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Discussion Starter · #1 ·
Hey all, I have a fresh built 391 in a 72 Newell Motor Coach with a Holley Sniper EFI and Hyperspark Ignition. Ignition timing is currently set to 10 degrees and I'm about to adjust to 5 in hopes of it helping. It only overheats when sitting idle. New fan cluch, new custom shroud, radiator was cleaned. I'm thinking Ignition timing and maybe the radiator cap is the wrong psi. Any 391 guys out there that can help? I've made 4 trips in this Newell since I restored all of it and it really only gets hot when sitting idle after a long drive. What should I be looking at as far an Ignition curve?
 

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you want more timing at idle, not less.
are you using manifold vacuum to add anything to that 10-degree base setting?

think about a "normal" motor.
you'd have 10 degree base, then add whatever the vacuum canister (using manifold vacuum) is contributing.
so at idle, you'd be sitting around 18 degrees or so.

that's what you need to emulate with the Sniper.
 

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Discussion Starter · #3 ·
you want more timing at idle, not less.
are you using manifold vacuum to add anything to that 10-degree base setting?

think about a "normal" motor.
you'd have 10 degree base, then add whatever the vacuum canister (using manifold vacuum) is contributing.
so at idle, you'd be sitting around 18 degrees or so.

that's what you need to emulate with the Sniper.
I have set it to 10 degrees as a base @ 550 rpm. But the 391 originally was set to 5 degrees.
 

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Discussion Starter · #6 ·
how much (if any) timing are you pulling in based off the engine vacuum?
I'm not sure if there is a default setting for vacuum advance, I didn't make any adjustments to that. I take it you have some experience with the Sniper setup? I'll pull the settings up later and check. I've been running in Learn Mode and have yet to do any tweaks to the basemap for fuel or ignition.
 

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i'm only vaguely familiar with the Sniper. the concepts are all the same, just the application is different. :)

let's back up a bit - i've been assuming that you're running fully computer-controlled timing, but that may not be the case.
what distributor are you running? does it have vacuum advance, and if so, what is it connected to?
 

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Discussion Starter · #9 ·
i'm only vaguely familiar with the Sniper. the concepts are all the same, just the application is different. :)

let's back up a bit - i've been assuming that you're running fully computer-controlled timing, but that may not be the case.
what distributor are you running? does it have vacuum advance, and if so, what is it connected to?
I'm running the Holley Hyperspark Distributor
 

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check this thread out - guy having a very similar issue/question as yours:
 

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Hey all, I have a fresh built 391 in a 72 Newell Motor Coach with a Holley Sniper EFI and Hyperspark Ignition. Ignition timing is currently set to 10 degrees and I'm about to adjust to 5 in hopes of it helping. It only overheats when sitting idle. New fan cluch, new custom shroud, radiator was cleaned. I'm thinking Ignition timing and maybe the radiator cap is the wrong psi. Any 391 guys out there that can help? I've made 4 trips in this Newell since I restored all of it and it really only gets hot when sitting idle after a long drive. What should I be looking at as far an Ignition curve?
Hello Bartonj3,

Since you mentioned it only overheats at idle and you have a decent base timing, assuming it's not a question of low dynamic compression from mismatched static compression ratio and camshaft specifications, the one glaring point you made is a custom fan shroud. Do you have a picture of it? What size and how many blades are the fan itself? Can you hear the fan clutch increasing speed to the fan when hot? (it should sound like a big leaf blower and very noticeable)

A rule of thumb for shroud to fan spacing is you want half the edge of the end portion of the blades sticking out of the shroud. A fan too far inside the shroud or too far outside the shroud just creates turbulent air flow and destroys any laminar air flow through the radiator, which of course laminar air flow through the radiator and condenser (if air con equipped) is what you want.

Cheers
 

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Discussion Starter · #12 ·
Hello Bartonj3,

Since you mentioned it only overheats at idle and you have a decent base timing, assuming it's not a question of low dynamic compression from mismatched static compression ratio and camshaft specifications, the one glaring point you made is a custom fan shroud. Do you have a picture of it? What size and how many blades are the fan itself? Can you hear the fan clutch increasing speed to the fan when hot? (it should sound like a big leaf blower and very noticeable)

A rule of thumb for shroud to fan spacing is you want half the edge of the end portion of the blades sticking out of the shroud. A fan too far inside the shroud or too far outside the shroud just creates turbulent air flow and destroys any laminar air flow through the radiator, which of course laminar air flow through the radiator and condenser (if air con equipped) is what you want.

Cheers
I have about 1 inch from being centered in the radiator shroud. I replaced the fan clutch and kept the old fan. I haven't heard the fan clutch kick in loud like a leaf blower as of yet necessarily.
169712


169713
 

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I have about 1 inch from being centered in the radiator shroud. I replaced the fan clutch and kept the old fan. I haven't heard the fan clutch kick in loud like a leaf blower as of yet necessarily.
View attachment 169712

View attachment 169713
Hello Bartonj3,

It's hard to tell for sure, but the fan looks like it's a bit too far out of the fan shroud. The clutch should really engage if the coolant temperature is high and heating the air on the bimetal spring in front of the fan clutch. If the fan is too far out of the shroud, it sucks air through the easier path of the small space between the fan and the shroud rather than pulling it through the radiator core.

Again assuming it's not an engine mismatch in parts, timing or mixture, you may want to shim the clutch and fan closer to the radiator to get half the blade width inside the shroud. Either that or modify the shroud to extend half way over the fan blade end.

It could also be a defective fan clutch. The amount of poorly made brand new parts is staggering now-a-days. What is the temperature of the engine when it overheats at idle? Where are you measuring this?

Cheers
 

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Discussion Starter · #14 ·
Hello Bartonj3,

It's hard to tell for sure, but the fan looks like it's a bit too far out of the fan shroud. The clutch should really engage if the coolant temperature is high and heating the air on the bimetal spring in front of the fan clutch. If the fan is too far out of the shroud, it sucks air through the easier path of the small space between the fan and the shroud rather than pulling it through the radiator core.

Again assuming it's not an engine mismatch in parts, timing or mixture, you may want to shim the clutch and fan closer to the radiator to get half the blade width inside the shroud. Either that or modify the shroud to extend half way over the fan blade end.

It could also be a defective fan clutch. The amount of poorly made brand new parts is staggering now-a-days. What is the temperature of the engine when it overheats at idle? Where are you measuring this?

Cheers
I replaced what was on there when I got it, it was this one Hayden Automotive 2710 Hayden Fan Clutches | Summit Racing
This Coach was on it's way to a recycling center so there wasn't much left of it and I'm sure parts have been replaced and mismatched over it's lifetime.

Could I add a metal band off the shroud to extend it? I think I've seen that before.

I wonder if there is another fan clutch that extends further and maybe a heavy duty type. I'm gonna have to start pulling spec sheets again.

When I come to a stop and the temp is 210 or so, it will start to increase to 230 then I shut her down. But I don't ever hear the fan clutch kick in loudly or notice a bump in the engine rpm as if the clutch engages.

I also haven't had much luck finding parts diagrams of the thermostat housing and what's the best thermostat to use on this old setup. Some say it has 2 thermostats but the engine shop only put 1 in there and we couldn't figure out where a 2nd one would possibly go...
 

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can you put a timing light on it?
i'm very interested to see if you only have the 10 degree base timing on it while idling.
 

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I replaced what was on there when I got it, it was this one Hayden Automotive 2710 Hayden Fan Clutches | Summit Racing
This Coach was on it's way to a recycling center so there wasn't much left of it and I'm sure parts have been replaced and mismatched over it's lifetime.

Could I add a metal band off the shroud to extend it? I think I've seen that before.

I wonder if there is another fan clutch that extends further and maybe a heavy duty type. I'm gonna have to start pulling spec sheets again.

When I come to a stop and the temp is 210 or so, it will start to increase to 230 then I shut her down. But I don't ever hear the fan clutch kick in loudly or notice a bump in the engine rpm as if the clutch engages.

I also haven't had much luck finding parts diagrams of the thermostat housing and what's the best thermostat to use on this old setup. Some say it has 2 thermostats but the engine shop only put 1 in there and we couldn't figure out where a 2nd one would possibly go...
Hello Bartonj3,

On the fan clutch, it's a viscous type and has a smooth engagement and disengagement. It looks like a typical large 7 blade fan and you should definitely hear it when the fan clutch engages. In looking over the pictures again, it isn't that far out of ideal position. I'm wondering if this isn't a case of several little things adding up to the overheating.

What is the exact condition of the radiator? It looks like it's possibly an original piece. Could there be mineral build up on the interior of the cores? You mentioned it could have had 2 thermostats, is that verified? If it's just the one thermostat, is it the high flow model or a standard replacement? Are you using a standard coolant pump or the higher flow variety?

Another question is are the sides, top and bottom of the radiator reasonably baffled off to prevent hot fan air from recirculating through the front of the radiator when ram air from vehicle movement is stopped.

I know it's a lot of questions but since it sounds like possibly original pieces are missing or changed you have to cover all your bases sort of thing. Again this is assuming the engine was put together correctly and your air fuel ratio isn't too lean nor timing too retarded.

Cheers
 

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I agree it may be radiator/fan related related the fan tips if I remember correctly should be about a 1/2 from the shroud, I would rule out the fan clutch and just put a new one on, 7 blades are great at idle, also could have sediment building in the radiator, flush it first and put at least a 18lb cap, if you replace the radiator get a dual pass, I run one from afco it took 25 degrees out of mine at idle, I say that because keep in mind when you bore it you essentially are making the motor bigger, and that makes more heat or btu, and different cam specs and timing all play with the btu the motor is making, take a heat gun and check you coolant temp coming out of the motor, and out of the radiator, if you don't see much difference I would suspect the cooling system efficiency, seeing as it stays cooler when driving because there is more airflow entering the front of the radiator, and at idle you are dependent on the clutch and fan.
 

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All really good suggestions. 👍 But I want to re-zero on your symptoms for some additional suggestions. From your description, the issue occurs after cruising (heat accumulated), and at low speed (idle). This indicates either excessive heat accumulated or the failure to shed the excess heat at low speed — or both. That should remain the focus for a moment. Most of the suggestions have.

As you are diagnosing an issue(s) I will suggest a diagnostic approach of setting a baseline, allowing you to directly address the remaining symptoms. In order to clear the table to see issues, you need a good baseline. Do you have excessive heat to remove, or poor removal, or something else? How can we tell? Verify everything is functioning properly for the application, bring everything to optimum tune, and look again.

Ford originally installed thermostats based on region. Except in very cold climates, "Heavy Duty" applications (trucks, police, taxi, etc) had fewer emissions restrictions and received 162°F thermostats, until emission regulations forced higher temps. This isn't your Mom's car. Operating temperature was targeted at 170-177° and radiator pressure was 13 psi, which determined maximum temperature rise before relief. As a base issue, you can imagine it would be much easier to cool-down from that lower temperature in everything from the block to the transmission.

Electronic timing control is much more capable than mechanical/vacuum, as it may be manipulated outside of mechanical constraints in order to provide optimal timing for any conditions. As an example of it, the original TFI electronic systems in Mustangs had 22°BTC timing at idle (10 base + 12 idle advance). Modern trucks are similar. Of course, the timing would drop instantly to handle the added load of acceleration, to the base 10°. Newer systems can drop even further as-required, and even change at the same speed to meet current conditions.

This transitions us to your high cruise heat that needs to be shed at idle afterwards. With electronic timing, you can run cool when lean, but that means it needs more timing or it will be effectively retarded (heat). So, we need to add timing to correct the slow burn or it will accumulate heat in your drivetrain.* Reduced cruise heat coupled with lower accumulated heat (thermostat) will force less stress on the idle cooling.

TL;DR: Without going further at this point, I hope you can see that tuning is required in order set the baseline operation, allowing you to clearly identify any remaining system weaknesses. While I could suggest that first in order to see the remaining issues and address them directly; your fuel and ignition tuning depends directly on operating conditions, which is why I brought the thermostat up first. If your idle cooling (fan or other stuff) is still weak, you'll easily see that after the baseline is set.

* If you have not done economy tuning before, it has specific concepts that should be understood in order to make the gains you're after. Do some good reading on economy tuning. You'll find that simply leaning the burn in your tune isn't how you save fuel. ;) Good luck and have fun with it as you make progress!
 
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