Is 51° total OK?

General engine tech -- Drag Racing to Circle Track

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Re: Is 51° total OK?

Post by kirkwoodken » Sun Sep 17, 2017 1:58 am

There is a huge/helpful list of GM vacuum canisters on the Chevelle website. Everyone should have that info. Covers HEI's!
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Re: Is 51° total OK?

Post by Hrdlx62 » Mon Sep 18, 2017 2:44 pm

Hey thanks for the responses, lots of good stuff here. Those b28 and b26 cans are not the easiest to source. I work for a big auto parts supplier and we have to order them direct. I'm gonna see if I can get a couple on a stock order as they want as much for shipping as the part.

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Re: Is 51° total OK?

Post by Racerdad916 » Tue Oct 10, 2017 10:20 pm

I read earlier in this thread about tuning the vacuum advance by watching the Air/Fuel ratios. I'm curious what's safe and what isn't. On my Gen 1 SBC with Dart 10x heads, solid roller 268@.050" in the motor at 100.5, large cap HEI and a Jegs Adjustable VA. The timing is 35 total, all in at 2700 with 45 degrees w/VA. At 70 mph (2900 rpm), I have 13" of vacuum and the A/F ratio on the wideband is floating in the mid 15s to low 16s. With no VA, it runs in the low 14s. The pistons are Speedpro hypeutectics and the CR is 10.68:1 and I run 93 octane. At 2600 I have 10" of vacuum and the wideband runs from the mid 14s to low 15s. There is no audible ping (stupid loud side exit exhaust), but the motor seems to be less happy than when I run no VA. Is it possible that I need little to no VA?
Idle quality is good at 1200 rpm. So this is the long winded way of asking how lean is too lean on the wide band if the motor seems happy?
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Re: Is 51° total OK?

Post by Geoff2 » Wed Oct 11, 2017 4:46 am

45* is not excessive cruise timing. Better to slightly richen cruise timing than to reduce VA.

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Re: Is 51° total OK?

Post by bigblockmopar » Wed Oct 11, 2017 4:14 pm

Belgian1979 wrote:
Wed Aug 23, 2017 8:09 am
Without a dyno there is no good way to actually tell what best idle advance is. You can be in a 'good' range but that's about as far as you can get.

On my efi engine I just did it with vacuum and rpm in mind : crank timing way up to see what the lowest vacuum is, then adjust downward untill you see it drop. Set it like that. I started with 30° and had to adjust downward to 22-24° where it is now.

The issue is that at startup and when cold, the engine tends to run slower and when you have lets say 600 rpm instead of 1100 what my normal range would be you have way to much timing even at 22°. I solved that by creating a 600 rpm row with 12° which in an efi system is a no brainer. On a regular carb about the only thing you can do is use the choke. However on my old carb the engine always died when closing the choke some
I found the idle advance was one of the easiest things to set and get right (at least on my automatic transmission car with MS3x timing control). But it's also the least important setting, unless you're doing a lot of idling between daily stop and go-traffic.
With trans in Drive the engine is 'braking' against the torqueconvertor so the 'perfect' advance can found in a seconds by adjusting for the least amount of engine load (most torque).

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Re: Is 51° total OK?

Post by Belgian1979 » Sat Oct 14, 2017 1:36 pm

Idle timing makes a lot of difference in the way it idles. Maybe not a fairly moderate cam, but not on anything that has an agressive cam.

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