Anyone else notice the 800 pound gorilla?
10-17-2007, 07:56 PM
#61
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An 84 US is a completely different animal than a GT. Your CR is under 8.5:1 and your ignition advance can be set without a computer. It's almost idiot-proof to moderately boost your car.
10-17-2007, 08:13 PM
#62
In Your Face, Ace
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Some of the newer factory boosted engines are running the higher compression ratios, using head gaskets with no "O" ringing, using cast pistons, and are doing it with a 14.7:1 full throttle air/fuel ratio. I'm not talking about just the relatively new direct injection engines either.
Just curious.
10-17-2007, 08:58 PM
#63
Three Wheelin'
December will be 3 years with an Keel-type SC system on my 928. I've only tracked it once, and my boost orgies are mostly limited to the highway on-ramp. The car is my daily driver.
I do run a Cometic gasket and it might seep just a little bit, but not much. My low coolant light comes on every few months, and I add a couple cups of tap water. The MLS gasket is thicker than the stock gasket.
The car is due to go in for a checkup/tuneup. I should probably have the car SharkTuned while I'm at it. I fall into the category of somebody that really didn't know what he was doing, but hasn't suffered any obvious setback.
With the thicker gasket and boost adjustable RRFPR, I'm the guy that unscientifically lowered his CR and unscientifically runs the car a bit rich. I did begin the process on a complete engine rebuild, which has probably helped in terms of not having old, worn items give up the ghost that much faster.
And to add to the heresy, I run plain old conventional oil, chaging the oil every 6 months or so.
I really mistreated my 944, and it was at 250k miles at the time I got rid of it. I think sometimes there's an awful lot of hand wringing and dangers get blown out of proportion. Although if I wreck an engine, my tune would probably change.
I do run a Cometic gasket and it might seep just a little bit, but not much. My low coolant light comes on every few months, and I add a couple cups of tap water. The MLS gasket is thicker than the stock gasket.
The car is due to go in for a checkup/tuneup. I should probably have the car SharkTuned while I'm at it. I fall into the category of somebody that really didn't know what he was doing, but hasn't suffered any obvious setback.
With the thicker gasket and boost adjustable RRFPR, I'm the guy that unscientifically lowered his CR and unscientifically runs the car a bit rich. I did begin the process on a complete engine rebuild, which has probably helped in terms of not having old, worn items give up the ghost that much faster.
And to add to the heresy, I run plain old conventional oil, chaging the oil every 6 months or so.
I really mistreated my 944, and it was at 250k miles at the time I got rid of it. I think sometimes there's an awful lot of hand wringing and dangers get blown out of proportion. Although if I wreck an engine, my tune would probably change.
10-17-2007, 09:10 PM
#64
BTW, it is not uncommon for 928 head gaskets to deteriorate from age. I had to replace them on my US spec 80 at 54K. Car sat a lot. This is not the same type of failure as Andrew is experiencing, but gaskets can weaken over time.
I wouldn't hesitate to SC that engine now.
10-17-2007, 09:40 PM
#65
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I love to see things on which I know absolutely nothing about, which in turn provides the impetus for research. In so doing, I found this interesting piece by Allen Cline. Not a peep about gasket failure in his article. More at:
http://www.enginelogics.com/detonation.html
"One thing to understand is that detonation is not necessarily destructive. Many engines run under light levels of detonation, even moderate levels. Some engines can sustain very long periods of heavy detonation without incurring any damage. If you've driven a car that has a lot of spark advance on the freeway, you'll hear it pinging. It can run that way for thousands and thousands of miles. It's not an optimum situation but it is not a guaranteed instant failure. The higher the specific output (HP/in3) of the engine, the greater the sensitivity to detonation. An engine that is making 0.5 HP/in3 or less can sustain moderate levels of detonation without any damage; but an engine that is making 1.5 HP/in3, if it detonates, it will probably be damaged fairly quickly, here I mean within minutes.
Detonation causes three types of failure:
1. Mechanical damage (broken ring lands, rod bearing failure)
2. Abrasion (pitting of the piston crown)
3. Overheating (scuffed piston skirts due to excess heat input or high coolant temperatures)
The high impact nature of the spike can cause fractures; it can break the spark plug electrodes, the porcelain around the plug, cause a clean fracture of the ring land and can actually cause fracture of valves-intake or exhaust. The piston ring land, either top or second depending on the piston design, is susceptible to fracture type failures. If I were to look at a piston with a second broken ring land, my immediate suspicion would be detonation.
Another thing detonation can cause is a sandblasted appearance to the top of the piston. The piston near the perimeter will typically have that kind of look if detonation occurs. It is a swiss-cheesy look on a microscopic basis. The detonation, the mechanical pounding, actually mechanically erodes or fatigues material out of the piston. You can typically expect to see that sanded look in the part of the chamber most distant from the spark plug, because if you think about it, you would ignite the flame front at the plug, it would travel across the chamber before it got to the farthest reaches of the chamber where the end gas spontaneously combusted. That's where you will see the effects of the detonation. You might see it at the hottest part of the chamber in some engines, possibly by the exhaust valves. In that case the end gas was heated to detonation by the residual heat in the valve.
In a four valve engine with a pent roof chamber with a spark plug in the center, the chamber is fairly uniform in distance around the spark plug. But one may still may see detonation by the exhaust valves because that area is usually the hottest part of the chamber. Where the end gas is going to be hottest is where the damage, if any, will occur.
Because this pressure spike is very severe and of very short duration, it can actually shock the boundary layer of gas that surrounds the piston. Combustion temperatures exceed 1800 degrees. If you subjected an aluminum piston to that temperature, it would just melt. The reason it doesn't melt is because of thermal inertia and because there is a boundary layer of a few molecules thick next to the piston top. This thin layer isolates the flame and causes it to be quenched as the flame approaches this relatively cold material. That combination of actions normally protects the piston and chamber from absorbing that much heat. However, under extreme conditions the shock wave from the detonation spike can cause that boundary layer to breakdown which then lets a lot of heat transfer into those surfaces.
Engines that are detonating will tend to overheat, because the boundary layer of gas gets interrupted against the cylinder head and heat gets transferred from the combustion chamber into the cylinder head and into the coolant. So it starts to overheat. The more it overheats, the hotter the engine, the hotter the end gas, the more it wants to detonate, the more it wants to overheat. It's a snowball effect. That's why an overheating engine wants to detonate and that's why engine detonation tends to cause overheating."
http://www.enginelogics.com/detonation.html
"One thing to understand is that detonation is not necessarily destructive. Many engines run under light levels of detonation, even moderate levels. Some engines can sustain very long periods of heavy detonation without incurring any damage. If you've driven a car that has a lot of spark advance on the freeway, you'll hear it pinging. It can run that way for thousands and thousands of miles. It's not an optimum situation but it is not a guaranteed instant failure. The higher the specific output (HP/in3) of the engine, the greater the sensitivity to detonation. An engine that is making 0.5 HP/in3 or less can sustain moderate levels of detonation without any damage; but an engine that is making 1.5 HP/in3, if it detonates, it will probably be damaged fairly quickly, here I mean within minutes.
Detonation causes three types of failure:
1. Mechanical damage (broken ring lands, rod bearing failure)
2. Abrasion (pitting of the piston crown)
3. Overheating (scuffed piston skirts due to excess heat input or high coolant temperatures)
The high impact nature of the spike can cause fractures; it can break the spark plug electrodes, the porcelain around the plug, cause a clean fracture of the ring land and can actually cause fracture of valves-intake or exhaust. The piston ring land, either top or second depending on the piston design, is susceptible to fracture type failures. If I were to look at a piston with a second broken ring land, my immediate suspicion would be detonation.
Another thing detonation can cause is a sandblasted appearance to the top of the piston. The piston near the perimeter will typically have that kind of look if detonation occurs. It is a swiss-cheesy look on a microscopic basis. The detonation, the mechanical pounding, actually mechanically erodes or fatigues material out of the piston. You can typically expect to see that sanded look in the part of the chamber most distant from the spark plug, because if you think about it, you would ignite the flame front at the plug, it would travel across the chamber before it got to the farthest reaches of the chamber where the end gas spontaneously combusted. That's where you will see the effects of the detonation. You might see it at the hottest part of the chamber in some engines, possibly by the exhaust valves. In that case the end gas was heated to detonation by the residual heat in the valve.
In a four valve engine with a pent roof chamber with a spark plug in the center, the chamber is fairly uniform in distance around the spark plug. But one may still may see detonation by the exhaust valves because that area is usually the hottest part of the chamber. Where the end gas is going to be hottest is where the damage, if any, will occur.
Because this pressure spike is very severe and of very short duration, it can actually shock the boundary layer of gas that surrounds the piston. Combustion temperatures exceed 1800 degrees. If you subjected an aluminum piston to that temperature, it would just melt. The reason it doesn't melt is because of thermal inertia and because there is a boundary layer of a few molecules thick next to the piston top. This thin layer isolates the flame and causes it to be quenched as the flame approaches this relatively cold material. That combination of actions normally protects the piston and chamber from absorbing that much heat. However, under extreme conditions the shock wave from the detonation spike can cause that boundary layer to breakdown which then lets a lot of heat transfer into those surfaces.
Engines that are detonating will tend to overheat, because the boundary layer of gas gets interrupted against the cylinder head and heat gets transferred from the combustion chamber into the cylinder head and into the coolant. So it starts to overheat. The more it overheats, the hotter the engine, the hotter the end gas, the more it wants to detonate, the more it wants to overheat. It's a snowball effect. That's why an overheating engine wants to detonate and that's why engine detonation tends to cause overheating."
10-17-2007, 10:01 PM
#66
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All these modern cars with high C/R and boosting are fine. Because they ARE DESIGNED to take that pressure. Extra studs in the head, thicker sealing surface, bigger journals, it's just approaching what has been around in the diesel world for many years. I never said you couldn't have high C/R and boosting together, I said it's all about the combustion chamber, and the F/A ratio, and the cam profile, and the ignition map. The 928 could be very successfully boosted with a C/R over 9:1. Just investigate the cam timing and duration, overlap, fuel maps, and ignition. You may also have to change head gaskets, and add estra studs to the engine, but hey, it'll eventually do the job.
Just adding a blower and retarding the ignition a few degrees isn't the answer. The head gaskets will fail at a higher rate than the same age, use factor, and design as the non-SC engine. More pressure, more probability of detonation. More detonation = more head gasket failures. My poor old Avanti really wakes up with 6PSI of boost. By todays standards that is marginal.
Just adding a blower and retarding the ignition a few degrees isn't the answer. The head gaskets will fail at a higher rate than the same age, use factor, and design as the non-SC engine. More pressure, more probability of detonation. More detonation = more head gasket failures. My poor old Avanti really wakes up with 6PSI of boost. By todays standards that is marginal.
10-17-2007, 10:26 PM
#67
We checked Adams block as it came apart and it was 9.3:1..so lots of room for S/C ing..
later,
Tom
89GT
10-17-2007, 11:09 PM
#68
We are working with a 4 stud per cylinder block here people. I mean the new LSX small block (buildable to 500ci btw) is *6* bolts per stud.
We are on the Tee-ball field and the rest of the world is in the big leagues.
We are on the Tee-ball field and the rest of the world is in the big leagues.
10-17-2007, 11:13 PM
#69
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Some examples of factory boosted engines with higher compression than a 928 has have already been mentioned. The first that I heard of a factory boosted engine running at 14.7:1 air/fuel ratio with cast pistons was the engine used in the PT Cruiser turbo and SRT-4. I guess stock boost is around 14-15psi, with factory performance parts availability consisting of a different turbo and management parts to go to around 350hp, at somewhere beyond 20psi.
The Ecotec 2.0 turbo engine is supposed to be the highest output per cubic inch engine GM has ever produced. It makes 18-20psi of boost and uses cast pistons.
The new Mitsubishi Evo engine is 2 liters, around 300hp, an open deck block, and uses the MLS gaskets and no "O" ring. The same open deck design and MLS head gasket was used by Dodge for their NASCAR engines.
I believe all the factory boosted Fords, including the Ford GT and 500GT Mustang use MLS head gaskets and no "O" rings. I don't know if I've ever heard of a factory stock configuration using "O" ringed heads.
The Ecotec 2.0 turbo engine is supposed to be the highest output per cubic inch engine GM has ever produced. It makes 18-20psi of boost and uses cast pistons.
The new Mitsubishi Evo engine is 2 liters, around 300hp, an open deck block, and uses the MLS gaskets and no "O" ring. The same open deck design and MLS head gasket was used by Dodge for their NASCAR engines.
I believe all the factory boosted Fords, including the Ford GT and 500GT Mustang use MLS head gaskets and no "O" rings. I don't know if I've ever heard of a factory stock configuration using "O" ringed heads.
10-17-2007, 11:31 PM
#70
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Well, coming up on 3 years here with a boosted GT. On the original head gaskets. And loving every minute I drive the car. Recently upped the boost and made some major tuning changes for both fueling and ignition mapping done by some well known experts. Knock count is lower than stock, and we tuned for 93 octane pump gas.
Now, when you modify a car from stock, there are risks to reliability and longevity of the stock components above a certain power level. If you choose to venture above that level, you need to understand the risks and the proper way to reduce them. I am waiting for my 18 year old head gaskets to fail, I'm sure they will at some point, and I will either replace them when they do, or replace them before they do as part of my pm schedule.
Personally, I feel very confident that even with my 10:1 compression ratio and the GT cams I will get years of further enjoyment from this engine before needing any major work.......because it is tuned right. Z and Murf spent many hours with sharktuning and datalogging to make it not only safe, but wicked fast. Since most of the time while driving the BOV is open and the engine sees no boost, the amount of added stress on a street car is not really as bad as some here believe.
Some of us believe in boosting our cars, some here do not. Personal choice, but a PROPERLY tuned, properly SC'd 928 should last a good long time............
Jim
10-18-2007, 01:40 AM
#71
Three Wheelin'
One other thought occurred to me, and I think it's a testament to the kind of mission creep that can't help but pervade the thoughts of people who run boost--
--and that is, while I don't want anything to happen to my engine, if it DID have to come apart again, I think there are some interesting upgrades that I wish I would've had the presence of mind (and cash) to explore the first time it was apart.
I don't know why someone wouldn't consider building a mild stroker using lighter/stronger materials and THEN throwing some boost on top.
Some of the AMG cars make incredible amounts of torque. I recognize that getting it on the ground is a different matter altogether, but there seems to be a common complaint--or at least observation--that there are specific circumstances that work to the 928's advantage in an acceleration contest (i.e., just wait until you hit 100, then look out!)--and other circumstances that do not (began at a rolling 2nd gear start). With even more torque available earlier, I'm sure it would be enjoyable.
--and that is, while I don't want anything to happen to my engine, if it DID have to come apart again, I think there are some interesting upgrades that I wish I would've had the presence of mind (and cash) to explore the first time it was apart.
I don't know why someone wouldn't consider building a mild stroker using lighter/stronger materials and THEN throwing some boost on top.
Some of the AMG cars make incredible amounts of torque. I recognize that getting it on the ground is a different matter altogether, but there seems to be a common complaint--or at least observation--that there are specific circumstances that work to the 928's advantage in an acceleration contest (i.e., just wait until you hit 100, then look out!)--and other circumstances that do not (began at a rolling 2nd gear start). With even more torque available earlier, I'm sure it would be enjoyable.
10-18-2007, 03:13 AM
#72
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Try and find a factory car that has a supercharger and 10 to 1 compression (or more, in the case of later cars). Throw in cast pistons with 4 bolts around each cylinder and it isn't too tough to predict what is going to happen to every supercharged engine out there. The SAE guys must know something! Sure you can "patch" the immediate issue with a stronger head gasket, but eventually the rings will blow off the cast pistons or a connecting rod will magically appear on the outside of the engine.
Other than that, I have no opinion.
Other than that, I have no opinion.
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Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
greg brown
714 879 9072
GregBBRD@aol.com
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
10-18-2007, 06:21 AM
#73
I'm intrigued by the following by quotation from above:
"Andrew's is the SECOND supercharged car to experience head gasket failure in the last year (the other one was in Australia)".
Not many S/C'd 928s in Australia. One S/C'd that has had a problem was not due to head gasket failure. So whose Australian 928 is being referred to? Can anyone advise, or is the quotation incorrect?
"Andrew's is the SECOND supercharged car to experience head gasket failure in the last year (the other one was in Australia)".
Not many S/C'd 928s in Australia. One S/C'd that has had a problem was not due to head gasket failure. So whose Australian 928 is being referred to? Can anyone advise, or is the quotation incorrect?
10-18-2007, 11:27 AM
#74
Three Wheelin'
I have one of the Andy Keel Roots blower kits. I knew from the beginning that the AFR was phucked up in the original configuration (using stock GT chip) and was pleased to be an early user of John Speake's SharkTuner. I was able to clean up the fuel mapping early, and have experienced no issues in 2+ years, 20k+ miles. I mean no pinging / head gasket issues since. I did have some PS Steering belt issues that have been resolved - but that is completely unrelated.
I have had my car on the track many times since the blower was installed - probably about 8 track days including some brutally hot Sebring days in Florida in the summer. No probs.
I have not done any EZK tuning, perhaps I should. But I have not gone crazy on the boost either at 7 psi. Befor I SharkTuned, I had put the Aussie Coding plug in whick IIRC takes 3 degrees of timing out under heavy load. I did this because I was afraid of predetonation in the way Andy had designed the system. Basically, there was a lean condition under tip in that scared the hell out of me - since it happened frequently as a daily driving in real world driving situations. I SharkTuned with the stock plug back in and have not worried since. When I did all of this, there was no EZK tuner available, so you guys that are supercharging now should have no fear. Tune your ignition and AFR and you will be fine.
I have had my car on the track many times since the blower was installed - probably about 8 track days including some brutally hot Sebring days in Florida in the summer. No probs.
I have not done any EZK tuning, perhaps I should. But I have not gone crazy on the boost either at 7 psi. Befor I SharkTuned, I had put the Aussie Coding plug in whick IIRC takes 3 degrees of timing out under heavy load. I did this because I was afraid of predetonation in the way Andy had designed the system. Basically, there was a lean condition under tip in that scared the hell out of me - since it happened frequently as a daily driving in real world driving situations. I SharkTuned with the stock plug back in and have not worried since. When I did all of this, there was no EZK tuner available, so you guys that are supercharging now should have no fear. Tune your ignition and AFR and you will be fine.
10-18-2007, 01:23 PM
#75
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Niklas has reminded me that the stock EZK will reatrd the igntion in 3 deg steps up to its max of 9 degrees. This is quite sufficient to protect an engine running at around 6-7 psi.
He found when remapping his EZK that he needed to take out 10 degrees at WOT, when running 9-10psi boost.
So if you run these higher boosts, and don't remap, then the stock system will leave you exposed to detonation.
It seems that the old rule of thumb that you need to take out 1 degree of advance for every pound of boost is a good one.
Obviously, when you have SharkTuned your EZK, you then regain the 9 degree safety margin if any knock occurrs. If that isn't sufficient to let you sleep easy at night, then the SharkTuner allows you to increase the maximum retard range available, and the step size it takes to get there.
He found when remapping his EZK that he needed to take out 10 degrees at WOT, when running 9-10psi boost.
So if you run these higher boosts, and don't remap, then the stock system will leave you exposed to detonation.
It seems that the old rule of thumb that you need to take out 1 degree of advance for every pound of boost is a good one.
Obviously, when you have SharkTuned your EZK, you then regain the 9 degree safety margin if any knock occurrs. If that isn't sufficient to let you sleep easy at night, then the SharkTuner allows you to increase the maximum retard range available, and the step size it takes to get there.