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Old 09-02-2010, 11:43 PM
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502 HO idle RPM

Does anyone know the factory recommended idle RPM for a 502 HO motor is?

I was told that the initial timing should be 14 degrees. I am trying to find out at what RPM should I get 14 degrees at.

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Old 09-03-2010, 05:24 AM
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I have a 454HO in the 59
Disconnect the vacuum advance and set to 16 as low an RPM as possible

I run 16 initial and at 2500 I am about 36-38-----with the vacuum advance hooked up I am close to 50* at 2500.

I have a manual trans so my idle is set at 750, altho many set their
BBC idles at 900-1000 with automatics

What kind of vacuum are you getting at idle-----me--only about 10
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Old 09-03-2010, 08:19 AM
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Quote:
Originally Posted by rajsid
Does anyone know the factory recommended idle RPM for a 502 HO motor is?

I was told that the initial timing should be 14 degrees. I am trying to find out at what RPM should I get 14 degrees at.
If you are using a mechanical advance distributor, just be sure that the rpm is low enough that the mech. adv. is not adding any advance. Disconnect and plug the vacuum advance like Brian said and you will be fine.

If for some reason you find that there's advance coming in even at a low idle, you will need to add a heavier advance spring to prevent this.

The Ramjet 502 is:

Ignition timing: 8 BTDC @ 800 rpm; 36 total @ 5000 rpm.

You can compare your cam, etc. to it to see if the 800 rpm should also apply- but I would say it should be a good rpm to use.

FWIW, the 502 HO uses (what is shown in this particular spec chart):

36 total @ 3600 rpm, with vacuum advance disconnected.

The HT 502 uses:

Ignition timing: 4 BTDC
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Old 09-03-2010, 08:53 AM
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Quote:
Originally Posted by cobalt327
If for some reason you find that there's advance coming in even at a low idle, you will need to add a heavier advance spring to prevent this.
You are right, I think the key is to keep the rpm low enough that there is no mechanical advance.

Quote:
Originally Posted by cobalt327
36 total @ 3600 rpm, with vacuum advance disconnected.
The engine is timed at the 502 HO spec. I am getting 3600 @ 36 with the vacuum disconnected. The dist that I am running gives 22 degrees of mechanical. So 36-22=14 is the initial timing that I have set to. Nowhere can I find any reference of the initial RPM.
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Old 09-03-2010, 09:46 AM
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Quote:
Originally Posted by rajsid
You are right, I think the key is to keep the rpm low enough that there is no mechanical advance.



The engine is timed at the 502 HO spec. I am getting 3600 @ 36 with the vacuum disconnected. The dist that I am running gives 22 degrees of mechanical. So 36-22=14 is the initial timing that I have set to. Nowhere can I find any reference of the initial RPM.
14 BTDC sounds pretty good. You might even go for a little more, but you'd need to limit the mechanical to keep the total where it needs to be.

I didn't find an initial timing rpm spec, either. That's why I thought the 800 rpm spec that the Ramjet uses might suffice.

But as long as there's no mechanical advance sneaking in on you and the vacuum is disconnected and plugged, you're good to go.

Good luck.
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Old 09-03-2010, 10:30 AM
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Quote:
Originally Posted by cobalt327
14 BTDC sounds pretty good. You might even go for a little more, but you'd need to limit the mechanical to keep the total where it needs to be.

I didn't find an initial timing rpm spec, either. That's why I thought the 800 rpm spec that the Ramjet uses might suffice.

But as long as there's no mechanical advance sneaking in on you and the vacuum is disconnected and plugged, you're good to go.

Good luck.
Cobalt and Bryan,
Thank you so much.





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Old 09-03-2010, 11:14 AM
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Nice ride! My brother in law had a '68 4-speed w/o T-tops about that same color, w/a 400 HP 427. I had basically the same engine in a '69 Impala SS 427 w/TH400 rated @ 390 HP.

I bet that 502 woke it up- what trans do you have?
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Old 09-03-2010, 03:23 PM
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Quote:
Originally Posted by cobalt327
I bet that 502 woke it up- what trans do you have?
M21 4-Speed Manual (close ratio)
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