Carb tuning AFR, and 10-1 compression

[Ziegelsteinfaust]

My truck is running, and I am tuning it slowly. Today I got the 700R4 adjusted so I can now do full throttle tuning. Along with dialing in the part throttle.

My engine is a 383 LT1 I did not build, and I only got to see the cam card once.
It has 10.5-1 compression, AFR 195 heads, 4 barrel carb, and 1 3/4 long tube headers. The cam I can only go off of memory, and general smog legal cam specs. Though it is about .525 lift, and 210/220 duration on a 116 lsa.

BTW timing is a bigger mystery then women, but it starts + runs good. I can only fix that at rebuild time.

With that out of the way now the question. Engine is only run on 91 octane gas, and equipped with a Holley 750 vacuum secondary.

Obviously 12.5-13.0 afr will be the goldilocks zone for afr with a carb at full throttle.

But where does high compression, and a short duration cam like part throttle afr? Since I am running a carb with no knock sensors I am running a touch fat at 13.0-13.7 afr as I cruise around or hit the freeway. Should I be relatively safe with 14.0 to 14.7 afr cruising at part throttle?

Thoughts?

[MySons68C20]

What is your total mechanical timing (vacuum disconnected) at 3000 rpm?
I would start at 34 deg total and see where initial is as a baseline.
As far as afr at cruise light throttle 14 ish is fine.

[Ziegelsteinfaust]

Originally Posted by MySons68C20

What is your total mechanical timing (vacuum disconnected) at 3000 rpm?
I would start at 34 deg total and see where initial is as a baseline.
As far as afr at cruise light throttle 14 ish is fine.

I can't set timing like that till I atleast pull a head to get true TDC. Right now I have it approximated.

Timing is all in by 2400rpm. I used spare hei weights on my early distributor.

I need a new vacuum pot as it is no longer working.

[Tom]

10.5:1 with aluminum heads is not an issue.
The old rule of thumb is aluminum heads can run 1 point more compression then iron. Vortec 350's run 9.5:1 with a much smaller cam and iron heads on 87 octane. You'll be just fine.

[geezer#99]

You don’t need to pull the head to find true tdc.
A piston stop is 10 bucks or less.

https://www.summitracing.com/parts/s...yABEgIC1PD_BwE

[MySons68C20]

This ^^^^^^

[Ziegelsteinfaust]

Originally Posted by geezer#99

You don’t need to pull the head to find true tdc.
A piston stop is 10 bucks or less.

https://www.summitracing.com/parts/s...yABEgIC1PD_BwE

I used one, and it bent. When I rolled the engine around with a 3/8's rachet.

[geezer#99]

Should have a nice gouge in your piston too!

[Dead Parrot]

You can adjust your timing without knowing true TDC since it already starts and runs.
Note what the readings are now as a base reference point. The numbers don't matter, just a point you can return to. Might temporarily put in a heavier mechanical spring while setting initial to avoid having mechanical start to come in while setting initial. Then get it hot and start adding in initial timing a couple of degrees at a time. If it gets hard to start or starts pinging, you have found max initial and back off a couple of degrees. Note where the timing mark is now. You can now adjust your mechanical to taste or where the motor tells you to stop.

While you won't know the true numbers, the timing will be close and even when you know TDC, you will likely set your timing this way anyway.

IIRC, for a daily driver, you want the AFR lean for part throttle cruising. Somewhere in the 16~18 range. That is one of the reasons for the vacuum pot. To add more timing advance to allow the slower burning lean mix to properly burn.

[Ziegelsteinfaust]

You really can get away with it that lean.

I never went above 14.7 at part throttle highway. Although these were low compression stockish 350's that were built for driving.

Most of my performance I built were for fun, and I jetted then richer for power. Also the cam choices I made liked it better too.

[Ziegelsteinfaust]

Originally Posted by geezer#99

Should have a nice gouge in your piston too!

Engine has 280,000 roughly. So I will wait for rebuild time to correct it with better pistons.