[77TA406]

Hello all. I need someone to educate me on the theory of proper timing in relation to cam size. I already understand how initial, mechanical and vacuum timing work together. My current 406 engine combination, which is fairly mild my most standards, uses a hydraulic cam that is 222/228@ .050 and .465/.474 lift. The engine runs great with 11 degrees of initial and 24 degrees of mechanical for a total of 35 degrees. My vac can pulls an additional 14 degrees at cruise. I can't remember the stamping number on the centerplate, but according to my chart, that centerplate gives 12 distributor degrees which equals 24 crank degrees. Therefore 11 plus 24 equals 35 degrees plus 14 vacuum degrees equals 49 degrees total running down the hiway. Am I understanding this correctly so far? Last year, I installed a much larger solid flat tappet cam. The bad news is that cam lost four lobes after 12 miles of driving, but before it went south, I learned a few things that I need some help with. The new cam needed approx 20 degrees of inital timing to idle properly. My problem is becoming obvious now. Add 20 degrees initial to the 24 deg mechanical, and I have 44 degrees. The engine wouldn't last long with that kind of timing.Am I correct in saying, that the answer is to limit the mechanical travel down to about 15 degrees, so that on full advance, I will be back at 35 degrees. (20+15). How does limiting mechanical advance affect performance? What is the best way to limit the mechanical advance? My chart says that a #371 centerplate will give 9 deg in the distributor, or 18 deg at the crank, of mechanical advance. That still isn't low enough, and I can't find a centerplate that will limit it enough. Will the moroso kit that comes with a new centerplate, weights, springs and bushings solve this problem? How big does a cam need to be before this becomes an issue? I hope I haven't asked too many questions at once, and I really need someone to set me in the right direction before the new engine goes together. Thanks to anyone who can help.
Chris

[Geoff]

There are many factors other than cam duration that will affect ign timing. Comp ratio, chamber shape, intake type, headers, etc. For best throttle response, you want initial timing as high as the starter will tolerate. If the starter doesn't labor with 20 deg, leave it there & add the remaining 15 deg with centr advance. You will probably have to experiment with weights/springs to get the rate correct & you may have to braze/weld/bush the adv/ret slot in the dist to get 15 crank deg. Add vac adv to a manifold source for best & smoothest idle.

[Cliff R]

There is no need to ever run that much initial timing with well chosen components. When you find yourself having to run more than about 14 degrees initial timing, try adding some more idle fuel. If the vacuum reading at idle speed is low, 12" or less, most carburetors will not supply enough idle fuel to make the engine happy. We recomend improving idle fuel capabilities first, before shortening up the timing curve and/or adding in the vacuum unit to increase initial timing off the scale.

Even if you end up using manifold vacuum to the advance unit, the additional idle fuel will help idle quality in and out of gear, as many engines will loose enough vacuum when the trans is placed in gear, to cause the vacuum advance unit to drop out the timing, then the engine stalls out immediately. For this reason, an adjustable vacuum advance unit is prefered if you choose to use it for tuning idle quality.

I can say this with great confidence, IF the carburetor is up to par for the engines idle fuel requirements, MOST applications will not require a lot of initial timing at idle. A quick test to confirm that you need more idle fuel is to get the engine idling and fully warmed up, then instead of advancing the timing, gently place your hand over the carburetor, or partially close the choke flap. If the engine speeds up and/or smooths out, you need more idle fuel.

I would also add, that the additional idle speed typically required for engines with performance camshafts installed, often brings in part of the mechanical advance curve at idle speed. This makes idle tuning difficult, if not near impossible, as the timing will fall out when the trans is placed in gear, or engine loaded a bit, and usually slow WAY down or die out. We see this ALL the time, as most folks think that you need ultra light distributor springs and a really quick advance curve for "high performance" engines. In most cases, the aftermarket springs and weights are HOPELESS, being of poor quality and do not provide a good spark curve anyhow.

We run stock Pontiac center cam, weights and springs in our HEI unit. It works flawlessly at every rpm, and runs equally as well as an MSD billet unit in back to back dyno testing. The only modification we made to it, was to limit the vacuum can to 10 degrees, and weld in a positive stop for the mechanical advance, as outlined in our book and the Pontiac book from Jim Hand......Cliff

[steve]

Originally Posted by Cliff R

We run stock Pontiac center cam, weights and springs in our HEI unit. It works flawlessly at every rpm, and runs equally as well as an MSD billet unit in back to back dyno testing. The only modification we made to it, was to limit the vacuum can to 10 degrees, and weld in a positive stop for the mechanical advance, as outlined in our book and the Pontiac book from Jim Hand......Cliff

who can build me such an HEI ?

[Cliff R]

We are completely OUT of center cams and weights. I've stripped down every distributor in the shop to steal them to replace the POS aftermarket stuff everyone installs, doesn't work, then sends them here.

I heard that someone in the distributor business was having these parts made, but not sure if they are in production yet? Until we can get good parts, we're not taking on any more HEI work.......Cliff

[steve]

i have the core !

[Geoff]

Cliff,
You better speak to your friends at KRE, & tell them they are using too much initial timing. I was present, & checked, initial timing on a KRE 600+HP engine that was shipped here. KRE built & dynoed the engine. Comp ratio is over 11:1. The dist was untouched from when it was removed from the shipping crate & fired up in the car. It has an 850 HP Holley on it, 4 corner idle, hardly likely to be 'lean' at idle. When I demonstrated what MVA would do, idle rpm increased over 250 rpm & vacuum increased from 7" to 11". This was in N, so #s will be less in D. We couldn't test in Drive because the car had no brakes & we couldn't rely on chocks under the wheels, putting it into D at 1100 rpm idle with no brakes!!

[Geoff]

Should have added that the engine in post #7 had 20 deg initial timing.

[Cliff R]

Sorry Geoff, I'm have no association with KRE other than getting them to supply us with some parts, some machining, and an occassional dyno test. They are in the business to build powerful engines, with out of the box components, ie engine "packages".

Since our carburetors and distributors are custom tuned EXACTLY for the application, our results are likely to deviated from theirs.

You will find with this initial timing thing that you could walk thru a car show with 300 cars of various make/manufacturer/engine size/combination of parts, etc, idling, and twist the distributors in them in the advanced timing direction, and see an idle speed increase on every single on of them. Really doesn't mean much in the big scheme of things, as any engine responds with increased rpms as the timing is increased at idle speed.

We've covered this ground many, many times, and we still recomend to tune each and every engine combination by using the carburetor and distributor to find the ideal setting(s). It doesn't mean that we start adding tons of timing to any or all of them at idle to overcome deficiencies in carburetor fuel delivery, combined with poorly chosen internal components. We ALWAYS set everything up that we work on here with the most desirable timing and fuel curves, and find that once we get the carburetors dialed in exactly for the application, 99 times out of a 100, LESS initial timing is required for a smooth idle in and out of gear......Cliff

PS: I would not assume than an HP950 Holley is "optimum" anyplace for any particular application. I've obtained and tested quite a few of them here, and they were WAY off for my engine, clear across the engine speed/load range, and at idle!

[Half-Inch Stud]

CLIFF, an interesting post on idle enrichment to enable idle timing back off.

I've done gone used to advancing my idle timing to attain idle with oversized cams so much that I forgots to add idle fuel. Been finding out the ever carb I gots now is too lean at idle and loves a nearly closed choke so it can pull fuel from the Q-JET "PTE" holes...what a crude waste of gasoline I gots for cold-start.

So, I may set an HEI for such dynamic MECH ADV (Idle to 300RPM) and see how it goes.

[Cliff R]

HIS, keep in mind when using the terms "rich" and "lean" at idle, that we are NOT making the A/F "rich" at idle, we are simply modifying the idle system so it can supply sufficient fuel for a good stable idle in and out of gear. It's only running "rough", because it can't get enough fuel with the stock setting(s). Some folks immediately think or assume that we are supplying the engine with an overly "rich" mixture, which is NOT what we are doing at all, it simply needs the fuel to idle well. Same thing with advancing the timing, we are having to advance the timing because there isn't enough idle fuel avaiable for whatever vacuum reading the engine is producing. Basically, it's not able to pull down enough fuel, it can't idle well, so it's now time to optimize the timing and fuel curves to help the engine out. Like we've always recomended, give the engine what it wants to make it happy, this includes both timing and fuel, and can/will vary dramatically between various applications.........Cliff

[Motor Daddy]

I'm not arguing one way or the other, but I do use MVA on my car.

As with everything in this universe, changing one thing affects the other because it's a system.

Using no vacuum advance at idle and increasing idle fuel to satisfy idle requirements at a good A/F ratio often will make cruise too rich, because the idle/transfer circuit is really what you cruise on because the throttle is barely cracked at cruise, and the mains are basically not active at the low throttle positions.

It's very easy to get the idle A/F ratio dialed in with an wideband, but as MANY people have discovered, that's when the problems begin. When they find out idle is good and cruise is rich, then what? Make the cruise more lean with smaller IFR's, lower idle screw settings, transfer jets, or larger IAB's and you just screwed your perfect idle/tip-in AFR.

After getting my wideband, I had the idle AFR tuned in 2 minutes.

I finally got the entire deal dialed in 2-3 months later, working on it almost every day for hours. It drove me insane, so you don't have to wonder why I'm such a fruit loop. That's what caused it, the damn wideband and the never ending pursuit of perfection.

[77TA406]

Well, I'm definatly seeing two different ends of the opinion spectrum.
Cliff...The carb was built by you a few years back. It works great and runs like a scalded cat on my 406. It's a bit exhaust stinky at idle, so I can't help but think it was getting enough fuel, even with the new cam. It soulds like a lot of members were doing the same thing as me...increasing initial and limiting mechanical. What method did you all use to limit mechanical...aftermarket kits...welding the slots, or find certian# stock centerplates?
Chris

[Cliff R]

Chris, we MIG up the "window" for the mechanical advance, then use a round file to open it up to give us the amount of advance we require, and provide a positive stop for the advance at the same time. Takes minutes to do the entire operation.

Two things to consider with "stinky" idle. The cam is the BIG player here, and a lean mixture will run you right out of the shop, it "stinks" ten times as bad as a rich mixture. Why, one might ask? When you fail to provide enough fuel for complete combustion, the stuff that comes out the tailpipes stinks BAD.

Cams with "tight" LSA's, and/or lots of overlap tend to have the exhaust smelling pretty bad, even with good timing and carb settings.

"I finally got the entire deal dialed in 2-3 months later, working on it almost every day for hours. It drove me insane, so you don't have to wonder why I'm such a fruit loop."

This is EXACTLY why some of us get paid to custom tune carburetors, the learning curve is NOT easy, and we are still not done learning......FWIW.....Cliff

[Region Warrior]

Originally Posted by Cliff R

We are completely OUT of center cams and weights. I've stripped down every distributor in the shop to steal them to replace the POS aftermarket stuff everyone installs, doesn't work, then sends them here.

I heard that someone in the distributor business was having these parts made, but not sure if they are in production yet? Until we can get good parts, we're not taking on any more HEI work.......Cliff

Ya, we need stock springs and wieghts now!

Questions;
Have you tried to make wieghts?
And if you(me) had no choice but Crane/MSD springs, what have you found best?
2 heavy, 1 heavy/1 med, or...?

[77TA406]

Thanks again Cliff, for sharing all your knowledge with us. The smell isn't bad enough to drive me out of the garage, but will burn the eyes a bit. Cam is a comp 222/228@.050 .465/.474 110 lsa installed at 106.
I guess I'll deal with the issue when the new 461 goes together. Considering I don't have a cam picked yet...although I'm leaning toward the Crower 60919. Thanks for everyones help.
Chris

[Cliff R]

Chris, the 60919 is a very good choice. In made peak power at 5600rpm's in our 455, and over 500ft lbs torque from 3000rpms to 5100rpm's. I am still hearing about lobe failures just about daily on the boards, you may want to consider moving up to a roller cam, if the budget will allow for it?

Haven't tried to make any weights, springs are easy, we can get them made, but the minimum order is 1000 peices, 10,000 if you really want a bargain! I was told the weights and cams were nearly done 6 months ago, but haven't heard anything since?........Cliff

[77TA406]

Cliff.... Other than cost, is there a downside to running a hydraulic roller? Does it present it's own set of problems regarding set up, tuning, carburetion, etc, or is controlling all the extra hp and torque the biggest thing to deal with.
Chris

Originally Posted by Cliff R

Chris, the 60919 is a very good choice. In made peak power at 5600rpm's in our 455, and over 500ft lbs torque from 3000rpms to 5100rpm's. I am still hearing about lobe failures just about daily on the boards, you may want to consider moving up to a roller cam, if the budget will allow for it?

Haven't tried to make any weights, springs are easy, we can get them made, but the minimum order is 1000 peices, 10,000 if you really want a bargain! I was told the weights and cams were nearly done 6 months ago, but haven't heard anything since?........Cliff

[Cliff R]

The only downside I've seen so far is having to run a bronze or polymor distributor gear, unless you have the cam made with an iron gear on it.

The benefits of the roller are many, the engine will rev up quicker, put less heat in the oil, and make a tad bit more power than a comparable flat grind.

Don't get me wrong here, there is NOTHING at all wrong with flat cams, we have used them and ran well with them for many years. It's the lobe going "flat" deal that I'm not comfortable with these days. We've seen engine builders do EVERYTHING right, and still had a cam go "flat". I've personally never had a problem, but went to roller cams about the time this cam lobe failure thing started.....Cliff