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Discussion Starter · #1 ·
Thermostat:
For the 25 years I drove my 72’ GT, I was always overly concerned with the engine temperature, I thought since these engines had such a bad reputation for cracked cylinder heads living in a warm climate I thought cooler was better, the first GT I bought and the one I’ve had since 87’, the head cracked right away with no indication (it was probably cracked before I purchased it). Rebuilt back then with a good head I still use today. I also purchased a 70’ GT for a parts car a few years later and it’s head was cracked. I used the 160° thermostat for over 25 years. If the temperature gauge got past the 1/3 or 175°F mark I used to start getting nervous. If it crept up to the halfway mark (on those 90+° F days) I’d pamper it and was afraid to push it.

I’ve become more assertive and also discovered that it ran better with the 180° thermostat over the last year or so, the car ran @ 175° mostly just below the 180°.

Recently I started reading more on the subject Bob’s old post mentioned 185° (210° oil temp) was optimum performance temperature. Opel claimed 193° was their findings for best performance according to this post.

Realizing I very infrequently got up to those temperatures I picked up the 189°F/87°C OEM temperature thermostat.
With the cooler winter temperatures right now I felt it was harmless to try. I was surprised how difficult it is to get. 160°, 180°, 195° & 205° are the standard aftermarket temperature.
I realize now our thermostat is common to many other makes. During my search I used this site if you scroll down there’s 1,000’s of different car manufacturers that use the same thermostat. That’s how I eventually tracked down the 87°c T-stat. I noticed Splendid also carries itI was looking for something more local.
Further reading on this subject indicates that the clearances, crank, piston rod, everything on up to the cylinder head were designed for this temperature.
You wouldn’t think it makes that big of a difference but I’m experiencing a definite improvement.
Engine idle is smoother, acceleration throughout more consistent now, better power using less throttle (gas) easy feel on the gas pedal going up steep grades & there to see on my afr gauge, up above the 14’s instead of the upper 12:1’s. Best of all, it’s early but I think I’m paying slightly less at the pump.

Is spark plug heat range related:
I’ve done some further reading on the definition of spark plug heat range to see if it had anything to do with the engine temperature. 🤔 How was it chosen by the manufacture, in our case GM or Opel?

From what I’ve read, spark plug heat range is not related to the engine temperature, a hotter plug transfer’s less heat through the insulator to the cylinder head and more energy/heat to the electrode at the tip of the plug. The colder plug does the opposite. If running rich, use hotter, running lean go colder etc. all related to what comes in the combustion chamber more so than actual engine temperature. Nothing to do with coolant running 160°F temperature at the thermostat vs. 190° F.

I don’t know why but I thought I’d throw the spark plug subject out there for curiosity sake, hoping to learn more.
 

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The Young One
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When I drive my GT it is usually at or a little bit below the half of the temp gauge. But i am running it pretty hard. But no problems yet I didn’t know that cracked heads happen a lot are they really popular?

Thanks Sam
 

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Can Opeler
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I’ve been meaning to swap to a higher temp thermostat on my Gt too. My Gt is most happy around 195-200° too.

In the winter I never get above 176°F
In the summer above 100° outside my GT cruises at about 210°F at 80mph on the highway I will slow down to maintain this temp if I have to.

At the track I’ve seen sustained 215°+F with oil temps likely around 300°F. I’ve never had a issue yet. Worse thing that happens is the DCOE will boil and make my idle go dog rich randomly when it gets this hot.
 
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Kyler, have you tried a gas cooler? Not a cool can, but that would work for the track. Are a little pricey though.
I used one like the one below to stop vapor lock on the hotter days.
.
 

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Discussion Starter · #5 ·
When I drive my GT it is usually at or a little bit below the half of the temp gauge. But i am running it pretty hard. But no problems yet I didn’t know that cracked heads happen a lot are they really popular?

Thanks Sam
Yours should be fine, it was the 72-73 GT heads that had issues. Most of those are long gone by now.
That 70’ GT I had was an automatic with A/C, rear window defrost (I think) and everything else, the PO may have neglected it badly and probably subjected it to enough that would do in any good engine or cylinder head is my guess.
 

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Discussion Starter · #6 · (Edited)
I’ve been meaning to swap to a higher temp thermostat on my Gt too. My Gt is most happy around 195-200° too.

In the winter I never get above 176°F
In the summer above 100° outside my GT cruises at about 210°F at 80mph on the highway I will slow down to maintain this temp if I have to.

At the track I’ve seen sustained 215°+F with oil temps likely around 300°F. I’ve never had a issue yet. Worse thing that happens is the DCOE will boil and make my idle go dog rich randomly when it gets this hot.
I’m so relaxed now about keeping it installed in the summer LOL.

I would suggest to anyone who has a good cooling system to at least give it a try. Now we’re into the aluminum radiators and slightly higher velocity water pumps OGTS sells. Some of the members here are using the 195° thermostat.

I’ve been meaning to swap to a higher temp thermostat on my Gt too. My Gt is most happy around 195-200° too.

In the winter I never get above 176°F
In the summer above 100° outside my GT cruises at about 210°F at 80mph on the highway I will slow down to maintain this temp if I have to.

At the track I’ve seen sustained 215°+F with oil temps likely around 300°F. I’ve never had a issue yet. Worse thing that happens is the DCOE will boil and make my idle go dog rich randomly when it gets this hot.
That sounds about right. Generally speaking I never take the GT out when it’s over 95° and it usually stays under half way. I’ve been stuck in over 100° temps, when driving 80 and it’s probably gotten up to 210°.

I’ve been meaning to swap to a higher temp thermostat on my Gt too. My Gt is most happy around 195-200° too.

In the winter I never get above 176°F
In the summer above 100° outside my GT cruises at about 210°F at 80mph on the highway I will slow down to maintain this temp if I have to.

At the track I’ve seen sustained 215°+F with oil temps likely around 300°F. I’ve never had a issue yet. Worse thing that happens is the DCOE will boil and make my idle go dog rich randomly when it gets this hot.
I have a pusher fan for back up, it works brilliantly under low RPM conditions in hot weather but at 4k on the highway it’s useless. At those speeds you can typically add 90°F to the ambient temperature in my experience when it gets hot outside regardless of the thermostat in place.
I don’t have A/C like you do so I’m rarely driving when it’s above 90° but that’s good to hear how well everything holds up for you. It’ll be uncharted territory for me to see how steady it is with this thermostat this summer, it rarely gets above the 195° mark so my hope is that it’ll just stay more steady like all of our other cars do.

I copied this to my Google drive for future reference:
Motorcraft 190° T-stat O’Rielly’s
Part #: RT1139 Line: MOT
$16.99
That little Kadett is amazing, keep up the good work (y)

Edit:
FWIW my apologies. I cleaned this post up a little to get rid of the redundancy, it saved an old reply I started yesterday and merged it with my reply this morning, I was completely unaware until just now.
 

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OH BOY I'm sure I read it here before, but how would one tell if their head is one of the little trouble makers?
All the 12-bolt North American market 1.9 heads were problematic. This includes 1972-1975 Opels.
 

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Can Opeler
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Perfect, no, not really, lol
Don’t worry about it. Many or most of them are cracked, but most opel owners never notice the issue. It’s only worth worrying about when you decide to rebuild the engine.
 

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Discussion Starter · #11 · (Edited)
Perfect, no, not really, lol

Good advice, there are some survivors. Lindsay has another good 73’ cylinder head that he’s been driving around in for years. I’m sure there’s probably a few more out there. If you get around to rebuilding, bigger valves etc. you would be careful at that point. Keep your eyes out and you’ll be able to find a good 10 bolt 4 bearing head very inexpensively. Then when the time comes you’ll be all ready to go.

Just a tip, I was just looking for a 4-speed transmission and it came with an engine and rear end from a 73’ GT with a good 71’ cylinder head for $300. So don’t turn your nose up just because you find a 72-75’ engine up for grabs, there’s still a good chance the PO replaced the head years ago.
 

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The head cracking became pretty common, almost guranteed, with induction hardened valve seats in the heads to tolerate the then-newly mandated No-Lead gas. This problem with induction hardened seat cracks occured in lots of heads, not just Opel or GM, and continued for a couple of decades! Mopar Magnum heads from the 90's still commonly have cracks around the seats.... and you guessed it: the seats were induction hardened. It is just a problematic method to locally harden cast iron that sets up a lot of stress in the seat material versus the softer surrounding material, and may start the cracks in the hardening process AFAIK.

Hence the later heads being the big issue. I have not ever had a crack in an early head. Don't recall that I have ever had a later head that didn't crack or have a crank when I got it. That is pretty definitive IMHO....

The cracks can be a real issue. I had overheating when racing repeatedly... not severe but an added 15-20 degrees F everytime we cranked hard on the engine on a stage. Yep.... '73 head with a crack. Got it out of the car before it took us out of a long rally. And the last one I found cracked was so bad that it would leak coolant into the #2 cylinder when it set for a day or 2. That was a '75 head.
 

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But there is always some cylinder heads that do not give any problems ....... If you have the exhaust seats installed it can negate the cracked cylinder head issue.
When I worked at the Dealership we only had to replace 3 cylinder heads (69' - 75') and parts only kept 1 cylinder head in parts stock.
All the cylinder heads we replaced were on cars that had a major overheat issue (water pump failure or coolant leak).
I have been running a 1975 cylinder head for 25 years (had the exhaust seats replaced and Chevy guides and valves)
but I still look for 71' to 72' 4 cam bearing cylinder heads ....
Back to the start of this .... what spark plug do people use and the heat range.
 
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Discussion Starter · #14 ·
But there is always some cylinder heads that do not give any problems ....... If you have the exhaust seats installed it can negate the cracked cylinder head issue.
When I worked at the Dealership we only had to replace 3 cylinder heads (69' - 75') and parts only kept 1 cylinder head in parts stock.
All the cylinder heads we replaced were on cars that had a major overheat issue (water pump failure or coolant leak).
I have been running a 1975 cylinder head for 25 years (had the exhaust seats replaced and Chevy guides and valves)
but I still look for 71' to 72' 4 cam bearing cylinder heads ....
Back to the start of this .... what spark plug do people use and the heat range.
It’s interesting to hear that about the cylinder heads. Thank you for sharing that bit of history on them. It’s a real treat to hear from your experience working on these at the dealership. I always enjoy reading the posts you provide from those days.

As far as the spark plugs are concerned. I’m currently running the NGK Iridium 4085 and very pleased with their performance. For years I’ve been used to running the Bosch Platinum WR7’s, then the NGK 7022 copper plugs the last few years. All excellent spark plugs, the iridium just seem to run a bit smoother. Nothing special for my near stock 2.0. Just the Pertronix 2 ignition paired with the recommended Flamethrower .6 ohm coil & gapping the plugs at .035”.

It would be great if everybody chimes in here with what spark plugs they’re running these days.
 

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It would be great if everybody chimes in here with what spark plugs they’re running these days.
My preference is NGK BPR6HS for typical street engines.

For naturally aspirated competition use (or heavily modified high compression street engines), I’ll go to BPR7HS (one step colder). I’ll also use the BPR7HS for forced induction or nitrous use.

For competition use with forced induction the preferred plug would be BPR8HS.

Not a fan of any plugs with precious metals (platinum/iridium, etc) in a CIH Opel. The sole exception for me being the Bosch WR7BP in a bone-stock 1.9 CIH, they always seemed to run the best.
 
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Discussion Starter · #16 ·
Here’s some updated information based on my recent experience. This may be common knowledge to the more seasoned experts on this site but news to me. Standard vs. high flow. The Victor Rienz 189° thermostat I mentioned earlier got removed and tossed as it started making me uncomfortable, driving when the outdoor temperature was a little higher in the upper 60°’s the gauge started climbing to the 212° mark and started going back and forth between it and the 194° mark on the gauge.
Headgear Cap Line Font Rectangle


I then purchased the 192° Motorcraft thermostat that I posted earlier on this thread since they are a good product, thinking that it would run just a bit cooler than the 195° stat. Unfortunately it ran about the same. Here’s a little movie clip of my temperature gauge showing the Motorcraft 192° thermostat.


Fortunately Harold let me know what he was running and recommended an alternative 195° t-stat much easier to get, Mr. Gasket high performance thermostat pt. #4365 .
What I learned is that my GT needs to have the high flow thermostat. The temperature gauge is much more stable and hovers right around 195° or lower, NOT higher temperatures. Here’s a another little movie I made showing the difference.


There’s also some other good brands that offer a high flow, high performance thermostat, I’m not promoting this one it’s just easy to get. When I look at the temperature gauge now it’s back to being consistent and close to the rating :)
 
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