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Modifying the stock LS1 style thermostat

Old 12-05-2015, 08:33 PM
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Default Modifying the stock LS1 style thermostat

This was originally done and posted by vettntuts, thank to him for his efforts!

There have been two threads recently on modifying the stock thermostat (a modification that isn’t discussed much anymore) and I recently got to check the new thermostat I just bought out of the box and then with a few different thickness shims. My goal is not to get a real cold thermostat, but rather get one to open around 180 degrees with good flow by the time the thermostat is in the low to mid-190’s. I am hoping that with the DeWitts radiator I have on order and this slight modification to the thermostat, I will be able to keep the engine temperature in a narrow range from 185 – 195 degrees from spring right through fall while ensuring my oil temps are over 200. That’s the goal; guess we’ll see how things work out in the coming months and if the oil doesn’t heat up enough I will re-install the stock unit. The other thing worth noting, the temperatures I am trying to achieve are based on how I typically drive the car as I don’t race the car so extended high RPM runs are simply not in the cards.

In the first photo the thermostat has been disassembled. You can see one of the shims that I made and the hole into which it is installed. The thickness of the shim will alter the opening point by lowering it, or as is the case if you get to approximately 0.100” thick shim, the thermostat will barely close.

One of the problems I ran into was how to measure data so it wasn’t subjective and I could repeat the test fairly well between the different shims and get a good comparison. After some messing around with the test, I decided to take the temperature at three points in the opening sequence. First, by hanging the thermostat such that the neck was downward, it was easy to trap air behind the rubber door that opens in the thermostat. The first data point would then be when the thermostat opened enough to release the trapped air. This doesn’t take much movement at all and you can barely see the rubber door move at all when this occurs since the movement is so slight. The second point of measurement would be when the rubber lip on the seal was even with the flat mounting surface of the thermostat (see the photo). Finally, I took a third measurement when the rubber lip was 0.2” from the same flat mounting surface. While this seems to be a precise number, it is actually the width of my flat screwdriver that I could hold in the water and use for a measuring tool. With one edge of the screwdriver against the flat mounting surface of the thermostat, when the lip was even with the other edge of the screwdriver blade that is when I wrote down the temperature.

Below is a photo of the shim installed prior to re-assembly of the thermostat.

Next was the method of heating up the thermostat so it was immersed in water. In the next photo you can see I bent up a coat hanger and hung it from a small stick across a large pot of water (my wife thinks I should be committed at this point ). The wire next to the thermostat is the thermocouple that was tied into a Fluke Meter for measuring the temperature.

Below is the Fluke Meter that was used to monitor the temperature, which is accurate to 1/10th of a degree.

Here is the data that I obtained, noting each shim was tested several times to obtain these average values:

It appears that with the 0.098” shim, the thermostat is barely closed. The 0.058” shim didn’t quite get me to where I wanted to be. The performance of the 0.075” shim, which seems to seal the thermostat better when it closes down again, will hopefully provide me the temperatures I am trying to achieve in the motor. However, if it is too cool the data will be used to make adjustments as necessary to get to my final goals.

I ran the 0.075" shim for a while but in the motor it was too cold. Further experimentation in the motor brought me to a final value of 0.045" shim. My motor runs between 180-185 while cruising (depends on outside temperature and what the motor is doing, i.e., higher RPM cruise vs. loafing on the highway). Oil is typically 198 or higher depending on outside temperature while driving. In the summer it will get to 208.

Last edited by Paul Bell; 07-17-2017 at 10:21 PM.
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Old 12-10-2015, 07:39 PM
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Just a note here, I went through this way back when
(before I knew I didn't want a stock 'stat, but better
radiator) and found that modding it made the control
range swing a lot wider. Also I gather some newer
"stock" thermostats are differently constructed and
the shim point may not be accessible.

I think this is one of those "free mods" everybody was
in love with 10 years ago, but the aftermarket and
tribal lore has moved on.
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Old 01-09-2017, 06:20 PM
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never heard of this mod but seems interesting.....where does one get that size shim stock ?
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