L78 Specifications

[Joel T.]

Hi;

Can I impose upon someone to post the tune up specifications for the 1965 L78 motor?? I cannot find this data in any printed material I have... Most interested in the initial timing.

Thanks,

Joel

[Wayne M.]

Re: L78 Specifications

Hi;

Can I impose upon someone to post the tune up specifications for the 1965 L78 motor?? I cannot find this data in any printed material I have... Most interested in the initial timing.

Thanks,

Joel

Joel -- the '65 AIM has some info in the K-66 section (Sheet 4) where it calls for 8 degrees (+/- 1 degree) at 800 RPM with vac line disconnected (and plugged). However, the interim '65 shop manual shows somewhat different numbers for the 425 hp version (10 deg nominal, 8-14 range).

[Joel T.]

Re: L78 Specifications

Wayne;

Thanks... I have the '65 supplement but not that extra booklet on the 396.. Right now the motor has about 400 miles on it since the build, running a 10.7 CR with custom pistons.. It will run a water temp of about 180-190 degrees and not complain. It was a nice warm day here today in NJ so I took it out for about a 100 mile continued break in run. Cruising around 45-55 MPH (varied speeds per Duke) it stayed nice and cool with basically no knock. Once I ran it up a bit, on the highway, 65-72 MPH the temp went up to about 210-220 degrees and it knocked like a bandit...

It's time to start digging in and find out how the guys in the shop set her up... It also has a "new" TI system from Dave Fiedler so I need to assume at this point that the advance curve and vacuum can are correct.

Thanks,

Joel

[Duke W.]

Re: L78 Specifications

Never assume anything! In a previous line of work assuming things were what you thought they were supposed to be could leave a lot of dead bodies.

Also note that in many previous posts I have said that the OE 15.5" VAC is not suitable to the L-78.

A CR of 10.7 is beyond my recommended limit for a small block if you want to run pump gas and definitely too much for a big block. I'd keep the SHP versions to about 10.25,which should be doable with the OE pistons and a thicker than OE head gasket.

Duke

[Bill M.]

Re: L78 Specifications

from Corvette News Vol. 8, no. 3

initial timing: 10 degrees
centrifugal: 0 at 1000 engine rpm
15 at 1800
28 at 4600
vacuum: 0 at 8 in. Hg.
15 at 15.5 in. Hg.

[Joel T.]

Re: L78 Specifications

Duke;

Thanks as always. I will start digging in there today and let you know what I find. The CR is what it is going to be at this point so I will need to eliminate the knock based upon fuel additives and timing adjustments.. I have extra B22 and B28 cans in the garage to work with as well as some octane booster.. the stuff that Clem mentioned a couple of times... just need to figure out how much to use per tank.

Joel

[Joel T.]

Re: L78 Specifications

Bill....

Thanks!!

Joel

[Duke W.]

Re: L78 Specifications

The B22 is what you should install. Idle vacuum at 900 should be about 14" and the B22 is pulled to the limit at 12". The OE 15.5" VAC is stamped "201 15". You should also check that the centrifugal curve meets OE specs, and has not been made more aggressive with light springs.

If the engine will run detonation free with the initial timing at 6, then keep it there and don't mess with octane boosters or high octane fuel.

All this assumes it has an OE camshaft or exact replacement.

Based on reports from other L-78 owners, I'm pretty sure that L-78 has full time vacuum advance, but you should check to be sure. Total idle timing should be the sum of initial plus 16 with the B22 at 900 idle speed.

Duke