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Old 08-09-2022, 09:53 AM
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i82much i82much is offline
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Default idle speed and engine temp

I am curious how idle speed affects engine temp. my idle when fully heat soaked is pretty high, 1150 rpm or so. Hyd roller manual trans. Electric fans, so faster idle does not mean more airflow.

Sitting still at idle for 20 minutes or so on a 90 degree day recently, temps creeped up just a smidge over 200 and stayed put. 505 ia2 with round port edelbrocks and aluminum radiator, 185 thermostat. has vacuum advance, basic pump gas build.

Would my engine likely run cooler at idle if I dropped the idle speed 300 rpm or so?

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Old 08-09-2022, 10:04 AM
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Doubtfull. I have our 69Z idling around 1100 rpm (manual trans) and daily drive it that way. It doesn't care as far as engine temps go, I mainly have it idling that high to help the engine produce a bit more vacuum for the brakes since we are up at 5000 ft elevation.

The biggest affect on idle temps I've found is whether you have vacuum advance hooked up to a ported source or manifold vacuum. Manifold vacuum, with the advance working at idle, has had a much more noticeable difference on engine temps, running cooler, in that configuration.

The trick though is having that vacuum advance dialed in so it's working at around 6 inches of vacuum and all in by 10 inches or so, and only adding about 10-12 degrees. I modify them to do so. If you run a can that adds 20 degrees and doesn't start working until 12 inches of vacuum it's not going to do anything for you at idle when hooked to manifold vacuum.

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Old 08-09-2022, 11:23 AM
Steve C. Steve C. is offline
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Can we presume the fans have proper shrouding so they pull air thru the entire radiator ?

Examples:

http://www.rondavisradiators.com/ass..._COPYRIGHT.gif

https://www.becool.com/sites/default...ts/82168_0.jpg


.

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Old 08-09-2022, 11:24 AM
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I have the stock MSD Ready to Run canister hooked to manifold vacuum. It pulls 15" or so at the current idle speed and the idle speed drops noticeably if you disconnect it. The engine does not ping at light throttle so I can't see a strong reason to limit the vacuum advance.

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Old 08-09-2022, 11:28 AM
jwhit jwhit is offline
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had basic same issue when i was running electric fan with shroud
went back to stock fan and shroud and with 160 t stat idle temp never goes
over 193 and driving temps about 178

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Old 08-09-2022, 11:30 AM
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Quote:
Originally Posted by i82much View Post
I have the stock MSD Ready to Run canister hooked to manifold vacuum. It pulls 15" or so at the current idle speed and the idle speed drops noticeably if you disconnect it. The engine does not ping at light throttle so I can't see a strong reason to limit the vacuum advance.
That'll probably work just fine. I've found the MSD distributors usually run a vacuum can very similar to the old B30 can. That works just fine without limiting anything.


The problem these days is finding those cans. So I end up taking an adjustable can, start the limit at about 6 inches of vacuum, and then weld the shaft to limit the rod travel to about 12 degrees of advance at the crank. An engine not as sensitive to pump gas you can add another 2 or 3 degrees.

Then it works just like a B30 can.

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Old 08-09-2022, 11:38 AM
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Quote:
Originally Posted by Steve C. View Post
Can we presume the fans have proper shrouding so they pull air thru the entire radiator ?

Examples:

http://www.rondavisradiators.com/ass..._COPYRIGHT.gif

https://www.becool.com/sites/default...ts/82168_0.jpg


.
yes these are Flex a Lite dual fans with a reasonably well-sealed shroud.

I don't think the temps are overly concerning.

https://knowhow.napaonline.com/tempe...ps-and-tricks/

"For example, a 180 degree thermostat begins to open within three degrees of 180, so 177 to 183. By 200 degrees, the thermostat is fully opened. The operation of the thermostat, open to close, is a range of about 20 degrees, so with different temp ratings, you can fine-tune the run temp of your vehicle."

So if my gauge is just a little off and reads 205, maybe that means the temps are fully controlled and the thermostat is just fully open at that temp.

I like having some heat since I drive on cooler days frequently and it also seems to keep the oil a little cleaner to run a warmer thermostat, but it would be interesting to see if I tried a 160 whether it ran any cooler or still floated up to 205 or so.

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Old 08-09-2022, 11:43 AM
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Getting back to the cooling situation, it's starting to sound like a fan or radiator issue.

My electric fan adventures are hit and miss but most of the time.....miss. They really get dependent on how well the shrouding is made around the fan like Steve eluded to. When dad tried dual electric fans with a full shroud on his car, it just wouldn't cut the mustard. Air flow was a problem and the engine would eventually run above the turn off temps all the time, keeping the fans running non stop and the engine would just sit on 190-195 after a while, didn't matter if we were cruising or sitting still, it wouldn't budge. The fans were completely shrouded but I surmised the shrouding wasn't deep enough for one. It was only an inch off the radiator. So the areas of the radiator that the fans didn't cover were just simply blocked by the shroud, inhibiting air flow. Cutting holes and installing flaps gave mixed results, and 2 different types of fan controllers didn't help. I also think that even with dual fans, it just didn't move enough air.

Went back to the stock 69 GTO shroud and clutch fan setup and now I can't get this thing hot at all. Most of the time it runs 175 degrees on the highway in the Arizona summer heat and actually runs even cooler when putting around town at slow speeds and traffic. It will sit and idle all day and never get above that temp. Still using the same radiator as well.
On cooler days like 80 degrees outside or less, it's hilarious how cool that thing runs.

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Old 08-09-2022, 11:48 AM
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Quote:
Originally Posted by i82much View Post
yes these are Flex a Lite dual fans with a reasonably well-sealed shroud.

I don't think the temps are overly concerning.

https://knowhow.napaonline.com/tempe...ps-and-tricks/

"For example, a 180 degree thermostat begins to open within three degrees of 180, so 177 to 183. By 200 degrees, the thermostat is fully opened. The operation of the thermostat, open to close, is a range of about 20 degrees, so with different temp ratings, you can fine-tune the run temp of your vehicle."

So if my gauge is just a little off and reads 205, maybe that means the temps are fully controlled and the thermostat is just fully open at that temp.

I like having some heat since I drive on cooler days frequently and it also seems to keep the oil a little cleaner to run a warmer thermostat, but it would be interesting to see if I tried a 160 whether it ran any cooler or still floated up to 205 or so.
Cooler thermostat likely won't do anything for you. If it's running 205 with a 185 stat, it'll run 205 with a 160 stat.

Technically, if the cooling system is up to par, the engine tune is good, and the engine vitals are healthy, it should run right at the thermostat rating most all the time. If there is a deficiency somewhere in the mix then they will tend to run hotter than the thermostat rating. All the thermostat does is make sure the engine runs at least that hot and not colder than it's rated at, but nothing about the stat keeps the engine from getting hotter. That gets into either radiator efficiency, cooling fan efficiency, or engine tune, or engine being healthy and happy, or even engine components happy that play well with each other, (ie: meaning camshaft and compression are a good match for instance) Not saying this is your case, but when you have an engine that isn't very efficient with mismatched parts, it's much more of a struggle to get that combo to run cooler.

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Last edited by Formulajones; 08-09-2022 at 11:58 AM.
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Old 08-09-2022, 11:54 AM
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To help explain that further, using dad's car as an example.

I run a 160 stat in it, but as I mentioned, during our summer months when it's 100+ degrees, the car typically runs 175-ish. It pretty much just stays in that area cruising on the highway. It actually cools down a few degrees as soon as you take an exit and putt around 30-35 mph on back roads.

All that means is that the current radiator and shroud setup, and the current engine combo, that's about as cool as I can get the engine to run on hot days. That's all the system is capable of. When it's hot outside, it's just not going to run at the 160 stat rating.

Now on cooler days below 80 degrees or so, it's not uncommon to see it stick right around 160. It can sometimes see 157-158 as the thermostat just cracks open, and will fluctuate right there. That's the thermostat now doing it's job making sure the engine stays at that temperature. It can get hotter, but it won't get colder.

Now if I put a 180 stat in it, it would no longer run 175 on the hot days obviously, the thermostat would force the engine to run at least 180, and since I know the cooling system is capable of keeping it colder than that I suspect it would stay right at the thermostat rating. Slowing down on the backroads wouldn't cool the engine down any further either, as that thermostat would force it to run there.

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Last edited by Formulajones; 08-09-2022 at 12:01 PM.
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Old 08-09-2022, 12:51 PM
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I don’t really think you can compare the Ring friction at idle of a 302 cid 69 Z , and that of a 500 plus cid motor!
I mean come on your talking about a motor that has 65% more cid.

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Old 08-09-2022, 01:29 PM
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Quote:
Originally Posted by 25stevem View Post
I don’t really think you can compare the Ring friction at idle of a 302 cid 69 Z , and that of a 500 plus cid motor!
I mean come on your talking about a motor that has 65% more cid.
I don't think ring friction is going to play into it Steve LOL

But if it makes you feel better, dad's 571 sits there and lopes along at 950 rpm and it idles all the way down to 650 rpm. It doesn't care, the engine temps are the same.

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Old 08-09-2022, 09:10 PM
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Quote:
Originally Posted by i82much View Post
I am curious how idle speed affects engine temp. my idle when fully heat soaked is pretty high, 1150 rpm or so. Hyd roller manual trans.
Idle speed seems unreasonably high.

IF (big IF) this is compensation for poor idle quality at lower speed, I'd be looking into idle mixture, fuel distribution, and idle timing. Maybe valve seal.

Quote:
Originally Posted by i82much View Post
Electric fans, so faster idle does not mean more airflow.
But it does mean substantially-more water-pump flow, which offsets substantially-more heat load from the faster firing impulses.

Quote:
Originally Posted by i82much View Post
Sitting still at idle for 20 minutes or so on a 90 degree day recently, temps creeped up just a smidge over 200 and stayed put. 505 ia2 with round port edelbrocks and aluminum radiator, 185 thermostat. has vacuum advance, basic pump gas build.

Would my engine likely run cooler at idle if I dropped the idle speed 300 rpm or so?
Running that far beyond the thermostat rating at idle or cruise shows a basic deficiency of the cooling system.

However...try dropping the idle speed, see what happens.

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Old 08-13-2022, 09:59 AM
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What size water pump pulley is on the 505? I don’t have AC but found running the smaller AC pulley with a non AC crank pulley knocks the idle speed temps down on hot days. Been running that way for years and haven’t had any waterpump issues from the extra rpm.

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