I have a ZZ454 that I'm tweaking the timing on. I have the initial timing at idle at 17*. At 2500 RPM it is at 36* and at 3400 it is at 43*.
From what I understand, I subtract the idle timing of these to get my actual timing which is 19* at 2500 and 26* at 3400.
I run an MSD with no vac advance, all mechanical. I'm currently running the 2 light blue springs and I have the red Stop Bushing in to limit the advance, which is 28* per MSD. How does that limit it to 28*?
Per other info, everyone says it runs the best with 18/36.
Thanks, Allen

 

Allen, don't take this personally but you are incorrect.
Subtracting base timing from max timing only tells you the amount of advance. That's what the stop bushing are for, to limit the amount of advance. At 43 degrees total timing ( initial + advance) you are risking breaking the ring lands off the pistons. 36 degrees total is plenty for a ZZ454.
Mike

 

What timing light are you using, MSD is notorious for giving wrong readings with certain digital timing light. In theory you should be aiming for 36 total so with a 28 degree stop your initial should be 8 which is low. Do you have a 24 degree stop so you can set initial to 12 for the total of 36?

 

Mike ,just another question from a n00b!
On a small block, people say the optimum is 36 degrees all in by around 3000rpm give or take with vac disconnected and plugged.
if you then add the vacuum into the equation, as long as its less than 52 then that's a pretty good place to start performance wise, obviously all engines are different and so will need tweaking.

On an engine not running a vacuum advance such as the OP, would the 43 all in not be ok? I know its a 454 but would it make that much difference?

To the OP, just wondering as I know little about it but what are the pros/cons of not running a vacuum advance?

 

43* of timing is going to be really hard on the motor and actually make less power than less timing.
Unless you're at 10,000 feet or something it's going to want far less timing than that.
Thing is, is that your cylinder pressure may be peaking at or prior to TDC as the RPM's rise. You want it to peak ATDC about 15* or so after. Doing otherwise is forcing the piston to rise against too much pressure and something is gonna give eventually.

Without vacuum advance you will loose the advantage of better MPG at cruise.

 

Please forgive me if I'm reading this wrong, but this how I'm understanding your question.

If at idle, it's set for 17*, and at 3400 it's 43*, it appears that your getting 26* out of your mechanical advance (43-17=26). This also assumes that your fully advanced at 3400.

If your goal is for 36 degrees total, set the initial to 10*. Then, when all in at 3400 it will be at 36* (10+26=36).

I hope this helps.

 

This is from the ZZ454 GM Manual

"Start the engine and adjust the initial timing. If using the HEI distributor P/N 93440806, set the ignition timing to 4º before
top dead center (BTDC) at 650 rpm with the vacuum advance line to the distributor disconnected and plugged. This
setting will produce 26º of total advance at wide-open throttle (WOT) when using the HEI distributor P/N 93440806. The
HEI vacuum advance canister should remain disconnected. This engine is designed to operate using only the internal
centrifugal advance to achieve the correct timing curve. Rotate the distributor counterclockwise to advance the timing.
Rotate the distributor clockwise to retard the timing.
Once the engine is warm, Double check the total advance timing is 26°at 4000 RPM if using the HEI distributor P/N
93440806.
"

So the total mechanical advance should be 22 degree's

4 @ idle + 22 total mechanical advance curve for 26 total timing at 4000 RPM

you might be able to use 3100 - 3200 RPM but you won't see much improvment with a ZZ454. Don't use the stock GM 350 rules most quote and setting all in timing to 36 total around 2200-2800 RPM. This is not good for the GM ZZ engine line.

 

Going by your numbers, the vac advance is adding an additional 16 degrees to the total timing. Even though the vac advance drops when you accelerate, it is still too much timing.
Ever since 1966 emissions have had a lot to do with timing. More timing adds power, increases gas mileage, and engines run cooler. But it also creates HC and NOX gases. Your average chevy timing set at 8 degrees initial (4 degrees for standard trans) and would advance 14-16 degrees very slowly. The vac advance would add another 12-14 degrees on a base motor. Cars like the 71 corvette had systems in place that would not allow any vac advance until it was in 4th gear (manual) or 3rd gear (auto). This was strictly and solely for emissions. So at best they were barely getting 24-26 total base + mechanical. A vac advance pulling another 14 degrees made a big difference in driveability.
Camshafts and stroke also determine how much total timing an engine needs. A moderately built 355 will like 38 degrees but a 383 usually prefers 32-34. Take two engines, built exactly the same, same pistons, same heads, etc, except one has a 256 cam and the other has a 278. Both have the initial timing set at 14 degrees. The 256 motor may not like 14 degrees but the 278 may want more. Because the 278 motor has a much lower DCR because of the cam.
So the better the timing curve in the distributor, the less vac advance you need. This is a common problem for many well tuned motors. The vac advance advances the timing too much and requires puting a travel limiter on it.
But 38 degrees is the max for your average 350. Usually a little less for stroker motors. And a 454 is a stroked 427.
When you get into the computerized systems that started in 1981 such as GM's CCC and C3I, Fords MCU, EEC3, and EEC4, and Cadillac DFI, they would pull 52 degrees total on decel. They had no vac advance or mechanical advance and the base timing was always set on 10 degrees. But those systems controlled timing by sensor inputs and ran off a program. Plus they added a knock sensor. 52 degrees on a non-computerized distributor is way too much.
There are several members here that say: before you go buying carbs and intakes, recurve the distributor first" and they are correct. There is a lot of power to be gained just from a recurve.
A vac advance unit has no business on a race car but on the street those extra 10 degrees are nice to have when you are just cruising.

This reminds me of a cartoon I saw many years ago about a scientist on the phone with the president. The caption read: Mr President, we got the emisions down to zero..... but the car doesn't run.
Mike

 

Unless Chevrolet is asking their customers to operate this engine with a ton of retarded spark to keep the horsepower and warranty costs down, what possibly is Chevy claiming they've done in this engine design to produce an engine whose combustion burn rate is completely independent of cylinder pressure?

Call me skeptical here.

 

We are not talking a lot of difference. The head design does effect burn rate on these ZZ line compared to stock 350.


I just listed what came with the engine. GM also lists the parts used to rate the build

If you don't use there listed setup and parts then you will adjust an test, but you may find adding a lot of timing to this engine doesn't add much in performance once you get past 26- 32 degrees total and at a lower RPM than 3100-3200

The best you can do is tune for best mph over 1/4 mile at the track vs the seat of the pants sensor which is not very reliable

 

I understand that you listed Chevy's instructions (that's why I highlighted the user manual part in my earlier post). I just don't see anything in the build of this engine that somehow allows it, versus the other 99.9% of engines in the world, to run a timing curve that is independent of load and cylinder pressure.

 

My new ZZ454 would not run at 4 degrees initial, or not like it should. I trusted the GM guidance of 4 and finally worked my way to 12 initial. I have an Davis HEI with 22 degrees of mechanical advance so my total was 34 at 2500 rpm. Vac advance gives me another 13 or so degrees for cruise. I still had a little lag going to WOT from low rpm so I moved initial up to 14 degrees which fixed that. There is no reason this engine should be set up as they suggest.

 

Not that it makes much of a difference but that timing spec is for 92 octane gas

 

Is this what you have

http://www.chevrolet.com/performance...ck-zz-454.html

or a different setup.

If this is the engine as shown start with there recommed timing and go from there.

Also your calculations are not what you are looking for.

Inital timing is your idle speed timing. Your mechanical advance will start based on the weights and springs you use, and will top out at some point.

Again measure the timing or note the RPM where you no longer get any advance.

This is the total timing @ some RPM. The difference between the 2 is the total mechanical advance your distributor allows for. ( from your info this should be 28 - MSD red stop bushing )

If you have the above engine as spec'd. They recommend 4 init and 26 total. So a total mechanical advance of 22 degree's.

Using your MSD as you have it setup now. Starting at 17 + 28 from the MSD would put your total timing @ 45 degrees at some RPM. This is outside of what you want.

I would bump the initial back to 4 + the MSD's 28 with a total = 32 at some RPM

Then add additional limit to the MSD for 22 degrees and compare.

You are always safer with less timing than more. I ususally go to the track and start with less advance and add 1 - 2 per pass until my 1/4 mile speed stops increasing. At some point adding timing will slow the car down.

You then can add vacuum advance to improve street driving and cruising if you like.

 

The specs are a starting point. The problem with running 4 degree's at idle you need the carb setup to deliver the proper AFR. This is easiest with EFI. Unless you have a way of measuring AFR you will not be able to get the advance down to 4 degree's. I think this is why most run 10+ @ idle.

I also found running a lean idle makes it more difficult to smooth out off idle throttle response. I think to be able to use the spec'd timing you would need EFI or a perfectly tuned carb to match.

there are many factors to look at

example

From MSD

http://www.msdperformance.com/setyourtiming/faq.html

 

Thanks for all the replies. The ZZ series needs to be between 16-18* at idle. I know it sounds crazy, but anything below 16* and it is just not responsive at all. I've read tons of articles on the ZZ454 and the GM directions are wrong.
I'm still confused about timing answers. Did I do the math correctly? I don't understand the stop bushing in the MSD making it stop at 28*...is that 28* + the 17 from idle?

 

Your observations sound entirely logical.

 

The stop bushing should set the total amount of advance the distributor will add. Meaning from 0 to a max of 28 degree's at some RPM based on the weights and springs used.



example: distributor advance curve from 0 - 28 degree's (bushing setting limiting 28) starting at 1100 RPM - 3000 RPM ( weights and springs control this )

so if you set the initial idle @ 17 any RPM below 1100 will remain @ 17 degrees timing. Once the RPM goes above 1100 the distributor will advance the timing based on a curve. Once the RPM reaches 3000 RPM in this example the ditributor will no longer add timing and therefor reach the max of 45 degrees (17 + 28 = 45) ( total max timing )

 

Ahhh, now I got it! Thanks everyone. Cagotzmann, that is nice and clear.