Cylinder Head Temps, My Experience + lessons learned Options

[rwilner]

I drove down to Chris Foley's shop to get his shift linkage installed this weekend. At the charlton plaza on I-90, I stopped to fill up. I usually put premium in the 914 but this time put regular in because it's what I put in my other cars and I was on autopilot. Oh well, I thought...let's see how she runs on 87, maybe I'm wasting my $$ on the good stuff.

Before the fillup, I was running between 300 and 325 F in 5th gear around 70-75 mph.

After the fillup, I was running between 380 and 400! (IMG:style_emoticons/default/ohmy.gif)

I spent the rest of the trip in 4th gear at no less than 3500 rpm to try and keep the head temps down. When I got to Chris' shop, we put the car in the air and drained out 5 gallons. I replaced them with 93 octane before getting on the highway for the ride home. My cyl head temps were back down to 310-350 for the return trip.

Here's what I learned:

• A cylinder head temp sensor and gauge are critical to know what's happening in your engine and should be on everyone's short list. Without it I would never have known of the dangerous operating conditions for the engine. The aircraft spruce gauge/probe are short $$ and a quick install.
• The state of tune has a major impact on running temps (duh). I am still tuning my car with the microsquirt and it's clear I have work to do.
• Higher octane gas makes your car run cooler.

As always YMMV but thought I'd share my experience.
Rich

[ww914]

Hey Rich

Your post is very timely with me putting the engine back in my car. A couple of questions: The A/C Spruce website shows a spark plug probe and a 14mm probe. Does the 14mm probe go where the existng probe is in the head or do you buy the spark plug one. The third one looks like an egt probe.

(Edit) I think, after doing a little more research, I answered my own question. The 14mm dimension is the size of the spark plug, right. Also, do you think it would be worth it to buy the dual guage to read both head temperatures? Not sure we need egt info like you would on a Lycoming engine, right? That might be going a little too far. I do notice that Chris sells these things. Probably wouldn't hurt on a race car though.

Thanks, Warren

[ChrisFoley]

Not sure we need egt info like you would on a Lycoming engine, right? That might be going a little too far. I do notice that Chris sells these things. Probably wouldn't hurt on a race car though.

Thanks, Warren

EGT is definitely not much value on a street car. Exhaust temps fluctuate too much with changing throttle position.
A good (single) AFR meter is fine for tuning.
CHT on each cylinder of an aircooled street engine is more valuable than you might think though.

[ww914]

[/quote]
EGT is definitely not much value on a street car. Exhaust temps fluctuate too much with changing throttle position.
A good (single) AFR meter is fine for tuning.
CHT on each cylinder of an aircooled street engine is more valuable than you might think though.
[/quote]

Chris

Are you suggesting a single AFR meter and 4 cht probes?

Warren

[ChrisFoley]

Chris

Are you suggesting a single AFR meter and 4 cht probes?

Warren

Yes.

As Rich Wilner's first post points out, without any CHT he would have no idea the engine ran so hot on low octane fuel.
And at the moment he doesn't know just how lean the mixture is because of the broken LM1.
With hi-test he was getting away with the lean mixture, but not with 87 octane.

Most everyone assumes cyl 3 is the hottest so they put a single CHT on #3.
My car has a hot #4 cylinder right now. If I had a single CHT on #3 I wouldn't know that.
Injector problems, spark problems, etc. might only affect one cylinder. With a 4 channel CHT its easy to identify and resolve problems before they become serious.

[jsayre914]

With a 4 channel CHT its easy to identify and resolve problems before they become serious.

What guage do you reccomend Chris (4 channel) ??

[ChrisFoley]

With a 4 channel CHT its easy to identify and resolve problems before they become serious.

What guage do you reccomend Chris (4 channel) ??

I sell this one
I have the same one in my street car and my race car.
The gauge has several modes of operation. The standard mode automatically cycles through each cylinder so it only takes 4 seconds to see all 4 head temps.
The display shows cylinder, digital temp and a green/yellow/red bar graph for quick reference.