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engine timing

12K views 44 replies 13 participants last post by  Desert Deuce 
#1 ·
I have a stock 2002 ZZ4 350/385HP. I checked the timing per the manual and found it was advanced beyond the 10 degree mark. I reset to the 10 degree mark and it seems to have a flat spot at low rpm that didn't use to be there. Is everyone running 10 degree above TDC or is there a better spot for lowend torque?
 
#2 ·
Lonestar Rider said:
I have a stock 2002 ZZ4 350/385HP. I checked the timing per the manual and found it was advanced beyond the 10 degree mark. I reset to the 10 degree mark and it seems to have a flat spot at low rpm that didn't use to be there. Is everyone running 10 degree above TDC or is there a better spot for lowend torque?
You will get the best results if you set the total advanced at 36 degrees. If the engine is idling a little fast the distributor will be already advancing and give you a bum reading. Slow timing will cause a flat spot.

Adrian
 
#3 ·
I have found 14 degrees at idle to be optimal. Try it, you'll like it.
Also, I have found 38 degrees total mechanical advance to be even better than the 36 degrees recommended by almost everyone else. This is a question of getting the most advance you can without pinging and each motor seems to like a little more or less advance. These engines have very little load on them due to the comparatively light weight the engine has to pull and can, therefore, take a lot more advance much sooner in the RPM curve. I advanced mine until pinging under load and then backed off two degrees at a time until it stopped. Trial and error.....Joe
 
#4 ·
Timing Marks

I recently bought a 2002 SB 385HP and don't have the manual. Where are the timing marks on the engine as it doesn't have a traditional harmonic balancer? I have added the #7 converter and nitrous. Have not had the nitrous on yet as I havn't gotten comfortable with how hard the converter hits yet. Probably tomorrow :twisted: !! Yes there will be nice weather in Atlanta tomorrow!

Phil
 
#7 ·
Found Timing Marks

Thanks Guys. Found the marks and now just need to find my old Snap On timing light. Think I left it at the hanger while timing an electronic ignition on an experimental airplane 10 years ago!

I assume from what I have read on the board and from the mark I see on the pulley that the "proper" timing is 10 degrees btdc at low idle or 36 degrees @ 2500 rpm +. All with vaccum disconnected and plugged.

Thanks,

Phil
 
#8 ·
Phil,
First, you may want to get a timng light with an adjustable scale. Makes things easier.
Yes, vacuum advance disconnected and pluged, the mark is 10 degrees. But I think most have found 12-14 gives the best response. Your total cenrifugal advance is
good @ 36 degrees, although some like Joe, have found 38 to work well for them. All depends on what your motor likes. Some of the guys (myself included) have installed the Crane Vacuum Advance kit, and like it a lot. It gives you a choice of spring strengths, which will bring the centrifugal advance in a little faster, and an adjustable vacuum advance diaphram.

I'll send an article that is pretty good on the GM HEI ignition to your e-mail address. Good luck.......Don
 
#9 ·
Phil,

I've got a 385 small block & I run 14 degrees, 38 degrees and 50 total with 125hp NOS kit & #7 converter......works awesome and no detonation. I do run a progressive computer on my system though so it doesn't get 100% initially on the button normally.

You're gonna love it :lol: 8)

Tim
 
#10 ·
Thanks

Thanks again. Got the article and have ordered the Crane advance kit from Summit. Will install and time asap. The converter change brought this puppy to life, with some additional and more quickly advanced timing it should be a real handfull when the nitrous kicks in. I do not have the progressive computer, it justs come on 100% at full throttle after arming. Is this a problem?

Phil
 
#12 ·
Phil,

I would think not, I really added the controller 'cause I want ed be able to control the wheel spin a bit..........I actually wanted to beat somebody :D

Lots of guys running your combination and are very happy :lol: ]

The converters make a ton of difference and really make the bike feel strong.

Tim
 
#13 ·
GRUMPY said:
Phil,

I would think not, I really added the controller 'cause I want ed be able to control the wheel spin a bit..........I actually wanted to beat somebody :D

Lots of guys running your combination and are very happy :lol: ]

The converters make a ton of difference and really make the bike feel strong.

Tim
Hi Tim

The real reason to add the controller is to get consistent runs with your NOS (such as bracket racing). It also looks good. The way we use Nitrous is not trying to get the same run within a few thousands of a seconds each time but to beat your buddy or light the tire up at all speeds and so forth.

Adrian
 
#14 ·
GM HEI Ignition article

Don;

How's about shooting that GM HEI article my way as well please.

I just got a good set of Taylor 8 MM wires, and hi-temp socks for their ends (puppies were CHARRED!), and am ordering the MSD Upgrade kit, and wish to put it all on @ once.

I have never timed an engine with multiple advance features, so I'm looking for a local guru (small block smart) to nurse me thru it the first time.

Thanks!

Ride Safe

HAWK88
Jack Phillips
PSJLP@HOTMAIL.COM
 
#15 ·
Jack,
I sent it to your e-mail. It's the part that deals with the advance mechanism.
You, and anyone else that might be interested, can also go to Wag's site to see the complete article. I'll include a link to it.
Scroll down just past half way, and you'll see the "Help" files. On the left, under "Engine" you'll see "HEI Performance." We are concerned with the Non-computer controlled distributors.....Don

http://webpages.charter.net/wag1/
 
#16 ·
Howdy Adrian,

How you & Cozaey doing?? Say "Hi" for us.

I've tried mine at 100% & no delay, and it will light the tire up to any speed I've tried so far :lol: I think the fastest is about 70 that I tried it with no delay, and it spins the tire pretty hard.

With the delay & stepped down to about 30% to 40%, you can just nail it and the tire stays pretty well hooked up....hazes just a bit when the NOS kicks in according to the guys behind me.....then stays glued until you let off. Not sure what it would do with the stock 230, think it'd probably go up in smoke unless the "hit" was 25% or less on the initial.

I only tried getting into it once with the tire really smoking, and it just buried the tach as soon as I hit the button 8) Way cool :lol:

Tim
 
#20 ·
Re: Crane Vacuum Advance Kit

Lonestar Rider said:
Hey guys, I just received my kit from Summit. Think I can install per the instructions in the article on wags site. Is everyone using the limiter plate that comes with the kit or not.
I have done 5 Boss Hoss's and I tried it both ways. It works best for me by leaving the limiter out.

Adrian
 
#21 ·
Alright I took my Boss apart today got yhe new Crane vac can installed. I have decided to use the limiter plate which I assume goes just under the front screw and pushes against the advance rod coming from the can. Is this correct, and did you use the maximum setting (i.e. pushing rod into can as much as possible) or something less? I appreciate all the help as this is the first time I've been this far into my bike.

Lonestar Rider.

Don Lantz
 
#22 ·
For what it`s worth... I took Adrian`s advice and installed a yellow and a silver spring to get full advance around 1800-2000 rpm. Left the limiter out and backed the allen screw in the advance diaphram out 5 turns from bottom... Set my total to 36 degrees and my bike runs like a bat out of hell. No pinging and very responsive.
Adrian... Thanks buddy.
 
#23 ·
Thought I better mention....... Make sure you have ported vacuum to your advance diaphram. Mine was hooked up to manifold vacuum from the factory. With the lighter springs you will need to change that if yours is not already.
 
#25 ·
You will find a capped vacuum port on the right front corner of the carb. the port is a little bigger but a 5/32 vacuum line will still go on there. Use that one and cap the rear one it is hooked up to now.
 
#26 ·
We had a real good debate on here a while back or it may have been at Yahoo regarding ported vs manifold vacuum. I run manifold. This is a real good article on the subject for those interested. It is long.

Chris

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
 
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