'66 427/425 cooling

**Joe L.** · September 9, 2002, 12:53 PM

Re: '66 427/425 cooling

Kurt-----

A fan clutch problem usually evidences itself mostly at idle and during low speed operation. Usually, when the car is at highway speed, the vast majority of the cooling air flow is provided by motion of the car. On high ambient temp days, though, the fan may need to provide some supplemental cooling. It's a very good idea to replace the fan clutch, though. If it's the original unit, I can virtually guarantee you that it's at least partially disfunctional. With the rest of your cooling system being, apparently, "up-to-snuff", you might as well have this component in new condition.

Although your 1966 L-72 did not originally use fan shroud or radiator support seals, if I were you I would add the seals used on 1966 427 with A/C applications. Excellent reproductions are available from Dr. Rebuild. Some of those folks with 66 big blocks who are not having cooling problems may have added these seals (or some previous owner may have).

If you fan clutch is, indeed, the original fan clutch (or an exact original replacement installed early in the car's life) it's worth having it rebuilt. Fred Oliva is the primary, if not only, source for this work. You can find his e-mail address in the "profiles" section of this board.

**R N.** · September 9, 2002, 01:23 PM

Re: '66 427/425 cooling

Joe:

Thanks for your reply. These seal sound like they will put more air through the rad than around it. I've noticed there are some good sized air spaces top and sides of the rad support. Any idea what the temp. range should be for this engine with a correct operating cooling system with "normal" summertime temp. (75-80 degrees). I'm in southern Ontario (Toronto area) and it does get hot and humid here. When driving after work on warm days (85-90 degrees plus) it's hard to keep the engine below the 210 mark, whick seems a tad on the high side.
Today it's 94 degrees and humid so I know if I take the car out....
Thanks agian,
Kurt

**Duke W.** · September 9, 2002, 01:47 PM

Re: '66 427/425 cooling

Several recent discussions of the L-72 indicate that all have a "ported" vacuum advance in addition to an insufficiently aggressive vacuum advance map.

The lack of vacuum advance to achieve total idle timing of 25-30 degrees will make any SHP engine run hot at idle.

The cure is to install the vacuum can from the '65 SHP/FI small block, which provides 16*@8" and tee into the choke vacuum break line for the vacuum can signal line. The choke vacuum break always sees full manifold vacuum at idle, whereas the normal vacuum can signal line off the carburetor will not have enough vacuum at idle to even start pulling the vacuum can, much less pulling it to the maximum, which is best to foster a smooth stable idle on a SHP engines without throwing excess heat into the cooling system.

You might want to check the recent archives on this issue.

Duke

**Clem Z.** · September 9, 2002, 02:31 PM

Re: '66 427/425 cooling

get a IR temp. reader and check your temp. at the gooseneck where the water enters the top hose as your gauge or sender may be off of calabration and you are not running hot.

**John Lokay** · September 9, 2002, 02:43 PM

Re: '66 427/425 cooling

Very interesting comments from Duke. I also own a 66' 427/425hp vette with about 6,000 miles on the rebuilt original engine (fatory clone type cam; bored 40 over; 3:70 rear) While the car runs good, it will get hot at idle and climb over the 210 mark on the temperature gauge. It has the orignal vacuum advance unit. The fan clutch appears to be okay, I think the sending unit is okay and I back-flushed the cooling system last year. I was ready to buy a new radiator from DeWitt as I question the cooling capability of my factory correct radiator even though it flow tested okay a number of years ago. I think I will follow Duke's advice, but I have a few questions for him as follows:1) where can I get the 65' SHP vacuum can?; 2) just to confirm, you stated that the factory vacuum advance set-up is receiving ported vacuum of the carburetor base not full manifold vacuum?; 3) to obtain full manifold vacuum could you attach the vacuum advance line to the plug opening on the back of the intake manifold instead of cutting into the choke vacuum line?; 4) do you have to take the distributor out of the engine to replace the vacuum can; 5) total vacuum advance at idle with the vacuum advance can attached for the L 72 engine should be between 25 and 30 degrees? Thanks.

**R N.** · September 9, 2002, 03:16 PM

Re: '66 427/425 cooling

Thanks for the thoughts Clem, however the engine is runninghot as I do get overflow out the the overflow hase of the rad at times.

**Duke W.** · September 9, 2002, 04:01 PM

Re: '66 427/425 cooling

1. You can buy the '65 SHP/FI vacuum can from any NAPA store. The Echlin/NAPA part number is VC1810

2. According to measurements taken by two other L-72 owners and reported in public forums or on private e-mails to me. The OEM vacuum can signal line has little or no vacuum at idle, and in any event, not enough to begin to pull the vacuum plunger. I recommend you tee into you vacuum can signal line and check with a vacuum gage to see if your engine conforms to this observation. You can also check idle timing with the can connected and see if it shows vacuum advacne at idle, which would be any advance over and above the initial timing.

3. You can probably find an adaptor to install in the manifold in place of the pipe plug that will allow you to connect 1/8" tubing to the vacuum can.
The other quick alterntive is to remove the original choke vacuum break hose and buy a 1/8" tee and a length of 1/8" vacuum tubing to fabricate a new line to the choke vacuum break and vacuum can.

4. At lease one L-72 owner has reported that the can can be removed and replaced from the TI dist. while it is on the engine. It's tricky, but doable.

**Duke W.** · September 9, 2002, 04:11 PM

Re: '66 427/425 cooling (continued)

(Sorry, I hit the wrong button and posted before I intended too.)

5. The total "idle timing" is the initial. plus vacuum advance, plus any centrifugal. It is the amount of TOTAL timing that the engine has at normal idle speed, and for best idle quality and stability it should be in the range of 25-30 degrees on a SHP engine. Also, to insure idle stability the advacne must be constant which requires a vacuum can that provides full advance at least 2" below what the engine idles at. The '65 SHP/FI can, which provides its full 16* at 8" will fulfill this requirement with any Chevrolet SHP cam - big or small block. (Note: racing cames not included - just the original factory option SHP engine cams.)

Since your engine is probably very original, do not modify and existing parts including any hoses and the vacuum can. Save everything you remove and obtain the new can and whatever new hose and hardware you need. This way you can always return to the original configuration for judging.

Duke

**John H.** · September 9, 2002, 05:17 PM

Re: '66 427/425 cooling

Kurt -

Does it "puke" coolant while you're driving, or after you shut it down and it heat-soaks for a few minutes? In any event, I'd find a shop with an I.R. gun (or buy one) and "shoot" the thermostat housing and compare the reading against what the temp gauge shows at the same time; Corvette temp gauges are notorious for reading high, especially if the sending unit has been changed.

Your fan clutch definitely needs a rebuild; Fred Oliva (who rebuilds them) says they should turn no more than three revolutions when shut down from hot.

**R N.** · September 9, 2002, 05:49 PM

Re: '66 427/425 cooling

John:

It doesn't puke, just bubbles out after I shut it down, I guess this is caused by the heat soking of the engine. Your advise and Clems is good. Any ideas where to purchase such an item.

Thanks also to John for additional questions and to Duke for the answers to resolve my problems.

I didn't release when I purchased my Corvette there would be so many other things to purchase. It's a good thing my wife didn't know either.
Thanks.

**Clem Z.** · September 9, 2002, 11:37 PM

Re: '66 427/425 cooling

also check you rad cap and if less than 12/15# get a new one. BB rads with no external expansion tanks the water level must be left a least 3" below the filler neck or it will puke every time due to water expansion.

**John H.** · September 10, 2002, 12:40 PM

Re: '66 427/425 cooling

Kurt -

Lots of the mail-order catalogs have the Raytek MT-4 I.R. Gun - best diagnostic tool I own. Try Griot's Garage (www.griotsgarage.com) or AutoTest Systems, www.autotestsys.com/ .

**David Z.** · September 10, 2002, 01:16 PM

Re: '66 427/425 cooling

I picked up a Raytek MT4 (laser sighted) IR gun from an outfit called 3DCOOL.com (http://www.3dcool.com/?module=product&sku=MiniTempMT4)for $59.95. Neat toy for shooting temps. Also useful in the house for monitoring AC performance.

**John Lokay** · September 10, 2002, 03:00 PM

Re: '66 427/425 cooling

Dave, Can you once again provide the 3dcool website info? I could not pull it up on the earlier address you gave.

'66 427/425 cooling

'66 427/425 cooling

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Debug Information