I have been playing with the timing curve for my 380 stroker. It's a stock HEI from a 403 where I've been modifying the weights. What's the optimum curve for my engine?

Currently it's as follows:
- 24 degrees of mechanical advance.
- 16 degrees of vacuum advance from an adjustable vacuum can
- 36 degrees total, in at 2800 RPM.

This puts the initial at 12 degrees.


Car specs:
380 Stroker build by Mark Remmel
3.5" stroke, 4.155 bore
9.5:1 compression
Iron 7a heads with polished chambers and large valves.
Hydraulic roller cam, advertised 279/286, 221/228@.050" lift, 112 LSA. .542/.536 lift with the rockers I have.
Performer RPM intake.
'77 Qjet, tuned for the engine.
TR3550 5 speed transmission, 3.27/1.98/1.34/1.00/0.68 gears
2.56 posi rear end.

Thanks
- Mark

 

I am surprised it has so much mechanical advance, one Autolite reman HEI had the same curve, most have less mechanical advance. I find with most HEI weight combos, around 3000 is the quickest the curve will come in, no matter what springs are used. How does it run? How stable is the idle? How do you like that manual trans and what was it out of?

 

How does it run with those settings? If its doggy, increase your initial and remove some mechanical. There are bushings or you can fab a stop.

 

 

It had 17 degrees of advance before I swapped for different weights.

Idles at 700 RPM, with 14" of vacuum and a slight lope. Off idle is okay - typically worse when the vacuum advance is hooked up to manifold vacuum as opposed to ported.

I like the transmission - it's the first version of TKO transmission, so pretty heavy duty. The deep first gear makes my 2.56 rear end feel like a 3.08 rear&M21 Muncie combo, which is positive. Freeway speed is 80 MPH over here, puts me at 1850 RPM in 5th.

Runs decent - as mentioned above, ported seems to work better for getting moving.

I'm running 99 RON with 5% ethanol, as mandated by law over here. (That makes it 99.48 octane, technically).

I haven't run it hard yet, as I was unsure if the curve was unsafe - that's why I started the thread, to hear some thoughts on the curve - before I start blasting away. Before the recurve, it ran 12.5:1 at WOT.

 

Ah, that explains it. I also have an HEI from a 403.

Some years ago I did a lot of distributor modifications and after much experimenting I ended up back with the original center bar and weights along with one light and one medium spring from a Moroso HEI advance curve kit. The original weights/center bar gave ~18º mechanical advance so I could set the initial higher than with the parts from the kit (they gave 24º mechanical) and this gave a smoother idle, higher vacuum, and more off-idle torque. Now when I turn the AC on at idle (~700 RPM), the RPM barely drops.

 

Very interesting. When was it all in? And did you run ported or manifold vacuum?

 

 

I'll try some stiffer springs and see if I can hit 3000-3200. Thanks.

 

I checked my notes as this was done in 2003. Wow, how time flies.


Timing Parameters 11/09/2003

Moroso weights/center plate removed
Factory weights/center plate installed
Weights stamped 139, plate stamped 446

One light and one medium spring - total in at 1800 RPM
Two medium springs - total in at 2200 RPM
Vacuum advance connected to manifold
Idle set to 800 RPM in Neutral, 650 RPM in Drive
Vacuum = 16 in Hg @ 800 RPM; 18 in Hg @ 1400 RPM

 

Gotcha. 1800 RPM seems early?

 

That sure is early, almost all my HEI's even with light springs are 3000+. This 260 distributor is the first that I could get in before 3000, all in by 2800 maybe sooner. I forgot my 260 doesn't have a timing tab on it but the Chinese HEI actually has a quick curve, probably similar to Seff's curve but I have it set over 60 degrees and it likes it. I have to say for price, it was well machined and I put a quality module, Accel coil and low resistance contact button. Hopefully, it lasts.

 

Yes, very early, which is why I used stiffer springs.

When I started playing around with parts, I discovered the Moroso weights/center bar with the lightest springs had mechanical advance coming in at idle speeds. When I turned the AC on, the idle RPM would drop, which caused the timing to decrease, which caused the idle RPM to drop, and this downward spiral continued until the engine died.

 

 

 

I'll get working on that. The cruise/APT has never worked properly on this carb - I suspect a (maybe partially) clogged vacuum channel to the power piston.

 

Have you adjusted the APT?

 

No, I haven't been near the car since Monday.