69 L-71 427 Compression Ratio Question
I'm restoring a 69 L-71. I am doing a complete rebuild on the engine. I cc'd the #1 cylinder, and came up with only 9.52 CR. The pistons are Speed Pro L2268F's, which according to the piston specs should yield 11.00- 1 compression with the 840 heads, which are 106.8 cc's. The piston specs say the the piston dome should take up 36.3cc's, which would give about 11.00-1. However the pistons only cc at 21.87. My question is, are there better pistons to use, which will yield 11.00-1? Mark
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Re: 69 L-71 427 Compression Ratio Question
Mark------
If it were me and I intended to actually drive the car, I'd be happy with the 9.52:1 compression ratio. That's about the absolute highest I would even consider using for a big block.
Think of it this way: if you build it at a true 11.0:1 and you end up with detonation that you can't control with any tuning tricks, then what? Tear it down and lower the compression? Live with a miserable to drive car?In Appreciation of John Hinckley -
Re: 69 L-71 427 Compression Ratio Question
Joe,
I'm well aware of the problems of going above 9.5-1 with cast iron heads on todays fuel.However with the correct additives, the engine can and will run at 11.00-1. If a person builds a detuned L-71, it's not really a L-71 is it? I want the engine correct, just like the rest of the car. So I'm still looking for an answer to my question as to is there a different piston that will yield 11.00-1?Comment
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Re: 69 L-71 427 Compression Ratio Question
Mark------
The Speed Pro L2268F pistons should have a dome volume of 36.3 cc. If yours don't, I see these possibilities:
1) The pistons are mid-identified and are not actually L2268F;
2) Someone has modified the pistons by shaving the dome;
3) You're not measuring the dome volume correctly.In Appreciation of John HinckleyComment
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Re: 69 L-71 427 Compression Ratio Question
I basically agree with Joe, but I think you can get away with 10.25 (with the OE cam) if you have 93 PON fuel available.
Most L-71s left Tonawanda with CR closer to 10.5 than 11. GM's advertised CRs were optimistic. John McRae had a problem with too high CR on a L-71 that had been decked and solved the problem with thick copper head gasket. He also measured the dome volume of the Speed Pro OE replacement pistons and it came in lower than spec, so you should talk to him.
DukeComment
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Re: 69 L-71 427 Compression Ratio Question
Mark,
Likewise, I would build the L-71 to its original specs as you intend to do. Since the L2268F is a replacement piston do you have an original piston to compare the dome height? As Joe has suggested, it is possible that someone has modified the dome and that would give a higher cc volume as well as influence the CR. I do not have a piston out of a block to tell you what that dome height should be. The next piston selection takes you to the 12.5 compression ratio as used in the H.D. 427 engine. You might consider using a fiber composition head gasket similar to those used with the aluminum head engines rather than a steel shim gasket as used originally with this engine.
RayComment
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1969 Riverside Gold Coupe, L71, 14,000 miles. Top Flight, 2 Star Bowtie.Comment
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Re: 69 L-71 427 Compression Ratio Question
All-----
As I've said before, GM builds current crate big block engines with cast iron heads with a compression ratio of 8.25:1 and specifies 91 minimum octane. Most crate big blocks with aluminum heads are built with a compression ratio of 9.6:1 and specify 91 octane minimum. What do you suppose that GM knows that we don't know?In Appreciation of John HinckleyComment
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1969 Riverside Gold Coupe, L71, 14,000 miles. Top Flight, 2 Star Bowtie.Comment
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Re: 69 L-71 427 Compression Ratio Question
If your block has not been decked and you used the much thicker head gaskets as Duke said , does your chamber volume not increase lowering your compression ratio?Comment
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Re: 69 L-71 427 Compression Ratio Question
The piston was checked exactly 1" down in the bore and cc'd as usual, then subtracted from the volume of a 1" cylinder of the same diameter. So I know the piston dome is not what the manufacturer claims. By the naked eye, the pistons look untouched. Now the engine had a composite gasket with a compressed thickness of .042. I did not know that the L71 used a steel shim gasket. That will change things a bit. I'll recalculate with the shim gasket, and see where it winds up. I plan on using the stock L71 cam, so I'll need all the compression I can get.Comment
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Re: 69 L-71 427 Compression Ratio Question
Mark------
All Corvette big blocks except L-89, L-88, ZL-1, and LS-6 used steel shim type head gaskets.
The exact same cam used for 1969 L-71 was also used for 1971 LS-6. The LS-6 had 9.0:1 compression ratio. In my opinion, the LS-6 is the best sounding and best running of all Corvette mechanical lifter big blocks.In Appreciation of John HinckleyComment
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Re: 69 L-71 427 Compression Ratio Question
I recently restored a 69 L71 Coupe and used the same pistons you mentioned. Speed Pro advertised a .100 dome on these pistons. I did not check the pistons , but I used the steel gaskets to help insure an OEM compression ratio. I have driven the car about 2500 miles and use Marathon no lead premium. Timing is set to stock TI specs using the K and K ignition board. To date no pre ignition and other similar problems. I have taken the engine only to between 5000 and 5500 RPMS. I recently took the car to a local drag strip test and tune to play. I put in ten gallons of 100 octane low lead gas from a local small plane airport. The engine liked that gas very much and it was pretty easy to get into the upper 13's coming off the line at 2500 with street tires and shifting at 5500. Hope this helps CurtComment
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Re: 69 L-71 427 Compression Ratio Question
Keep in mind that the '71 LS-6 was designed to operate on 91 RON unleaded fuel, which is essentially the same in terms of detonation resistance as today's 87 PON.
I think GM's 91 PON minimum fuel requirement for the moderate compression ratio big block crate engines is very conservative - probably for warranty purposes.
DukeComment
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