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Apollo77

Z20 build

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Some later modules have 4 connections. One for power, two for the negative side of the coil and a 4th that when grounded by a vacuum switch disables the exhaust side plugs to reduce engine noise under heavy load. Now normally a dual plug system needs less advance so suddenly switching to single plug would also reduce power, it would run retarded, but the module somehow increases the timing on the remaining intake plugs to compensate.

 

Seems like you could wire this is in regular advance, set your timing and have a vacuum switch that turns on the retard mode during hard acceleration or boost applications to prevent pinging. The difference is built in I would immagine and may be 5-8 retard degrees?

 

The Mileage Option Z20 has a special knock sensor in the block and a detector circuit that retards the timing. This one may vary the retard, I don't know.

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OK, so I have to shorten them slots a little on the outside as doing it on the inside could throw other things out of sync like the idle timing.

If I were to guess I would say around a third of the slots need to be welded up, maybe a quarter, and then the metal faces need to be cleaned up, nice and smooth as possible, then use some kind of degree device to see where I am, I want 12 to 15 degrees welded up.

This is likely something I could do to the work truck also, it knocks if I have it advanced any more than 2 degrees when I time it, if I were to weld up maybe 8?/10 degrees I could likely go back to the stock timing specs.

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Why not just turn the timing retard a couple of degrees?

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Why not just turn the timing retard a couple of degrees?

 

Were talking about 10.9 to 1 compression ratio on the LZ20 according to you, most on here that have raced or do race say I am going to have to do something as it is going to knock/ping.

The work truck just barely knocks sometimes/intermittently, that is what I can hear, I expect it knocks/pings and I don't hear it and I am at 2 degrees before TDC now, when the cam was timed wrong I didn't have these issues except when it was very hot outside, then it would barely knock when going up slight grades, if I floored it or let up it would quit knocking/pinging, now when I floor it it just keeps on knocking/pinging, I suppose I could dial it back to 1 degree BTDC or TDC, or maybe I can take some of the mechanical advance out of it and be able to set the timing back to where I am used to it being, 6/8 degrees BTDC, as it doesn't seem to be working all that well the way it is, as I drove it with the cam timed wrong for several years without issues.

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Well I pulled apart a Z24 distributor this evening and it didn't come apart easily, likely because it has been out in the scrap pile for a while.

Lot of stuff in there but I finally made it to the Plate/weights/springs, even with the springs loose that plate don't like to turn, but I would imagine when it is spinning it worked fine as the weights were easy to move with my finger.

That is not a lot room to work with in them oval holes, they are 3/8ths of an inch long and the pin is an 1/8th by itself, so that is a 1/4 inch movement of which I have to fill just slightly over a 1/16th inch of it.

Also when the weights move out do they change the timing evenly or is there a curve?

Looking at it, to me it looks like the farther it moves the less/slower the timing changes.

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Some have dual springs wayno. A shorter one that adds to the main spring as it extends so the advance isn't linear. Also the weight and shape of the weights affects the advance curve. As the weights move they rotate and swing a lesser? mass outward.

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High compression like that I'd set it up for 16 BTDC at idle, 30-32 total mechanical by 3200 or so rpm.  Its difficult to get that right without an accurate Sun machine that has a recently calibrated tachometer.  

Likely you'll need to weld about .040" in the shorter slot.  Maybe a hair more. 

Our race engine runs 24 at idle, 28 total.  The practice motor at 11:1 is set up as I described above.  Scalded cat!  

 

Quick calculation:

the advance weight pin travels in the slot .0143" per degree of distributor advance.  The distributor will advance half of what you see at the crank due to the doubling effect of the timing gear set.  So for 8 degrees in the distributor, = 16 mechanical at the crank multiply that figure by 8 and weld up the difference from your slot with the weight in still in it.    

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High compression needs a shorter burn time so less advance. Or do you mean 8 crank/16 distributor degrees?

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High compression like that I'd set it up for 16 BTDC at idle, 30-32 total mechanical by 3200 or so rpm.  Its difficult to get that right without an accurate Sun machine that has a recently calibrated tachometer.  

Likely you'll need to weld about .040" in the shorter slot.  Maybe a hair more. 

Our race engine runs 24 at idle, 28 total.  The practice motor at 11:1 is set up as I described above.  Scalded cat!  

 

Quick calculation:

the advance weight pin travels in the slot .0143" per degree of distributor advance.  The distributor will advance half of what you see at the crank due to the doubling effect of the timing gear set.  So for 8 degrees in the distributor, = 16 mechanical at the crank multiply that figure by 8 and weld up the difference from your slot with the weight in still in it.    

So what you are saying is I basically need to fill in around 1mm to the short hole, and I do nothing to the long hole?

To me it would seem like I need to do both sides, but that is what I see in my head, I have never done anything like this before.

Is there a reason for only doing one side, does it lock it in better?

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I wanted to chime in earlier on but been busy with the kids at home.

 

In my '68 510 I had built a 81 Z20s that was a long rod motor and I used the NAPZ 3 terminal retimed dizzy directly to the I/E coils with dual SUs and was right at 11:1 comp (race gas).

 

It was fun and was about a radical as could be for a NA street/strip Z motor, only 130hp.

 

The timing thing is a game of power versus ping pong boom with the dizzy.

 

I feel with old school performance motors, they should take advantage of the use of Knock Sensors and Megasquirt EDIS atleast for spark control by using SR20 coil on plugs.

 

Its cheap and full spark control, and can use a regular electronic dizzy to a pulse modifier so you dont need a 36 -1 crank trigger.

 

68_510_Z20.jpg

 

68_510-_Z20_2.jpg

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Wayne, the other side may add 1 degree of timing above 4000 rpm, but that's a good thing.  It makes up for timing lost by the the module so its a wash.  

The whole assembly is triangulated, so you really only need one slot. None of this really matters if you don't get the springs correct.  

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Well I just dismantled the only L block Matchbox I had left, it appeared to be broken as it has this plastic 3 armed piece with bearings on the end of each arm, one end was broken that held the small bearing, but that was not the real issue, I could not get the the piece of metal that slides on the shaft that has the slots in it off the shaft, I wasn't able to move it at all.

I don't have any kind of puller that will fit in that small of a place, I tried clamping on with a pair of vice grips and then using the edges of the distributor for leverage with 2 screwdrivers on each side of the vice grips, didn't work, I held the distributor upside down, put the end of the vice grips on my leg and then tapped fairly hard on the vice grips right next to the distributor without results, all I am really doing is scarring the outside of the shaft with the vice grips, I gave up and put it back together.

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Feel free to send it to me Wayne.  I feel like I owe you one for the locker you sold me last year.  You really saved our butts!

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Feel free to send it to me Wayne.  I feel like I owe you one for the locker you sold me last year.  You really saved our butts!

 

I will figure it out eventually.

I will make or buy a small puller to get that part off the shaft of that Matchbox, I have a steering wheel puller, all I will have to make is the lower piece.

The Matchbox has way more parts in there than I thought they would have, I just want the part modified before I pull the one apart on the engine that is in the truck.

 

Did you even use that locker, it didn't seem like it had the gearing you needed.

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I used the Detroit Locker with 3.89 gears.  Truck truck acted like we were on solid pavement instead of slimy salt.  

Next year we are using a 10-bolt Chevy with 2.72 gears and a spool.  All or nothing.  The locker will go into our spares pile to bring along, or into the parts truck we're assembling to look like a Matchbox version of the 620 I was given as a gift a few weeks ago.  

 

DO NOT use a puller on the distributor.  You will damage it.  Period.  I have a system to get them apart without damage.  Its simple and effective.  I'd like to offer this to you.  I'd like more than one other person on this forum to know how big of a difference a properly recurved distributor can make.  Its the most overlooked tuning parameter, and nearly the most important.  

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On my Z20, this is how I wired up the coils. Also used my 86 Z24 carbie truck as a future reference.

 

720_Coil_Config.png

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question on the z20 pistons, if i wanted to take them up a size, do i need to worry about any hot spots from enlarging the pistons? i was talking to a friend about it and he brought up this very good point considering the l20b always pops between 2 and 3 from overheating. 

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Z20 is a different block, just how much are you going to over bore?

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Increasing from 85mm to 86mm bore adds just 0.7cu inch per cylinder! In total the Z20 is still under 2 liters in displacement. You and the engine will never know or feel the difference this 2.82 cu in makes.

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If you do overbore, I'd be interested in the stock 85mm pistons.

 

 

These are flat top pistons, right?

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You'll need the 6" rods that go with them.

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L18 crank with L16 rods in an L18 block. I'm covered....just missing the pistons.

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