1960 469C tune up

[Douglas C.]

Going to help a friend tune up his 270 H P. Can someone please answer a few questions below.

Set points each @ 29 degrees dwell, overall should wind up around 34 degrees dwell?

What is best all in timing advance target?

Coil primary & secondary ohms target numbers, 1.5 primary, 8k to 10k secondary?

Proper ohms test of condenser?

A few drops of oil down distributor shaft to lube centrafugal weights?

Any other suggestions appreciated as I want to make sure all is in proper order, and is their an existing NCRS document I can access. I have a ST-12, etc. someplace which I just can't locate right now.

Tks. Doug

[Duke W.]

Re: 1960 469C tune up

Download the applicable year vehicle information package from www.gmheritage.com.

"What is best all in timing advance target?"

It's not clear to me what you are asking. The centrifugal advance specs should be in ST-12, and it's also in the GM Heritage pdf. It's not adjustable unless you remove and disassemble the distributor since the fly weights and springs are under the breaker plate assuming the engine still has the OE dual point non-vacuum advance distributor.

The above will probably also recommend an initial timing value, which you can adjust, and it's probably on the conservative side.

GM Power Manuals from the seventies say optimum total WOT advance is 36-40 and you want to get it is a early as possible, the limiting factor being detonation.

283s might be at the lower end of this range due to the smaller bore and combustion chamber.

Duke

[Douglas C.]

Re: 1960 469C tune up

I did find the AMA. See pic. Says 12 crankshaft degrees initial and 28 centrafugal @ 3,700 rpm. So if reading that right that would be all in timing of 40 degrees @ 3,700. Does sound somewhat high, although I see you indicating that 40 was not unheard of.


Tks. Doug

[Duke W.]

Re: 1960 469C tune up

It's not that aggressive. The 365/375 HP 327s OE centrifugal is 24 @ 2350 with an initial recommendation of 12-16.

Advertised CR of the 283/270 is only 9.5 and the average out of Flint was probably closer to 9.25:1

If it doesn't detonate it's okay and may even run detonation-free on 89 or even 87 PON.

Duke

[Douglas C.]

Re: 1960 469C tune up

It's not that aggressive. The 365/375 HP 327s OE centrifugal is 24 @ 2350 with an initial recommendation of 12-16.

Advertised CR of the 283/270 is only 9.5 and the average out of Flint was probably closer to 9.25:1

If it doesn't detonate it's okay and may even run detonation-free on 89 or even 87 PON.

Duke

I agree, the AMA numbers sound way out of line for a low compression 283. Owner has had the car since 1980 and always ran very well. Don't imagine anything on that original distributor has been altered. After tune up and verifying correct dwell I'll check initial and all in timing with my dialback light. Just curious what it will show.

Tks. Doug

[Duke W.]

Re: 1960 469C tune up

One of the things you should test is the start point of the centrifugal curve. Starting at, say 1000, lower idle speed until the timing stops retarding, but this may be tough to do because the Duntov cam will destabilize the idle by about 700 and the engine may stall. The AMA specs say centrifugal starts as low as 600.

The other way is to use you dial back light and check maximum initial plus centrifugal advance at about 4000, but before you do that use the light to find out the engine speed that max centrifugal is all in.
If it appears to be all in by the OE spec of 3700, then set advance at 39-40 at about 4000, and if it doesn't detonate on a test drive, you're good to go. If it does detonate reduce in 2 degree increments until the detonation goes away.

On an engine with a vacuum advance distributor, the vacuum hose to the VAC must be disconnected and plugged. This simulates WOT even with no load on the engine because the vacuum advance is disabled.

On a dual point distributor without a VAC you don't have to deal with it.

Duke

[Douglas C.]

Re: 1960 469C tune up

One of the things you should test is the start point of the centrifugal curve. Starting at, say 1000, lower idle speed until the timing stops retarding, but this may be tough to do because the Duntov cam will destabilize the idle by about 700 and the engine may stall. The AMA specs say centrifugal starts as low as 600.

The other way is to use you dial back light and check maximum initial plus centrifugal advance at about 4000, but before you do that use the light to find out the engine speed that max centrifugal is all in.
If it appears to be all in by the OE spec of 3700, then set advance at 39-40 at about 4000, and if it doesn't detonate on a test drive, you're good to go. If it does detonate reduce in 2 degree increments until the detonation goes away.

On an engine with a vacuum advance distributor, the vacuum hose to the VAC must be disconnected and plugged. This simulates WOT even with no load on the engine because the vacuum advance is disabled.

On a dual point distributor without a VAC you don't have to deal with it.

Duke

Yes, will do. I'll back rpm down low as possible and using my dialback see at what rpm the timing begins to move. Slightly below that point I'll see if it's really at 12 initial. Then see what it reads at 3,700. See if it stops advancing there. Overall I guess with probably closer to 9.25:1as you said, you can probably get away with a lot of timing. Will be interesting to see how this will all wind up

Tks. Doug

[Duke W.]

Re: 1960 469C tune up

The lower the CR the more advance an engine can tolerate on a given octane fuel, all other things equal.

It's explained in more detail in my tuning seminar.

Web search: "tuning vintage corvette engines for maximum performance and fuel economy"

Duke

[Douglas C.]

Re: 1960 469C tune up

One of the things you should test is the start point of the centrifugal curve. Starting at, say 1000, lower idle speed until the timing stops retarding, but this may be tough to do because the Duntov cam will destabilize the idle by about 700 and the engine may stall. The AMA specs say centrifugal starts as low as 600.

The other way is to use you dial back light and check maximum initial plus centrifugal advance at about 4000, but before you do that use the light to find out the engine speed that max centrifugal is all in.
If it appears to be all in by the OE spec of 3700, then set advance at 39-40 at about 4000, and if it doesn't detonate on a test drive, you're good to go. If it does detonate reduce in 2 degree increments until the detonation goes away.

On an engine with a vacuum advance distributor, the vacuum hose to the VAC must be disconnected and plugged. This simulates WOT even with no load on the engine because the vacuum advance is disabled.

On a dual point distributor without a VAC you don't have to deal with it.

Duke

Ok, put in the dual points and dwell at 33 degrees. Exactly what is listed in the AMA.

Prior to this the issue was backfiring thru exhaust at high rpm. Checked timing and looks like the timing tab is not correct. So could not dial in exact timing, but adding and reducing timing by around 8 degrees did not change the backfiring. Timing light shows no bouncing around at high rpm so looks like balancer is good. I'll pick another day and use my piston stop to find close as I can TDC, then check timing. Otherwise car starts up on one click of the key and runs smooth.

Your thoughts? I'm thinking the backfiring thru exhaust at high rpm is fuel related, float level?

So next will be fine tuning timing, checking ignition wires, and fresh plugs, see if any backfire change.

Doug

[Dan D.]

Re: 1960 469C tune up

Doug, what rotor do you have? Back in the late 60s GM shortened the tip of the rotor, resulting in increased distance from the rotor tip to the distributor cap contact. This was done to create a hotter spark to help burn off emissions.

But this meant the coil voltage had to be higher in order to fire the plugs. At high RPM there is just not enough time for the coil to charge enough to bridge the gap. The total gap is the rotor gap plus the plug gap.

This is a known problem and there are rotors advertised with having the longer rotor tip.

You may be experiencing marginal firing and loading the exhaust up with un-burned air/fuel charge.

Something to look at. I don't know what RPM you are experiencing this, but it usually shows up around 6500 RPMs.

Dannnnnn

[Douglas C.]

Re: 1960 469C tune up

Doug, what rotor do you have? Back in the late 60s GM shortened the tip of the rotor, resulting in increased distance from the rotor tip to the distributor cap contact. This was done to create a hotter spark to help burn off emissions.

But this meant the coil voltage had to be higher in order to fire the plugs. At high RPM there is just not enough time for the coil to charge enough to bridge the gap. The total gap is the rotor gap plus the plug gap.

This is a known problem and there are rotors advertised with having the longer rotor tip.

You may be experiencing marginal firing and loading the exhaust up with un-burned air/fuel charge.

Something to look at. I don't know what RPM you are experiencing this, but it usually shows up around 6500 RPMs.

Dannnnnn

Very interesting. The backfiring happens around 4,500, it's a 270 H P dual point distributor. How exactly would I check the rotor part number or measurement of gap as you mentioned

Doug

[Duke W.]

Re: 1960 469C tune up

You need to verify that the plug wires are indexed properly on the cap and the distributor is installed correctly.

Duke

[Douglas C.]

Re: 1960 469C tune up

You need to verify that the plug wires are indexed properly on the cap and the distributor is installed correctly.

Duke

Yes, will do that after piston stop tool and verifying TDC on compression stroke.

Doug

[Dan D.]

Re: 1960 469C tune up

Very interesting. The backfiring happens around 4,500, it's a 270 H P dual point distributor. How exactly would I check the rotor part number or measurement of gap as you mentioned

Doug

Doug, the Corvette Central P/N is 301032 in stock at $14.

For $14 I would buy it anyhow. You can then measure the tip length and compare it to what you have now. I did not check any other suppliers.Screenshot 2022-03-28 at 08-34-14 58-74 Distributor Rotor - 58-74 All Except 58-61 2 X 4.jpg

Dannnnnnn

[Douglas C.]

Re: 1960 469C tune up

Doug, the Corvette Central P/N is 301032 in stock at $14.

For $14 I would buy it anyhow. You can then measure the tip length and compare it to what you have now. I did not check any other suppliers.[ATTACH=CONFIG]110996[/ATTACH]

Dannnnnnn

Ok, interesting, I'll check the measurement. Tks. Doug

Tks. Doug