Civic Vtec-E lean burn: Better burn on lower octane fuel? - Fuel Economy, Hypermiling, EcoModding News and Forum

tim3058 · 03-03-2010, 08:53 AM

A question to those who paid more attention in chemistry classes than I did:

The 92-95 Civic Vtec-e engine uses a "lean-burn" mode during cruising, increasing the air/fuel ratio considerably, giving the car its considerable mpg. One of the drawbacks is a severe drop in power once the car is in lean-burn, generally limiting hill-climbing and restricting 65mph cruising to level-terrain (unless you step on it and kick the car out of lean-burn, and get 40mpg instead of 50). Elsewhere I recall reading lean-burn mixtures ignite and burn slowly, thus my question: Would 86 octane fuel promote lean-burn mixes more than 87, and 87 more than 91, etc?? (faster ignition, faster flame front, etc). I just started running 86 octane this tank (it's cheaper, plus the station that sells it discounts another 5 cents/gal if you pay with cash, so its like 40 miles extra each tank for free), so I don't have any of my own data yet (seat-of-the-pants doesn't notice a difference yet), was wondering if any of you already studied this or maybe know more from the chemical side. If there is a slight bump in power, then thats a bonus to the savings. If the consensus is that it would hurt mpg, then its an issue of lost mpg vs. savings at the pump. Thanks.

Ryland · 03-03-2010, 11:28 AM

I avoid low grade as my mileage tends to drop, but that is hard to tell if it's the low grade gas or the ethanol that is in our low grade gas.
I also notice that my car tends to stumble more with low grade gas.

tim3058 · 03-03-2010, 12:09 PM

Quote:

Originally Posted by Ryland

I avoid low grade as my mileage tends to drop, but that is hard to tell if it's the low grade gas or the ethanol that is in our low grade gas.
I also notice that my car tends to stumble more with low grade gas.

Hmm, here in NY we have 10% in all the mixes as far as I know. I see what you are saying, ethanol may be more of an issue in low grade though. Are you running advanced timing?

TomO · 03-03-2010, 03:27 PM

I know in my VX it makes a nearly 2mpg difference using oxygenated (ethanol) or non-oxy gas (non-Oxy results in higher mpg). Only problem is: the non-Oxy is only available in 91 Octane. so my input is moot in the Octane level making a difference in mpgs.

Old Tele man · 03-03-2010, 08:58 PM

...I'm only mumbling an educated "guess" here, but I believe that lean-burn prefers higher-octane to ensure 'smooth-constant' flame propagation across the cylinder without creating combustion-induced "knock/ping" as the flame-front crowds across the cylinder.

Old Mechanic · 03-03-2010, 09:56 PM

I guess I'll have to try premium some day.

Nothing here but E10 regardless of the grade.

I run the cheapest gas I can buy.

I did try a tank of premium in my Insight and it seemed like the mileage dropped.

regards
Mech

redline5th · 03-04-2010, 01:14 AM

Make sure you run what is recommended in it. I think your motor is a SOHC--read up on what it takes. Don't try to run anything less than recommended.

I say this because some VTEC motors require higher levels of octane. I know mine runs on at least 91. I keep 93 in it most of the time.

tim3058 · 03-04-2010, 01:53 PM

Good point redline, I'm guessing it's 87 but I didn't actually check. Will do tonight. I had the head shaved .030 and the shop said it'd be more than fine on regular pump gas (didn't spec 86 specifically though). I'm driving pretty conservatively so I'd imagine pinging should be very minimal if at all.

Old tele man: Good thought, sounds like you know more than I do about it. Tank mpg is looking great although I'm taking my first noob steps at P&G also.

TomO · 03-04-2010, 01:57 PM

Octane Level Requirements are 87 minimum as per the Owner's Manual for the Civic VX, just an FYI

Old Mechanic · 03-04-2010, 05:04 PM

I read the lean burn threshold is 6 inches of manifold vacuum. If that is true then the effective compression would be atmospheric minus 6 inches, so I would tend to think spark knock would not be an issue.

I have never heard any trace of spark knock on my VX. At 53k miles there should not be any wear issues or EGR deterioration. I do run a can of BG 44 k occasionally to try to keep things clean. The inside of the engine (seen through the oil filler cap) is immaculate.

Last tank was 55 MPG with average temps around 40 degrees.

regards
Mech