V8 to V4

[aggie2k4]

I own a Ford Bronco with a 351 V8 and I am trying to improve gas mileage. I know the first remark is that I shouldn't have gotten an SUV, but it was given to me and the cab is big enough for me to fit into comfortably. My question is I know that some older Cadillac's had V8's with switchs to turn them into a V4 by cutting the fuel into four of the cylinders. Is there any gas mileage advantage to doing this, and would there be any detrimental effects other than a loss of power (ie. increased wear, ect.)? Thanks

 

[schmidtj86]

Well, one obstacle is the fuel injection system is running in closed loop with the O2 sensor in the exhaust. If it gets a lot of air pumped at it, it'll think the engine is lean, and add fuel, probably to the limit of the calibration. Then, you'll have 4 really rich running cylinders, and 4 air pumps. You'd probably want to disable every other cylinder in the firing order, for smooth operation, so, if you built a dual exhuast system that contained the firing cylinders in one group, you could put the O2 sensor there, but that's a complicated mess. The speed density or mass air flow system, otherwise, should still be able to accurately (enough) meter fuel, even without knowing how many cylinders are working.

But, there's another issue. You'll still be wasting power by pumping air through the throttle and valves for 4 cylinders that aren't producing any power. If you were to disable the valves, then you wouldn't be - and the compression and expansion of the trapped air would roughly balance out (sure, there's heat transfer, but that's not my point, and it still probably balances out in the end). The *only* gain in your equation then is that the throttle is now slightly more open for the 4 cylinders that are producing work, thus reducing some of the throttling losses. I would imagine that the net gain would be negligable for the hassle.

 

[GregLocock]

It's called displacement on demand (DoD). There's a lot of interest in it at the moment. It does work, efficiency wise, but there are a whole bunch of implementation problems. I can't see wear being an isssue, but throttle response, noise and vibration, and emissions are all known issues.

If I had 10 cars and an infinite amount of spare time I might try it on the car I hated most.



Cheers

Greg Locock

 

[PSlem]

I recall when this came out, there was a write up with a dual plane intake and an adapter to mount the carb sideways so four cylinders were shut off except when the secondaries opened. I don't think results were good.

 

[dwedel]

Isn't this the same thing that Dodge does with the new Hemi engine? I thought I read another article about chevy doing something similar.

I agree that the valves would be the nicest (fuel efficient) way to make this work.

You could install an new Hemi in your Ford. ;-)

dwedel