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octane and timing

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Old 05-08-2007, 12:23 PM
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Default octane and timing

So i'm told that I can run 87 octane in my motor, no problem. Most of the gas docks on our lake only have 89 or 93, but I generally fill up on shore anyway.

Last year I ran 89, and had the timing bumped to 11 degrees base, 31 total (stock is 8 base, 28 total). This year I rebuilt the carb, filled it up with regular, and set the timing back to stock.

My question is, can I safely run any extra timing with 87 octane? Should I stick with the stock timing and run 87, run 2-3 degrees extra and run 89, or run 2-3 degrees extra and stick with 87?

Motor is a 502 mag, carbed. totally stock except a rebuild a couple years ago. 20 degree t-bolt IV module. stock timing is 8 degrees base, 28 degrees total. When I bumped the timing up to 31 total, i picked up a couple mph, but that was running 89 octane.

Also, under what circumstances do I need to be concerned about running E10? I understand the engine will run leaner on 10% ethanol, is the carb set up rich enough from the factory to deal with this?

Last edited by sleeper_dave; 05-08-2007 at 12:36 PM.
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Old 05-08-2007, 01:28 PM
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If you have Thunderbolt V ignition with knock control, I'd say give the extra timing a try. You will know if there is a problem because if she knocks I believe it will retard 10...

If you have Thunderbolt IV ignition, I'd stay with stock timing. I cooked a Gen IV engine running 12, 22, 34 total...
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Old 05-08-2007, 03:08 PM
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20 degree t-bolt IV module, are you sure its not a 24?

I have run 34 for years, last year I ran 36 all year with aluminum heads.
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Old 05-08-2007, 03:46 PM
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Guessing it's a Tbolt IV with NO knock sensor.
Not sure why that motor was set up with only 28 total. The 454 got 32 total (no knock sensor), and T Bolt V 502 can run up to 34 total.
Play it safe and leave at 28 and buy 87 (save a couple bucks). Merc usually does conservative stuff for a reason.
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Old 05-08-2007, 08:47 PM
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My Mark IV 502 Mag came through with a 20* module in it. I changed it to the 24* module all in at 3750RPM I believe. I set that base timing at 10* and run only 93 octane with no issues. I have tried the base timimg at 12* but didn't really notice a change in topend so I thought I would play it safe and bring it back to 10*.
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Old 05-09-2007, 02:03 AM
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I see no reason why you can't run total timing at 32* and run 87 octane.

The 502/390hp has the same internals and specs as the carbed 454mag. The only difference I'm aware of is the cubic inches.
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Old 05-09-2007, 09:04 AM
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Hmm. A mixed bag of responses.

It is a 20 degree module, and from reading here that is stock on the 502's. 20T I believe, i downloaded the advance curve from OSO last year. T-bolt IV, so no knock sensing.

I switched to a hotter thermostat yesterday (160F) to see if i could gain some efficiency and reduce wear. The boat ran 20 hotter, which should help efficiency and wear, and I don't think that should affect the tuning of the engine significantly. From reading here it seems that closed cooling motors get the hotter thermostat, and the only reason theorized for this is salt cooking out of the water and clogging up the motor. I run in freshwater only so i'm not so worried about that, we'll see how that works out with my next oil analysis.

The boat is running a bit slower now, down about 3 mph. But I also added 80 lbs of ballast (sand) to the front of the boat to improve planing and ride in light swells, so that probably cost speed too. I didn't trim up as much as I have in the past, either, given sea conditions. So I can probably get a little more out of it as-is.

I'm thinking maybe i'll go back to running 89 and bump the timing up to 10-11 base, 30-31 total. When I bumped the timing up before, i picked up a couple mph. I am scared of detonation, though, so I think I'll stick with 89 if i bump the timing up. The 10 difference in price at $3.15 a gallon is negligable, and if I'm picking up power in the midrange i'm picking up efficiency too.

I did some reading about my ethanol question as well. It seems that even without ethanol, the stoiciometric a/f ratio for gas with MTBE is about the same as gas with ethanol. I believe all fuel contains one or the other, and ethanol is more prevalent now due to environmental issues with MTBE.

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Old 05-09-2007, 12:46 PM
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You will not detonate that engine with 32* timing and 87 octane. Its 8.75 to 1 CR and has a very mild cam for a 502. The 454 mag has the same CR and same cam and runs 32*.
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Old 05-09-2007, 01:31 PM
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Any ideas why only 28 degrees total on the 502?
Anyone?
Thought maybe because siamesed blocks didn't cool as well.
Gary

Quote:
Originally Posted by Griff View Post
You will not detonate that engine with 32* timing and 87 octane. Its 8.75 to 1 CR and has a very mild cam for a 502. The 454 mag has the same CR and same cam and runs 32*.
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Old 05-09-2007, 02:08 PM
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Quote:
Originally Posted by Griff View Post
You will not detonate that engine with 32* timing and 87 octane. Its 8.75 to 1 CR and has a very mild cam for a 502. The 454 mag has the same CR and same cam and runs 32*.


Yes, the 454mag and 502mag have the same cam profile, but the initial timing of the two is 5 different.
Also, the compression ratio of the 454mag is slightly less because of the smaller bore. I've read the 454mag is closer to 8.5:1.
These reasons may be why Merc ran a different advance module on the two engines.
IMHO, I would not increase the timing without knock control or running higher octane.

Like I said in my earlier post, I did cook the pistons in a Gen IV running 34. Not the same engine though, it was a 1989 7.4 with cast pistons.
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Last edited by Pat McPherson; 05-09-2007 at 02:15 PM.
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