# Engine Timing Advance



## WhittP (Jun 19, 2013)

My engine is in the process of being built right now and I have a couple of questions for you guys. I've always read that you make sure all 3 parts of the advance curve are dialed in right to make the car run well without detonation. My engine builder is suggesting I run 15 degrees or so initial with 20 degrees or so mechanical and locking out the vacuum advance. I was under the impression this needed to be used on a street car for idle cooling and idle quality, and cruising economy.

The engine will be in my 67 GTO conv. It's a 461 stroker that should be around 500HP/550 TQ. It has Edelbrock round ports and a roller cam set up. The engine builder says 10.4:1 compression and it will run fine on ~91 octane pump gas. Is he worried about detonation with the vacuum advance or is there something I'm missing here?


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## geeteeohguy (Feb 2, 2008)

Running no vacuum advance on the street is not a good idea. It's a race-track set-up, and works for consistency in performance with no regard for fuel efficiency or engine temps over an extended run period. The very fact that your builder is recommending this approach sets off red flares to me. It's a quick, down and dirty way of getting the car out the door without doing any homework or figuring. 15 degrees initial is a bit high, to me. Should be no more than 12, and stock is about 6 to 9. So, you are correct: you need all 3 parts set/adjusted. Be advised that with manifold vacuum, at WOT, there is no vacuum advance. It's all mechanical. Where the vacuum advance comes in on a street car is cooler operating temps at idle and drastically increased fuel economy at cruise. The oem connection on most Pontiacs is direct manifold vacuum, and the maximum advance at about 2500-3000 'all in' is about 40 degrees.


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## chuckha62 (Apr 5, 2010)

I hear guys recommend this all the time and it always strikes me as lazy and lacking understanding of exactly how timing works. There's a reason that GM (and all other manufacturers) set up the advance curves the way they did. These wannabe experts make these recommendations because they don't know what they're doing and don't want to take the time to set it up right.


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## WhittP (Jun 19, 2013)

So if he dials in 34 degrees total - initial plus mechanical (which he says is his rough target) do you think I should worry about detonation at high vacuum situations like cruising or idling? I'd like the benefit of the vacuum advance for idle quality and fuel economy but I understand I'd need to limit it to 10 or 12 degrees or so.


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## BearGFR (Aug 25, 2008)

There's a ton more that goes into the detonation equation than just timing. In fact, using timing to control detonation is a bald-faced admission that you "blew it" building your engine and now the only way to keep it together is to use timing as a Band-Aid. Definitely NOT the way to be thinking.

How much total timing your engine needs to make max power is, more than anything else, a function of combustion chamber efficiency and flame propagation properties. Chambers that are less efficient tend to need more "time" to get the burn going and thus need more advance. 'Better' chambers don't need as much. Closed chamber cast iron 670's flow really well for factory heads, but the chambers aren't very efficient so they tend to "like" lots of timing - 37 degrees or more, sometimes as much as 40. Open chamber heads don't "need" as much. When I was running my 722's I ran them at 35 degrees, which seemed to be the happy spot.

So, which heads are you running? How much compression?

Bear


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## WhittP (Jun 19, 2013)

Its the edelbrock round port 87cc heads at 10.4:1.


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## BearGFR (Aug 25, 2008)

Alrighty then.... 33-34 degrees total is a reasonable safe starting point for those particular aluminum heads at 10.4:1 in "our neck of the woods" on 93 octane, but the motor might "like" more. If you want to really nail it, the best way is to take a day/night and your tools including timing light and go to the track (you need an accurate set of e.t. clocks). Start there, make a pass or three to get a base line, then bump it up a degree and make another pass or two. Make sure as best you can that all the conditions are the same for each one (same engine temp, same starting line procedures, etc.) - repeatable consistency on each run is more important for this than trying to lay down the absolute 'best' time. Chances are as you add timing, you'll start seeing better e.t.'s. Then you'll reach a point where the car either doesn't improve or maybe even falls back a little. The previous setting then is the optimum for your combination.

I'm running round port 72cc E-heads on mine at 10.0:1, and 36 degrees total seems to be what it likes. The DUI HEI I'm running has 20 degrees "in" it so that means my initial is at 16 degrees. The car starts and runs fine, doesn't overheat, etc.

Bear


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## WhittP (Jun 19, 2013)

Thanks for the advise. So al8ng with your 16 degrees initial plus 20 mechanical do ypu also use vacuum? If so how much?


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## BearGFR (Aug 25, 2008)

Yes I use vacuum - I have no idea how much. It doesn't really matter. Like GeeTee said, the purpose of vacuum advance is to help with engine cooling and economy under _part throttle, lightly loaded, cruise_ conditions. As soon as you hang your big toe over in the carburetor all the vacuum goes away so it's out of the picture, and stays out of the picture until you let off. Since detonation risk really only exists under high load, low rpm conditions that means that presence/absence of vacuum advance (and how much) has zero effect on detonation. So...... I don't know and I don't care 

Bear


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## 666bbl (Apr 13, 2014)

Vacuum advance is almost a misnomer because it's actually a LOAD COMPENSATION device. It drops timing at load (zero vac) and pulls it back in slowly as vac increases. Yes, it really is that simple.


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## geeteeohguy (Feb 2, 2008)

Very well put, 666bbl.


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## WhittP (Jun 19, 2013)

Well it sounds like my initial and mechanical will be set up just fine then I'll just hook up the vacuum when I install the engine. I already have the adjustable vacuum canister and an adjustable limiting cam installed so I should be able to set it up to work right. I'm guessing it will need quite an adjustment to come in at 10-12" of vacuum where right now it works at about 18". If I see or hear any signs of detonation at part throttle I can dial it back.


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## GIgtoGuy (Jun 24, 2021)

this thread has caught my attention as I have just completed my breaking in. I have a 70 GTO with a 461 stroker and number 13 heads. I’m getting a lot of spark knock under hard acceleration unless I use 50/50 91 and 110 Is this a common issue with 10 to 1 compression? Is race gas the only answer or are there other alternatives Other than pulling timing out?I see the engine builder has removed the vacuum advance as well. I apologize if any of this has been covered as I am new here and new to trying to tune my own engine. Just would like some input from knowledgeable Pontiac people.


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## Lemans guy (Oct 14, 2014)

Race engine builders often pull the vac advance a bad idea for a streetcar. A properly set up vac advance will not be the cause of detonation. An improperly set up one can on acceleration, but at WOT there is no vacumn and therefore no vac advance.

Detonation comes from your built in high compression, running too lean, or too much heat in your engine ( a poor cooling system) not enough octane and or too much timing advance.

So your engine is built, now you have to work on octane, AFR mixture, timing and heat or ......change the heads and reduce the compression.

Most distributors I see have way too much spark advance. Racers generally are better at watching total and set up an 18 base /18 Centrifigal. Ditching the vac advance and giving up 10 more degrees at idle for cooling,.....and also the benefit of that 10 degrees at light throttle which burns the leaner mixtures more efficiently.

yes use high octane, dial in your carb, make sure cooling system is at peak performance and find out your base and Centrifigal timing.

The gang here can help you determine those timing numbers and go from there. What kind of ignition system are you running? HEI, MSD with 6A? Points size dizzy and external coil? Etc


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## GIgtoGuy (Jun 24, 2021)

I’m still running the original distributor with external coil yes with perTronics ignition instead of points. I have an advance curve kit that I am going to install and then attempt to tune the car. The advance timing light will be here in a couple days. From my understanding the first place to start is set total timing with advance unplugged somewhere around 36. And then do I set initial timing with vacuum advance unplugged as well? Around 12-26 is what I read. Will that change total timing in the process? The engine builder told me he unplugged advance because it was adding 20 and he wanted more control over the timing. I’m gonna plug it back in and try to do it right. The car has a TCS solenoid but from what I read I should just unhook that and bypass it
Thanks for any advice it’s nice to have access to knowledge!!🙏😀


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## Lemans guy (Oct 14, 2014)

You are well on your way. So their are key numbers that you want to discover.

Don’t put on any new parts yet. Warm up the engine for smooth running. Start with checking how your timing light works. Now Check base timing.

Now shut the engine off remove the distributor cap and the two screws on the rotor, remove the rotor. With a small pick and your safety glasses, remove one of the springs on the Centrifigal weights. Put the rotor back on, remember the tabs underneath the rotor, one square one round. Yes they can be forced on wrong.

replace the distributor cap. Set your timing light to 40 degrees of advance. Now start it up. Make sure you have parking brake on, wheels chocked, park or neutral, good to have a helper as well, with foot on the brake. Now With timing light hooked up rev the engine steadily and watch the timing mark as it advances.

you will not have to rev the engine up too high because you have one spring removed and you will reach mechanical limit quickly. When the timing mark stops advancing note that number. If your engine builder set it at 36, it should read 36. But it could be 34 or 38 or whatever. Shut it off, remove dist cap and rotor and replace the spring. Don’t Forget!!

Now subtract the base timing reading that you previously determined from your new total number. This is your centrifigal advance number. Write both down.

then we can move on to fixing your vac advance correct and checking the springs.


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## Lemans guy (Oct 14, 2014)

Do all this first with the vacumn left plugged.... you need these numbers first.


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## Lemans guy (Oct 14, 2014)

And don’t plug that vac can in…20 degrees is too much…one step at a time


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## GIgtoGuy (Jun 24, 2021)

Thank you very much! As soon as the timing light shows up I’m gonna jump in and I will post results.. i’m excited about getting this done myself with the help of you rather than pay someone. Knowledge is power ! You guys are awesome! Thanks 👍


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## Lemans guy (Oct 14, 2014)

Once that you do this Gig, hook up a vacumn gauge and make note of your vacumn reading at hot idle. Automatic in drive brake on, manual in neutral.

Then note, also hot your idle RPM’s. Those numbers will help get it right. You will get it and all the gang here will jump in and help you out.


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## PontiacJim (Dec 29, 2012)

GIgtoGuy said:


> Thank you very much! As soon as the timing light shows up I’m gonna jump in and I will post results.. i’m excited about getting this done myself with the help of you rather than pay someone. Knowledge is power ! You guys are awesome! Thanks 👍


It will be to your advantage, and ours, to start a new post for your timing issues and engine combo - as they are all different.

Give us some specs on your car and engine build as this is always an important list of items so anyone with a similar combo can see what you have done to dial in the timing.

Then we can follow and guide you along and your post will not get lost in another's and will be eventually added to the "archives" where another future member can access it and read your experience.


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## GIgtoGuy (Jun 24, 2021)

OK I started a new thread with my engine specs as the title and added some new information 








1970 GTO #13 Heads YS Block 461 Stroker Kit Comp HYD Cam...


Started this new thread to try to get some assistance timing and carburetor tuning etc. Engine has 300 miles and Oil and filter installed…but extremely sluggish nothing near what I expected. It’s the original 1970 YS block with the original distributor and external coil, points have been removed...




www.gtoforum.com


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## GIgtoGuy (Jun 24, 2021)

OK got the spark curve to come in around 3000 RPMs and total timing is now 36. So timing at idle at 22° at 830 RPMs. Plugged the vacuum advance back in and took it for a drive and it runs much better pinging has stopped. Power is up but seems nowhere near I feel it should be. What about carburetor adjustments? Any and all advice would be appreciated.


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## Lemans guy (Oct 14, 2014)

Your numbers sound good, that 22 at idle is with vac advance operating connected to full manifold….if that is correct you are good.


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## Lemans guy (Oct 14, 2014)

Carb tuning can be very basic, vac gauge and string the idle jets, driving checking plugs, smelling observing, feeling performance. You can add in an AFR gauge to tune in each circuit of the carb.

idle circuit, power valve, accelerator pump, main jets and secondary jets. So many ways to go depends how much you want to do.


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## GIgtoGuy (Jun 24, 2021)

Actually that 22 was with the advance unplugged at idle. I never measured timing with the advance plugged in. Why would this number being so high?


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## GIgtoGuy (Jun 24, 2021)

GIgtoGuy said:


> Actually that 22 was with the advance unplugged at idle.


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## Lemans guy (Oct 14, 2014)

Well that would mean that your centrifigal advance inside the dist is 14, or your springs are too light adding Centrifigal timing at idle. You could have 14 Centrifigal but you should check as most are not that low. More likely what you believe your total advance to be is not

Distributors will have advances commonly up to 4000 to 5000 Rpm’s….But you won’t see that advance until you rev it that high.

Did you follow the procedure that I outlined in post #16 above? If so you should be positive of what your Centrifigal number is. Then vac pulled and plugged set base by subtracting that number from 36 or 34 if you want to be conservative. Always check for pinging.

Your mechanical number is fixed, now add the correct 10 degrees of vacumn….base should read your base number + 10 degrees


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## PontiacJim (Dec 29, 2012)

GIgtoGuy said:


> Actually that 22 was with the advance unplugged at idle. I never measured timing with the advance plugged in. Why would this number being so high?


The engine builder may have set it at 22 as that is what you would want WITHOUT any vacuum advance. So 22 degrees Initial WITH the vacuum can hooked up is where you want to be. So with the vacuum can disconnected, loosen the distributor, back your timing on the balancer down to around 9-10 degrees to start. Then connect your vacuum can and your timing should jump up near the 22 degrees. Then whatever that number is, let's say 19-23, just move the distributor to get that 22 degrees again with the vacuum can connected. Then snug down the distributor and that should be a good start for the rest of your adjusting.

Now this may change, or need to be changed, but it is a starting point. Do not do too many changes all at once or you will not get things dialed in. You make 1 adjustment and see what the results are. If not good, then you can reverse/adjust it knowing where the issue is.

Idle speeds will also have bearing on adjustments, vacuum, and centrifigal advance from the weights. You will want to note at what RPM your mechanical advance begins. Some can begin early at lower RPM's while others can begin to advance later - depending on springs/weights. Your 830 Idle RPM is probably about right. If the engine fires up easily and does not labor to start, then that too is a good sign. Too much advance will cause a labored cranking/start. You also want the engine to stop when the key is shut off, not continue to run and buck - called "dieselling" or "run on." I suspect you may have the electric solenoid at the carb that is used for this purpose?

I see in one of your posts the engine is now running good - no detonation. Was the 22 degrees without vacuum hooked up a typo? 

The TCS should work off the transmission? It is used to retard the timing through the vacuum advance and was an emissions item. It prevented vacuum to reach the vacuum can so it would not advance the timing. The only time the vacuum advance was operated was in high gear (4-speed or automatic). You should not be using this on your engine.

Keep in mind that a Pontiac engine may "feel" like it is down on power, but is not. They rely mainly on torque and depending on cam, the torque curve can be flat and long, not "peaky" like some engines. So the power pull is smooth and you may think it doesn't have the power you think it should, but track results can prove otherwise.

Gearing can also be a big factor, so maybe that is where the power loss is feeling?


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## GIgtoGuy (Jun 24, 2021)

I will double check all these numbers just to verify they are correct. But I do feel it is down on power I installed a 355 posi rear end and I feel like it should break the tires loose from a very slow roll which it does not.


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## Lemans guy (Oct 14, 2014)

Yes the first number to start any good timing curve is the total centrifugal advance…..not when springs bring it in, not vac, not base….

total centrifugal advance….is the starting place.


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## GIgtoGuy (Jun 24, 2021)

Got it. I’m 100% sure I have that set at 36°. I will double check the other numbers


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## Lemans guy (Oct 14, 2014)

Ok so what is your Centrifigal advance number?


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## Lemans guy (Oct 14, 2014)

36 is your total advance…..Base + Centrifigal = Total


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## GIgtoGuy (Jun 24, 2021)

OK double checked my numbers. Base timing is 20° at 800 RPMs. Total timing at 36°. Total timing is all in around 3000 RPM. All numbers were done with vacuum advance unplugged. So centrifugal would be 16° 15 Hg of vacuum at idle. 12-14 Hg with the brake on in gear.

So wondering if this means anything when I plug the vacuum advance in the idle jumps to about 950 RPMs and then the base timing jumps to about 40°. And with the vac can plugged in and I when I rev past the 3000 RPM mark my timing light is showing about 60° of total timing. What the hell is going on here ?? I thought the vacuum advance didn’t add timing when total mechanical in the distributor took over? Is my vacuum adv can bad? Also the timing mark jumps around a bit is this a sign of distributor malfunction or normal?


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## GIgtoGuy (Jun 24, 2021)

So confused


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## BearGFR (Aug 25, 2008)

GIgtoGuy said:


> So confused


Do you have two threads going on this topic?? I swear I just wrote an answer to the same question.


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