Questionable L-72 and L-71 CR and other specifications

[Samuel J.]

thatreplacementQUESTION #1QUESTION #2: What am I missing here?? In other words, how did the CR increase, and how did the compression height increase?


Also, the stated weights for the piston and connecting rods are equally confounding to me. For example, the L-72 piston (GM#3888342) is listed at 25.46oz in 1966, while the seemingly identical L-71 piston (same part, GM#3888342) is listed at 24.67oz. in both 1967 and 1968 Vehicle Info Kits. The L-72 connecting rod is listed at 27.84oz, yet the seemingly identical rod for the L-71 is listed at 24.67oz in 1967 and then magically back to 27.84oz again in 1968!!

QUESTION #3: Are the pistons, connecting rods, and cranks for the L-72 actually different from the L-71 despite having the same GM part numbers and dimensions??


Finally, I've read many times that when plotting horsepower and torque curves, they necessarily cross on the graph at exactly 5252 RPMs.

QUESTION #4:Why do the dyno graphs in the Vehicle Info Kits show the curves crossing at what appears to be only 2,600 RPMs?

Thanks in advance!!

Sam

[Joe R.]

Re: Questionable L-72 and L-71 CR and other specifications

Hi Sam,

I don't have answers to your questions, but would do have some newsletter articles that may help you on this page. Look in the 2012 time frame. Both have Duke Williams as the author.

http://www.stlouisncrs.org/news.htm

I would recommend contacting him for help with your rebuild. What are your goals for the engine? How far along in the rebuild are you? What parts do you plan to use?

Joe

[Duke W.]

Re: Questionable L-72 and L-71 CR and other specifications

Documents have errors, and there are production variations, so you have to take measurements on each specific engine. John McRae is a very meticulous engineer, and I would take his measurements to the bank if they differ from published specs. We co-authored a paper on CR management that I have attached. You start by measuring deck clearance of all eight cylinders prior to disassembling the short block. I recommend a max, measured CR of 10.25 for SHP big blocks, and the rest of the engine is good as is, including the OE design cam, however it may be cheaper to buy new aftermarket rods than investing in inspection and rework of the OE rods. You can probably get the CR down to 10.25 or a little below with the OE replacement forged pistons from Federal Mogul, but you should also look at what's available in the Keith Black hypereutectic line. For sure the latter will be less expensive. If you want "more power" massage the heads. I believe the production stell shim gasket is .028" thick.

[Samuel J.]

Re: Questionable L-72 and L-71 CR and other specifications

Thanks Joe and Duke! I'm appreciative and very familiar with the articles cited as well as the recommendations on CR, fuel, and replacement parts. The OE steel shim gasket is reportedly .021" compressed.

Joe, as far as my intentions with my engine, I'm trying to return it, as close as I reasonably can, to its original glory in terms of replacement components (OE spec cam, carb, etc.), as well as the 425 advertised gross HP output and 11:1 compression ratio, and I plan on only driving it 1,000 to 1,500 miles per year. Larger valves were already installed in the prior rebuild, but the heads are being massaged extensively to correct some shoddy valve seat cutting. Although it's relatively expensive, I plan on using Sunoco 260 GTX because it's as close as I can get to the original Sunoco 260 that the L-72 was designed for, with the added benefit of being lead-free and ethanol-free (there is no ethanol-free gas near me). I know that the engine has only had one prior rebuild (done in 2006, 8 years before I bought the car) and I am now discovering the shop's workmanship was mediocre at best.

Getting back to my original topic, has anybody cc'd a piston dome, especially an original L72/L71 piston, and if so was it significantly different from the published spec?

Perhaps when time allows, Joe Lucia or a SHP aficionado can chime in on my L-72/L-71 part numbers questions? Are those pistons identical?

Duke, I was secretly hoping you would have a simple explanation for the Corvette dyno graphs showing torque and HP crossing at ~2,800 rather than 5,252 RPMs.

[Duke W.]

Re: Questionable L-72 and L-71 CR and other specifications

Duke, I was secretly hoping you would have a simple explanation for the Corvette dyno graphs showing torque and HP crossing at ~2,800 rather than 5,252 RPMs.

Horsepower, which is work per unit time was defined by James Watt as 33,000 lb-ft per minute. Work is a force acting through a distance, and torque is a force acting through a circumference - one engine revolution, so work and torque are the same thing. Do the math and unit conversions and horsepower is torque in lb-ft times RPM divided by 5252. So torque in lb-ft and horsepower are the same numeric value at 5252 RPM, and if the numeric scales for both are the same the curves will cross at 5252 RPM. If the scales are not the same the crossover point is meaningless, but if you look at the value of each on their respective scales, they will be the same.

[Samuel J.]

Re: Questionable L-72 and L-71 CR and other specifications

Ahh! Well, I just found a graph from Corvette News (see attachment) that is more legible than the fuzzy one in the Vehicle Info Kit, and just as you said, the HP scale on the left is not the same as the torque scale on the right side--in fact, it's exactly 1/2, so that explains why the curves cross at 2626 RPMs rather than 5252.

Thanks for solving that mystery, Duke!

[Joe L.]

Re: Questionable L-72 and L-71 CR and other specifications

Thanks Joe and Duke! I'm appreciative and very familiar with the articles cited as well as the recommendations on CR, fuel, and replacement parts. The OE steel shim gasket is reportedly .021" compressed.

Joe, as far as my intentions with my engine, I'm trying to return it, as close as I reasonably can, to its original glory in terms of replacement components (OE spec cam, carb, etc.), as well as the 425 advertised gross HP output and 11:1 compression ratio, and I plan on only driving it 1,000 to 1,500 miles per year. Larger valves were already installed in the prior rebuild, but the heads are being massaged extensively to correct some shoddy valve seat cutting. Although it's relatively expensive, I plan on using Sunoco 260 GTX because it's as close as I can get to the original Sunoco 260 that the L-72 was designed for, with the added benefit of being lead-free and ethanol-free (there is no ethanol-free gas near me). I know that the engine has only had one prior rebuild (done in 2006, 8 years before I bought the car) and I am now discovering the shop's workmanship was mediocre at best.

Getting back to my original topic, has anybody cc'd a piston dome, especially an original L72/L71 piston, and if so was it significantly different from the published spec?

Perhaps when time allows, Joe Lucia or a SHP aficionado can chime in on my L-72/L-71 part numbers questions? Are those pistons identical?

Duke, I was secretly hoping you would have a simple explanation for the Corvette dyno graphs showing torque and HP crossing at ~2,800 rather than 5,252 RPMs.

Samuel------


Yes, the part numbers for the L-72 and L-71 pistons were the same---GM #3888342. Are the pistons the same? Well, given that they have the same part number I would expect them to be identical. However, sometimes GM revises a part and does not change the part number, especially if there is no change in the functionality of the revised part. I have some information that the dome configuration of the 3878231 piston did change between 1966 and 1967 but I do not know if it's accurate. My hunch is that it's not accurate and that the pistons were identical.

By the way, if you are using AMA specs for some of your information, I caution you not to rely on those. As I've said many times before, while those specs SEEM to be authoritative, I have found many errors in them. As a result, I consider them suspect.

[Samuel J.]

Re: Questionable L-72 and L-71 CR and other specifications

Thanks for your insights, Joe! To get closer to solving this mystery, I am going to have one of my original, standard bore L-72 pistons cc'd by a well-known piston manufacturer, and I will share the results here.

[By the way, I believe you'll find that the #3878231 piston you mentioned is for the 396 SHP engine used in '65. #3888342 is for the '66 to '68 SHP engines, and may have been used again in '69.]

Sam

[Joe L.]

Re: Questionable L-72 and L-71 CR and other specifications

Thanks for your insights, Joe! To get closer to solving this mystery, I am going to have one of my original, standard bore L-72 pistons cc'd by a well-known piston manufacturer, and I will share the results here.

[By the way, I believe you'll find that the #3878231 piston you mentioned is for the 396 SHP engine used in '65. #3888342 is for the '66 to '68 SHP engines, and may have been used again in '69.]

Sam

Sam------


Yes, I mis-spoke when I said that the part number was 3878231. I was also dealing with a 396 SHP question and I got the numbers crossed. I've corrected it in my previous post.

[Samuel J.]

Re: Questionable L-72 and L-71 CR and other specifications

Joe, I'm consistently amazed by your ability to make sense of the myriad parts and their respective numbers. Thanks again for your help!

[Samuel J.]

Re: Questionable L-72 and L-71 CR and other specifications

UPDATE: I've sent off my original L-72 piston to have the dome volume precisely measured. If it comes back around the commonly cited 36.3cc spec, then it's seems likely to me that the advertised 11:1 represents the ABSOLUTE MINIMUM CR, and the only plausible way to get it that is for the deck height to be .015 higher than nominal and the heads to have 111cc combustion chambers rather than nominal 109. If the block and heads were on spec, the CR is over 11.5, and could be as high as 11.75 if the block were only .005 shorter than nominal. Stay tuned...

[Samuel J.]

Re: Questionable L-72 and L-71 CR and other specifications

I just received the dome measurements of my original, standard bore L-72 piston (GM part #3888342, GM casting #3888304, TRW part #L2268F):

35.5cc dome volume (with a .300" dome height)

The piston mfr. who measured this piston added that did to 60's-era manufacturing tolerances, pistons with the same part number from the same engine rarely had identical dome volumes.

So, when you plug the 35.5cc dome volume into the the csgnetwork.com CR calculator, along with the other (i.e. 4.251 bore, 3.766 stroke, .021 steel gasket, 4.375 gasket bore, 108.9cc combustion chamber, as well as the average .020" piston-to-deck clearance), you get a CR from the factory of 11.52. Of course, if you're lucky enough to have a block that is .010" higher than nominal spec, combine with combustion chambers that are 2cc larger than the spec, then you'll have a true 11:1 CR from the factory. But, if your pistons are truly 36.3cc, and the block is .005" shorter than nominal (yes, they exist), and the combustion chambers are a mere 1cc smaller than spec (that happens too), you're at about 11.9:1, and probably no stranger in the land called "Deto-Nation."

[John M.]

Re: Questionable L-72 and L-71 CR and other specifications

Sam, I will get back to you on my L71 piston measurements when I get back to the office but my reason for measuring the pistons in the engine was based on the same big numbers you got. That's when I got the 30cc instead of the 35 that you got. Maybe they were machined to reduce the CR to the stock 11:1. I thought they were untouched. I have photos and numbers to check. Seems to me that the "correct" pistons make for a very high CR.
John

[Samuel J.]

Re: Questionable L-72 and L-71 CR and other specifications

Yes, it sure seems that way--much higher than most commonly believe.

If I'm not mistaken, I was thinking that another reason why the pistons you cc'd years ago would be lower than mine has to do with them being oversized--weren't they .030 overbore?

Below is a photo of my standard bore OE L-72 piston. Look forward to hearing about yours and seeing a photo if you have one.

[Joe L.]

Re: Questionable L-72 and L-71 CR and other specifications

Samuel------


Do these pistons have a forging number on the bottom?