L71 L72 427 Running on Pump Gas

[Duke W.]

Re: L71 L72 427 Running on Pump Gas

4.28" x 3.76"
zero deck clearance (nominal OE is .020", NHRA mininum is .008")
.070" x 4.35" gasket (bore hole dimension is an estimate)
106.9 cc head chamber (NHRA minimum - I don't know the OE nominal or your actuals)
30 cc net dome volume

The cgnetwork calculator yields 10.436:1 - everything the same with .035" gasket thickness is 11.377.

I didn't see that you listed the head chamber volumes you measured. What were they?

I suggest that you measure one more piston dome. If the results are consistent with the first one you measured, I would go with your measurements rather than the 35.5 cc "spec".

Duke

[John M.]

Re: L71 L72 427 Running on Pump Gas

Thanks Duke,
The chamber volumes vary from 110.8 to 112.4. Most are 110.8 to 111.4 (1 at 112.2, 1 at 112.4) With this 30 CC dome it seems to me that I should be close to OK as is. I'll measure another piston. Why is this never simple?
John

[Michael H.]

Re: L71 L72 427 Running on Pump Gas

Why is this never simple?
John

It is simple if you use the system I talked about earlier. Install the crankshaft, one rod and one piston with only a top ring. Pack white lube into the area above the ring to seal it. Install a used head gasket and the cyl head.
Roll the engine on it's side and fill the chamber with fluid from your CC burrett. Do a LITTLE math and ya got it.

That eliminates all the chance for errors or incorrect information from vendors of pistons and even the published volumes from GM.

Like I mentioned way earlier... you absolutely can not trust the dome volume numbers from the piston mfg's.

[Duke W.]

Re: L71 L72 427 Running on Pump Gas

Thanks Duke,
The chamber volumes vary from 110.8 to 112.4. Most are 110.8 to 111.4 (1 at 112.2, 1 at 112.4) With this 30 CC dome it seems to me that I should be close to OK as is. I'll measure another piston. Why is this never simple?
John

Okay, so using your 112.4/110.8 chamber volumes I get 10.060/9.914, so IMHO the .070" gasket will solve the detonation problem, and you will probably be able to run a more aggressive than OE spark advance map with pump premium, which will increase low end torque and throttle response. You could even consider a slightly thinner gasket to yield an average 10.25 and grind the smallest chambers to reduce the spread.

This is a good example of why real measurements on each engine are critical rather than relying on advertised specifications.

Run the numbers on the cgnetwork calculator to check my calculations.

Duke

[Joe C.]

Re: L71 L72 427 Running on Pump Gas

Ted,
I was one of the guys who's engine will not run on 93 octane. The engine is on a stand now with heads off. I've measured deck clearance and CC'd one head. The CR values range from a low of 11.32 to a high of 11.59.
This is with a deck clearance that is basically zero and a head gasket compressed thickness of 0.035" My plan is to have Cometic make me a 0.070" head gasket which will bring the CR down to about 10.5. I may do a little chamber grinding to even up the cylinders. Pistons are standard 0.030 over. Engine was professionally built 10 years ago (15-20k miles) and looks great inside.
John McRae 30025

If you are running zero deck with a .035 gasket, then you're running .035 quench, which is where I'd want to be. Some report running as low as .025 using cast pistons, tight clearances, and max RPM's of say, 6000, with strong rods. Anything less than the ideal quench of (about) .040 has no further effect, so I wouldn't be looking to tighten more than the current .035.

I'm sure that you are aware of the fact, that by opening your engine's quench to zero + .070 = .070, you will make the engine much more prone to detonation (hypothetically, if all else were equal, including the SCR).

If it were me, I'd mill some of the dome off of the pistons, have the assembly re-balanced, cc the chambers to the largest size of the existing 8, and use another .035 gasket. I'd be looking for an SCR of 11.0 MAX, with 10.75 ideal. With .035 - .040 quench, the engine should not detonate @ 11.0, but with .070 quench, it probably will.

So, you're saying: "who cares, we're only talking about .50 SCR difference". Well, that's a few foot-pounds, and 5-10 horsepower that I'd pay the extra hundred or so dollars for.

[Ted S.]

Re: L71 L72 427 Running on Pump Gas

Duke,

Some choices are

.064 @ 4.38 dia
.062 @ 4.38 dia
.060 @ 4.39 dia
.053 @ 4.38 dia

Some are copper the .053 is a felpro composite.

Ted

[John M.]

Re: L71 L72 427 Running on Pump Gas

Duke, Joe, Michael,
I ran the cr calculator numbers and confirm that they agree with yours. The thing that bugs me is the dome volume. The piston has the number right on it, has not been machined and measures 5cc different than advertised. I hope I made the mistake (twice)
I'm trying to avoid a lot of work here, like machining pistons rebalancing etc and am now on information overload. I'm going to remeasure the dome volume and then summarize again.
Something's amiss; this engine has this low quench height and less than 11 to 1 SCR as is. I must have made a mistake somewhere.
John

[Joe C.]

Re: L71 L72 427 Running on Pump Gas

John,

From an earlier post, it looks to me like the engine is right "on the edge" of detonation. In fact, only 1, 2, 3 or 4 cylinders might be detonating.

You can handle this in a number of ways, but I would not do so by ADDING quench, which is self defeating.

At this point in time, your actual, calculated SCR is rather meaningless, since you are right at the "ragged edge".

One can ask questions like:
" Did you check that all of the compression heights are exactly the same on all 8 pistons?" Or ensure that your piston-to-deck clearances are measured accurately by taking the average of the distances at the upper and lower thrust positions (piston 0* and 180* circumferential).
"Does your SCR calculator factor in ring land volume? Head gasket bore diameter?"

I would carefully equalize all 8 chambers to the size of the largest. Verify all 8 piston-to-deck clearances, and verify all 8 dome volumes. If the engine still detonates, I would tighten the intake valve lash to the absolute minimum safe limit. Richen the idle mixture by backing out the mixture screws slightly. Check the primary float level, and raise it 1/16" above spec, if needed. Block the exhaust crossover port in the intake manifold. Change to a 170 degree thermostat. I assume that your heat riser valve is disabled fully open. I assume that you are using a full manifold vac signal to your distributor advance diaphragm. As a last resort, as I assume that the detonation occurs during part throttle operation, I would limit vacuum advance travel by a couple degrees, while maintaining total WOT advance @ 10 + 26

[John M.]

Re: L71 L72 427 Running on Pump Gas

Joe,
Before taking the engine out of the car I did almost all of the things you suggest; valves tightly lashed, mixture changes, timing changes, full manifold vacuum on a limited travel advance can, 170 therm., different distributor weights, springs, swapped a Pertonix equipped distributor for the TI and I don't know what else.

I have all the BB numbers now, I measured the dome volume in 2 cylinders two times and the spread in my measurements was from 29.86 to 30.1 so I think that's it. The dome height is 0.266". The gasket thickness is 0.035" with a diameter of 3.375". So here's a tabulation derived from the calculator. Neg deck clearance is above the deck. I assumed a 3.76 stroke, the rest I measured.

Cylinder, Deck clear. CC Chamber, CC Piston Dome, SCR
1 -0.001 110.8 (30.0) 10.94
3 +0.003 110.8 (30.0) 10.82
5 0.000 112.4 29.86 10.74
7 -0.002 112.3 (30.0) 10.80
2 -0.001 111.0 (30.0) 10.92
4 -0.004 110.6 (30.0) 11.04
6 -0.003 111.4 (30.0) 10.93
8 +0.001 111.2 30.1 10.84

I will even up the combustion chambers and take off the edges but do not want to disassemble, machine and rebalance, etc.
John

[Ted S.]

Re: L71 L72 427 Running on Pump Gas

John, Do you know if the cam is installed straight up or by any chance was it installed in an advanced position (i.e. +2* or +4*)? Ted

[Joe C.]

Re: L71 L72 427 Running on Pump Gas

Joe,
Before taking the engine out of the car I did almost all of the things you suggest; valves tightly lashed, mixture changes, timing changes, full manifold vacuum on a limited travel advance can, 170 therm., different distributor weights, springs, swapped a Pertonix equipped distributor for the TI and I don't know what else.

I have all the BB numbers now, I measured the dome volume in 2 cylinders two times and the spread in my measurements was from 29.86 to 30.1 so I think that's it. The dome height is 0.266". The gasket thickness is 0.035" with a diameter of 3.375". So here's a tabulation derived from the calculator. Neg deck clearance is above the deck. I assumed a 3.76 stroke, the rest I measured.

Cylinder, Deck clear. CC Chamber, CC Piston Dome, SCR
1 -0.001 110.8 (30.0) 10.94
3 +0.003 110.8 (30.0) 10.82
5 0.000 112.4 29.86 10.74
7 -0.002 112.3 (30.0) 10.80
2 -0.001 111.0 (30.0) 10.92
4 -0.004 110.6 (30.0) 11.04
6 -0.003 111.4 (30.0) 10.93
8 +0.001 111.2 30.1 10.84

I will even up the combustion chambers and take off the edges but do not want to disassemble, machine and rebalance, etc.
John

John,

Does your car have sidepipes?

I was convinced that my engine was detonating, until I realized that the inner tubing in my GM sidepipes was resonating, and sounded like detonation!

I also quieted a buzzing shifter handle, which contributed to the problem.
You may have already been there, but if not, I'd look at those items.

Well, you can cure the problem by increasing octane level, so I suppose that it can't be either of these, then.

If you don't drive your car in "cold" weather, then I'd block the exhaust crossover port in the intake.

Yes, equalize the chambers as best you can, but I would not, immediately jump to such a drastic thickness change in the gasket.

If you'd rather be done with it, then go ahead and use the .070 gasket. If it were me, I'd increase thickness in steps, until issue resolved. I would take some more vac advance out before I went to such a thick gasket.

[John M.]

Re: L71 L72 427 Running on Pump Gas

Ted, Joe,
The cam is a Crane copy of the original L71 installed straight up.
Yes, the car has sidepipes and I know the sound you're talking about but it's not that one. It pings under low throttle (low) load; I'm always down shifting to avoid it. I've got time and equipment to do chamber grinding and will try to choose a thinner gasket after I do the work and remeasure. Maybe a little of both will do the trick, little thicker gasket and increased chamber size. I hope I get it right the first time. I should add that I've been dealing with this for 10 years; used to get 94 octane and mix 110, then 93 plus Avgas but it's a pain. Also, from my experience most of those octane boosters are useless (and expensive).
John

[Ted S.]

Re: L71 L72 427 Running on Pump Gas

John, was the cam degreed when it was installed? Ted

[John M.]

Re: L71 L72 427 Running on Pump Gas

Ted,
No I installed it, 10 years ago and at the time did not have the equipment to do this.
John

[John M.]

Re: L71 L72 427 Running on Pump Gas

Regarding the pinging problem, here's what I decided to do. I'm going to use an 0.062" head gasket, switch heads from one side to the other to match volumes up closer on 5 and 4, gring the chambers a bit, take off sharp edges and put it back together. It won't be on the road til June or so but I will report then.
I learned a lot here and thanks to all who responded. The dome volume thing is the most interesting; thanks to Michael for encouraging me to actually measure it.
Best regards,
John McRae 30025