Scarab P-29

Just wondered if anyone could tell me how to set the timing on a MPI motor? Does having aluminum heads have anything to do with it? Thanxs

mcollinstn

Should be no big deal. Aluminum heads - why would that have anything to do with it?

Some electronic advance ignitions require a wire to be disconnected or a jumper put across some terminals to disable the advancer which then allows timing to be set normally (otherwise, the advancer is frying to mess with the timing while you are setting it).

I'm sure somebody will know the exact procedure for you.

Scarab P-29

I was looking for overall timing. Someone told me I could give it a Little more timing because the chamber cools faster? Does that sound right? 34 was my original overall.

mcollinstn

yeah, i think aluminum os good for a couple of degrees. Some Merc motors have a knock sensor, though, that can pull your timing back out. Make sure you have good plug wires and solid grounds or else electrical noise will fool the computer into backing down your timing.

Audiofn

I always thought that advancing your timing was only good for bottom end power not top end?????

Jon

mopower

Nooooooooooooo, the flame burn rate remains constant. As the rpm increases you need to give the spark more of a head start to ignite the fuel because the piston will be at tdc quicker. If you didn''t do this and left it at ,say, 8* adv. at 5000 the flame would be reaching its maximim explosion at around 26* after tdc. The idea is to advance iit enough that the max. combustion occurs when the pistons at the top Now if you have too much initial timing...when you crank the motor over ,it reaches max.while the piston is still coming up and as a result it's very hard to start(sounds like a bad battery). Once the engine is running if it's too far advance you get that crunching noise also now as "pinging" now get off of here and get back to work on yu boat

gofastpilot

P-29 Scarab, To set the timing on a MPI,TBI or any other Mercruiser ECM controlled engine you need to borrow or buy a plug in timing tool. It gets plugg into the wiring harness.I for the sake of me cannot remember the name of the company you can buy the tool from but you can also use it as a trouble scanning device by counting the number of flashes on its l.e.d. I own one of these but its in my shop and not in front of me as we speak. Also depending on the year of your setup there are different tools for different years...............gofast

mcollinstn

Audio

the way I understand it is that any given "combustion chamber shape/compression ratio/piston configuration" will have a fairly consistent optimum spark advance for best power. Obviously, if a particular cam spec or fuel requirement is out of range for that optimum then the best results may be met with more spark advance. This, though, is more of an indication of a less than optimal combination of parts.

As far as the "less advance" for top end, this is more of a dragrace thing. Let's think of combustion pressures. With a given fuel, there is a specific peak combustion pressure before knock occurs. Above that pressure, knock occurs (detonation).

In a drag motor, optimal scavenging may occur at a high rpm. At that high rpm, the engine can be running at well over 100% volumetric efficiency (which is kind of like "momentum" supercharging). At this point of best scavenging, combustion pressures will rise due to the increase in cylinder charge.

This may require the spark to be no greater advance than "x" BTDC. In order to squeeze the best acceleration from the motor, the spark might can be advanced to "X plus 3" through the mid to upper midrange rpm due to less than optimal scavenging at those rpm. For this reason, you can easily find dragrace ignitions that allow topend retard functions.

It's not that topend requires less advance. It's more of a midrange can tolerate and possibly benefit from more advance.

At least this is the way I understand it to work.

**
As far as the guy who started the thread, he is already running a couple of degrees less than usual (34 vs 36). He is asking if he should feel safe enough to go back to 36 since he has installed aluminum heads.

M

FindMe

service bulletin
No. 97-15 TO: SERVICE MANAGER TECHNICIANS
PARTS MANAGER
Timing 1998 350 Mag MPI and 7.4L MPI Models
Models
MCM 350 Mag MPI Alpha and Bravo Sterndrive Engines: Serial Number 0L000001 and above.
MIE 350 Mag MPI Ski and 350 Mag MPI Inboard Engines: Serial Number 0L000001 and above.
MCM 7.4L MPI Sterndrive Engines: Serial Number 0L000001 and above.
MIE 7.4L MPI Inboard Engines: Serial Number 0L000001 and above.
NOTE: This does not apply to any other 1998 model EFI or MPI engines.
Timing Procedure Change
The Delco ECM used on these 6 engines is different from the other engines for 1998 and for 1997 and older models. Because of this, there is a slight change to the ignition timing procedure.
1. With all other ECMs, the engine’s speed has to be manually advanced to 1800 RPM before setting the ignition timing.
2. The ECMs used on the 1998 engines listed above have the RPM advance programmed in them. The
engine speed will automatically increase to a predetermined RPM when the timing tool is connected to the DLC harness connector or the scan tool is put in the ’service/timing mode’.
3. The rest of the timing procedure remains the same as before.