L76 Performance tuning, detonation, timing

[Dennis K.]

Trying to get my 1965 L76 (30-30 cam solid lifter, Holley Carb) engine running at its best. I am witnessing minor detontation problems... and it is also running a little hot. The engine has been restored to original spec, and the distributor is all in at 3 degrees Mg. Valves are also adjusted as per Duke's recommendation. (Thanks Duke)

However, I still get light detonation on midl acceleration (detonation goes away under harder acceleration) and the engine runs a little warm (even at idle or mild cruising speed). Any advice?

I THINK that I probably have more work to do on the distributor advance (perhaps I need to limit the advance)... do I need to use fuel additives? should I swap out my 180 degree thermostat?

Advice greatly appreciated.

Cheers,
Dennis

[Clem Z.]

Re: L76 Performance tuning, detonation, timing

try a higher # power valve so the carb richens up at a higher engine vacuum

[Dennis K.]

Re: L76 Performance tuning, detonation, timing

Hi Clem,

This cam doesn't create a whole lot of vaccuum. I assume that's why Chevy wants the ignition advance all in at 3 degrees... not sure how a power valve will address my problem (engine appears to be running OK from a fuel perspective)??? Can you please elaborate? I am wondering if I am still getting too much advance out of the distributor? I am thinking that I probably need to re-curve the distributor and limit the advance at some point... Has anyone experimented with this? Or fis anyone amiliar with how to setup a distributor for the L76? Also - what thermostats are you guys running? Fuel additives?

thx,
-d

[Clem Z.]

Re: L76 Performance tuning, detonation, timing

a higher # power,85 vs 65 will richen up the fuel curve sooner,at 8.5 inch of vacuum vs 6.5 inches of engine vacuum and the richer mixture at that part throttle may stop your pinging. you said it did not ping at full throttle and that is when your present power valve would be full open. your mechanical advance should not be in full till at least 3000 RPM. anything sooner combined with you vacuum advance could be too much at certain RPMs. you should power time your engine,mark off 36 degrees on your damper and with the vacuum disconnected set the 36 degree mark at the 0 mark on your timing tab.you should shoot for having this happen at about 3000 RPM. you may need to dick with the springs and weights to get this to happen.

[Duke W.]

Re: L76 Performance tuning, detonation, timing

"...and the distributor is all in at 3 degrees Mg."

This statement makes no sense. All the centrifugal advance - 24 degrees should be in at 2350, and the range of initial timing is 10-14, so the WOT advance will top out at 34-38.

How are you setting the timing?

The vacuum can should begin to add advance at 4" Hg. (mercury) and provide the full 16 degrees at 8" Hg.

Part throttle acceleration should reduce vacuum to below 4", which will release all the vacuum advance, but you should run some experiments with a vacuum gage in the cockpit as you drive to see if the detonation occurs at greater than 4" manifold vacuum.

If the centrifugal is too aggressive there are two approaches. First is to limit the rate of advance with stiffer springs. The other is to reduce initial then grind out the slot to allow more centrifugal so the total WOT advance is in the range of 36-38, but below 2350 it will be less.

I would recommend juggling the advance map to find a combination that works before you change the power vavle or use some snake oil fuel additive. Keep the 180 degree thermostat.

Tell us what you mean by "a little warm". What is the temp? How measured? And what are the driving conditions and ambient temperatures.

With a 15 psi cap and a 50/50 glycol mix the boiling point is 265F.

Duke

[Dennis K.]

Re: L76 Performance tuning, detonation, timing

Hi Duke/Clem,

Sorry about the odd response... My vaccuum can is starting to add advance at 3 degrees Mg. However, based on your data - maybe I need to change springs?

The engine is running at 195-200 when I am at freeway cruising speed... and around 210 if I cruise at 75mph or sit in traffic. I am driving in 80 degree weather in Northern California, so ambient temp/bar pressure shouldn't be an issue. ??

I am not overheating (using 50/50 glycol), but I would feel more comfortable if the temp were closer to 180... and the car didn't detonate.

thx,
-d

[Joe L.]

Re: L76 Performance tuning, detonation, timing

Dennis-----

A thermostat change is not going to significantly affect your engine operating temperature. A thermostat only controls an engine's MINIMUM operating temperature. Once that temperature is reached, the thermostat is full open and has no effect, whatsoever, on how much hotter the engine gets.

[Duke W.]

Re: L76 Performance tuning, detonation, timing

What kind of radiator do you have, and how old is it?

I would highly recommend that you verify temperature readings with a IR gun pointed at the the water outlet. Many replacment temp sending units are not properly calibrated for the gage, which will cause erroneous readings.

Any OE small block with a properly functioning "316" radiator, properly functioning OE thermo-modulated fan hub, and a proper ignition advance map should only rarely run over 180 in extreme conditions such as extensive idling in hot weather or triple digit cruising speeds in triple digit temperatures.

Duke

[Joe C.]

Re: L76 Performance tuning, detonation, timing

Dennis:

Follow Duke and Joe's good advice. I would like to emphasize a couple things, and add a few more.

1. Do not change your 180 thermostat. As Joe says, it only controls the low limit of your engine temp. It will absolutely have no effect on an "overtemp" condition.

2. Don't rely on your temp gauge for accurate readings. You should verify engine temp with an IR thermometer. You can always tweak your gauge with a trim resistor if that makes you feel better.

3. I don't recommend changing your "65" power valve. Going to an "85" puts it dangerously close to enrichment at idle (about 10 in-hg.).

4. Make sure your heat riser is free, and working properly. I would wire it wide open. Unless you do a lot of winter driving, this will not hurt driveability.

5. If you rebuilt your distributor with "as original" parts, then you should have a "236" vacuum can. I think that an incorrect vac can is the most likely culprit. If you want a cheaper version of the 236, then Duke can fix you up with the NAPA part number.

My '65 'Vert with L76 was recently restored. Just about everything was replaced with "as original" parts. I rebuilt the engine same as it left the factory (with some trick headwork), and it neither detonates, nor overtemp at cruise, or stop-and-go. It will, however, boil over if I let it. This happens if I idle the engine for anything longer than 50 minutes, without moving, in ambient temps exceeding about 80 degrees. I only did this for test purposes, and will probably never do it again. My feeling is that, even with a brand new Harrison (DeWitt's), Chevrolet did not design in very much excess cooling capacity. Good luck.

Joe

[Dennis K.]

Re: L76 Performance tuning, detonation, timing

Hi guys,

Thanks. These are all great points. In answer to Joe's questions:

1. I suspected that a thermostat change would not do much... interesting though how lowering the thermostat in my 2001 Vette made a difference. (I suppose that this is due to the 2001 computer's ability to dial in exact times when the fans kick in... and excess cooling capacity in the new Vette's cooling system)

2. I have tested engine temp with an IR thermometer. My prior estimates of engine temp are accurate. I am least happy to know that my temp gauge is accurate! ;}

3. Good. will stay with the #65.

4. This is a CA (warm weather) car... so maybe this is what I should do. Is there anything to be gained by leaving it operational (e.g. easier starts in cold weather?... in the event that I ever do drive it on a 45 degree day)

5. Thanks to a prior post by Duke - I think I am running the proper NAPA alternative can.

Joe - have you ever dyno'd your L76? My engine sounds very similar to yours (I too did some trick head work and added roller rockers). I would be curious to know your torque and hp curves... I will take my to a dyno once I resolve these issues...

thx,
Dennis

[Clem Z.]

you do not have to go to #85 there are #75 PV

this is a very easy thing to try compared to finding someone with a distributor machine to play with the curve. about the C-5 engine,from my observations of chassis dyno runs the C-5 make the most HP between 190 to 210 degrees,anything under or over causes a HP loss. heat make HP and if you remove too much heat from the combustion chambers you lose HP. the oil temp is the big factor in HP not water temp. the TB coolant bypass is also a waste of time and money.the older corvettes with a aluminum FI "doghouse" needed this because the TB transferred heat to the "doghouse" causing a HP loss but with the composite intake manifold of the C-5s this is not the case. inlet air temp is what you need to lower,1% gain in HP for every 10 degrees you lower the inlet air temp.

[Joe C.]

Re: L76 Performance tuning, detonation, timing

Dennis:

As long as the heat riser is working OK, and you are willing to keep after it, then leave it alone. If it eventually seizes up from rust, then you will have heating, and right side exhaust corrosion problems.

I have never had my engine dyno tested, but I suspect that I would be very disappointed at the numbers----Chevrolet was very generous (creative) with their horsepower ratings during this period. Additionally, engines were rated at SAE gross horsepower, which means that the engine was run on a test stand with open manifolds, and no parasitic losses such as P/S, alternator, A/C compressor, etc. Power was measured at the flywheel.
I am satisfied that my mill is producing the output that was normal in "the day". The engine runs excellent, has little low end torque below approx 3K as expected with the 30-30, sounds great, and SCREAMS above 3.5K. I doubt if she will shut down a new Z06, but I love her for the "visceral" sensation that she imparts.
I would be interested in knowing your dyno results when you get them. Don't be too surprised if they prove to be 20-30 percent less than 365.

Joe

[Duke W.]

Re: L76 Performance tuning, detonation, timing

Back in '63 S&S Research - the outfit that did the prep for Alan Green's #7 AP '63 Z06 driven by Jerry Grant - told my that a good AP 'Vette, which would mean a 360 HP FI engine would make about 250 HP at the rear wheels. Don't know what conditions they corrected to back then but it would probably have been standard sea level. Standard "SAE condtions" used today would be a couple of percent less.

Back is '63 production cars were virtually showroom stock. Essentially no modifications were allowed to the engines other than open exhaust - no headers, no "metal removal from the head". About the only allowed tuning parameters were the fuel and igntion maps.

A rebuilt SHP/FI engine with a 30-30 or LT-1 cam with a nice three angle valve job, pocket ported port matched heads, through an average set of OE type mufflers and with the thermomodulated fan would probably make a little over 250 HP at the rear wheels. If it was a warm day and the engine heated up enought to tighten up the fan clutch it would cost some power.

If I could get a vintage SHP/FI 327 to make 275 RWHP using blueprinting/tuning with an OE solid lifter cam, I would do backflips.

Like Joe said, these engines are not about brute power or torque. A C5 LS1 automatic will smoke most of them in the quarter mile. The LS1/6 are great modern engines, but just don't have that nervous, high strung, visceral feel and mechanical cacophony of a SHP mechanical lifter 327.

There's much more to life than 0-60 and quarter mile times!

Duke