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MG MGB Technical - Higher octain/higer idle

I have recently gone from using a 89/91 octaine to a 93. Ever since then I have noticed the idle has gone up about 200 rpm's higher.

Has anyone come cross this before? I think when the tank gets low again I will go back to a lower one and see if the idle drops
GG Ginty

Octane simply makes gasoline harder to ignite, permitting higher compression without detonation. The fuel enables the engine to make more power, but is not more powerful than lower grade gasoline in and of itself.

You've changed fuel - did you change from straight gas to "E10" - that is, gasoline with 10% ethanol? Or the other way around?

I've noticed that my fuel economy on my "regular" car is now about 31-32 mpg, when it had been 29-30 for years. The "10% ethanol" stickers are no longer on the pumps at the station I use - I wonder if there's a connection as ethanol contains less energy than gasoline.

Perhaps the car runs better with higher octane, as it's "tuned" for it - but I'm surprised about a change in idle, as the engine is not under load.

Report back when you switch back - but be mindful of other factors, too.

John Z

I question the ethanol possibility too. Usually, your gas mileage will decrease with added ethanol because the required air/fuel ratio is almost half of gasoline meaning you must burn much more ethanol to get the energy out. This means with a fixed jet system, you would run leaner and leaner as more and more ethanol is added to the gas. A lot of times your carb adjustment is set so that slightly leaner will make the idle faster-- this is how you check you SUs by raising the piston slightly to see if the idle speeds up...
CD Dewey

I will have to check to see what the petrol station has listed in terms of ethanol etc. My B once warmed up in the morning runs very strong. Just a little alarmed at the increase in idle. It does sounds as though its working "overtime" when I complete my drive into work(3 miles) and I come to a stop to park.

Not sure what you mean about raising the piston slighty. Please accept my ignorance!
GG Ginty

Reformulated fuel (10% ethanol) may drop the idle as it has a lower energy content then normal fuel
John H

Change the octane, change the timing.
Dan Robinson

Where is the problem? A higher octane increases engine output. Keep it on and adjust the carbs consequently.
R.G
Renou

PLease read my post, Renou. Higher octane does NOT increase output. It's a common misperception. Higher octane permits higher compression - that's what increases engine output.

In today's engines with knock sensors and electronically controlled advance, many engines do get a "bump" in output with premium fuel - but its due to the car's ability to adjust the timing on the fly.

An MG, unless it has such an advanced system would not benefit.

The make up of the fuel itself might account for the change. The nature of the drive and underhood heat may have something do with it, too. After a short drive (a few miles) my idle is somewhat stable. After a long drive, the idle tends to move up a bit - I have always put it down to cleaner plugs and perhaps higher temps permitting more active vaporization. Never worried about it, though.
John Z.

"Change the octane, change the timing"

Exactly. Whilst changing to a higher octane in an MGB in itself won't give you more power, on a high compression engine at least higher octane will allow you to run with more advanced timing, which *will* give more power, and better economy when you aren't using the power. The formulation and grade of fuel available today is so very different from that available in the 60s and 70s that the manufacturers specs for timing and advance curves are not really relevant any more, even if every mechanical component on your engine were to spec which probably very few if any are anyway. Short of spending a lot of time on a rolling road designing centrifugal and vacuum advance curves and optimum timing for *your* fuel in *your* engine the best approximation is the highest advance it will run short of pinking at any combination of throttle opening, revs and load. Even then you will get different effects at different altitudes and terrain.

The holy grail would be a knock-sensing retard distributor, then you would have the ideal timing under any conditions including fuel grade.
Paul Hunt 2

Hi GG,

Just wondering.. the weather over here has just turned a little colder. I was wondering whether your change in fuel happened to coincide with a drop in temperature, hence increased charge?

Still, 200 rpm would be quite a lot.
Alec

Not sure about the drop in temperature as it has been quite mild of late. I have been paying attention to signs at the station to see if they say 10% ethanol. Must admit that it is listed at two places I stop at, not sure about my regular place.

My idle was running quite high towards the end of last week. When fully warm and had been running for couple of miles, it was about 1250-1300 at stop signs and the engine sounded like it was working overtime. So I got her back and adjusted the idle until it was just below 1000. So far when full warm it does not go above 1000 at stop signs etc, engine doesn't sound like its working overtime etc. The only trouble is starting when cold. The idle is quite low, about 600 when I first start her up and have to hold the choke out a little to prevent stalling, at this stage it sounds a little "bumpy" until the temperature rises and then off I go.

When I bought the 74 a little over two years ago I was told that it might have to be tweeked occasionally as the engine got "broke" in. The engine has been completely redone, bored over, new everything. I've put on a little under 3000 miles in that time.

Guess I am going through a learning phase!

Cheers and thanks
GG Ginty

Heck, my idle will vary by as much as 200 rpm from one stoplight to the next. It's probably time to rebuild/fine-tune my HIFs - a trip to Joe Curto is in mind. But it seems all my MGBs have been this way over the years. I guess the thing common to all of them was the owner/mechanic. Hmmm...

FWIW,
Allen
Allen Bachelder

"The only trouble is starting when cold"

But the 'trouble' you describe sounds normal to me, and is why we have the choke and fast idle control. I start my roadster on full choke then immediately push it back to half. Then I drive off and as the temp gauge starts rising I can push the choke in bit by bit, until by the time the gauge is 2/3rds the way to N it is fully back. The V8 is a little different in that it also needs full choke to start, but I can only push that in a little way immediately or it stalls, otherwise that also goes back in bit by bit as the temp gauge rises. How much choke you need also depends on how much throttle you use, I only use gentle throttle until it is getting up to N, if I used more it would need more choke.

It is probably normal for a fully rebuilt engine to be a bit tight initially, then as it loosens up the idle *will* go up, and need turning down.
Paul Hunt 2

3 MILES TO WORK!! For that kind of distance walk or take a bike. Better for you, better for your MG. Your poor engine does not even get up to normal running temperature.
Lee

The premium stuff is a little bit denser (0.78 kg/L instead of 0.75) so you will be running a tiny bit richer. My V8 will sometimes idle a little faster if I use 98 octane. Could be that you got a particularly good batch of petrol. The blenders in the refineries try not to give you too much of the good stuff (light alkylate etc), but sometime they have no choice. It could also be that you did have a little bit of preignition going on, due to carbon buildup etc.
Peter Sherman

>3 MILES TO WORK!! For that kind of distance walk or take a bike. Better for you, better for your MG.

Agreed! Your engine won't heat up in 3 miles, so the water (water is a byproduct of combustion) and fuel that make it into the oil won't get driven out, so you'll get accelerated engine wear. The exhaust won't get heated up, so it will rust out pronto. Likewise, the trans and diff won't heat up so if there is any condesation in them, it will remain there.

At 10 mph on a bike (which is a slow, comfortable pace), it'll take only 18 minutes to ride 3 miles, and you'll be better off for having done it! ;-)
Rob Edwards

"3 MILES TO WORK!! For that kind of distance walk or take a bike"

NOT agreed. Find a longer route to drive your MGB to work and enjoy.
Paul Hunt 2

Paul,

Thanks. Must admit that the car is driving nice and the idle is now below 1000 when fully warm. As a matter of a fact a friend of mine lives above my "local" and said that he heard me start up and take off the other day and commented on how much better it sounded since I turned the idle down.
GG Ginty

This thread was discussed between 25/09/2007 and 04/10/2007

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