318 #302 head timing question

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1972dodgedemon

1972dodgedemon

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Recently I overhauled my 318 and in the process installed the 85-91 cast #302 heads, a comp cams H268 http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=619&sb=0, an Edelbrock performer intake, and an Edelbrock 500 cfm carb part number 1404. I've calculated the compression ratio to be around 9.5:1(head gasket: thickness compressed .028in, bore 4.14in. Piston(flat top) to deck clearance .058in. Combustion chamber: heart shaped swirl port 59cc).

Has anyone had experience tuning an engine with these heads? Hot Rod magazine built one that cranked out over 400 hp and torque at the fly wheel. Their engine had closer to 10:1 compression, 2.02/1.60 valves, and very ported #302 heads. With this setup, they ended up making the most power with 30 degrees of total advance timing.

The instruction manual that came with my Mopar Performance electronic ignition kit says to set the maximum initial+mechanical for 35 degrees with a total of 50 degrees BTDC including vacuum advance for pre '89 ,motors. The heads I'm running were produced from 85-91.

I don't know whether the hot rod guys had additional advance from vacuum or just stayed with 30 degrees maximum.

If 30 degrees advanced is the maximum I should run, I don't want to risk detonation on my fresh motor by setting initial+mechanical+vacuum for 50 degrees.

What do you guys think? Should I leave vacuum advance out of the equation or go with the mopar performance instructions @ 50 degrees total?
 
With the fast burn 302 heads, you likely won't need more than 34* total timing by 3000rpm...you'll need to recurve the dizzy to limit it.

When the editors say they only had 30* total timing, you have to realize, the vacuum advance only works when there's "less" than 0" hg in the manifold...under WOT, there should not be any less than 0" because the throttle is open. Under a lighter load, you create lowered manifold pressure, ie, cruising down the boulevard, you may have 22" hg (and your total advance may be as high as 50* when the engine is low demand, high manifold vacuum), but when you mat the pedal, the manifold pressure goes "up" to 0" hg (and your dizzy reverts to mechanical only)

In other words, dyno runs are WOT flog events, vacuum advance is essentially non-functional, so the dizzy is operating purely on the mechanical advance (weights and springs)
 
318 with a 4.14 bore huh? Stroke must be tiny...bet that thing revs like a 2stroke, lol
:)

Never mind....read it again and realized that was the gasket bore, lol
 
This is the number 1 mistake a lot of people make. They are told were to time the car, they set the distrib. to where they were instructed, and that's that! I've had performance machine shops, etc, etc, all instruct where to put the timing. Some have been close, but the E.T doesn't lie. I go up (or down) 3-4 degrees at a time to see how the car responds. If it likes it, I continue to head in that direction till max achievement is accomplished. Even trial and error out of town can help with this. Let YOUR car tell you what the timing should be....
 
txstang84 said:
With the fast burn 302 heads, you likely won't need more than 34* total timing by 3000rpm...you'll need to recurve the dizzy to limit it.

When the editors say they only had 30* total timing, you have to realize, the vacuum advance only works when there's "less" than 0" hg in the manifold...under WOT, there should not be any less than 0" because the throttle is open. Under a lighter load, you create lowered manifold pressure, ie, cruising down the boulevard, you may have 22" hg (and your total advance may be as high as 50* when the engine is low demand, high manifold vacuum), but when you mat the pedal, the manifold pressure goes "up" to 0" hg (and your dizzy reverts to mechanical only)

In other words, dyno runs are WOT flog events, vacuum advance is essentially non-functional, so the dizzy is operating purely on the mechanical advance (weights and springs)
Click to expand...

Thanks for the insight. So, if I were to decide that the maximum initial+mechanical timing made the best power at 30* where would I start with adding in vacuum advance for street driving? Should I just, through trial and error, figure out what works best? Do you recommend a good starting point for vacuum advance?
 
In a nutshell, yes...trial and error is best. We can give you recommendations where to start, where you end up, is up to you

Set up your mechanical first. Most here would likely recommend about 14 initial and try to curve it to figure out what's best for your total...and that's just the beginning.

Go to summits website, look up their MP electronic distributors, click on and download the instructions and read about how to set the vacuum advance. It involves a 3/32" Allen key, a friend, and a mity vac hand vacuum pump
 
For what it's worth;

When I broke in the 318 in the Scamp, with KB107's, the same .028 gaskets and heads that you have, I was at 9.68:1 static with a 62cc chamber. I put stainless valves in that had bowl faces, so that may have been to the added CC, but I also shaved them .010" to clean them up.

I was at 61.5cc, so I radiused the sharp edges of the hart shape chamber and rechecked to see them back at 62cc. I think they're closer to 60cc with the flat faced valves, stock.

When I fired it up, it was at 36°, broke in great. It didn't like it when I shut it off after and started it, kicked back, backed it off to 34° total. Vacuum doesn't do anything to total. The best way to hit max advance is to listen for starter kickback. Fire it, advance it, shut it off, start it... If it starts without fighting/ sounds normal, give it more, 2° at a time, until you can hear a change in the starter, then come back 2°.

Again, I ended up with 34° by doing this, so you might start there and mess with it to find what yours likes.

I still have some jetting to do, but I'm not going to touch it without an AFR, so I know which way to go. I'm almost positive that it's not getting enough fuel. That engine caps at 4400 until I put my hand over the air horn and starts climbing. I followed jetting charts for 5400ft, but it's got 3° cam advance and a lot of ramp on an RV cam, so maybe it needs it.

The reason I bring it up, is because after you get yours tuned, if you are experiencing any hesitation, you may end up going with a bigger carb. These heads flow better than I anticipated. I'm running a 625 Street Demon. All I did to the heads was deburr and clean the garbage up in the runners, as well as any obvious port volume changes between the runners from casting flaws.

I also centered my intake and I'm running a 318 port size intake, but now that I have it apart again, I've got the urge to gasket match them with the new small runner gaskets, because I can. I'm not sure it will have any big effect on flow, but it could help stabilize firing. It idles beautiful, already. I was able to trim within 30rpm at idle, once I got the timing down to 34° and locked it.
 
I'm running vacuum advance, because it's a street engine, but again, that doesn't do anything to full timing. I have it fed on a timed vacuum port from the carb, so it does what it's supposed to do at idle and other high vacuum areas of throttle/ load.

Once you've got the timing where it likes to stay, 2° retarded from starter kickback, work on primary to secondary transition, if it stumbles, with accelerator pump setting and jetting if you can't get rid of it with the pump. After that, if you can, read the exhaust and get a good AFR reading. I used to take my cars to the emissions clinic to do it, before getting my hands on an AFR o2 reader. I put o2 sensor fittings in my cars, now.

After you've got the engine dialed, double check and reset your kickdown linkage to make sure you are getting the correct full and part throttle activation for enough band and clutch apply pressures. You may want to climb down there and adjust the low/reverse and kickdown bands, if it doesn't hold enough for you.

Other than that, it's a matter of playing with stall, tire size and gearing, but a good carb tuning and timing should make it drive clean, without fouling plugs.

Just check your plugs after initial run and again at your first oil change, to be sure everything looks good.
 
Thanks for all the help guys. I ended up driving it to the local burger drive up joint the way it was and it ran pretty well. This weekend, I'll spend some time on the timing to get it just right. As it sits, it likes about 18 degrees initial and I may go ahead and turn it up to 20, but if that's the case, I'll need to pull the distributor and reduce the max mechanical advance.

On another note, I'm trying to decide on what thermostat I'm going to put in it. I'm getting a new aluminum radiator from summit and I just want to make sure that it doesn't run cool. I've looked around a bit and it seems as though the optimal operating temperature is around 195. What do you guys think?

Edit: spelling
 
I'm running a 195° in mine with an aluminum radiator. Gets to operating temp like it should and stays there.

Should work for yours, seeing as how you've got a very similar engine.

The initial timing set with vacuum at 18 is a good starting point, but you really should check total timing with a dial back light, or some timing tape on your balancer to show you what you are getting at.

Did you try dialing the timing back from starter kickback condition? That's really the best way to do it. Advance it until you can hear a difference when you start the engine and trim it back 2° or until it doesn't change how the starter sounds when cranking.
 
I've got the mopar performance timing tape that goes to 60 degrees before and after TDC. I gather that a dial back timing light accomplishes the same thing?

In the likely event that I have to limit the mechanical advance on my distributor, do you know how to do that?
 
It's a matter of stopping the weights sooner. Some take the plate out and weld it, then open it with a file using a template that some websites have available.

This should be done with some good springs to help tune. I'm using the lighter stock spring and another super light spring, so the full advance comes in on my engine at around 2600 and stays at 34°, where I have the distributor body set.

The advantage of trimming total advance on the curve by welding and trimming the plate inside the distributor is mostly for guys running it on the track, that want full advance really quickly on super light weight return springs. You can gain some there, as well as with a better electronic ignition module than the stock replacement ones, or even the Mopar Performance orange and chrome boxes. It also helps to phase the distributor cap, if you're looking for more, after recurving the distributor advance with springs and stopping the weight.

I found that just using the factory light spring that is installed, with another lighter spring on the other weight and no weight stop adjustment works good for my street engine. I can't remember what my initial/ vacuum is set at. I'd have to check, but I set total timing with tape, like you have, at 34°. I got that by dialing it back from 36°, because I was hearing starter kickback. It's happy there.
 
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