ignition curves for turbo engines
 

Ive been toying with the idea of changing the curve in my distributors. The engine is a 454, iron stock square port heads, 7.5 compression with a draw thru twin turbo system. I have a complete MSD ignition on the boat now. (billet distributors, 6AL boxes, and boost timing retard) I pull 2 degrees for every lb of boost. I am setting my boost at 10 lbs. My initial should be about 8 degrees with total a total of 36 degrees before pulling timing. I'm thinking of recurving the distributor with the lightest springs to give me more advance before boost kicks in. According to MSD directions, if I use the silver springs and the silver advance stop bushing. I should have 26 distributor degrees plus my 8 degrees if initial advance for a total of 34 degrees all in by 2100 rpm. What I am trying to do is get the engine to idle better, especially when cold.

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Your timing curve should look like a smile with lower timing at peak torque and higher before and after peak torque.

Twin turbo small block I have runs at 36* under boost 10lbs or better its 7to1. Think your shooting a little low on your timing. Think if it was me I would lock it at 32*

Im running 93 octane fuel. The thing I am afraid of is detonation.

I think I will up it. TurboJack had the same setup as me and he was pulling 1 degree per lb of boost and said that I should go no lower than 30 degrees at full boost.

That's what I was running on my 400 race turbo set up aluminum heads. Normally the designee of the chamber dictates what the timing would be plus bore diameter. My 600 NA motors on the dyno ran 41* 10to1 and on pump gas 93 .. Make sure you get a knock meter.

If you really want to see how much timing it wants you need egt at the port another before the turbo and one after not enough timing the temp keeps getting higher on the way out. Just because it has boost don't mean it don't need timing. Plus the exhaust valves will live.

A msd or daytona sensors programmable box would be a good investment on this setup. Timing control via a map sensor. You can keep timing up when in vacuum or light load to help keep the engine efficient, and pull timing back when the turbo starts making boost in the lower rpm range. You can keep timing up when idling, or pull timing while cranking, etc etc

Recently tuned an engine on the dyno with one of these boxes. By pulling out 7 to 9 degrees of timing in the midrange, the engine GAINED torque, over it being locked at 34. This was on a roots blown setup making 10psi. To me, that proved we were firing the plug way to early with locked timing.

There are several things that contribute to what determines when the spark should be lit. Engine RPM/speed, mixture density, bore size, chamber design, rod length/stroke, etc. When cruising along at 3000RPM and light load, the engine may like say 34* of timing. When the throttle is down, and you're accelerating and the boost is coming in, the engine may want 24* of timing there (just examples).

To the OP:

For fuel to get to each cylinder in your engine must be a nightmare. Even more so during cold start up. Timing and a/f ratio must be way different than most motors for cold start up.

Are your intercoolers fed water during idle ?

Yes cold starts are hard. I have to keep the rpms up to about 1500 and get some heat into it before it will idle nice. Once the engines are warm, restarts are good. Right now it has the factory high rise dual plane intake, but I have a pair of single plane Team G intakes I might switch to. Really the ultimate set up for me would be a Hurricane single plane EFI intake from ebay so I can use my existing intercooler and Ls-1 throttle bodies on the front of each turbo, with a megasquirt controlling each engine. The intercoolers get their water from a pair of Mercruiser dual outlet water pickups on the transom. So they only get water when moving.

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