1972 LT-1 initial timing spec

[Patrick H.]

Hello,

I don't have my reference material at work.

Anyone have the initial timing spec for a 1972 LT-1 as per the sticker on the firewall OR a similar old GM manual. I know for a 200hp base motor car it is 8 degrees BTDC, but wanted to check on an LT-1.

Thanks,

Patrick

[Joe L.]

Re: 1972 LT-1 initial timing spec

Patrick------

It's 8 degrees BTDC. While they were not installed in Corvettes with LT-1, the THM-400 versions installed in some Camaros were to be set at 12 degrees BTDC.

[Duke W.]

Re: 1972 LT-1 initial timing spec

Remember that the ignition map for '72s was driven by emissions considerations. The optimum total WOT timing is still about 36-38 degrees (maybe even 40 since the chamber is bigger than previous high compression SBs), so the best initial is likely 38 minus the total centrifugal. Beyond that you can experiment, but if the timing is within 2-3 degrees of optimum, the power is within 1 percent of max, which is tough to feel SOTP.

Likewise, if emission testing is not an issue, install a VC1810 vacuum can connected to a source of full time manifold vacuum, then go through the sixties vintage idle speed and mixture adjustment procedure.

Duke

[Joe L.]

Correction

Patrick-----

I gave you the spec for a 1971 LT-1. I guess I "inextricably" associate you with 1971 Corvettes even though I know you also have a 1972. So, even when you clearly say 1972, I still think 1971.

Anyway, the spec for initial timing for a 1972 LT-1 is different than 1971. For 1972, it's 4 degrees BTDC. The base engine re,ains at * degrees BTDC, just like 1971. 1972 Camaros with LT-1 and THM-400 did have an initial timing spec of 8 degrees BTDC.

[Patrick H.]

Re: 1972 LT-1 initial timing spec

Duke,

Thanks. I've got my cars set up like this, but needed the "published" spec for another project I'm working on which actually has nothing to do with how the cars operate.

Patrick

[Patrick H.]

Re: Correction

Joe,

Thanks. That's what I needed.

And you're not the only one who makes that 71/72 confusion with me. I forget I have a 72 at times, too. What's worse is when I can't remember which car has which details. Oops.

Patrick

[Duke W.]

Re: 1972 LT-1 initial timing spec

The "low compression" LT-1s are rated at 9:1, but as built, they are probably closer to 8.5 if typical of other Chevrolet engines.

Unfortunately, low compression and high overlap are about as compatible as bleach and ammonia i.e. a very effective way to release toxic chorline gas or kill the torque curve across the entire rev range.

With such a low CR and late closing inlet valve the DCR is very low and can probably take a lot of timing before you hit the detonation limit.

You can probably recover some low end torque by getting all the centrifugal in as early as possible - certainlly by 3000 and maybe as low as less than 2500.

The centrifugal on my Cosworth Vega (8.5:1 rated, 8.0 actual) starts at 1000 and is all in by 2000. Of course this is MY curve, not OE. It operates detonation free on unleaded premium. Prior to reinidexing the cams to eliminate most the the OE overlap (for NOx control) it would exhibit some transient detonation at low revs on a hot day. Now that the valve timing is essentially the same as the L-79 cam, it won't detonate no matter how hard I try. The large exhaust gas residual with the OE cam indexing contributed to detonation by heating up the mixture to increase the propensity to detonate and KILLED the torque curve, particularly at the low end.

The 80 percent torque bandwidth now, as measured by a chassis dyno is 1900-7200, which is pretty good for an engine without variable cam indexing or induction tract length.

If I were to rebuild a low compression LT-1 I would use the 1970 domed pistons and select a head gasket to yield no more than a true 10.5:1 CR, which should be easy with off-the-shelf part and use unleaded premium.

If I ever rebuilt the CV bottom end I will use pistons to yield about 10:1 and use premium. That would raise the torque curve across the range by an average of about 10 percent.

Duke