[bazman]

Thought I'd post up my current project - trying to get 30mpg out of my 550hp LS1. That is about 36mpg imperial - quite a hard thing to get out of a 4 door family sedan with an auto. No problem in a 6sp Vette.

Car is an Australian Holden Commodore SS. Model is a VT series 2 with the LS1 5.7 V8 and the 4160e auto through a 3.07:1 rear end. Car weighs aprox 3750lbs (not weighed it but based on specs). They are like a 4 door GTO.

It has averaged 18.75mpg over the last 50,000 miles combined city and highway. (We work in metric or imperial down under but all figures I quote will be converted to US to ensure comparability).

Well, I've achieved the first goal (550hp) thanks to a low boost turbo set up at the back (STS custom setup). Now I'm working on the 2nd part (30mpg at cruise). That bit seems a tad harder.

Best so far of 25.8mpg under pretty much ideal cruise conditions and it is as lean as it will run without stumbling on itself - so no more tuning is going to help (about 20 hours on dyno just doing drivability etc so I'm not exaggerating).

Before I changed the cam the car could get 27.5mpg on a long trip. The cam has cost me about 15% in lost mpg overall but the extra 50rwhp is not coming off anytime soon as I like it too much, so I have to get creative now.

Here are my thoughts to get it done:

1 - Fit Rhoads variable duration roller lifters. This will bleed off (depending on how much adjustment we give it) from 5 - 15 degrees of cam duration at low rpms. This will lift vacuum and make the cam behave like a stock one - lifting mpg and power at low revs we hope. As the rpms climb the cam will retain its current specs so this mod should retain top end while improving the bottom end.

The down side will be a requirement to run adjustable roller rockers and hence noise and lash adjustment checks every oil change - but I can live with that.

The lower duration at low rpms should enable us to run less idle and maybe a little leaner.

I'm hoping this alone will get me back to 27.5mpg at cruise (av 62mph or 100kph). That would be 33mpg imperial.

2 - Fit a Plasma ignition system - the Blue Phoenix unit tested by David Vizard. The bigger spark is worth a few extra hp I likely won't notice, but what it will do is greatly enhance our ability to run leaner - we should now be able to get into the 20's A/F without power loss at cruise....

3 - Fit another water/meth injection system (the one I have only comes on under boost and is not mappable). They can share tanks but I'm thinking of using a modern ultra accurate EFI feul injector and adapting any old ECU to run it then inject water/meth under lean burn conditions.

e.g. the GM computer we have set to bring in lean burn after a few seconds of steady light throttle cruise, if we could pick up the signal when it is triggered we could simply tell the water ecu to follow suit and start injecting.

We could run water/meth all the time in direct proportion to gas (about 20% by volume) but we'd then need to refill the 15L tank every tank of gas - and that'd get tricky seeing as methanol is not exactly at most service stations. Yes we could run 100% water, but at 10.5:1 compression with boost, I'd rather have the added insurance of 50% meth.

I'm hoping the plasma and water/meth will get me past 30mpg by allowing a much leaner mix than the engine can now tolerate while maintaining adequate power to hold cruise speeds.

A later project will involve a new built engine and then we'll go for 40mpg and 600rwhp, but I need to walk before I can run.

I believe we can have our cake and eat it too - we do not need small engines to get acceptable mileage. I know of a 500 cubic inch twin turbo Viper that makes over 1000hp (at crank) and can get 30mpg under steady cruise. That is no longer technically difficult. Getting to average 35mpg IS difficult.

Welcome anyones thoughts