Compression Ratio

[Mike H.]

I am rebuilding a 327-300HP with 10 1/2 compression ratio and a 427-400HP with 10 1/2 ratio. What compression ratio do you all recommend taking into consideration modern fuels. I have been using premium fuel and plane on continuing to do so. I drive these cars 2 or 3000 miles per year. The reason I am asking is the BB had broken ring lands on one cyl. piston. I am told it was caused by detonation, but I heard no pinging. I took this same engine down in 1978 and it had broken piston rings in 5 cyl. with leaded fuel. The pistons were good at that time and I reused them.I was told at that time the broken rings were probably caused by over heating. The car was new to me then so it could have been ran hot but not since. I would not like to repeat either of these situations again.
CORDially Mike

[Duke W.]

Re: Compression Ratio

Unless you hear sustained audible detonation, you can run the same CR as the engines have now. The actual CRs are likely at least 0.5 below the OE specs, which are highly optimistic.

Did you read the recent Corvette Restorer article on how to measure actual compression ratio?

Broken rings/lands can be caused by detonation, but unless the crowns show signs of detonation that probably wasn't the cause. I believe rings can break as you extract the piston if the bore has a significant ridge due to wear. Overheating is more likely to scuff the pistons or cylinder walls than break a ring.

Rings can break if they are installed with too little end gap for thermal expansion. If the ends butt, they break like sticks. This is especially critical with the top ring on Keith Black hypereutectic pistons because the top ring is mounted very high and runs hotter than conventional designs. KB has specific instructions on minimum top ring gap, and if the installed gap is below minimum, a broken ring is almost guaranteed!

Detonation usually shows up first as little craters in the piston crowns. Detonation increases heat tranfer to the combustion chamber boundaries by an order of magnitude. This can cause localized overheating of the crown manifested by local melting that leaves little craters. The crowns can look like someone took and ice pick to them. Clean up the crowns and inspect them carefully with a magnifying glass.

What is the mfg./part number of the installed pistons, and what type of material? Maybe it would be wise to use a different pistons design.

Duke

[Mike H.]

Re: Compression Ratio

The pistons are orig. pistons in a standard bore engine. I tore down the engine because coming back from Georga to Mi. no. one cyl. was pumping 2 Qts. of oil every 50 miles when I stopped and trailered it home. There is no signs of detonation on the piston top. The lands were broken severely and not from removal as there is only .002 ridge and all the pistons dropped out easly. The only piston with a problem was # 1. I am disinclined to rebuild with new pistons without knowing what happened.
CORDially Mike

[Duke W.]

Re: Compression Ratio

If you want the TDB to give you an informed failure analysis, I suggest the following.

Clean up the #1 piston and post close up photos of the crowns and land failure area.

If the #1 rings are intact, measure the end clearances in the bore and land clearances. Post this data along with the OE specs from the COM or AMA specs.

Post the same data for two or three undamaged pistons so we can if the dimensions are in spec.

The quality of any analysis from the TDB is directly proportional to the amount and quality of data you can provide.

Since the problem is in only one cylinder, I suspect too tight ring gap or land clearnace, and measurments should reveal the culprit.

Even if the #1 ring is broken you can use the remants to measure land clearance, but measuring end clearance of that ring may be impossible.

You may be able to get away with replacing just one piston, but how ever many you replace, measure end gap and land clearances for all rings and make sure they are all within OE spec.

Duke

[William F.]

Re: Compression Ratio

A lot has appeared in thes posts from time to time stating emphatically"if you can't HEAR pinging-detonation, you're ok." This goes contrary to an article appearing several years ago in Corvette News stating that there is a form of preignition that is not audible but detectible only with special instrumentation, but non the less increases piston ring wear significantly.This goes against the often given advice herein that 93 octane pump gas if fine for all Corvette engines, even those with the higher compression ratios.

[Duke W.]

Re: Compression Ratio

Very light detonation can be inaudible depending on the engine and car, but it usually doesn't cause any long term damage. Likewise, audible transient detonation usually doesn't cause any damage, but damage can be caused by sustained detonation.

Preignition is not the same thing as detonation, and preignition may or may not lead to detonation.

Recall that many seventies vintage cars detonated under part throttle acceleration or climbing hills, which was certainly more than "transient detontation". I think Ford or some other company ran a TV commercial calling it "the sound of fuel economy", which was a bit over the top. Catalytic converters allowed more part load spark advance without busting emission standards, and the OEMs pushed it to the limit to meet CAFE mandates.

Modern cars with detonation sensors often run at the ragged edge of detonation or allow some light detonation, which may be inaudible because this is the most efficient operating point that yields highest fuel economy, but whether a modern car or a vintage car, if you can't hear detonation, it is likely not doing any damage.

I doubt if the piston failure referenced in this thread had anything to do with detonation, but without more info from the OP...

I've never seen any research that suggests long term damage from inaudible detonation can cause catastrophic damage. How about scaning a posting the article?

Duke

[Mike H.]

Re: Compression Ratio

Here is the information requested on my piston damage on a 1967 427 400HP. Hopefully you will find photos posted. The casting no. of the pistons is 8869967 the ring lad clearance was .002, end gap of the rings was .028. The spec. for land clearance is .0012-.0032 and for end gap is .010-.020. This car has been driven for 30 years since the last time I honed the cyl. and reringed it with no problems to this point. The order of failure looks to be bottom land first then second land followed by top land. I don't now how long this took to occure. I checked the timing mark on the balancer with the timing mark on timing chain cover and it is right at TDC. What are your thoughts?
CORDially Mike









CORDially Mike

[Mike H.]

Re: Compression Ratio

Here's the additional photo.

[Duke W.]

Re: Compression Ratio

Very good information!

The piston crown shows no signs of detonation or significant skirt scuffing that I can see, but it has a dome. Is that an OE or correct OE replacment 390/400 HP piston?

From what I can see it appears that the second compression or oil ring initiated the failure. Does the end gap/land clearance you measured apply to to the top ring or second ring?

I assume the broken ring is what you found when you pulled the piston. Is it the top or second ring and what was the condition of the oil control ring and other compression ring.

I'm sure others can contribute or will have other questions.

Duke

[Dick W.]

Re: Compression Ratio

Ever had ether used to assist starting the engine? Not used properly, it will destroy ring lands.

[Timothy B.]

Re: Compression Ratio

Just a suggestion, could the ring set that was used be not as thick as what the piston calls for.

[Duke W.]

Re: Compression Ratio

For the ring (or rings) he measured, the land clearance is in spec. Too thin a ring would be way over spec.

Since the lands above and below the oil control ring are both broken, that ring is the no. one suspect and needs more analysis.

Duke

[Mike H.]

Re: Compression Ratio

The top comp. ring was intact and stuck in the land. The second comp. ring was broken. These are oem pistons for this engine. The pieces you see in the pictures are the broken lands. Because of the damage on this piston the land clearances on this piston would be suspect. On the back side of the damage the land clearance on the second comp. ring was still .0015. The top[ comp. ring was tight. The oil ring was distorted because of the damage but I saw nothing glaringly wrong with it. No ether has been used on the engine or other starting aids during my ownership.
CORDially Mike

[Duke W.]

Re: Compression Ratio

Was the top ring stuck due to carbon buildup or tight/no land clearance at some point around the circumference?

You can use a dial caliper to measure the piston ring groove widths around the circumference of each groove in undamaged areas and compare to spec. Then if you measure ring widths around the circumferences you can subtract from the groove widths to get a min/max range of clearance.

The ring widths should be well within a thou around the circumference. Same with the ring grooves in the piston, but there may have been some material yield near the damaged area, which would alter the original as machined dimensions.

There's still no smoking gun, but if you take enough measurements of the ring and groove widths and analyze the data you might find a possible tight land clearance issue, which could cause the type of damage that is evident.

One working hypothesis might that the top ring ran out of land clearance, which overstressed the section between the first and second ring grooves and broke it subsequently fouling the second ring, which lead to the failure of the section between the #2 compression and oil rings, and finally the bottom land via the same process.

Often an initial failure will cause significant consequential damage. The trick is to find the most probable cause of the initial failure, then trace it through the consequential damage.

A good example is a broken connecting rod. If the rod is significantly bent at the break point it usually means that the rod bearing seized, so the broken rod is consequential damage. The primary failure is likely oil starvation, and a spun bearing will always leave evidence on the rod big end bearing bore.

When a bearing seizes it puts a huge bending load on the rod, which can snap it like a match stick, regardless of how stout it is,

A few years ago I reviewed a case as above on a C2 with a built NOM 350 that had the base engine oil pan. The suspension was well set up with sticky tires. The rod broke on the way back from a track event at Willow Springs. It was well bent at the break point, with clear evidence of a spun bearing. I was about 99 percent certain that the rod bearing suffered damage during the event due to oil starvation and subsequently seized on the trip home, breaking the rod.

Duke

[Timothy B.]

Re: Compression Ratio

Mike,

Are you going to stay with the standard bore and reuse the other seven pistons? If so I would make sure those cylinders get a nice machine shop hone and those ring lands are cleaned with a tool to remove any carbon.

If the bore is out just a bit I have seen pistons that are knurled to keep from piston slap. Also, do some research on pin oiling as these original pistons may benifit from small modifications.

Could this have possibly been a air filter problem or carburetor flooding problem..