Octane

[Zippy]

Because of the pinging in my engine and the subsequent destruction of the head gasket I have been trying to figure out what to do. I posted what's below to my thread on the head gasket but decided to move it here.

The manual in my glove box for the 1800 Spider says to use 91 Octane (Research Method) That’s RON.

Okay, so I did a little research on octane. I searched the web and found the quote below attributed to Chevron Fuels Technical Service.

“The Antiknock index is defined as (RON+MON)/2 (RON + MON divided by 2). The RON is typically 97.5 for 91-92 AKI premium gasoline. MON for premium is about 87.3.”

Chevron uses the AKI rating on the pump. What this tells me is that I’m at 97.5 RON octane when I buy Chevron gas with a pump rating of 91 AKI

I found a chart on Wikipedia that shows the relationship between AKI and RON
From the chart: “regular” gasoline in Canada and the US RON 91-92 MON 82-83 AKI 87
An AKI of 87 equals a 91-92 RON so should we all be using regular in our cars? Am I missing something here? Does this mean I need a 100 or 110 octane fuel for my high compression engine?

Source of Chevron Quote: http://forums.nasioc.com/forums/archive ... 92996.html

Source of octane chart: http://en.wikipedia.org/wiki/Octane_rating

Any ideas or suggestions?

[So Cal Mark]

when does pinging occur? low rpm, heavy load; high rpm, light load, or some other circumstance.
What is your compression? What is your initial timing? What is your total maximum timing? What rpm is your total advance all in? What a/f ratio do you have throughout the rpm range? How hot does your engine run?
All of these questions need answers to determine the cause of your trouble

[maytag]

Yeah, I'm with Mark on this one. I would spend all that time working-out WHY you're getting detonatio (pinging) and resolve that, rather than trying to understand the aki/ron/mon-doola.

I've run some seriously 'hot' motors (upwards of 12:1, with Iron heads) on the street before, using pump gas. I've had race motors that actually developed MORE hp on the dyno using regular (87) than with premium (91).
Until we absolutely know otherwise, I would consider this is more a tuning and build issue than an octane-rating issue.

Along with the questions Mark asked, I'd like to know the state/condition of the valves and head, as well as the piston tops. It's very common for people to spend a lot of money on parts, but if they miss a small burr, or knick a piston in the "quench" area and develop a hot-spot, they're gonna have pinging issues. just gonna. no matter the timing or the octane.

[Zippy]

Mark,

Thanks for the info. Here's what I have so far.

When does pinging occur? low rpm, heavy load; high rpm, light load, or some other circumstance.
Answer: above 4500 RPM under heavy load. It sounds like it does it more when the secondary opens
What is your compression?
Answer: Do you mean piston? 9.8:1 Or cylinder? 172, 180, 188, 183
What is your initial timing?
Answer: 2.5 BTC. After doing the head gasket replacement last year I set it at 10 BTC. That’s where the pinging is most noticeable. After driving the car for a couple of weeks this year the engine started sputtering and hard to start and had poor acceleration from standing. I checked the timing and found it at 0. I must have not tightened the bolt enough because since last year it moved. I reset it to 10 and since I was road testing I did some hard acceleration and that’s when I noticed the pinging. I can’t hear it above the radio or on the freeway so that’s probably why I didn’t notice it sooner. I reset it at 5 BTC and it was slightly improved so I set it a 2.5 BTC and its better still but now the acceleration from standing is getting worse. FYI at 0 it is very hard to start and sputters on acceleration.
What is your total maximum timing?
Answer: about 32
What rpm is your total advance all in?
Answer 4500 RPM
What a/f ratio do you have throughout the rpm range?
Answer: I don’t know. I will have to figure out how to check that. I have never done it before. Any tips?
How hot does your engine run?
Answer: Driving it sits pretty much at about 195 F. Idle in traffic fluctuates between 200 and 230 F.

Just so you know, I have no emissions on the car.

Thanks again for the help.

[So Cal Mark]

That's pretty good info, except the answer about timing is a little vague. You wrote that you have 32 degrees of total timing, yet your initial timing ranges from 0 to 10. Is it 32 at 4500 including your initial timing of 10?
Your engine temp is high, I'd get that radiator checked for flow. It shouldn't be running over 200. I also think you should check the a/f ratio throughout the rpm range, your secondary jet may be too small

[Zippy]

Okay let me see if I can clear this up.
"You wrote that you have 32 degrees of total timing, yet your initial timing ranges from 0 to 10. Is it 32 at 4500 including your initial timing of 10?"
Answer: I rechecked it this morning trying to be more accurate. The initial timing is set at 2.5 BTDC at idle. I put a mark at 36, as I rev the engine the new mark moves toward the TDC mark but stops short of 36 at 32 BTDC at 5000 RPM. At 3000 RPM its 11 BTDC, at 3500 RPM its 15 BTDC, at 4000 its 20 BTDC, at 4500 RPM its 30 BTDC, at 5000 RPM its 32 BTDC and thats where it stops.

"Your engine temp is high, I'd get that radiator checked for flow. It shouldn't be running over 200."
Answer: I will pull the radiator and have it checked out but do I need to do that before I figure out what to do about the pinging? Is that temperature high enough to cause the pinging?

"I also think you should check the a/f ratio throughout the rpm range, your secondary jet may be too small."
Answer: So too lean could be causing the pinging? Is this something I can check myself? Is there a gauge I can use without welding a bung on the exhaust? Should I take it back to the carb rebuilder and have him do it? What do I tell him I am looking for? I take it I will need to have the advance under control first?

We are getting into areas I have absolutly no experience in but I want to learn and try to do as much as I can myself . It is a lot more advanced than just maintaining a normal car because of the high performance mods the PO did.

Thanks again.

Al

[So Cal Mark]

it could just be a combination of things; too much advance, too high temp, too lean mixture. I'm surprised to hear your distributor has 30 degrees of advance. Is it all mechanical?

[Zippy]

it could just be a combination of things; too much advance, too high temp, too lean mixture. I'm surprised to hear your distributor has 30 degrees of advance. Is it all mechanical?

Yes it is mechanical. Could I be mistaken? I took the circumfrence of the crank and came up with 18.4 inches, divided by 10 and got 1.8 inches. I measured 1.8 inches from the TDC mark clockwise and put a new mark at that point.That should be 36 degrees.

How about checking air/fuel ratio. What should I do there? Next step.

I appreciate all your advice.

Thanks

Al

[Zippy]

So I guess my next step is to run down to the carb shop and have him check my air fuel ratio. What am I looking for here? Do I need to tell him what to do specifically or will he know?

Thanks

[So Cal Mark]

since you wrote that the pinging occurs when the secondaries open, you're probably going to have to check the a/f ratio on a dyno or with a wideband sensor while driving.
I'd sure take a look at the mechanical advance, I haven't seen one with that much advance. Just off the cuff, I'd say you'd be better off to reduce the advance and get more initial timing

[Zippy]

Mark,
Thanks for the help! I will try to find a way to get the a/f ratio but it may take awhile. I like the idea of reducing the overall advance and increasing the initial advance as it starts and runs better for most of my driving at 10 degrees. It's just when I really step on it that it pings and I don't want to have to do another head gasket for a while.
Al

[majicwrench]

Checking A/F ratio is pain in the butt, you are going to find it hard to find someone to do it, and is going to be hard to find someone who will get it accurate anyway. If it quits pinging when timing is retarded, is likely not the fuel ratio. I would work on everything else first, A/F ratio is going to be like chasing ghosts.
Is quick and easy to check and make sure the TDC mark is really TDC, and that needs to be done.
From a modern perspective, nothing wrong with an engine that gets over 200deg f. Heck the electric fans on 80's Chevs didn't come on till 230, and most other car makes basicly the same.
Keith

[So Cal Mark]

230 yes with a system designed to run that temp, but a properly working Spider cooling system should maintain a temp no higher than 200. If Fiat intended the Spider to run at 230, I'm sure they would have tuned the engine accordingly, i.e. less advance, richer mixture, etc