Of coure anyone with experience can chime in on this. It's just that Dan suggested I ask Doug about it. I am about to install an Acell Gen VII EMC on my EFI turbo slant, and at that point I will have avilable a 16x16 spark advance map. I can set this thing up to be what ever Ithe engine wants. Right now, with the initial set at 2 BTDC, the stock centrifigal creates big time preignition under 8 lbs of boost. So under boost I need to come back from whatever the stock centrifigal advance is. The distributor is a MP electronic ignition purchased about 12 years ago.

There will be a retard feature with a knock sensor which will dial back the timing automaticaly when preignition starts to happen so the boost timing is not ultra important, I guess. What do you recommend as a timing curve for a stock, naturally aspirated slant if you could idealize it with no concerns about the limits of the distributor advance, or vacuum advance, or even preignition, since that will be corrected for if it occurs. This means lower octane fuel if I am not racing, and it still will not hurt itself. Youcan discuss the boost aspect of it too. The cam is a custom grind done by a guy on the Eastern shore of Maryland here. It is about a stock grind with a couple of degrees less over lap to keep from pushing raw fuel/air mixture through the exhaust under boost.

The Gen VII comes with a program that asks for the bore, stroke, and cam specs of your engine, and sets a default for your engine to get it started. Thanks in advance for any thoughts you wish to give this project.

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The "suspect" item you have is the MP distributor, you need to find-out how it is set-up. Many of these MP distribs. came with only one light spring, not a good combo. for anything except WOT racing.

Here is how I would set up the curve to start:
- 6 to 8 degrees initial
- Light spring starts to move at 1000 rpm and pulls in 16 degrees by 2600
- "Looped" heavy spring slows down the curve for the next 1600 - 2000 rpm adding only 4-6 degrees (this spring will have thick coils)
(9X govenor)

8 + 16 + 6 = 30 degrees projected total with you initial setting allowing you to adjust in the 26 to 34 range. (example: set initial at 4 BTDC will give you 26 degrees total, initial set at 12 will give 34 degrees total)

From here, use the timing controller to take out more timing as the boost increases.
DD

Are you trying to combine a mechanical advance setup with a computer controlled advance? In that case, I'd recommend locking out the mechanical advance or putting in a Lean Burn distributor so the timing is just computer controlled. Just seems easier to have your timing curve directly visible on the computer screen instead of using the computer to make changes to a mechanical system. It won't necessarily perform better, but it just seems like a little less hassle and less math to me.

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
My blog - Mad Scientist Matt's Lair

I'm sorry, I should have said, the ignition timing curve is completely locked out of the distributor with this system. The centrifigal advance is locked out, and the vacuum advance is unplugged. All of the spark timing curve is built into the computer. It allows you to break up the RPM into 16 different steps from 500 to 6000 or what ever you think your engine will turn. It also allows you to specify both vacuum and boost figures in a range from 20 or so inches of mercury to 10 bls of boost, or what ever you specify in the matrix.

The computer sees the manifold vacuum or boost through the map sensor, and knows the RPM. It looks up the spark advance figure for that vacuum/boost condition, and RPM from the matrix where the values are entered, and sets the timing at that figure. If driving or tuning reveal timing problems in that RPM and vacuum range of the engine, you change it in the computer, and you keep doing this until your car runs its best.

The question is then, if you could design a perfect curve for a slant, what would be the degrees of advance you think would be ideal under varying load, and RPM conditiions. I know this is probably going beyond most people's experience here at this point, but maybe you can extrapolate from your knowledge of mechanical spark timing controls into an idealized digital control world, so-to-speak.

Doug, have you ever mapped a timing curve on one of your strong slants with a timing light to determine what the curve was after you got it set up to run well? What were you seeing if you did this? What was your total advance? How much vacuum advance did you run? Do you have any idea at what vacuum figure this began to drop out? Just from this, I might be able to figure out how close your engines compared to what the ACcel computer gins up as the default timing curve map. Thanks for giving this a go.

Matt, thanks for the heads up on the lean burn distributor. Does anyone know where I can get one? Does anybody have one for sale? I have a standard elctronic one I will trade. Sam
Sam

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So Doug, What are the trade-offs between running 4-26 degrees vrs, 12-34 degrees? When would you chose one over the other? What factors seemed to dictate which was going to be better? If you could get it to run from 4 to 34, would you? Or if you could get tje timing to go from 12 to 26 would you? And why? Anything is possible here. The neat thing, i can run it way advanced from cruise,(high vacuum) and back the timing way back for power,(boost, or low vacuum). Thanks again for putting your thinking cap on here.
Sam

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Sam,

I can only speak generally, but here goes. The more cam and the less compression you have the more initial advance you'll want. I'd start with 10°. The advance should increase in a pretty much linear fashion until you have 20 degrees more timing around 2800 RPM. Your engine may like more initial timing and may like the advance to come in faster or slower. With less manifold pressure you can use more advance. I would start adding a little timing around 4-6" vacuum and ramp it up to 10-15° at 12" Hg. Again, this is a starting point and will require testing and adjustment. As manifold pressure builds you'll have to take timing out. 1° timing for every psi of boost is a starting point.

Carovair turbos used a ton of initial advance (24°) to help with the low compression, but had no centrigugal advance until 3800 RPM where it would bring in 12° by 4500 RPM. The pressure retard would take out about 10° of timing at 2 psi, but this was with an air-cooled engine and no intercooler. It also had a primative carb without adequate enrichment under boost. The slant has cooler combustion chambers and with EFI can stand more timing at low speeds.

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Joshua

Josh, that is exactly the kind of info that will be useful here. I will print out the thread once we seem to have exhausted the tentative, theoretical ideas, and will apply them to the tuning process. I will also report back the computer's calculated spark map for digestion and comment. You an bet I will study all of the comments from experienced slant builders here. This is a gold mine of information about what slants like. You might comment on the technical reasons a slant likes this or that amount of advance or boost. I was especialy intrigued by the comparisons betrween the slant and the Corvair turbo. I had several friends with these,, as well as one with a turbo Oldsmobile Jet Fire. I wish I paid more attention to it then.
Sam

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Sam,

Can you tell me a little more about your engine. I'm especially concerned with the static compression ratio and the valve timing. I doubt you'll need as much initial timing as the Corvair, but 12° or more is expected. I had a Comp 264 cammed 225 without enough compression and it needed 12° initial timing to idle well.

The slant has a much better cooling system than the Corvair when it comes to the combustion chamber. You're also giving your engine much better fuel and ignition systems. I wouldn't look at the Corvair and Olds Jet-Fire for ideas so much as the limits engineers faced with turbocharging in the early '60s.

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Joshua

Josh, I built this engine two years ago, and working from memory here, but I think the compression ratio is about 8.5. It has a set of Federal Mogul forged pistons, that are stock in profile. The block was not decked at all, and the head was milled just enough to true it up. It has a mild cam, like I said, with just enough reduction in overlap to reduce the blowthrough some. The valves are Mopar performance oversized valves. The head was ported. The runners were left the same but gasket matched. The boss for the valve guide inside the bowel were cut way down and steamlined. This work was done by me, and based on a book about head work for inline engines written by a British author. I blueprinted two sets of stock rocker arms, and discovered they varied in ratio from 1.47 to 1.57. I put the more agressive ones all on the exhaust. The thinking was that higher lift would help the exhaust flow to get the turbo moving. I cc'd the combution chamber, and they were pretty much all the same. I can't remember what size they were, but 58CC sticks in my head. I may be way off on that one though. I have this written down somewhere, and will look for it later. I need the cam card for the initial set up program to work, so I will need to go searching for it. I advanced the cam 2 degrees above the cam grinder's spec. This was based on advice given me by this forum two years ago. I hope that helps.
Sam

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If the cam has less overlap than stock and is advanced 2° you might only need 5° initial timing. Initial timing is highly dependant upon cylinder pressure at idle speed and mixture quality. With an early intake closing you trap more in the cylinder and end up with a more dense, less diluted mixture that doesn't need as much ignition lead to get burning.

Overlap with street turbo engines usually ends up causing reversion rather than blowing fuel into the exhaust as turbine inlet pressure is almost always higher than intake manifold pressure. If you were to put on a large turbine and could stand a lot of lag then you could get intake pressure higher than the exhaust.

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Joshua

Why does an early intake closing trap more in the cylinder? Does some otherqise get pushed on through. But then reversion is fighting this is it not. I guess the earlier closing minimzes the reversion things as well. Does the reversion tend to trap more of the fuel mixture in the combustion chamber, or does it simply mess op the mixture? Could you explain reversion a little please? Is this is a pulse back from the exhaust, into the cylinder when the intake opens? Explain the dynamic of this and how it affects things, and how you deal with it.

I wish I could tell you more about my turbo. It is simply a stock unit from an 87 Buick Grand National. I don't know it's name rank and serial number. I am crrently running 8-9 pounds of boost, which comes on amazingly fast, but because of the preignition prblems to date, I have never run it hard. As soon as preignition enters the picture, which it does readily, I back off. Right now it requires high test fuel. I have a Kasper Industires knock gauge which Bob D recommended a few years back. Thanks again for sharing your knowledge. I should say that the engineering goal here is to build a street car that had mild manners, gets good mileage, and still has some power for performance. So far none of those gaols have been achieved. I think I am on my way to realizing them now. At least I hope so. Thanks again for thinking this thing through with me. It gives me comfort to know that there those with more experience than I have willing to help out and give advice. I don't know why I left this forum for awhile. It is nice to be back. Actually I do, I got sucked into my Corvette for awhile, but that is sold and gone now.
Sam

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