Extreme MPG from 91' LS400 ??

[spf_lexus]

Ok so I have owned two 1991" LS4's over the years and im about to grab another with the intent to leave the engine alone. Gas is going to hit over $4/gallon and any increase in range is welcome. The best I can achieve driving at 60-65mph on the freeways is about 23-24mpg (depending on the temps). This isnt too bad for a heavy 20 year old v8 sedan when compared to any comparable model in the 90's, (or even the ls460). I got to thinking about feasible ways to stretch that to maybe 26-27mpg. Here's my idea and please feel free to lay on the constructive criticism as im looking for all possible issues I may face:

I want to make my own cylinder deactivation system that will effectively run on 6 cylinders versus 8. Here's my thinking on how to pull it off:

-Remove 2 companion cylinders' spark plugs = less wasted effort thru unused compression cycles = less resistance at the crank.

-Leave spark leads attached to the distributors but re-attach them to a DIY chassis ground that can handle the system's ingition spark = igniton system is effectively untouched.

-Unplug both companion cylinders' injector clips = no injection on 2 cyls = 25% less injection.


I do realize that the car will have significantly less acceleration but im ok with that.. I plan to do a lot of freeway cruising under 70mph as is. I like the fact that the early UZ does not have COP and this would cause the ecu to throw a fit so I "think" im ok there. I feel that city driving would kill most of the gas savings because the engine would need more revs to bring the car up to speed.

Taking this to the extremes would be to unplug the ignitor to an entire coil, effectively running on 4 cyl instead of 8.



If any of you have thought about this or have attempted/seen it done please add your thoughts.

 

[4U2QUIK]

If it helps I'm working on the gm management system on the 1uz, and one of the options i noticed on the software when tuning was that it could turn on and off the cylinder shutdown features on vehicles that had this feature. I'm guessing you could probably find the rest of the parts and make it work on the 1uz also.

 

[4U2QUIK]

also, with the 5 spd. manual on my friends cressida he's getting 27 mpg....

 

[spf_lexus]

^^^ the 5 speed is a nice alternative. If I can source the parts for a reasonable price, I may just try that out. Ever more extreme would be a t-56 w/ a 0.5:1 6th gear. The engine could sit at 2,000rpm at 70mph or so.

I tried accepting the idea of a honda crx or an mr2 spyder but in CA they are death traps.

 

[spf_lexus]

4U2QUIK:

How exactly does the GM ecu deactivate? I would assume direct control over a COP setup and injector pulse width. Im really wondering how much the compression stroke w/o a following combustion event will affect the savings? Thanks for the reply btw.

 

[stevechumo]

You'll make the other 6 cylinders working harder to push down the crank.

 

[spf_lexus]

You'll make the other 6 cylinders working harder to push down the crank.

My real question about this is stress at cruising speeds. Its an early UZ that has low miles. I understand that GM does this all the time with their V8's but only at cruising speeds.

Also, an important aspect is the torque. Correct me if im wrong but im pretty sure the uz achieves 80% of its tq around 2,000rpm.. which is just under my cruising speed of 2300rpm so taking 25% off that should be 55%tq or more at 60-65mph. I have heard about owners losing a coil or ignitor and effecively running on 4.. so I know it can be done but the real question is how effective it will be.

I'll give this a try maybe next week when I have time. The fun part about this is that it can be done quickly and for zero cash. Im actually heading over to Davids' later today to pick up his white ls.

 

[XR8tt]

I don't think it's that effective on G.M engines in the REAL world...

 

[spf_lexus]

I don't think it's that effective on G.M engines in the REAL world...

The GM engines are pretty bad... horrible efficiency and their cars have brick-like drag coefficients. Im planning to lower my LS a decent amount which should drop my 0.29 cd to around 0.27 or so. With a slippery car like the LS and all the tq... im pretty sure i'll blow the GM system out of the water

 

[EyesofThunder]

I'm the new guy here, but I know many GM engines (Pontiac V8, Chevy LT1/LT4, Buick 38000 Supercharged) like the back of my hand. Just picked up a 98 GS4, great platform car so far (couple da)/.

But on the GM stuff, hp/L GM is doing almost as much as most anyone else is with DOHC engines. And GM does it with OHV underhead cam 2V.

The Northstar should by now be much much stronger than it is. General rule of thumb, 60-65 hp/L for 2V engines and 70 hp/L for 4V. The LS7 makes 505 hp with 2 valves per cyl and pushrods and only from 7L, and can spin to 7000 rpm, again, with PUSHRODS.

Back to the DOD (Displacement on Demand uses oil pressure to collapse half of the lifters to so there is no pump losses. So you can simple shut off a plug and a fuel injector, but it will have pretty high losses with egt. You need to somehow be able to crack open a valve and relieve some of the pressures.

Now, GM has a slew of patents on the DOD engine, one of which is the method to keep the gas pedal from dropping way down when the cyl decativation kicks in.

Read up on the patents, there is a lot of good info in them. We used to use Micropatent for a company to help us do research.

 

[spf_lexus]

You need to somehow be able to crack open a valve and relieve some of the pressures.

Thats why I remove the spark plugs from each deactivated cylinder. Removed spark plug = no pumping losses except for the piston itself. I tried it a few days ago and it worked great on 2 cylinders and still driveable. I want to try unplugging an entire coil and try on 4 cylinder mode to see if it can be gimped at 55mph on the freeways.

 

[EyesofThunder]

I was looking trying to do that to a Pontiac 400, but I couldn't easily solve the way to push up on a rocker arm or to force both valves to be closed. I was going to interrupt the injectors and just leave the distributor sparking into nothing (just a bit hard on the cap/rotor with no load on the wire).

Being underhead cam I could use GM's DOD parts, but at the time they were unobtainable. Now they are, but I am not driving the car anymore (I never did get EFI on it, was going to do it all at once).

I had also looked at drilling into the chamber and install a machine fit pin that I could open as a valve that could vent the incoming air during to the exhaust (increasing free O2 in the exhaust helps cat light off), but that has its own issues, but is somewhat easier. But you need a perfect fit, and ring seals on the pin like a bolt for a rifle, but needs to handle combustion chamber pressures.

Thats why I remove the spark plugs from each deactivated cylinder. Removed spark plug = no pumping losses except for the piston itself. I tried it a few days ago and it worked great on 2 cylinders and still driveable. I want to try unplugging an entire coil and try on 4 cylinder mode to see if it can be gimped at 55mph on the freeways.

 

[spf_lexus]

Sounds like you have given this some thought. Thanks for contributing. I would assume a carburated application would be hard to cut fuel to individual cylinders and your right, cats would throw a red hot fit from all the extra HC's. Of course taking it to the extreme, a way to disengage the connecting rod would be the ultimate way to make the system efficient but thats Mcgyver territory Lol.

My method is somewhat crude, in that it cannot be done "on the fly" but its simple and quick enough that if I wanted to, I could pull off the freeway and in 5 minutes, have all 8 contributing again. Once my car gets its official import title and I can drive it daily, i'll have to try leaving a coil unplugged. Its a 50% loss is power/torque for a heavy car but I have heard its possible at low speeds.

Also, i havent paid attention to the dash when I briefly too out 2 injector plugs but if I get a noticible CEL, does anyone know the resistor value that would satisfy the ECU? I do not want to unhook the battery after each time I deactivate..

 

[spf_lexus]

Just found some details off planet soarer regarding 4 cylinders:

Driving the car up a ramp on 4cyls should not be an issue.
I've had to limp mine home a couple of km's a few times on 4cyls. Sure its slow and has no power, but it has enough to get the car up to 60kph.
Unplug the 4 respective injectors if you want to stop the backfiring.

Its not factual... just an opinion of someone who has actually tried running the engine at half power. Wether or not I can do 55mph is unclear but I wont know until I try!

 

[EyesofThunder]

Yup, gave it a lot of thought. Was looking at EFIing the 400 Poncho for it. But alas, not enough time to get into doing it.

I guess another way (on the fly, if the way you are doing it does work out) is to install a butterfly valve in the intake port and close it and shut off the injector. It would need to be able to seal to prevent as much air as possible from coming in. Then there would be very little to compress and then very little friction compression loss when that cyl is down. You can do V8 to V4 pretty easy, V6 is sorta odd and might end up being too shaky to be tolerable.

Then you can set it up later to drop to 4 cyl on decel and at idle, V8 when accelerating and then 4 cyl when at light TPS when in OD. So then have it in normal V8 mode in 1st, 2nd and 3rd, but drop to 4 cyl mode in 4th and below say 50% TPS, or what you determine from the testing you are about to try to see where in 4 cyl mode the TPS position is at cruise. Expect a lot of throttle opening for it.

Sounds like you have given this some thought. Thanks for contributing. I would assume a carburated application would be hard to cut fuel to individual cylinders and your right, cats would throw a red hot fit from all the extra HC's. Of course taking it to the extreme, a way to disengage the connecting rod would be the ultimate way to make the system efficient but thats Mcgyver territory Lol.

My method is somewhat crude, in that it cannot be done "on the fly" but its simple and quick enough that if I wanted to, I could pull off the freeway and in 5 minutes, have all 8 contributing again. Once my car gets its official import title and I can drive it daily, i'll have to try leaving a coil unplugged. Its a 50% loss is power/torque for a heavy car but I have heard its possible at low speeds.

Also, i havent paid attention to the dash when I briefly too out 2 injector plugs but if I get a noticible CEL, does anyone know the resistor value that would satisfy the ECU? I do not want to unhook the battery after each time I deactivate..

 

[XR8tt]

I think a 250CI sized motor with well matched blower or turbo would easier and more efficient..On a family sized vehicle...
Especially with direct injection where theoretically is more detonation proof...
Can run leaner and power when you want it by the throttle..

 

[spf_lexus]

Like you said XR8TT, the weight of the big LS is the biggest issue I can forsee. 6 cyl is fine but 4 is pushing it for freeway use. If I can manage that magical 55mph cruise speed I think i'll live with the complete absence of passing ability

Im always sitting at 55-60mph knowing that about half the engines' power isnt even needed so I think it might just work. Also, the LS has an amazing coefficient of drag and since its lowered another 2", it should be even lower. there's also the debate of how much CD even matters at 55mph. Can anyone shine some light on this variable?

Actually, the best platform for this would be a 1995-1997 due to less weight and a stronger engine.

 

[XR8tt]

A good 6 speed auto or manual will save heaps where cruise can be at low rpm and kick-down available for passing,,
The is where a 4.0 engine either V8 or 6 cyl with well sorted turbo will work..
Talking fuel economy in heavy vehicles..
A modern diesel is VERY hard to beat ..
I noticed I tried hiring a camper to travel across U.S and most where V8 or V10 petrol !!!
One brand of vehicle down under has 6 speed auto while it's poverty pac models have the 4 speed..
The difference in fuel consumption between the two is heaps..
In Fact the engine with old tech injection with 6 speed beats new tech direct injection.. Driven over the same road at the same speeds...

I've found GOOD ECU and tuning would benefit more than cutting cylinders etc..
To a point this is just propaganda from some manufacturers so you buy their product...
A proper wide band 02 senser and good tune will net good fuel ecenomy and in most cases better EPA results..

 

[spf_lexus]

Good points XR8TT


I just wish I had the cheese to afford a t-56 w/ .50:1 spec'd 6th gear. That would shave a few hundred rpm at cruising easy. But being realistic about spending money that could easily go towards gas, I may just install a the 3.20:1 final out of the supra TT and make the mounts myself. That should be a 12% drop in rpm. Add the t-56 in there and I would think 28-29mpg could be obtainable. Add a cleaner tune and who knows, maybe 30?

 

[BlackUZZ31]

you can save more fuel by going to an aftermarket ecu and leaning out cruise and part throttle then you would driving around without a couple of spark plugs... not to mention the motor would not last long due to the imbalance caused by the dropped cylinders...

also put some 3.2 diff gears in mine cruses at 2100rpm at 110kph and loves the highway miles now!