Why does GM spec such low idle timing advance?

[zwede]

Something I've noticed and can't figure out is that GM seems to always spec very little idle timing advance, even on performance motors (I'm talking GEN I small-block and Mark IV-VI big-block).

I have the specs for 1971 here and idle advance is speced as:

350 cui Z28: 8 dgr (Manual), 12 dgr (Auto)
350 cui LT-1: 8 dgr (Manual), 12 dgr (Auto)
454 cui LS-5: 8 dgr
454 cui LS-6: 8 dgr (Manual), 12 dgr (Auto)

By 1971 ported vacuum was used for the vacuum advance so no additional advance was added at idle.

I guess idle sniffer tests were becoming more common back then so the low timing advance could be to keep emissions in check?

But where it gets more interesting (to me at least) is that GM is STILL recommending the same thing on their current crate engines that are not emissions compliant:

ZZ427 (480 hp): 10-12 dgr
ZZ572 (620 hp): 15 dgr

Both of the crate motors state "The HEI vacuum advance canister should remain disconnected", so no additional advance should be added according to GM.

I'm used to dialing in 25-30 dgrs at idle but can't help wondering why GM doesn't? Their ZZ crate motors are not emissions compliant so I wouldn't think that's the reason?

[69-CHVL]

I think they get the cyl temps/egr hotter for emissions. I too cant seem to run less than 18-20* at a minimum to get a decent idle. Although my LQ9 runs about 12* for best idle...must be a modern engine thing.

[RednGold86Z]

But where it gets more interesting (to me at least) is that GM is STILL recommending the same thing on their current crate engines that are not emissions compliant:

ZZ427 (480 hp): 10-12 dgr
ZZ572 (620 hp): 15 dgr

Both of the crate motors state "The HEI vacuum advance canister should remain disconnected", so no additional advance should be added according to GM.

I'm used to dialing in 25-30 dgrs at idle but can't help wondering why GM doesn't? Their ZZ crate motors are not emissions compliant so I wouldn't think that's the reason?

They don't recommend ideal settings. They recommend safe and simple settings that can pass 100 hour durability tests. If you tune a curve yourself, you'll get better results.

[JoePorting]

Probably has more to do with the cam timing.

[novadude]

I would ask the opposite: Why so many enthusiasts think they need 20 deg + BDC at idle on mild street engines.

My 9.6:1 350 idles with 15" hg @ 750 rpm with a 217/225 @ 0,050 cam on a 108 LSA with only 12 deg initial and ported vacuum advance. In playing with the curve, I never saw any real advantage to running more initial timing.

I think that a lot of the really low initial settings on the early 1970s engines (2 deg BTDC, for example) were strictly for emissions. However, If you look at early '60s, pre-emissions engines, they still didn't spec a ton of initial advance.

[DaveMcLain]

I would say that it was done to reduce or eliminate hot start problems while still having a simple and effective advance system. Also when you have 195 at .050 duration on about a 114 lobe sep you don't need much idle advance.

[zwede]

I would say that it was done to reduce or eliminate hot start problems while still having a simple and effective advance system. Also when you have 195 at .050 duration on about a 114 lobe sep you don't need much idle advance.

That's why I only picked performance engines in my original post.

Cam duration @050 (h-rollers):
ZZ427/480: 224/234
ZZ572/620: 254/264

Yet they only spec 10-12 dgr for the 427 and 15 dgr for the 572. And they could easily add 10 dgr with manifold vacuum, but they specifically state that the vacuum advance should be left disconnected.

I'll be doing some experiments on my 454 today and see how it runs with less idle advance. I used to run 27 dgr and so far I reduced it to 20 and it seems to idle a little better. Cam is a h-roller 231/239.

[DaveMcLain]

Yes but what lobe separation was used on those performance engines? I be it is between 112 and 114 and that has a tremendous impact on idle quality.

Several years ago I had a customer who had to make a vacuum rule for a mud racing class. 15 inches at 800rpm. He had a high compression 400 Ford with a cast iron intake etc as per the rules. After much fiddling with the tuneup we found the best manifold vacuum by running locked out timing on the engine using the same setting that made the best torque and HP which I think was around 35 degrees. The engine made about 440 horsepower ran very competitively and won a lot of races I think he turned it about 6000rpm.

[novadude]

a 224 deg @ 0.050 cam still falls into the "mild" category in a 427 ci engine, IMO. Especially with the typical GM wide LSAs.

IMO, anything that makes good idle vacuum, has reasonable CR, and closes the intake valve relatively early probably does a better job of vaporizing fuel. This likely results in faster burn, which means you don't need to light it off as early to achieve peak cylinder pressure at 15-20 deg ATDC.

[grandsport51]

I' i've worked on hundreds of those engines back when I started in the business from 1970 On I never heard of any engine having direct vacuum advance at idle unless the hoses were hooked up incorrectly additionally putting in 25 or 30° initial timing at idle in a low performance car is certainly counterproductive and the damn thing won't crank up when it's hot . Obviously with a high performance camshaft and high compression pistons different specs are required but in reality Road timing the vehicle while making sure you don't over advance seems to always give me the best results for an older vehicle with not any yeah Euge compression issues nude possibility of Detonation.

[user-23911]

Too much idle timing makes them too hard to start.
They try to kick backwards when they first fire.

[bigfoot584]

Also don't forget guys the fuels available then had a
different burn rate also than what we have today.

[Geoff2]

Idle timing is related to cam overlap & to a lesser extent, comp ratio. More overlap, more timing reqd. Lower CR, also more timing reqd. Hence late model low compression engines had more initial timing generally. Both overlap & CR are related to the reqd burn time for most efficient combustion. Those who think a hot cammed engine idles best with 12* initial either don't know how to tune idle timing or don't understand the principles of combustion, or both.

Man vac adv was dropped in favour of the USELESS ported vac adv because MVA had higher HC emissions than PVA. With the reduced initial timing switching from MVA to PVA, the engine also ran hotter which reduced the oxides of nitrogen.
I find the HC reduction argument very spurious because reducing the idle timing using PVA reqd more throttle opening....& more fuel consumption because it made the engine less efficient. Soooo, the HC numbers would increase with more throttle opening & more fuel used.....

[Belgian1979]

I would ask the opposite: Why so many enthusiasts think they need 20 deg + BDC at idle on mild street engines.

My 9.6:1 350 idles with 15" hg @ 750 rpm with a 217/225 @ 0,050 cam on a 108 LSA with only 12 deg initial and ported vacuum advance. In playing with the curve, I never saw any real advantage to running more initial timing.

I think that a lot of the really low initial settings on the early 1970s engines (2 deg BTDC, for example) were strictly for emissions. However, If you look at early '60s, pre-emissions engines, they still didn't spec a ton of initial advance.

I've run anything from 12° up to 33° at idle. The engine changed from a vacuum of 71 kPa to 63 kPa at idle from the lowest timing to the highest. The idle rpm was kept the same by the IAC that is PID controlled, but it was obvious that it was closed more with a high idle timing vs a low timing at idle. Contrary to what I thought happened the engine drew less air which could be seen on the maf.
Engine ran considerably cooler with higher timing numbers

The cam is a high overlap cam and the engine is ITB's. Looking down the runners with a bore scope, I see no traces of exhaust reversion which I would have expected.

I do think reversion takes place but it might not be exhaust going back into the intake, but the intake charge backing up at low rpm.

[stealth]

The bigger question is WHY ARE YOU READING THE DIRECTIONS?......... man card may be revoked for this kind of behavior....