Pontiac GTO Forum banner

1 - 12 of 12 Posts

·
Registered
Joined
·
156 Posts
Discussion Starter #1
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?
 

·
64-67 Expert
Joined
·
8,561 Posts
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.
 

·
Registered
Joined
·
711 Posts
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.
 

·
Registered
Joined
·
156 Posts
Discussion Starter #4
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.
 

·
Super Moderator
Joined
·
5,744 Posts
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
 

·
Super Moderator
Joined
·
5,744 Posts
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
 

·
Registered
Joined
·
156 Posts
Discussion Starter #8
Thanks for the advise. So al8ng with your 16 degrees initial plus 20 mechanical do ypu also use vacuum? If so how much?
 

·
Super Moderator
Joined
·
5,744 Posts
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
 

·
Registered
Joined
·
217 Posts
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.
 

·
Registered
Joined
·
156 Posts
Discussion Starter #12
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.
 
1 - 12 of 12 Posts
Top