L71 L72 427 Running on Pump Gas

[Ted S.]

Duke and I had some disagreement on the ability to run a "factory configuration" L71 or L72 427 on currently available pump gas (i.e. 92 - 93 octane). That means "replacement" pistons (ie. advertised 11:1) and a GM 3863143 (or available blueprint version from Comp, Crane, etc.).

I got a little pointed with Duke so I'd like to extend an apology to him for that. So let's put a poll out see what comes back.

If you know what the true measured static compression ratio of your engine is that would be beneficial. Also if you know the quench on your engine I'd like to know that also. Also please state if it is an L71 or L72.

[Ted S.]

Re: L71 L72 427 Running on Pump Gas

All if you could give some specifics about the current build of you L71 or L72 engine and the octane required that would help. Thanks

[John C.]

Re: L71 L72 427 Running on Pump Gas

My 68 L71 with an unopened engine from the factory ran just fine on the 91 octane gas we get here in AZ. It would occasionally diesel on at shut down.

I had the cam sprocket strip a few months back and I an now having it rebuilt as close to stock as possible using 11 to 1 pistons and a original blueprint cam. It will be done in the next week or two. It will be interesting to see the results.

John

[Erik S.]

Re: L71 L72 427 Running on Pump Gas

my 68 L71 runs perfect on (European) 91 octane (plus some occasional lead additive).

Erik

[Duke W.]

Re: L71 L72 427 Running on Pump Gas

My 68 L71 with an unopened engine from the factory ran just fine on the 91 octane gas we get here in AZ. It would occasionally diesel on at shut down.

I had the cam sprocket strip a few months back and I an now having it rebuilt as close to stock as possible using 11 to 1 pistons and a original blueprint cam. It will be done in the next week or two. It will be interesting to see the results.

John

Very likely the "dieseling" will cease if you convert the ported vacuum advance to full time. You also might need to change the VAC. I'm not sure of the part no./specs used for the '68 L-71, but change it to a NAPA VC-1765 or equivalent, which is 0@6", 16@12".

Since this was an untouched OE L-71, what is the thickness of the original head gaskets and what is the thickness of the replacement gasket?

If the block has not yet been disassembled, measure the deck clearance, and this along with the OE gasket thickness can be used to compute the actual, true "as-built" SCR.

Also, upon assembly measure the deck clerance and compute the SCR for the refreshed engine.

I collect this type of data in order to build cases for the maximum SCR that various OE configurations can tolerate with today's availablel pump premium, but, unfortunately, most "engine builders" ignore this critical detail - measuring deck clearance, so the true SCR cannot be calculated.

I have a good body of evidence for SB configurations, but my BB data is spoty.

Duke

[Joe R.]

Re: L71 L72 427 Running on Pump Gas

Duke,

Why measure deck clearance before and after? Is that to make sure that the block has not been decked? How is deck clearance measured?

Joe

[John M.]

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

[Ted S.]

Re: L71 L72 427 Running on Pump Gas

John, Can you run the numbers using your 11.39 SCR and use .039 with a 4.370 bore (std felpro gasket) for your compressed head gasket thickness? Also have you ever blended gas to see what octane it takes to run it on? Ted

[Duke W.]

Re: L71 L72 427 Running on Pump Gas

Measuring deck clearance before block teardown allows you to compute the existing CR and CR variation between the eight cylinders. It also reveals any side to side difference in deck height, and I've seen up to.015" difference which is over half a compression point!

If the engine didn't detonate before teardown, you can reassemble it with the same CR, and equalize them to a maximum variation of 0.1 with a little chamber grinding on the highest cylinders before the heads are installed. Side to side deck height difference of more than .005" can be compensated with different gasket thicknesses, or, if the left deck is high, it can be cut down with no judging consequences.

If the engine did detonate, you can reduce the CR with a thicker gasket and equalize as required.

Deck clearance can be measured with a machinist's bar and a couple of feeler gages.

I think John McRae has a good photo of the setup (along with measuring chamber volumes, which is also required), and we cowrote an article on the subject, which was published in the New England chapter newletter earlier this year, and a re-edited version will be in the next issue of the Southern California chapter newsletter.

Duke

Duke,

Why measure deck clearance before and after? Is that to make sure that the block has not been decked? How is deck clearance measured?

Joe

[John C.]

Re: L71 L72 427 Running on Pump Gas

Duke

The engine is already apart so I'm not quite sure what measurements were taken before disassembly.

He did tell me the deck height on one side was .005 higher than the other. It was the stamp pad side so it can't be corrected with machining.

I will post the number I get when the engine is finished.

John

[John M.]

Re: L71 L72 427 Running on Pump Gas

Ted,
I think you meant the one at 11.59. Using your numbers for head gasket dimensions I get SCR of 11.47. Using the 0.070" thickness I get 10.61.
The car runs great on a 50-50 mix of 93 and 100 Low lead AV Gas.
It also runs great on a 8 gallons of the 93 and 2 gallons 110 Sunoco. I actually used to caryy 2 gallons of that stuff behind the seat but as you can imagine it's a real PIA. I never tried to fine tune the mix more than that. I've tried all kinds of timing thingsand it has full manifold vacuum advance.
John

[Ted S.]

Re: L71 L72 427 Running on Pump Gas

Actually I meant the 11.32 (i.e. the least SCR). I figured you would remove some material from the chamber to get the others to match. Also this assumes the variation is due to head cc's.

[Michael H.]

Re: L71 L72 427 Running on Pump Gas

Actually I meant the 11.32 (i.e. the least SCR). I figured you would remove some material from the chamber to get the others to match. Also this assumes the variation is due to head cc's.

I still don't trust the calculations found in the piston mfg's paperwork. If it were mine, I'd be using a CC burrett on an assembled engine and getting the real volumes/CR's.
There are too many variables in piston dome shape and the valve clearance machining to be consistant enough to conclude there is such a thing as an accurate TDC volume. It's not that difficult to do.

[John M.]

Re: L71 L72 427 Running on Pump Gas

Ted,
OK SCR from 11.32 to 11.202 with your gasket.

Michael,
I guess I could do one piston. Put the piston exactly one inch down in the bore, fill thru burette, run the calculation. How do I keep from losing fluid down past the piston and rings?
Thanks,
John

[Michael H.]

Re: L71 L72 427 Running on Pump Gas

Ted,
OK SCR from 11.32 to 11.202 with your gasket.

Michael,
I guess I could do one piston. Put the piston exactly one inch down in the bore, fill thru burette, run the calculation. How do I keep from losing fluid down past the piston and rings?
Thanks,
John

We used white lube to fill the area above the top ring. If I remember correctly, that will reduce the total volume by about .5 CC??
(if you give us the dimension from the piston top and top of the top ring, I can give you the exact area/volume in CC's)
I would set the piston down about 1/4" using a depth mike. That way you have exact numbers.

I always pre assembled the engine with the components that I was going to use, including the head gasket.
With the piston at TDC and the engine rotated about 80* on the engine stand, the chamber can be filled through the spark plug hole.
From the calculation of total chamber volume to cyl displacement, in CC's, the exact true C/R can be established. No guessing.

Piston mfg's numbers for piston dome volume is usually off by enough to toss your calculations out the window. At least that's what we've found on production pistons years ago. (maybe more accurate today??)
Expensive custom pistons were always a lot closer to the real advertised numbers.