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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locking the advance in the low 30's and pull it back for boost will be your best deal without a computer controlled timing setup...

This motor will want to be in the 30* range under full power.

14 Apache, TurboJack or anybody else that has run a twin turbo draw thru system. Has anybody run a wide band O2 sensor in your system? I have one on each right hand exhaust pipe about 6 inches after the turbo. Can anybody that has or had one of these systems tell me what you had for initial timing? total timing? spark plugs? idle speed? and even A/F ratio at idle. I am running a retracted tip plug, cant remember what number or heat range though. Currently my biggest problem is restarts. Id like the engines to restart easily and it seems like I have to crack the throttle one click on the throttle to get them to restart when the choke is completely open.

I just bought a pair of Gentry Twin Turbos for my Scarab. I am initially going to go carbs, just because I like them and the EFI that came with them is an old speed-density system that doesn't seem all that sound. May move to EFI later on, but the "retro-rod" build design on paper lends itself to it. ANYWAYS-----the guy I bought the sets from was a turbo guy, raced, built, etc. When we discussed my idea, he said he would lock it at 32* total and never look back.
Check with Carson Brummett, he purchased the Gentry rights. SUPER great guy and knows turbos! He's been a lot of help and kind of likes simplicity in setup, particularly if carbed.

Maybe set the initial at 16 or 18 to help your idle and not eat your starter and then limit the total to 36 all in by 3000 rpm, still using your boost retard so you don't melt the pistons. Just a thought.

If you are using a boost retard system, and you are taking out a significant amount of timing you will need to phase your rotor. otherwise you will get crossfire in the cap.

I find it incredible that these draw thru systems start at all. For a fuel droplet, especially large one's, to make all those angles, inclines and declines, intercooler fins, turbo rotors, and etc, is just plane mind boggling...again, especially at starter cranking speeds.

My mind is slipping and I have not had a turbo motor for 15 years.

I had tried the boost retard but I do not think I had good luck with it. I did end up locking the timing so it stayed only at one spot. When you have springs advancing the timing and boost retard taking timing out there are just too many things to go wrong. I believe that is the reason I just locked it in. I want to say I had it at 28 but I was able to dial boost up to 15. I never have torn up a motor at 10 lbs of boost, wish I could say the same for 15 lbs.

It is no big deal to go and lock the timing in. Why not go and try and see what that does.

What water temps are you running? I always had water temps at 100-120 max to help make sure I did not detonate the motors.

you are not running any kind of intercoolers right ? without those I still would lock dist and run 34 and pull back to about 26 under boost, or if not boost retard try 26-28 locked. We run a blowthru set up with intercooler and are at 34 locked, no retard, but without intercoolers or E85 ???

He has twin intercoolers.

Yes I am, 14 Apache is correct. I have dual core PFM intercoolers.

Take it from me (the previous owner that put all that stuff on) you need more initial advance. I agree with GPM. You're going to need initial of 14 I bet because of the poor volumetric efficiency at low rpm. Bump it 2 degrees at a time until you get your good starts then set your total advance accordingly with our bushings.

Hey Andy, I talked with Carson Brummett today and he is suggesting that I lock the timing at 30-32 degrees and not use the timing retard box. I'm testing it again very soon and will post some results.

Carson's the man!!!

Talked to Carson. He said lock it at 30 degrees. My starters have no problem spinning the engine because I have such low compression. The engine starts better now and idles cleaner too,

we run a procharger setup and 6psi timing locked at 32-34

I figured as much.
You know, going from number 2 Speedmasters to TRS's (1.33 to 1.5) means you just gained 600RPM so I bet at the end of the day those 21 clevers will be history and you'll have 25's Mirage or Hydromotives on that Scarab.