What you're saying is that a higher flow restriction is not a problem, but that's not a true statement. We could get into the diameter of the hoses, the pump size, the flow rate of the coolant, etc. The size of the thermostat DOES matter when you're keeping everything else the same outside of the size of the thermostat.

You cannot compare a V8 gas engine to a 3L diesel just by saying "same same," nor can you compare a daily driver with a car designed to drag race.

Om603
 

I guess I'm not be clear. I didn't say both engines were the same. My point was that they both create a lot of heat even thought they are different. The size of the Mercedes thermostat really doesn't matter because it doesn't open enough where its located because it gets hit with cold water and closes. The chevy thermostat gets hit with a more consistent water temp so it doesn't open and close as much. If they made an inline Thermostat that used the bigger Mercedes T-Stat then I would have used that. The location of the T-Stat makes more of a difference than the size.

well, yes I changed the thermostat already. I updated the T-housing too at that time.
I wasn't aware of the different cooling systems. Shows you what little I know.

I have some Motul coolant additive supposed to lower temps but haven't put it in yet, I'm sorta waiting to flush the system and replace antifreeze. I am skeptical of claims made by most additives but this might help a little. I used a similar Redine product in another car and didn't notice any difference.

Om603 cooling
 

With almost 20k on this setup I wanted to give an update on how the cooling system mods have worked so far. I have started towing my race car to Chuckwalla raceway from DelMar, Ca. It's roughly 200 miles each way, and I encounter some very steep grades. Last weekend it was 105 degrees out in the desert and making it up the last steep grade on the 10 Fwy before hitting Chuckwalla my engine temp peaked at 95c. Not bad considering the temp would get that hot or hotter with the stock thermostat location under full boost and a small hill. The race car and gear I'm hauling is around 2500-2600 pounds.
<a data-flickr-embed="true" href="https://www.flickr.com/photos/136374868@N03/49906731831/in/dateposted-public/" title="IMG_4101"><img src="https://live.staticflickr.com/65535/49906731831_1129823d62_w.jpg" width="400" height="300" alt="IMG_4101"></a><script async src="//embedr.flickr.com/assets/client-code.js" charset="utf-8"></script>

OM603 thermostat relocation and coolant bypass
 

After seeing JonW124's thread on this subject and a few helpful PMs from him, I made these same modifications to my 87 300TD last year. I have about 7500 miles on the mods, and my experience has basically mirrored his.

On Monday, I drove back to LA from camping in the Sequoias loaded to the roof with gear and bikes on the hitch rack. I climbed to 8000 feet and back down, ambient temps were up to 105F, and I was driving up to 95 mph depending on freeway flow. The coolant temperature stayed rock solid at 80-82C. It only ever climbed beyond that when climbing the Grapevine with the AC on, where I would have been over 100C with the heater on before these mods.

Om603
 

Glad to see these simple mods worked as well for you as they did for me!

coolant mods
 

Yes, I am as well! I appreciate you taking the plunge and opening up discussions on the boards so we could hear everyone's opinions and experiences.

I am on the original #14 head.

I'll make the same modification to my brother's 87 300D whenever he is down here next...got the parts in hand already.

That's a sweet tow rig. Tell me more about the e30 m3.

M3
 

Well It's been a track car for 20+ years now. When I got it the previous owner was going to convert it back to a street car and cut the cage out of it. I got it as a roller and put the original drivetrain back in the car and went through everything suspension and brake wise. Now I'm getting ready to build another cage for the car. I usually race out at Chuckwalla which is where I was headed when I snapped that pic above. Not sure if your familiar with the track but my PB is a 2.05.19. Not to shabby with the original 4 cylinder!

Also average 26-28 mpg towing my M3 out to the track. Total distance is around 400 miles round trip. Pretty happy about that. :)

PM'd you jon