Gas-octane rating 85,87,91-does it matter?

[knac1234]

Hello all,

So, I've noticed-and been reading-of gas prices going up.

I've been running premium (91 octane) in Colorado, simply because it was an 8:1 compression engine (rebuilt to 8.5:1 compression per the prev owner and records).

With this "higher" compression in mind, does it really matter whether I run Colorado's choices of 85,87, or 91 octane? I've read-maybe on here-that these engines were designed to run on pretty bad third world fuels even.

Keep in mind that I am at 9100 ft altitude, running a 2.25 petrol with Weber 34ich carb and the timing advanced. It is slow on hills of any kind, but on flat roads and downgrades "easily" hits 65.

Regards,
Julian

[Nium]

Less is more

Octane means detonation resistant. Therefor the higher the octane rating the less explosive the gasoline. Running octane 91 in an engine with 8:1 or 8.5:1 head is probably unnecessary. An octane rating of 85 will give you more bang for the buck and unless you get pinking across the power range there isn't a need to run higher octane fuel. Personally I run the lowest octane non-ethanol (Shell doesn't add ethanol around here in PA) fuel I can find.

Octane explanation links
http://www.btinternet.com/~madmole/Reference/RONMONPON.html

Keep in mind that I am at 9100 ft altitude, running a 2.25 petrol with Weber 34ich carb and the timing advanced. It is slow on hills of any kind, but on flat roads and downgrades "easily" hits 65.

Yeah, that sounds about right for a rover.

With the higher 91 octane fuel you would need to run the timing more advanced then with 85 octane.

My $0.02

[109 Pretender]

Niem answered your question accurately. Have you noticed "pinging" when going up a steep grade? If you have some pinging w/premium, it could be the result of the advanced timing. I would set the timing to stock and use the lowest grade of gas that doesn't cause pinging. If reg. gas works ok - no pinging - you could bump up the timing a couple degrees and retest.

Another good old "racers trick" is to set your timing with a vac. gauge. same as before - start w/stock settings on static timing. Adjust timing to achieve max vac. at idle. retard timing via distributor ~ 2" less than max vac. value. test drive on a hill under open throttle to see if it pings. (This method achieves max timing advance for YOUR engine in YOUR environment and accounts for the overall mech. state that YOUR engine is in). All factory settings are "normalized" to accomodate various states of driving styles/loads and different environmental conditions. It takes a couple tries usually but with a little work you can get it dialed in nicely.

Good Luck!

[siii8873]

Pretender,
when you say 2" do you mean 2" Hg or 2 psi? My vacuum guage is in psi.

[mongoswede]

8:1 compression is actually very low. The rover engines were designed to run on the crappiest fuel in the world and at 8:1 you should be able to get away easily with the lowest octane available. The flow characteristics and combustion chamber shape on the rover are not all that efficient and the head is not a cross flow design so its hard to get much efficiency from these engines. I believe the compression in my 650 dirt bike is 13:1 and its fuel injected and will run on 87 octane though I usually run high test. It sees less load and has less mass to move but it also has a much better design.

If you bumped the compression on your rover up into the 10:1 or 11:1 range then I'd start running the high test.

[knac1234]

Thanks for the tips. I was aware that with the lower octane (just by what the Haynes books says) that I should maybe have to retard the timing a bit.

As far as I can tell, no pinking in the current setup (can't hear anything...hopefully not due to all the other racket!!).

So, maybe I should try the 85 on the next fill up. The other thing that maybe has some bearing is that, unlike in PA, we have ethanol in the fuel in CO (I think about 10%).

Cheers,
Julian

[Jim-ME]

Try the lowest octane available. If you get pinging move up a grade. IMHO premium gas is an absolute waste of money in a Series Rover.
Jim

[Nium]

The other thing that maybe has some bearing is that, unlike in PA, we have ethanol in the fuel in CO (I think about 10%).

I just meant if you had any easy choice between a station that had 10% ethanol as opposed to one that didn't I'd use the gas without ethanol. In PA most stations have 10% ethanol added to the gas but for example Shell stations do not. I'll use either with or without ethanol just depends where I've stopped to fill up.

[stonefox]

Pinging

I've heard reference to ''pinging'' before ,have never experienced it and not sure I would know it if I did. Can someone explain the sound?

[Gilberto]

PING-RATTLE

Happy new year from Tampico, Mexico.
You will notice, it is like a baby'a rattle, no way to pass it undetected.
Be careful if pings, slow down.

Cheers,

Gilberto

[xsbowes]

Where in Colorado are you? This link provides ethanol free gas stations:

http://pure-gas.org/index.jsp?stateprov=CO

Read a news article this morning stating that ethanol price is now higher than gas. And it gives you lower mileage.

[Nium]

Pinging = spark knock

It's that kinda knocking/pinging sound you hear under acceleration when the gas/air mixture detonates prematurely in the combustion chamber. Due to the timing being to advanced or the fuel having to low of an octane rating for the compression ratio of the engine.

[luckyjoe]

[QUOTE=109
Another good old "racers trick" is to set your timing with a vac. gauge[/QUOTE]

I've not heard this before. What is "max. vac" for the 2.25p?

[meatblanket]

Go ahead and use the 85 octane fuel. Octane requirements are less with high altitude use. 85 worked fine for me at 5000 ft. with a similar set up, your octane requirements are even less.

Try to stay away from ethanol if you can, particularly in warmer weather as it will increase the likelihood of vapor lock.

[109 Pretender]

To Siii,

It sounds like your vac. gauge is a combo unit. Vacuum is measured in inches of mercury not psi. Engine vac is always less than ambient pressure.

As a general rule of thumb a good engine in mech. order and proper state of tune should run ~ 14" Hg at idle. (This is a general ballpark figure). It will of course drop to 0" Hg when the throttle is cracked open. As the rmp's are increased to about 3K, 3.5K rpm you should see a gradual rise in vac. to a max. around say 18 - 20+ inches vac. (All your centrifugal timing should be in by around these rpm ranges).

All tuning w/a vac. gauge does is optimize your setup to provide max. tune for the given fuel, altitude, temp, baro. pressure, and condition of engine mechanically as well as avail. spark quality and static timing settings.

The factory timing specs for all manufacturers are middle of the road because they have to account for various differences in the aforementioned variables.

A variable timing light is easier to use and most engines can be bumped a couple degrees w/o adverse effects. However, OP has a situation where he is high altitude and travels up long grades - THE single best situation to find timing and/or fuel problems because this condition stresses the system much more than steady state low load driving conditions. An 8:1 engine should run fine on 87 octane w/o pinging. If pinging when using 91 octane I'd suspect timing problems or stuck weights in cent. mechanism of distributor or cyl. combustion chamber carbon buildup. Actually, stuck weights usually stick closed (my experience) which means you are running very retarded above idle. This will cause the engine to have zero power and the exh. manifold will glow bright red which is about 2000+ degrees temp. You need a total of around 30-35 degrees advance on most engines at 3k rpm. With stuck weights you'd have only the static base of maybe 6 degrees.

I say ck timing w/good light - set to stock - run 87 octane and try it on a good steep hill and see if it pings.