Does anyone else with an underdrive crank pulley have a heating issue? I've got the 6 1/4" March crank pulley and I'm wondering if the lower water pump speed at idle is contributing to my heat problem. I've looked at almost all the other recommended fixes. Car runs 175-180 at highway speeds but gets up to 215 pretty quickly in stop and go driving. I've got a flow cooler water pump, 3300 cfm electric fan with shroud, radiator upper and lower baffle pans, 2 row aluminum radiator, running 50/50 coolant mix with water wetter. Car has KRE aluminum d-port heads. I'm in FL and running a 20w50 synthetic oil. At 220 she starts groaning and I've got to shut her down.

My next test will be to change back to an 8" crank pulley to see if that helps any. Any other ideas? Four row copper radiator?

FWIW, if you change to an 8" crank pulley, you'l have to change to a smaller w/p plly.
Which will speed up the w/p at all rpm's.

Sounds like everything is top shelf.

"2 row alum radiator"
Is the core at least 2.5" thick?

What degree thermostat?
Only other thing i can think of is timing at idle/low speeds.

I know this dosn't help much but being new to this whole Firebird project one of the first things that I noticed is how big the wp pulley is and how many overheating problems these cars have. To me the pulley is to big and does not move enough water. Just a guess.

Non a/c cars used 7-7/16" crank with 8" w/pump pllys.

a/c cars used 8" crk with 6-3/8 w/p pllys.

I have used non a/c pllys since 75.
And cast w/p impellars. Not stamped steel ones.

Also check impellar to divider plate clearence.
Some dont extend out far enough.

I already have the smaller w/p pulley. It's part of the March underdrive set up. So I will essentially be going to the size that the stock a/c cars had. It is a 2 row aluminum radiator around 2.5 inches thick. I am now running a restrictor plate with the smallest hole in lieu of a thermostat.

Try a larger r-plate 1st.
Thats the easest.