Timing question fe390?

[MaSnaka]

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What would be the correct setting for my timing with the car idling and my vacuum pluged. I have a fe390 with cam set for 230 intake and exhaust at .050 duration, .3063 lift and 110 seperation. Edelbrock 390 performer manifold and Holley 750 cfm double pumper. Engine is bored over to a 410 ci. Vacuum advance on the distributer. Any (intelegent) help would be great. Thanks

[tpiini]

Just an opinion mind you, but with that cam I'd go for about 15 degrees initial & shut down the mechanical advance so that you top out at about 36 degrees total mechanical. You can play a bit with the total and obviously the vacuum would hav to be tuned for whatever works for the best mileage without detonation.

[troll]

If your pulling a horse trailer up a big long mountain you will need the vacuam advance other then that dont use it.Weve lost some much of that over the years.Model T's had a timing lever on them.If you didnt turn it down while pulling you would over heat the pistons.The vacuam advance replaced that.If your just driving around drop the timing way down.If your looking for power then turn it up high,but dont leave it there.Get a mechanical that way you timing will stay with the RPM up to 2500 RPM even at WOT.WOT the vacuam drops off and goes back to static + mechanical anyway ,thats what they do and why they are there.Cool pistons all day,and hot pistons for racing.High advance will kill your starter motor it will start preigniting and you wont hear it.I had a 289 HiPo when I was younger it had no vacuam controls and no trialer hitch.

[Excaliber]

The biggest fear with vacuum advance is TO MUCH total advance (BASE, plus MECHANICAL plus VACUUM added together). But at what inches of vacuum does it start to advance the timing and by how much? Distributor 'curve' is what were talking about here, and it's tough to nail down.

Normally replica Cobras need more BASE timing advance than other engines because of the aggressive cam profile (compared to street cars). If you don't get enough BASE advance the carb will stumble and you'll have a lack of power until a certain rpm is reached (1800-2000 or so). Around 1800-2000 the mechanical advance starts working, so the motor starts running better.

The new MSD vacuum advance distributors don't even start working until 15" of vacuum. What YOUR distributor works at is a mystery, could be as low as 7", typically around 11" or 12". With an agressive cam profile you might not make 15" even at cruise rpm and light throttle. 11" or 12" is common with Cobra motors.

MSD, in consultation with their 'lawyers', are really conservative with their distributors 'as shipped'. Vacuum advance (non adjustable, assumes you have such a model) is WAY high at 15". Mechanical advance doesn't even start working before 4000 rpm (all units). Total advance is fairly limited (all units). To work properly they HAVE to be 'set up' and that shifts the responsibilty to the consumer! Wise people these MSD folks...

Rule of thumb is: Run as much advance as you can without detonation for maximum performance AND fuel economy. The FIRST thing manufacturers did to control emissions was retard the timing, the FIRST thing that did was reduce horse power!

You CAN run pretty advanced timing (beyond) 40 degrees utilizing the vacuum advance. This assumes crusing speed, light load, high intake manifold vacuum. This might be a GOOD thing, more advance, more efficiency, better mpg.

15 base, even more, may be required depending, again, on your cam\carb profile. There is no 'magic number', all custom engines vary in what they like.

I run about 18 base, 30 some total AND have a vacuum advance. But I got a fairly radical cam and twin carbs, I NEED a lot of base advance timing. Had to buy a special 'geared starter' to crank that engine over with the big base advance.

[MaSnaka]

You guys are making me realize I know very little about timing. I thought it was a simple adjustment. I purchased my car complete, I'm the third owner. The car was supposedly tuned for 6000 foot elevation. I live in So. California closer to 700 foot elevationl. I was chasing an off idle stumble and was following another thread here and so I am trying to start with the simple things first. By the way Excaliber you nailed it...slugish or stubled off the line until it hits 1800 rpms and then it goes like crazy. I replaced the plugs, distributor cap and rotor (Mallory Unilite with the MSD ignition). That didn't really help. Accelerator pump checks out but a slight delay in getting a squirt is due to a Acc. pump cam (green) installed backwards. There was still some play in the linkage even after I reversed it. I replaced it with the red cam and that made a big difference. But getting some backfires through the carb although nothing severe. So I checked the timing. It was idling at 6 degrees and I read a post concerning a 351 cleveland that suggested 18 would be good. I turned the distributor and got instant positive results, engine idles much smoother and seems happy. It is set at 16 degrees but that still isn't right, so that brought me to question the correct setting. I will turn it back some and see what 10 -12 gives me. I don't know and I hope your willing to enlighten me on some terminology you have used. I understand the vacuum advance, but there is also a mechanical advance? Built into the dist. I'm assuming. And base advance I'm assuming is what the engine has at idle no vacuum hooked up or is vacuum hooked up? What do you mean when you refer to detonation and fuel economy? How do I know when I have detonation? Sorry I don't mean to sound stupid but I need to learn this stuff. Thanks for your effort and patience with me.

[Excaliber]

Base timing is the 18 or so you have it set at now. As the rpm increases their are weights and springs just under the distributor ROTOR that begins to 'advance' the timing. Changing the springs (you could also change the weights) to a lighter weight spring makes the advance occur at an earlier rpm.

The vacuum advance should be disconnected and plugged to make sure it is NOT working when you set the base timing.

If this is a new MSD then it will NOT give you any mechanical advance until about 4000 rpm, unless it has all ready been modified (by changing to softer springs). There is also a little 'button' like thing that stops the mechanical advance from going TO far. Changing that 'button' moves the maximum allowed up or down the scale. Stock button should be fine, stock springs are NOT fine.

Detonation, ping, knock often mean the same thing. Detonation is the air\fuel ratio mixture 'igniting' either prematurly (to much advance) or in an uncontrolled (to fast) manner. This sudden pressure build-up of the fuel\air burning pushes the piston DOWN in the cylinder bore. THATS fundamentally how an engine works. IF the piston is still COMING UP and is not READY to GO DOWN and all that fuel\air burning pressure happens early, something is going to break! The piston will get extremely hot and actually start to break apart if it is severe OR prolonged detonation (ping, knock). Generally you can HEAR this "ping" sound and you know something is wrong. Cobra sidepipes are so loud you may not be able to hear it until it's to late. Detonation is common to high compression engines running low octane fuel. Higher octane inhibits detonation, thats WHY they make it and not all cars need it.

Timing is everything, it NEEDS to be set correctly some how.

[Excaliber]

One more time...

When the spark plug fires it ignites the mixture of fuel\air which has been COMPRESSED by the piston moving up in the cylinder bore and the valves are closed. There is no escape for the fuel\air, it is trapped and compressed and ignited by the plug. BOOM, big fire inside the motor, GREAT pressure now being exerted on the piston. But this is NOT an 'explosion', it is a very accellerated and yet 'controlled burn'.

Because it takes a little bit of 'time' for the 'burn' to reach maximum pressure we want to start the 'burn' a little bit before the piston actually reaches TDC (top dead center). We want to fire the spark plug maybe 15 to 18 degress BTDC (BEFORE top dead center). As the engine rpm increases we need to fire the fuel\air mixture even earlier than that. Because, well the piston is really flying now, things are happening REAL fast, we need to stay ahead of the game! 30-35 degrees TOTAL timing is in the ball park for max rpm operation. We want MAXIMUM cylinder pressure to be developed precisely at the point where the piston is at TDC and ready to go DOWN. BUT not before that, which is 'detonation'.

This is a simplified version. In fact we want the pressure build up to CONTINUE even AS the piston is moving down, so not 'exactly' at TDC. But I hope you get the point...

[MaSnaka]

Excaliber, Thanks for your input. You have certainly given me a clear explination on principals and a good starting point to super tune my timing. I am not sure at what my dist. is set at but it's a Mallory Unilite not a MSD. Are the pre-sets similar comming from the factory? And how will I know whats under the rotor...are the springs and weights identifiable as whats stock or modified. I don't know what the other guys mech. did to tweek my engine. What I am finding is not impressing me as to his knowledge. Thanks John

[Excaliber]

OH, Mallory not MSD. Mallory is much harder to work with and the mechanical advance unit is not at all user friendly or even visible.

You will need a timing light to watch the timing mark on the harmonic balancer to determine if\how much it advances at what rpm. This could be a job for a skilled mechanic at this point.

The questions are:
How much IS it advancing, the TOTAL advance amount?
At what rpm does it START to advance?
At what rpm does it STOP advancing?

If the answers are reasonable and in the ball park, your good to go. If not, see the Mallory Web Site for details on how to adjust the mechanical advance.

[MaSnaka]

Excaliber, I'll have to get back to you with specifics but right now I can tell you with the timing light at 800-900 rpms I'm at 12 degrees, reved to about 2500-3000 it advances to 30-35 degrees. I need a buddy or remote rpm gauge to be more specific. I can't read the harmonic balancer, hold the timing light, reach the carb. and watch the tach all at once. Maybe my wife will help...hmmmm. It might be a couple days or so before I get there. Don't give up on me I'll be back. Thanks!!

[Excaliber]

Your in the ball park right now, the numbers sound reasonable. IF you had a NEW MSD, you would have to make adjustments. No worries!

I ran a Mallory myself, until it wore out and I recently went to MSD. Either one is fine. I prefer MSD because it's much easier to 'dial in' to very specific numbers. Assuming you know WHAT you want for numbers.