Lower Shroud Doors - Time to Open?

[Wittsend]

When I got my Lakewood on the road it was late Spring and I'm in So. Cal.. I wanted to check for leaks, felt a tight engine might run hotter etc.. So I left the lower shrouds off. Tomorrow is going to be 39 degrees in the morning and I felt it a good time to put the shrouds on. Prior to installing them I tested with a heat gun and both doors opened though I had no way of determining at what temperature. There was no binding of the door movement.

No good deed goes unpunished and it seems my effort to straighten the shrouds actually didn't make for the best alignment. So, re-bending to fit, and chasing down bolts, cleaning/painting them etc., etc. had the job somehow stretch to 4 hours. When I was done I started the car (about 60 degrees at the time) and it idled about 20 minutes and I still didn't see the lower doors open. About the 25 minute mark the drivers side door started to open but going to 30 minutes still had a no open condition on the passenger side though I did feel slightly less resistance on that side door. The temp sensor never came on and I tested it for function prior.

One thing that might be a factor is the car has no heater. On each side where the heater hose connects I have screens to keep rodents out. The upper fan shroud does have a block off plate near the #5 cylinder so it wouldn't be an unequal air flow issue.

My questions:

Is having the "no heater" hose condition any different than having the heater turned on full?

How long should I expect on a 60 degree day (and the above stated lack of heater ducting) should I expect before the doors open while idling (I did blip the throttle a few times)?

Is it common for one door to open before the other, and if so how much longer is too long?

Can the doors be "tuned" (adjusted) to open in proximity to each other, or am I simple at the mercy of the response time of the bellows?

Thank you.

[Brizo]

There is no "time to open". The time will always differ depending on how hard the engine is working, ambient temps, etc. But all your heat is blowing out those screens making it take longer for the thermo doors to open....if ever. Block off the heater out lets with solid plates . Then they will open if it gets hot enough.... but probably not together.

[Brizo]

There is no "time to open". The time will always differ depending on how hard the engine is working, ambient temps, etc. But all your heat is blowing out those screens making it take longer for the thermo doors to open....if ever. Block off the heater out lets with solid plates . Then they will open if it gets hot enough.... but probably not together.

[66vairguy]

Wittsend - On my LM cars at idle it takes about 15-20 minutes for the driver side to open and after another 5 minutes for the passenger side to open. The passenger side always seems lag and I'm told this is normal. HOWEVER I have my heater hooked up and when it is off no air flows escapes. You have screens on your heater outlets and basically that is like the thermostat doors are always open! At idle with that configuration it would take some time for the thermostats to heat up - in my estimation.

I have thermocouples on each head with a digital readout and a circuit to drive two Covair temp gauges (I like to tinker with Arduino and C++).

Before starting I've turned on the power and the head readouts are within one degree of each other and typically 70-80F at my place (car has sat for a day and is cool).

Once the engine starts things warm up fast at first but over 150F it takes awhile to reach 250F on drivers side, 235F on passenger side AT IDLE. Sometimes the passenger door won't open fully. THAT is just letting the engine idle OFF choke. The heater OFF. On cold morning I move heat lever on, fan off, and before the thermostat doors open air blasts out of the heater/defroster at anything over 1,500 RPM until the thermostat doors open (this will NOT happen if the cold air hose is disconnected from the engine shroud as the hot air goes OUT the heater box cold inlet unless the heater lever is full down). With no cold air hose you must use the heater fan to pull engine heat into the passenger compartment.

Once I drive around town things get to about 280-300F depending on how I drive in about 5-10 minutes and stays there. At about 280F the doors are fully open!!

I hope this helps.

[Wittsend]

Thanks Brizo and 66vairguy. That confirms my hunch that the heater hose connection areas were bleeding a large amount of air. I just wasn't sure that it wouldn't matter if the heater was full on so thanks for clearing that up. Also good to know the "time to open" was not unusual nor the delay on one side. Will likely do the block off plates on the heater connection..., as well the down draft tube hole since I am not using the tube. Off to Cars & Coffee tomorrow. Will see how it goes.

[jimbrandberg]

Although it's probably better for the cooling air to exit out the rear to be free of the chassis as GM intended, it's not considered bad form to remove the lower shrouds and then the hot air is removed when the vehicle is underway.
When you blow the hot air out of the heater ducts with no hoses it doesn't really do any harm but the hot air does linger instead of going down and away. Down and away.
In broad sweeping terms, once the air gets through the head fins and oil cooler it doesn't matter where it goes.
On engines with minimal shrouding I'll cut the duct off the front corner shrouds and extend the right rear corner shroud to get below the head fins.

I just thought of something unrelated. They say a road draft tube has a scavenging effect when the car is underway. I wonder if that still holds true if it's not in close proximity to the lower shroud. A lone slanted tube in the breeze.

[66vairguy]

Although it's probably better for the cooling air to exit out the rear to be free of the chassis as GM intended, it's not considered bad form to remove the lower shrouds and then the hot air is removed when the vehicle is underway.
When you blow the hot air out of the heater ducts with no hoses it doesn't really do any harm but the hot air does linger instead of going down and away. Down and away.
In broad sweeping terms, once the air gets through the head fins and oil cooler it doesn't matter where it goes.
On engines with minimal shrouding I'll cut the duct off the front corner shrouds and extend the right rear corner shroud to get below the head fins.

I just thought of something unrelated. They say a road draft tube has a scavenging effect when the car is underway. I wonder if that still holds true if it's not in close proximity to the lower shroud. A lone slanted tube in the breeze.

As air moves over a round object the the side opposite the air flow direction will have a SLIGHT lower pressure. That is why that side is often cut at an angle to expose it to more low pressure, BUT the effect is slight for the speed a vehicle goes.

When I worked for a living we would design electronic hardware and in addition to electrical routing on a board, we had the thermal dynamics folks come in to check air flow over components and how things dissipated heat. What would seem simple was very complex.

[66vairguy]

About removing those lower shrouds ------ Years ago I went to one of my first Corvair shows. It was late summer and at a park were all the grass was dead. All the Corvairs were lined up and folks were polishing them and a fellow pulled in late with his Corvair that had NO lower shrouds and as it drove down the line of shiny cars it kicked up a huge cloud of dust and dead grass that landed on everyone's Corvairs. When he pulled into his spot and stopped ---- owners walked up to him and there was a lot of ugly language!!!!

[Wittsend]

I'm bringing that post back to relate my experience. Initially I tested my shroud doors to see if they would open with a heat gun. They both did open. I did not time the opening but I would say they opened "relative" to each other time wise.

As our "harsh" So. Cal. winter (48-52 average in the morning) was approaching I opted to install the shrouds. On the car I noticed a delay on the passenger side and that prompted me to start this post. It was about 10 minutes for the driver side to fully open but even at 20 minutes the passenger side was barely cracking. Some said this was typical and that because I had no heater ducting that the air had a way out anyway.

So the last Cars & Coffee event I drove the car the 4-ish miles taking about 12 minutes. Upon arrival the passenger door was still closed. On the return home (the day getting warmer) the passenger door still remained closed. I had to idle the car a few minutes to get the rear gate open and I noticed that the passenger door was just barely starting to open. As I pulled the Lakewood in and backed it up I noticed it seemed to have a knocking sound. Once parked I free revved the motor and heard no knock. But when I put it in gear and blipped the throttle I heard it.

It was my assumption that the passenger door not opening had (over) heated that side of the engine and thus the knocking sound. Thus I opted to removed the shrouds and this weeks trip to Cars & Coffee there was no knocking. Another telling aspect was that when I started the car this morning the the oil/temp light remained on. I removed the temp wiring and it was still lit. Because the Lakewood uses the FC type block the oil pressure switch was mounted on the block, not the rear cover as is common to most Corvairs (I have a mechanical gage at that position). And thus it seems the switch had been damaged by the heat.

Anyway, the car cold, the fan belt intact, 35+ PSI showing on the mechanical gage I drove it without issue regardless of the light being on. Previously the passenger door remaining shut after 15 or so minutes did cause issues of knocking and killing the pressure switch. And while that may be hard to believe with the air ducting out the heater hose connection point and a 80 HP motor with all of a 7.75 CR, - it did. The motor isn't running lean. I have #52 jets in it. Removing the shrouds removed the problem. The car seems to start/run OK on 45-50 degree mornings without them. And as it was Dec. 7th and the temperature 80 today, I' going to leave them off.

[66vairguy]

O.K. first lets get back to the initial post. You have NO hoses on the heater outlets, just screens. Well my old gray matter finally realized I did not ask about the FRESH AIR HOSE from the top of the shroud!! If that hose is ripped or the heater box flap is leaking (or the hose is simply gone with a hole in the top of the shroud) the passenger side will receive less cooling air, typically the #5 cylinder head area overheats (Note most, not all, shrouds have the fresh air outlet on passenger side). So is the top hose in place or is the opening blocked off???

Just theorizing here, but with poor air flow OVER the thermostat it only heats by radiation which is less efficient so the thermostat won't heat up as much to open the door. Anyway --- per the factory heater delete option, the two lower shroud heat outlets are CAPPED closed as well as the top fresh air shroud outlet.

As for the old style lower oil pressure switch, per forum comments over the years this switch is unreliable. Possibly due to the heat it is exposed to vs. the later top of engine location.

While possible, I have never heard of the bellows thermostat not opening the door (if the door is not binding). As they start to fail they open door too early, or do not close the door all the way. If you are not running the bellows thermostat let us know.

[Wittsend]

O.K. first lets get back to the initial post. You have NO hoses on the heater outlets, just screens. Well my old gray matter finally realized I did not ask about the FRESH AIR HOSE from the top of the shroud!! If that hose is ripped or the heater box flap is leaking (or the hose is simply gone with a hole in the top of the shroud) the passenger side will receive less cooling air, typically the #5 cylinder head area overheats (Note most, not all, shrouds have the fresh air outlet on passenger side). So is the top hose in place or is the opening blocked off???

Just theorizing here, but with poor air flow OVER the thermostat it only heats by radiation which is less efficient so the thermostat won't heat up as much to open the door. Anyway --- per the factory heater delete option, the two lower shroud heat outlets are CAPPED closed as well as the top fresh air shroud outlet.

As for the old style lower oil pressure switch, per forum comments over the years this switch is unreliable. Possibly due to the heat it is exposed to vs. the later top of engine location.

While possible, I have never heard of the bellows thermostat not opening the door (if the door is not binding). As they start to fail they open door too early, or do not close the door all the way. If you are not running the bellows thermostat let us know.

Yes, I made and installed the upper fresh air block off plate when I assembled the engine.

Thanks for letting me know about the capped openings with the heater delete. Should be easy to make.

Yes, I am using the bellows thermostats. I'll try and make a duct that will (hopefully) direct heat gun air evenly on both thermostats at the same time and see if they coincide closely with their opening off the car. Previously I did them individually just checking for function but not comparing respective opening time.

I'm still not opposed to leaving the lower shrouds removed - especially with my So. Cal. location. There seems to be no repercussions in doing so and it kept the pressure switch alive. I like having the mechanical gage on the upper rear cover. Not going to any cars shows on grass/dirt to shower with dust.

[joelsplace]

I run most of mine without shrouds. Engines run cooler and don't seem to have any issues except no heater.