Help with 396/425

[Richard M.]

I have never owned a big block car. Am considering a 65 396/425. Will appreciate any advice regarding these motors such whether there are difficult issues to resolve with overheating.

[Terry D.]

Re: Help with 396/425

Richard
The over heating problems with any Corvette are usually caused by an old cooling system. People think they can send that 50 year old radiator to some local shop, have it cleaned, throw some stop leak in it and be good to go. After all it worked on that old beater I had back in the day!..If you replace the old rad, with a new aluminum one, if it's judging points you are after go to DeWitts, have new hoses , a good water pump and (very important) the timing is set correctly you should have no problem with overheating in ever day driving. You don't say what you plan to do with the car. If it is to be judged, your options are limited, if all you want to do is drive it, there are lots of aftermarket options to help them run cooler if the engine is not stock. Hope this helps
Terry

[Richard M.]

Re: Help with 396/425

Thanks. Very helpful. I plan to drive and enjoy this one.

[Duke W.]

Re: Help with 396/425

Probably the most common cause of overheating, especially in low speed traffic is insufficient TOTAL IDLE SPARK ADVANCE, which is the sum of initial and full vacuum advance. (Total idle spark advance should be in the range of low twenties to low thirties degrees, and high overlap cam engines need to be in the upper half of this range.)

This can often be caused by a dead or incorrect vacuum advance, and the OE vacuum advance is not always "correct" because typical idle vacuum may not sufficient to pull the vacuum advance to the maximum and hold it there or the vacuum advance is "ported", which means there is no vacuum advance at idle.

Examples of the former are the '63 327/340 and '65 396/425.

Examples of the latter are '63 327/360, '66 427/425, and '67 427/435, and most emission controlled engines beginning in 1966.

Converting a ported vacuum advance to full time usually also requires a new VAC that meets the "Two-Inch Rule" in order to maintain full vacuum advance at idle.

Most of those who have installed a properly selected VAC in place of a poorly selected OE VAC or converted ported to full time advance with a suitable VAC for the particular engine configuration the meets the Two-Inch Rule report elimination or signifiant mitigation of low speed "hot running/overheating" episodes.

The Corvette cooling system was very well engineered with many models using premium parts such at the aluminum radiatore and clutch fan. If these components are performing to proper OE specification, overheating is rare, yet many owners cobble up the cooling system with aftermarket parts hoping to fix their problem.

This is a fool's game.

The way to fix "overheating" is to ensure that all OE cooling system components are performing to OE spec after correcting any issues with the spark advance map.

Duke

[Edward S.]

Re: Help with 396/425

I have never owned a big block car. Am considering a 65 396/425. Will appreciate any advice regarding these motors such whether there are difficult issues to resolve with overheating.

I have had my L78 for 12 years now, after the first year I bought a new DeWitts unit-
not because of overheating - because it had started to have small leaks. The DeWitt
unit runs about a steady 180, never lost a point and has never overheated. I have 2 friends here in Jersey who also have 396's and don't remember either one ever having
a problem. In the 40 years I have been in the hobby most overheating problems I have
ever heard about were 67 - 435's. If that's the only thing your worried about I would
buy the car - best vette I have ever owned. Good luck - if you buy it let's see some pictures. If you need some other info drop me a line.

[Chris H.]

Re: Help with 396/425

Richard, not to worry. I've had a couple 396s over the years and never had any cooling problems. If you wind up having problems, there are plenty of easy solutions out there. Your biggest concern with owning a 396 should be the local gendarmes.

[Carl N.]

Re: Help with 396/425

Richard,

I have been driving a stock L78 on a frequent basis for 40+ years. The first few years I owned it it was my only car. Follow the advise already posted and make sure all the components of the cooling system are in proper operational order and the timing is correct.

About the only thing I can ad to Duke's comments is to be sure the valve lash is correct if you retain the solid lifters. Should the lash be to tight or lose this can cause heating in stop and go traffic.

I drive in Texas where 100 degree days in May thru August are not uncommon. As long as I stay above 30 mph and keep the air flowing it will hold 180 degrees. Stop and go traffic and slow parades in 90 degree + temps will result in 210 to 220. I used to do these but I'v become paranoid and avoid the metroplex traffic and parades as much of fear of the other guy as the higher temps.

All of the 396/427/454 Chevy's I've owned over the past 40+ years will run fine (at or below 200) if everything is in good operating condition. Start changing cams, increasing comperssion, messing with timing and so forth to race them and heat can become a problem.

Be aware that if this car or any other which has sat idle for a long period of time without proper drain and flush of the cooling system may have a build up of sludge in the water jacket around the freeze plugs. IMO no amount of back flushing will remove the build up once it hardens. This build up can restrict coolant flow and result in over heating even when everything else is correct. There are two way to fix it - pop the freeze plugs and dig out what you can with your fingers and screwdrivers ( not what I'd do) or pull the motor - not all that hard, tear it down and have a competent machine shop vat the block and heads with the freeze plugs out. Install new brass freeze plugs and be sure to service the system every 3-5 years and you should be ok for a long long time.

The L78 is a fun car and less expensive to purchase than a '66 or '67 in equal condition.

Carl

[Jim T.]

Re: Help with 396/425

Carl does the 396 have drain plugs on the sides of the block like the SB's? Removing the SB's drain plugs is recommended in my 68's owners manual when draining the coolant system. It does not specify SB's only, and 427's were used. The first time I removed the block drain plugs there was a significant amount of debris that flowed out with the coolant. The spark plug shields have to be removed to have access to the drain plugs which most likely keeps the drain plugs from being removed. A lot of coolant is retained in the block until the drain plugs are removed.

[Carl N.]

Re: Help with 396/425

Jim,

Yes big blocks have the drain plugs. No shields need be removed.

If they have not been changed 9/16 pipe plug on passenger side between dip stick tube in front of starter above oil pan flange, driver side below middle freeze plug next to motor mount. I'll try to attach picture ---

Be very carefull removing these and make sure you have a good socket and take care not to round off the wrench flats - the factory plugs are a mild steel.

Carl