[jhein]

I have a stock DUI HEI dist. It has a vac adv of 12 with mechanical adv of 24 by 3000 rpm. That's set up for a stock motor. I called them and gave them the specs for my new motor and they sent me a set of springs and weights that will reduce the mechanical adv to 20 by 3000 rpm. Does that sound right?

The motor is 468 cid, E heads, 10:1 CR 236/242 @050, 520/540, 112 LSA. Fuel will be 92 octane.

[amcmike]

I believe the older generation D ports like 36 total and the newer like around 32 degrees. So makes sense.

[jhein]

So here I am looking at these mech adv curves and I have the DUI stock curve which is for a high compression, iron head motor which has 24 degrees by 3000 rpm. Base timing recommended at 12 by DUI. Then I have the "performance" curve recommended for the 10:1 aluminum head motor which is 20 degrees by 3000 rpm, also with a base of 12. Doesn't that seem kind of backwards? The problem I'm having right now with the stock RAIII motor and the stock DUI curve is detonation at high RPM. Wouldn't that be helped by the "high performance" curve they've recommended? Also, I don't doubt that the "performance" curve would be fine for the AL head motor at 10:1, but it would seem that combo may tolerate the factory DUI curve better than the stock high compression iron head motor. This actually has me tempted to put the new weights and springs in the current motor and see what happens. My guess is it will be better.

[soupman]

Originally Posted by amcmike

I believe the older generation D ports like 36 total and the newer like around 32 degrees. So makes sense.

Don’t mean to hijack your thread but what are considered old generation vs. new generation?

[25stevem]

Nothing will be 100% right for your motor / application until you get the car on a chassis dyno, or spend a test & tune day at the track.
For now all you have is a good ball park starting point which is all you can ask!

[Cliff R]

"The problem I'm having right now with the stock RAIII motor and the stock DUI curve is detonation at high RPM."

Forget what's been done to the distributor. Simply verify that the mechanical advance is actually stopping at a given RPM and no additional advance being added at a higher RPM.

Decide what fuel you are going to run in your RAIII engine, I'm assuming 91-93 octane pump gas but didn't see it mentioned anyplace?

Set the total timing at 30 degrees. Do some full throttle testing and see if it pings. If not move to 32, then 34, etc until you first experience detonation. When this happens to back to the last place you didn't have any.

I much prefer total timing tuning on a dyno or dragstrip, for either best power production or quickest ET/highest MPH, but that simply isn't an option for everyone.

Once you find the threshold for total timing, lets say in your case it's 32 degrees, you will be at 8 degrees initial timing. If the engine is happy there then no further distributor tuning is needed in terms of how much it adds. You may want to experiment with springs if you have a decent selection. In most cases those early high compression engines with stock Ram Air cams in them will not like, want or respond well to a super-quick timing curve. All-in by 3000-3400rpm's is usually soon enough.

IF your engine for some reason (unlikely IMHO) isn't happy at 8 degrees initial and prefers more, say 12 degrees they you will need to reduce the timing curve by 4 degrees so you can set the base timing at 12 and still have 32 degrees total. It's a simple as that and you'd be done with that part of the tuning.

Move on to the VA next. If it's adding 14 degrees, for example, that makes total timing at light load potentially 46 degrees. If the engine runs well there, no light pinging, surging, lack of performance, etc, for "normal" driving you are done with VA tuning. You can play around some there adding or taking away a couple degrees at a time and doing some testing. If you are close it may take several tanks of fuel and quite a few miles to nail down the very best number, but for most higher compression engines with the stock or close to stock cams 10-14 degrees is usually a good place to be.

For the new engine I'd start with the same parameters you were using on the old one and tune as recommended above. 10 to 1 compression, E-Heads and 236/242/112 HR cam will like about 12-14 initial, about 9 ( 18 at the crank) degrees mechanical advance. and another 12-14 from the VA. That's just enough cam in that build it will be happier with a bit more initial timing and shouldn't "buck" the starter on a hot restart.....IMHO......

[lust4speed]

Two factors in determining proper advance is compression ratio and fuel octane on a given cylinder head. Most all of the cast iron cylinder heads that we have set up at 9.2 to 9.3 compression ratio and our 91 octane pump gas like 38° total advance. If the compression ratio was higher then the total advance would need to be backed down. If it is a lot higher then you really need to back pedal on timing. Your iron head engine might need 32° or lower to keep detonation away. It really comes down to each engine and the fuel in the geographic area.

Old Edelbrocks needed a lot of timing. Even my first generation E-heads that were prepped by Dave at SD ended up producing best power on the dyno at 41°. The new generation of E-heads usually come in at 34° or a little less running on pump gas because of a more efficient combustion chamber.

Best thing to remember is that a tech sitting at the other end of the phone line is only making a guess at what your engine needs. It is up to you to listen to what it wants and make changes so that it runs at peak performance.

We had an interesting argument with one of the DUI techs a few months ago when one of my friends was placing an order. His engine had been on the dyno with one of my loaner distributors and we had mapped out an optimum advance curve. The tech wanted to set it up with his generic curve he had sitting in front of him, and it took awhile to convince him that we were dealing with tested parameters and that bested his generic one-size-fits-all curve.

edit-- Cliff posted while I was typing and I'll leave this, but do concur with him fully.

[389]

Disconnect the vacuum advance.

[jhein]

Thanks for the very helpful replies.

Just to review, what I had most recently was 8 base and 24 mechanical, total 32 @ 3000 rpm. It was better but still some detonation. So I thought I'd set the total at 30, but for that I'd be at 6 base which was low, didn't idle well. So I put in the new weights and springs from DUI. I set the base at 10 with 20 mechanical for 30 total @ 3000 rpm. Idles well, staying cool. I'm taking it out for a driveittle later so we'll see.

I still think the carb has to be the problem because with the previous carb I was at 10 base plus 24 mech with no problems. Anyway, this setup is only temporary so I'm not gonna be too finicky about it as long as it runs OK and no detonation.

[Brentco]

Is the carb leaning out at high RPM? With that cam, 10:1 compression, and 92 octane, you shouldn’t be getting detonation at 32 degrees total timing, IMO.

[jhein]

Originally Posted by Brentco

Is the carb leaning out at high RPM? With that cam, 10:1 compression, and 92 octane, you shouldn’t be getting detonation at 32 degrees total timing, IMO.

Sorry, it's a little confusing. I'm talking about 2 different motors (see post #3). The motor in the car now is the stock RAIII iron head motor.

[25stevem]

Compression is always talked about , but at best it’s a loose ball park guide when trying to steer clear of ping or worse detonation.

It’s mainly cylinder pressure that matters .

[Brentco]

Originally Posted by jhein

Sorry, it's a little confusing. I'm talking about 2 different motors (see post #3). The motor in the car now is the stock RAIII iron head motor.

Ah, gotcha. Well it sounds like you have it figured out now.

[amcmike]

Originally Posted by soupman

Don’t mean to hijack your thread but what are considered old generation vs. new generation?

When Edelbrock came out with the fast burn RPM cylinder heads, with chambers that more closely resembled the LS (after KRE had their version out for years. https://bangshift.com/general-news/n...c-455-engines/

[Kenth]

total 32 @ 3000 rpm or total 32 @ 4500 are two different worlds considering ping.

Start with a factory timing curve at all areas and you will be rewarded.

[Cliff R]

It is difficult in this business to find center cam, weights and spring combinations that will keep the timing out to 4500rpm's. Most timing curves on factory distributors, especially HEI's bring it in much sooner even with the stock springs on them.

I have no experience whatsoever with the DUI stuff so can't help out here with custom tuning one.

I've set-up many hundreds of factory points and HEI's at this point, and with few exceptions most aren't really that far off from an ideas curve so just need a few minor adjustments in a couple of places.

I'll add here that NOTHING in the aftermarket is going to make 1 more ounce of power than a well set-up factory distributor. Factory distributors are also dead solid reliable in long term service, and why I stick to them over anything in the aftermarket. For factory points units in almost all cases the rubber "sleeve" over the advance pin is rotten or missing. I make one here out of stainless steel and it's pressed on and crimped in place, so a permanent fix for that issue.

All adjustment to the timing curve are done with the MIG as far as how much timing it can add, so I know for sure there is a POSITIVE stop for the advance and we're not relying on weights, center cam and springs to do that for us.

Just some additional info on this topic that may be helpful to folks reading it........
[IMG]https://forums.maxperformanceinc.com/forums/attachment.php?

[389]

I prefer the older points distributors with a CD box. They came with performance curves built into the weights. Those weights are so important, you need to find the best ones. They not only control amount of advance but when the motor gets the advance. The old iron single point distributors are the best and when used to trigger a Capacitive Discharge ignition. You end up with one of the best ignitions going.

The inductive discharge ignition it too slow for todays performance engines. Its not so much the amount of power but how quickly it gets there and how fast the ignition can recover and ready for the next fire. Points, HEI, Pertronix and all the custom made distributors that you cant get replacement parts for are all Inductive Discharge. If keeping the ID ignition, I would just stay with the points. Summit sells a real nice set of Accel 32 ounce points that work well. I buy the vintage Made in USA points on e-Bay. I like the Vertex Z-6 Capacitive Discharge ignition. It is also made in the USA.