Engine disassembly quiz

[Frank C.]

Re: Engine disassembly quiz

Duke,

I know you will have more questions, but here is a practical exercise. I am having my original L79 rebuilt to stock specifications. The original block was not decked, I am using stock 462 heads, and a stock L79 cam. With the short block assembled, the builder has checked the deck height, ran the numbers using .040 head gaskets, and come up with a CR of 10:1.

What are your recommendations?

Frank

[Joe C.]

Re: Engine disassembly quiz

Duke,

I know you will have more questions, but here is a practical exercise. I am having my original L79 rebuilt to stock specifications. The original block was not decked, I am using stock 462 heads, and a stock L79 cam. With the short block assembled, the builder has checked the deck height, ran the numbers using .040 head gaskets, and come up with a CR of 10:1.

What are your recommendations?

Frank

Instead of the composite head gasket, use a shim steel gasket, as original, which measure around .021" compressed. This will move you from 10.07:1 to 10.57:1

The above uses an average piston-to-deck clearance of .025", per stock spec, a 5.3 cc piston dome volume, a 4.00" bore, and a 64cc chamber volume. Are these the dimensions the machinist used for your calculation? In order to have accuracy, the gasket bore volume must also be known. Did he measure the chamber volumes, because there is variation? Did he measure all 8 piston-to-deck clearances, because there should be significant variation here? These are not critical to your build, as I assume that you won't address any inconsistencies anyway. So long as your SCR is around 10.5:1, the engine will be near optimal as far as DCR is concerned, and you will have enough "margin" as far as detonation resistance, to enable the stock spark advance program.

[Michael D.]

Re: Engine disassembly quiz

...Once the heads are removed, what is the next operation?...

Open a congratulatory cold one.

[Duke W.]

Re: Engine disassembly quiz

Duke,

I know you will have more questions, but here is a practical exercise. I am having my original L79 rebuilt to stock specifications. The original block was not decked, I am using stock 462 heads, and a stock L79 cam. With the short block assembled, the builder has checked the deck height, ran the numbers using .040 head gaskets, and come up with a CR of 10:1.

What are your recommendations?

Frank

Deck height is not the what you use to measure CR. It's deck clearance. They are not the same. Did you read the article referenced in post #11? You need to understand the terminology and procedures, so we are on the same page.

What are the numbers? Are you using the OE replacement 5.3 cc dome pistons? Were the head chamber volumes measured? I also recommend you query the builder to determine what actual measurements he used and what standard specifications he used in lieu of actual measurements, what calculator he used, and then check his calculations with the calculator referenced in the article.

In any event I recommend a 10.3:1 NTE CR for the L-79 and this should allow an aggressive centrifugal spark advance curve depending on your available octane. The OE curve is extremely lazy (see a thread from a month ago: L-79 distributor blueprint overhaul.) A thinner head gasket will allow you to get closer to 10.3, but unless all the data to compute CR is actually measured data for your engine, you're just guessing, so better off to go conservative and stay closer to 10.

Pushing the CR too far is not worth the price of detonation that requires a retarded spark advance curve. The procedure to achieve the most aggressive spark advance map possible without detonation is in my San Diego presentation. Detonation onset is often sudden. A couple of tenths more CR and an aggressive spark advance map may have to be retarded several degrees, which will hurt low end torque, but there's no way to calculate the maximum and no computer program that will give you a single definitive answer. There are too many variables including inlet air temperature and density. My recommended maximums are based on experience and anecdotal evidence from users.

Also, as I previously stated, if the engine didn't detonate prior to disassembly and you obtain the data to measure the pre-disassembly CR, there is no reason to lower it, and OE assembled engines are usually several tenths lower than the advertised value.

If you haven't purchased the cam yet, I recommend the L-46 cam to replace the L-79 cam. The former has better dynamics, but must be installed on a L-79 four degrees advanced to bring the inlet POML back to 110 deg ATDC from the 114 as designed.

Duke

[Michael C.]

Re: Engine disassembly quiz

Find top dead center

[Patrick N.]

Re: Engine disassembly quiz

Once the heads are removed, what is the next operation?
Hint: It is NOT removing another part.

Bust out the check book and have it done by someone who knows what to do.

Duke[/QUOTE]

[Timothy B.]

Re: Engine disassembly quiz

When you check the deck height you will find TDC. My question is what harm is there if the new pistons are made with a taller compression height to allow a tighter deck clearance as the block can't be decked in most cases because of #'s.

[Jeff P.]

Re: Engine disassembly quiz

All the details along with photos are in the Fall 2009 Corvette Restorer article "Compression Ratio Explained" that I co-authored with John McRae.

I sure wish i could get a copy of this article !

[Duke W.]

Re: Engine disassembly quiz

If you are restoring an OE engine and don't want to touch the decks, the most important thing is to get the CR in the proper range for your engine configuration. Just let the quench clearance fall wherever.

Case in point. In mid-1962 production, Chevrolet began double gasketing 340/360 HP engines due to customer detonation complaints, and this continued through 1963. With the 60-61 cc chamber volume 461X heads and nominal deck height/clearance (.025") and the single .018" gasket the quench clearance was .043" and the true CR was close to the 11.25:1 advertised value.

Chevrolet's solution cut the CR about half a point and increased the deck clearance to a nominal .061" on a nominal 9.025" deck height block or more by the amount the deck(s) were high from nominal. My double gasketed '63 340 HP engine never detonated despite a very aggressive 24 degree centrifugal advance curve and up to 16-18 initial on the typical 98-99 RON leaded premium of the day.

There are guys on the Web that obsess about quench clearance, but Taylor says in his book that decreasing quench clearance doesn't help ward off detonation very much until it's less than .005 times the bore, which is .020" for a 4" bore, and Chevrolet's minimum recommended range is .035-.040" to prevent piston-head contact at very high revs.

If I was planning a max compression ratio engine and was not concerned about numbers and broach marks, I would design it to achieve the target CR with the deck clearnace in the above range, but that may be impossible if you don't want to touch the deck; however, if the left deck is higher than the right, it can be cut down with no visual trace once the engine is assembled, which helps a great deal to achieve near equal CR on all cylinders.

Some pistons have different compression distance values than OE. For example, KB hypereutectics are .003" more, which is not a great amount, but when doing a precision restoration seeking the maximum detonation-free CR, you should have as accurate data as possible to run the CR calcs.

Duke

[Duke W.]

Re: Engine disassembly quiz

All the details along with photos are in the Fall 2009 Corvette Restorer article "Compression Ratio Explained" that I co-authored with John McRae.

I sure wish i could get a copy of this article !

Left click on my name at the top of the post. This will give you a menu. Left click send Email (I have disabled the PM function since they are a WORTHLESS way to communicate), and state your request. Also let me know what engine you have and your plans for it.

Duke

[Rob M.]

Re: Engine disassembly quiz

If you are restoring an OE engine and don't want to touch the decks, the most important thing is to get the CR in the proper range for your engine configuration. Just let the quench clearance fall wherever.

Case in point. In mid-1962 production, Chevrolet began double gasketing 340/360 HP engines due to customer detonation complaints, and this continued through 1963. With the 60-61 cc chamber volume 461X heads and nominal deck height/clearance (.025") and the single .018" gasket the quench clearance was .043" and the true CR was close to the 11.25:1 advertised value.

Chevrolet's solution cut the CR about half a point and increased the deck clearance to a nominal .061" on a nominal 9.025" deck height block or more by the amount the deck(s) were high from nominal. My double gasketed '63 340 HP engine never detonated despite a very aggressive 24 degree centrifugal advance curve and up to 16-18 initial on the typical 98-99 RON leaded premium of the day.

There are guys on the Web that obsess about quench clearance, but Taylor says in his book that decreasing quench clearance doesn't help ward off detonation very much until it's less than .005 times the bore, which is .020" for a 4" bore, and Chevrolet's minimum recommended range is .035-.040" to prevent piston-head contact at very high revs.

If I was planning a max compression ratio engine and was not concerned about numbers and broach marks, I would design it to achieve the target CR with the deck clearnace in the above range, but that may be impossible if you don't want to touch the deck; however, if the left deck is higher than the right, it can be cut down with no visual trace once the engine is assembled, which helps a great deal to achieve near equal CR on all cylinders.

Some pistons have different compression distance values than OE. For example, KB hypereutectics are .003" more, which is not a great amount, but when doing a precision restoration seeking the maximum detonation-free CR, you should have as accurate data as possible to run the CR calcs.

Duke

I feel like I'm present at the birth of a future Regional or National event technical seminar...

[Frank C.]

Re: Engine disassembly quiz

Duke,

I am going to the machine shop today and will get the answers on how the CR was determined for my L79 build. I went back and re read the Williams/Mcrae article in the fall 2009 Corvette Restorer. I have posted a copy for reference. ( Note: If this is not appropriate, I or the administrators can/will delete). My intent has been to complete a precision, stock L79 rebuild. Before installation back in the car, I intend to test run the engine, and have the engine dyno'ed to tune and confirm horse power. I will share the results when complete.

Frank

Williams McRae CR Explained Page 3.jpgWilliams McRae CR Explained Page 1.jpgWilliams McRae CR Explained Page 2.jpg

[Duke W.]

Re: Engine disassembly quiz

Setting up a lab dyno test is expensive, and most don't simulate the "as-installed" condition. IMO you are better off installing the engine in the car, then breaking it in and road testing to find the most aggressive spark advance map and sorting out any carb problems.

Dynojet chassis dynos are common and can usually be scheduled at no more than $150 per hour, and if you have a good test plan, you can get a lot done in that hour. I have a test plan if you're interested, and this is what was done with John McRae's Special 300 HP engine and documented in the Fall 2010 Corvette Restorer.

If you do conduct a lab dyno test, run the OE manifolds and the complete OE exhaust system, if possible. Then use SAE air density rather than standard sea level air density correction. This will give you a pretty good estimate of SAE net as installed in the car.

Chassis dyno SAE corrected data divided by 0.85 for a manual transmission in direct drive is also a good estimate of SAE net.

Duke

[Duke W.]

Re: Engine disassembly quiz

I feel like I'm present at the birth of a future Regional or National event technical seminar...

Not really. The whole story is in the previously referenced Corvette Restorer article, and I emphasised the importance of CR in my San Diego presentation by reviewing the thermodyanamics, economics, and fuel octane facts that support high compression ratios.

This subject deserves a thorough discussion because so many vintage engines have been rebuilt with "lower compression" over the years, and this is still happening, so you end up with an engine that has less torque, power, and fuel efficiency than what Flint and Tonawanda built decades ago.

I found it interesting that the GM Powertrain engineer that Csabe interviewed emphasized the importance of CR in exactly the same way, so I decided to do the "quiz".

If you do your own engine work, you have what you need in the article and my San Diego presentation to get the best outcome possible. If you have the engine rebuilt by a shop, provide them this information. If the guy insists you need "lower compression", run like hell!

Duke