devilfish

hmm im just curious. I wounder how much more timing the LT1 heads allow vs stock L98 alu heads, due to the reversed cooling system??
When I look at a LT1 bin ( from a 94 vette ) and my L98 bin (1991 vette ). I first notice how "smooth" the spark tabel is in the lt1 bin compared to my L98 spark tabel,that got alot more variation in it.

And as I cant find a "PE spark tabel" in tunercat for the 94 bin i guess it dont use one?

I can also se that the lt1 bin as far more timing in the low rpm@high MAParea (around 10 degrees more ) . And my L98 bin as alot more timing on crusie speed areas, 1800-3200rpm @ 40-55Kpa area.

Can anyone verify this, and perhaps explain it to me..?

JPrevost

I can only tell you that the Lt1 combustion chamber and reverse cooling are both contributing to the higher advance. The cam also dictates some of it. With the tight LSA of the stock lt1 cam they can get away with more timing at high load low rpm.
That's all assumed so don't take my word for it.
The other reason you can't directly compare the 2 tables is like you said, there are other tables that are added and subtracted from the main table. PE spark, highway mode spark, low octane, and probably most importantly the cts table.
Too many people overlook the cts coolant table. With a "healthy" engine combination it's generally a good idea to nutralize a lot of tables to get a better tune faster. I tend to only add a few degrees at really cold temps and then remove a few degrees at temps where the engine is overheating (puts more energy into the exhaust than the cooling system).

devilfish

Thanks for your reply.

Ok another ide...

Lets play with the ide that we put on stock l98 alu heads on a lt1 ( iknow you cant...). Do you belive you have to change the spark tabel alot to keep the car happy? Could the desgin of the lt1 heads be so "good" (diffrent) that the car would run bad, or perhaps even develop knocks?

Z69

LT1's have a better chamber and are rev cooled.
So the chamber roof temp will be different.
Also, the LT1 uses a different intake. Makes the lo rpm cyl filling completely different.
So everything is different.

I have vortec heads which are basically LT1's w/o the rev cool.
In carbed apps, they work just fine with as little as 28* total adv.
On my car, I saw only about 5 hp going from 28* and 91 octane to
97 octane and 34*. A more typical sbc timing #.
Proving to me at least that my motor was port limited and not CR limited, as everyone said I was. It's a 10.9-1 iron head 383.

devilfish

im fishing here.

A miniramed/hotcamed car with aftermaket aluheads ( same comp as a stock lt1 ) L98, that would be simular to a lt1 (exept the cam )? I wounder how that engine would react on a stock lt1 spark tabel, (with all the PE spark advance zero out ). If the fact that a lt1 use optispark and r-cooling, would make the mod l98 develop spark knock and run bad?

Grumpy

The idea is using the least amount of timing consistant with best performance. Start low, and slowly add.

Don't worry about what the LT1s or other people do, do what makes your engine happy.

devilfish

True

Im just curious about the diffrent setups, and why things are like they are. And perhaps learn something on the same time.

As I try to build a complete new tune for my new engine, I have to understand the theory behind this. As unlike the VE where you have a WB to assist you, the sparktabel is somewhat a mystery to me...

cheers..

devilfish

im confused?? "everyone" say the lt1 can take more timing due to the reversd cooling system, right? But if I look at the super AUJP, and that bin has even more timing in to it? Im just curious about that..

anesthes

iTrader: (12)

See that ^^^^

Thats about the best advice you'll get. Start with a stock timing table and slowly increase your spark advance until you slow down. Then back it off 2d.


If your motor needs 40d of timing, you either don't have enough compression, or enough boost.



-- Joe

anesthes

iTrader: (12)

The LT1 is thermally more efficient. Sorta.

The LT1 takes more heat out of the combusion chamber, so you add more spark or more compression ratio.

They do however stay cooler, so you have less chance of detonation.

The LT1's real strongpoint is the small chamber.. HUGE quench area, and less area for thermal loss. More heat pushing on the piston, not absorbing into a huge chamber.

-- Joe

devilfish

Its just that, my stock timing tabel for the vette is OVER 46 degrees at part time throttel speeds! So I really dont know if I dare to use that bin with my "new" engine, as its nowhere near the stock config.

devilfish

So why is it then that a timing tabel for a stock LT1 is less aggresive then , for exampel the super AJUP?
Im just trying to understand this, im NOT going to use a lt1 tabel for my engine.

devilfish

...

anesthes

iTrader: (12)

I didn't jump on the super AJUP bandwagon sorry.

The vette timing table exceeds the mechanical limits of the distributor. I don't get that.

I'd start low and work your way up. Forget about what GM did, or what Tim did in super AJUP.

-- Joe

devilfish

I wounder what GM whas up to with that SA, if it like you say, that "The vette timing table exceeds the mechanical limits of the distributor"...

Yeah got it. Im going to start from the start. But cant se to find a good base timing tabel as my ""base" to start working from.

Grumpy

In all the codes, I've seen there is a max advance limit. So while the table might go 46d, when the processor comes up with it's final value, it'd be less then that, 46d that you see in the table.
At least for the purposes of the 3rd gen distributor applications.

devilfish

ok understand. So the 46 in the vette SA is just a way to MAX out the timing? Where can I se what the max advance limit for my setup?

HaulnA$$

While 45 degrees may be the mechanical limit of a V-8 distributor, add that to say 6 degrees base timing and you get a total available advance of 51 degrees. The ECM would remove the 6 degrees base from the table value and apply 45 degrees advance from the previous DRP. This is in theory only and does not take into consideration the max advance limit Grumpy mentioned, which is usually set to around 42 degrees from what I have seen.

anesthes

iTrader: (12)

I understand that. I was commenting on why GM would bother bringing the table up so high.. Makes no sense.

You have 45d between each terminal on the dizzy. And the higher the primary voltage, the more likely to fire that terminal from a farther distance away. It makes no sense.

-- Joe

RednGold86Z

iTrader: (1)

45 deg timing is only at the crank, which would be 22.5 at the dizzy. But most cals have the max SA set to around 42 in the constants. The rotor tip and the cap terminal overlap for quite a wide range, and is probably in the advanced position such that even if you fired it 45 degrees advanced, it'll still hit the correct terminal.

devilfish

i still dont understand why GM made the SA the way they did, I sure they got a good reason, but 46* at cruse speeds, that made me re-think about how to tune SA allover again..??

RednGold86Z

iTrader: (1)

I don't have my aluminum heads on yet, but that's probably the reason it's required. I know when I was tuning a 1.6L (I know, totally different), it wanted nearly 50 degrees in free revving neutral to run the smoothest (lowest MAP).

Gotta go

anesthes

iTrader: (12)

How do you figure?

Grumpy

If you running an LT1, fine.
If not you might as well study, VTEC, timing curves..

You need just to worry about what your doing, not what everyone else, or gm does.

Did you read the timing article I just posted a link to?.

Before you even think about trying to get even close to the extremes of how much timing you can run, you really need to study up on things, if your going with the max timing possible tuning chain of thought. Or have a thick wallet, you don't mind thinning out...

Grumpy

Dissy truns at 1/2 crank speed.