased cooling and the timing optimization has all been taken into account and factored in as a BSFC improvement at the same AFR as petrol.

# these are all approximations to show the scale of the disparity in the engine capacity limits for different fuel types.

# I would like to thank Graeme Bentink who worked the bulk of this out at my request, He does high performance turbos for Toyota D4D diesels. If you want a high performance variable nozzle turbo he's the man for the job.
> Graeme Bentink
> www.GTurbo.com.au
> Ph: 0401 524 399


BSFC of diesel at max power of 4000rpm is as follows:
*IDI - 300gms/kw/h
*DI - mechanical injectors < 1000 Bar pressure - 270gms/kW/h
*DI - electronic injector >1000Bar pressure - 250gms/kW/h

The petrols run up to 7000rpm for peak power. The BSFC of the petrol at peak power is ~ 315gms/kW/H, on the E85, this is better, 300gms/kW/h


The naturally aspirated petrol/diesel differential based on BSFC alone becomes for Diesel:
*IDI - 7000/4000 *300/300 = 1.75 times the swept volume required to make the same power.
*DI - 7000/4000 *270/300 = 1.58 times
*DI - 7000/4000 *250/300 = 1.46 times
These calcs exclude volumetric efficiency.

IDI are all (except the OM605/OM606 Mercs) 2 valve designs without swirl.
Early DI (12HT/1HDT) are swirl designs with poor volumetric efficiency.
Later DI designs flow 15% better.

So with the maximum potential VE of compression ignition engines it becomes:

*Otto 4 valve - 105% VE best case @ 7000rpm, tuned intake (like a VH45DE etc)

*IDI 2 valve - 80% VE; 1.75 * 105/80 = 2.30
*IDI 4 valve - 94% VE; 1.75 * 105/94 = 1.95
*DI 2 Valve - mechanical injectors < 1000 Bar pressure - 72% VE; 1.58 * 105/72 * = 2.30
*DI 4 Valve - mechanical injectors < 1000 Bar pressure - 85% VE; 1.58 * 105/85 = 1.95
*DI 2 Valve - electronic injector >1000Bar pressure - 72% VE; 1.46 * 105/72 = 2.13
*DI 4 Valve - electronic injector >1000Bar pressure - 85% VE; 1.46 * 105/85 = 1.80

And then, to all of that, you need to account for air/fuel ratio. Racing Diesel as low as 16:1 at the most extreme richness you'd sensibly run.
Racing Petrol 12:1 AFR.
To be honest, We should really allow a 1 point favour to the old DI (17:1 racing), but lets be conservative here because these numbers up ahead will rock your world.....

Thus an overriding factor of 16/12 needs then applied to the above values = 1.33.

*IDI 2 valve - 2.30 * 1.33 = 3.06 times the spark ignition swept volume to do the same work!
*IDI 4 valve - 1.95 * 1.33 = 2.59
*DI 2 Valve - mechanical injectors < 1000 Bar pressure - 2.30 * 1.33 = 3.06
*DI 4 Valve - mechanical injectors < 1000 Bar pressure - 1.95 * 1.33 = 2.59
*DI 2 Valve - electronic injector >1000Bar pressure - 2.13 * 1.33 = 2.83
*DI 4 Valve - electronic injector >1000Bar pressure - 1.80 * 1.33 = 2.39

Thus a 1HZ with 96kW * 3.06 = 291kW should be put out as a tuned petrol 7000rpm screamer and you know what..... that's about what you would expect

So there you go, they are the capacity upgrades and the same turbo factor can then be used between the two engines (1.7).
Now we don't want really complicated rules do let's assume people are only going to be racing high tech multivalve direct injected diesels and we'll stick with the LOWEST 2.39 factor.

A worked example:
Spark ignition NA: 6000 cc.
Spark ignition forced induction: 6000cc / 1.7 = 3529.4ccs
Diesel NA: 6000cc * 2.39 = 14340 ccs yes 14 litres - double the current rules which are clearly unfair.
Diesel forced induction: 6000 * 2.39 / 1.7 = 8435.3ccs

so for a diesel engine to be competitive in horsepower production against a turbo spark ignition engine it needs to be nearly 8 and a half litres swept volume.
Current rules work out at juse under 5.3 litres, not even close.