I have had my 65, 327 350HP for over 4 months. The car has spent the last 3 months in the shop. New suspention, power brakes, rebuilt trans., new interior etc. However one problem remains, ........constant overheating of the engine. The temp gage is always close to the 240*F mark. I have replaced the radiator, temp. sending unit, fan clutch, thermostat with 160*F, upper/lower hoses, new carb(holly street avenger 660cfm), also had the timing checked. Still overheating. My mechanic tells me the valves have been changed( now rollers). The valve covers sit higher because there is a spacer beween the block and the original valve covers. What is the correct timing for this engine? Is it 6*at 850RPM and 36* at 3000 RPM? What else could be causing the problem. I have also checked the upper and lower Radiator hose temps and there is a 30 degree difference. Could it be a water pump issue?

 

Starting with the basics, is it really running that warm? What temp reading is your IR gun giving you on the upper hose? Only a 30* drop through the radiator sounds low, should be more than that.

 

Question: Is the car puking antifreeze out of the overflow tube after you shut the car down while it is hot?

 

replaced the radiator with what?

your efforts / your mechanic's efforts to solve the overheating problem look a lot like the efforts the prev owner of my 65 went through (his mechanic went through) before he lost his taste for the car and sold it to me (not disclosing the chronic run hot problem, of course) - he kept every receipt and they sure told the story. Oh yeah, they tried a replacement water pump too.

Guess what? He was so quick to rule out the radiator as the issue since he had put in "a brand new one" - but that brand new one was a modern day brass replacement rad, which, even when new, starts out about 25% down on cooling ability (heat rejection, I guess) vs the aluminum Harrison stacked plate rad that the car originally came with.

Once I installed the proper original - style aluminum rad (from Dewitts) and took off all of the other "fixes" (the "better" fan, the electric fans) and returned all of that to stock (including the stock GM fan clutch) the problem was solved.

 

You use the words "roller" and "spacer". I'm guessing the engine is equppied with roller rockers and the spacers that are between the valve covers and the engine block are there to make room for those roller rockers. With these items mentioned I'd be willing to bet the camshaft has also been bumped up way beyond a stock camshaft in which case the stock timing specifications are no longer in effect. I'd also be willing to bet that part (if not all) of your over-heating problem is related to your ignition timing. I suggest you get a copy of Lars' ignition timing articles and read them diligently. Do a little searching here on the forum for those articles, they shouldn't be that hard to find.

Good Luck,

Mudbone64

 

I hope your are saying 6* with the VAC disconnected and vacuum line plugged. If the VAC is connected (+16) you should be around 24* at idle. Ignition timing that retarded would cause elevated EGT, run-on, and overheating. The 36* is also late for 3000rpm....should be closer to 48-50* with the combination of 8* initial, 16* VAC, and around 24* centrifugal.

 

I never quote a differential temperature (deltaT) as "normal" because that could vary a lot depending on ambient temperatures. Outside air temperature is the source of cooling and depending on if that is 100F or 60F will have a huge impact on the delta T. I'd say 20 degrees is typcial but that doesn't mean a radiator with 10 degree isn't working correctly. It might just mean there isn't enough cold air to increase the delta T. 30 degrees drop sound great to me, so I'm going to bet that's an aluminum radiator in there.

 

Full centrifugal advance isn't all in until somewhere around 5000 rpm. I don't remember the exact rpm or total but it's probably around 27-28*. The initial spec was around 8*. That's a guess. There's nothing there that will cause you to overheat. I'm reading quite a bit of overstatement in this thread about the impact of timing on engine cooling. If you're not careful in what you do here and start jacking the timing up, aviation gas won't keep it from knocking when you should be able to run it on the mid grade gas.

You never indicated what the temp of the coolant coming out of the thermostat was. Never mind the guage. Shoot it with an IR gun and post the reading.

 

I don't know, an odd mix of precise and imprecise info here, is able to give us the temp differential between the upper and lower rad hose, but quotes the temp gauge as if he has no other source of temp info; doesn't doesn't know the timing spec for the very popular L79, and to tell you the truth, out of spec timing or an improper vac advance can is not the huge overheat driver, at best it could push you over the edge if you have very little cooling margin. I'm betting this is all info his mechanic is supplying (not that there's anything wrong with that); I also have a pretty good idea what rad even a good local mechanic would usually install into a "basic chebby 327" car.

Joseph the dentist, what rad is in there now?

 

There is much confusion in this thread due to some less than precise terms. At idle (750rpm), with NO centrifugal advance, and the VAC fully pegged, timing should be near 24 degrees (8 initial + 16 VAC). At 3000rpm, OE timing should be in the neighborhood around the 44 mark with 8 initial plus 16 VAC and 20 centrifugal. Per the 65 shop manual the L79 OE curve has a centrifugal max of 30 at 5100rpm. It is true that some expert tuners recommend "jacking the timing up" for a quicker more aggressive centrifugal curve with centrifugal all-in around 3000rpm. Of course pushing the curve up may exceed the detonation limits of your fuel depending on a number of build factors including dynamic compression ratio. Sure.....there can be many causal factors contributing to your overheating but I would start with getting the timing OE right..

 

Thanx everyone for the quick responce. Yes Jack, I am not a mechanic and most of this info is coming from my mechanic. During my "starving student" days I worked on my cars, but that was a long time ago. My mechanic is actually a Mercedes mechanic and a close friend. He was reluctant to work on the Corvette. I figured if he can handle the complicated German engineering then the Corvette should be a breeze for him. ....And boy was I wrong. Now the shop is visited daily by many self proclaimed Chevy experts. That is why I have been all over this fourm just cross checking all the info.
The radiator was replaced with the stock copper/brass high efficiency radiator. The temp reading at the upper hose was 210*F and the lower at 180*F. The ambient temp was around 80*. The temperature at the temp. sending unit was 220*F. The Carb. was rebuilt and will be installed next week. I will have the timing corrected and will keep you guys posted.
Thank you everyone for you support.

 

The stock radiator was aluminum, so the copper/brass replacement in your car now may in fact be a key source to your issues.

 

[QUOTE=Talisman51;1562574275]There is much confusion in this thread due to some less than precise terms. At idle (750rpm), with NO centrifugal advance, and the VAC fully pegged, timing should be near 24 degrees (8 initial + 16 VAC). At 3000rpm, OE timing should be in the neighborhood around the 44 mark with 8 initial plus 16 VAC and 20 centrifugal. Per the 65 shop manual the L79 OE curve has a centrifugal max of 30 at 5100rpm.



The only thing I see confusing is the way you presented your numbers.


This is what I responded to: "The 36* is also late for 3000rpm....should be closer to 48-50* with the combination of 8* initial, 16* VAC, and around 24* centrifugal"



It's not necessary to be precise on those timing numbers or the curve. Why? Well, the 365 horse engine times different than a 350 horse. Way different. The initial, the curve and I believe the vacuum can is all totally different. Yet, they will both run well and not overheat if you swap the ignition tune from one engine to the other. Let's throw another ignition tune into the equation. The '63 fuel injection engine. It's mechanical curve is slow, similar to the L-79 plus it has a vacuum advance that runs off venturi vacuum and takes a lot of vacuum to start moving it. The result is you have no vacuum advance at idle with a lazy mechanical curve and slow pulling in vacuum advance. Those engines use the same cooling system as the others. Same compression and they don't overheat either.

The copper replacement radiator in my '65 keeps the engine about 190 on the highway, 200 around town with ambient air temp 90* or under. Borderline as far as I'm concerned but I don't plan on changing it anyways soon. If I lived in So. Calif. with lots of heavy traffic, I'd change it in a heartbeat. When the car was newer and still have the original Harrison radiator in it, it ran 180 regardless. As the radiator aged and started to plug, the temp started rising and finally got to where it wouldn't stay under 220 and started sprouting tiny leaks.

Same old story you read on this forum at least once a week during hot weather. Now when you get this borderline like my '65, I can see where SLOW engine timing will push one over the edge and keep it boiling hot.

 

Been through this on both my mid-years. Need to upgrade your radiator to all-aluminum rad. Dewitts has them and they look just like the original brass/copper one used by Chevrolet. Also install an electric fan which will pull air through radiator more efficiently when radiator temp. starts to get warm. Have both mine come on automatically at 195degrees. Both mid-years now run 180s on highway and 200 setting in traffic in the middle of summer. You will not believe how much more efficient the alluminum rad. is than stock brass/copper. James

 

please don't take my effort to help you by extracting more info the wrong way, really it is all towards getting you behind the wheel of a C2 that you can trust to run reliably, I know the frustration of having one that doesn't.

As for your overheating issues (assuming you have one, shoot the top rad hose with an IR gun when it is "running too hot" to confirm what the true temp is, or ask you mechanic to do so - the temp gauge itself is not a reliable source of such info, although seeing it in the red DOES get your attention!) - you are now considering things like water pump efficiency, timing, valve-train mods, etc. as the source of the [assumed] overheat problem. You are nibbling at little things that could contribute to a run hot condition,

You have also advised of what I suspected to be the case; you have a brandy new copper-brass replacement rad. THAT is a major issue, you have started at more than 25% down on cooling capacity, and perhaps with some other minor issues your cooling system simply can't cool your engine.

Just like mine.

It's gonna hurt, but order thyself a resto-rad from Dewitt, it is EXACTLY like the one that your car came with when new. And remove the brand new copper-brass rad you have in there now. I said it's gonna hurt, but you are chasing items that might be 2% contributors to your problem, why not get more than 25% more cooling capacity?

here's the rad, Dewitt pn 941:

http://www.dewitts.com/pages/productdetail.asp?ProdID=2

or get their pn 942B for $75 less. (not painted balck, not date coded, but same cooling capacity)

I walked the exact same path, my brother.

 

The "original one" was an Aluminum stacked plate Harrison radiator. Likely he won't be needing electric fans and such once he installs the original equipment, his car ran just fine when brand new without them.

 

James
You have a BB and the radiator was originally copper/brass. So what you say here is true, for a big block. But this guy has a small block and they came with aluminum already from the factory, so just like you upgraded to aluminum, he downgraded to copper.

 

My 68 "pukes" when I shut it down. What causes this?

 

ddsjoseph,

As others mentioned, temp sending units may not be accurate depending on the gauges & resistance. The true test is an IR gun on the top & bottom hose. You mentioned dropping 30 degrees but we need to know what the actual temperature was. Mine drops from 190 to 145 but I'm not sure if 45 degrees is common.

I overheating my 62 once by being an idiot when refilling the antifreeze. I had an air bubble in the system that restricted the amount of fluid. I'd make sure that you don't have that problem.

 

I'm sorry, I didn't mean to reopen the thread..just get an answer to one of the points made my a member. I was seeking the reason a car would expell coolant thru the overflow when you shut down. My 68 is doing it at the present.