compression and copper head gaskets
08-13-2003, 01:08 AM
#16
Three Wheelin'
By Chris:
Nobody that has answered seems to believe I really have too much compression. So here is the number. The compression is 12.1.
Nobody that has answered seems to believe I really have too much compression. So here is the number. The compression is 12.1.
Alright, back to the task at hand.
It is generally not a good idea to add a thicker gasket as it will more than likely upset the squish area. The squish area keeps charge turbulence high and is a strong deterrent to detonation. Upsetting this aspect of an engine will compound the detonation problem.
The 'proper' way to effectively lower the compression ratio is to:
get new pistons with the required dish, or,
remachining the original piston to create a dish, and/or
remove material from the combustion chamber.
Again, care must be taken when dishing the pistons so as to not upset the squish area. Likewise, removing material from the head is tricky too as one does not know it’s thickness.
Since you have brand new pistons, your best bet then is to remachine them; you're taking the engine apart anyway. The rule of thumb is to not let the minimum crown thickness drop below 6% of the cylinder bore, and the dish should not exceed 70% of the piston’s diameter.
Good luck, and let us know what you decide.
08-13-2003, 03:58 AM
#17
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Chris,
Do you care to share with us what shop built this motor for you?
Or will you make us wonder?
Lets see if we can figure it out:
Project started in Oct
Probably done in SD area
Previously was a 5.0
Maybe you will tell us the color of the car?
Andy K
Do you care to share with us what shop built this motor for you?
Or will you make us wonder?
Lets see if we can figure it out:
Project started in Oct
Probably done in SD area
Previously was a 5.0
Maybe you will tell us the color of the car?
Andy K
08-13-2003, 04:50 AM
#18
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"Do you care to share with us what shop built this motor for you? "
No. I am trying to work with them to get it resolved and bad mouthing them in a public forum is counter productive to that goal. If they do the right thing and fix it under warranty, I will give them props for it. If not, I won't be very nice.
I only posted to try to gain some technical knowledge and advice on copper head gaskets. Not to fling mud around the Internet.
No. I am trying to work with them to get it resolved and bad mouthing them in a public forum is counter productive to that goal. If they do the right thing and fix it under warranty, I will give them props for it. If not, I won't be very nice.
I only posted to try to gain some technical knowledge and advice on copper head gaskets. Not to fling mud around the Internet.
08-13-2003, 05:12 AM
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Lagavulin,
"It goes without saying that that's a lot of compression for a street engine! I bet it runs great on race gas though. What displacement is it? Did you dyno it? "
Yes it runs very strong on race gas. It is 6.5L (399 cu in.). Bore is 104.5mm (968 oversize) and 95.25mm stroke (Scat crank).
It has not been dyno'd yet. Dynos are hard on an engine and I wanted to fully break in he motor before subjecting it to that.
I agree on the 'proper' way to lower compression, but those steps mean a complete rebuild on a brand new motor. And dishing pistons means rebalancing everything all over again. Back to square one basically.
"Since you have brand new pistons, your best bet then is to remachine them; you're taking the engine apart anyway."
Not necessarilly. Heads can be removed without pulling the motor. The copper gasket trick, if it works, could get it back up quickly. Boosted engines have done this successfully. The head chambers could also be enlarged at the same time.
"It goes without saying that that's a lot of compression for a street engine! I bet it runs great on race gas though. What displacement is it? Did you dyno it? "
Yes it runs very strong on race gas. It is 6.5L (399 cu in.). Bore is 104.5mm (968 oversize) and 95.25mm stroke (Scat crank).
It has not been dyno'd yet. Dynos are hard on an engine and I wanted to fully break in he motor before subjecting it to that.
I agree on the 'proper' way to lower compression, but those steps mean a complete rebuild on a brand new motor. And dishing pistons means rebalancing everything all over again. Back to square one basically.
"Since you have brand new pistons, your best bet then is to remachine them; you're taking the engine apart anyway."
Not necessarilly. Heads can be removed without pulling the motor. The copper gasket trick, if it works, could get it back up quickly. Boosted engines have done this successfully. The head chambers could also be enlarged at the same time.
08-13-2003, 04:01 PM
#20
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Where did you source the 968 pistons....rods..or are they JE type replicas. Many of us are planning on doing mods like this to our engines of one degreee or another...any info shared on here is worh its weight in gold!
For someone to muck up the CR calc. isnt too good. Forgot to subrtact the 1 huh!
For someone to muck up the CR calc. isnt too good. Forgot to subrtact the 1 huh!
08-13-2003, 06:07 PM
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The 968 pistons are genuine Porsche sourced by 928 Intl. It seemed a safer bet than coating JE pistons. But 968 pistons are getting hard to acquire. 928 Intl could not locate any 104mm 968 pistons, but they found 104.5mm oversize pistons. 4 came from an in-state supplier (SSF?) and 4 from PCNA.
Rods are off-the-shelf chevy parts as the stroker crank uses an chevy size 2.100" journal. I went with a 4340 H-beam design, 5.85" length. Part of the problem arose from the available chevy rod lengths. They come in sizes of 5.7", 5.85", 6.0" and up. The 5.7" is a bit too short. I estimate it yields a CR in the mid 8's with a 968 piston and the rod angle is poor. The 5.85" is a bit too long.
Rods are off-the-shelf chevy parts as the stroker crank uses an chevy size 2.100" journal. I went with a 4340 H-beam design, 5.85" length. Part of the problem arose from the available chevy rod lengths. They come in sizes of 5.7", 5.85", 6.0" and up. The 5.7" is a bit too short. I estimate it yields a CR in the mid 8's with a 968 piston and the rod angle is poor. The 5.85" is a bit too long.
08-13-2003, 07:37 PM
#23
Three Wheelin'
By Mike:
Wonder what 6.5L at 8ish:1 on boost would make...
Wonder what 6.5L at 8ish:1 on boost would make...
Assuming a 4% loss of hp per point of compression ratio, 88% Vol Eff, 85% Aftercooler Eff, and air/fuel management is spot on.
Since the compression ratio is so low, we can go wild with the following boost numbers without worrying about detonation:
15 psi = 740.80 crank hp
20 psi = 856.62 crank hp
25 psi = 971.05 crank hp
30 psi = 1,084.26 crank hp
These numbers show that building an engine with an 8.0:1 compression ratio is really unnecessary unless one intends to really crank up the boost to take advantage of it.
Of course, at these hp levels, the engine must be built accordingly.
08-13-2003, 09:50 PM
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Chris:
If you decide on the head gasket option which I still don't think is necessary with re-mapping of the stock injection, take a look at our Full Metal Jacket Gaskets which I can make pretty thick for you on a custom basis. The copper gaskets will leak when cold sometimes where these will not.
If you decide on the head gasket option which I still don't think is necessary with re-mapping of the stock injection, take a look at our Full Metal Jacket Gaskets which I can make pretty thick for you on a custom basis. The copper gaskets will leak when cold sometimes where these will not.
Last edited by Derrek Khajavi; 08-14-2003 at 01:07 PM.
08-14-2003, 04:26 AM
#26
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dynos are hard on engines?????????????? huh?
If your engine was built right, it is ready for the dyno in about 20 minutes of running.
In fact, 99% of 928 engine built are sealed by the time the cams are timed just by hand turning! And after a few minutes of running, the bearings are "right".
Solve the compression first...is it similar to the GTS "knock"..if so, I have never seen any damage to a GTS engien due to knock of any kind.
What rpm does it occur...how long does it last, and what it the engine ign computer doing about the timing...how much retard is it dialing in?
What are the specs on the cams...we use stock cams all the time on strokers and all is well...98% of the time...
And if you use copper, you will have leaks....it is a truth.
Marc
DEVEK
If your engine was built right, it is ready for the dyno in about 20 minutes of running.
In fact, 99% of 928 engine built are sealed by the time the cams are timed just by hand turning! And after a few minutes of running, the bearings are "right".
Solve the compression first...is it similar to the GTS "knock"..if so, I have never seen any damage to a GTS engien due to knock of any kind.
What rpm does it occur...how long does it last, and what it the engine ign computer doing about the timing...how much retard is it dialing in?
What are the specs on the cams...we use stock cams all the time on strokers and all is well...98% of the time...
And if you use copper, you will have leaks....it is a truth.
Marc
DEVEK
08-14-2003, 05:24 PM
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"dynos are hard on engines?????????????? huh? "
I guess I have that conception because on dynos I am used to seeing full throttle runs to high rpm, which I considered a bad thing to do prior to a proper break in. That would be bad on the street or a dyno. So yeah there's nothing inherent about a dyno that is hard on an engine, it's all in how the engine is operated. I was led to believe the break in period was longer than you are saying. But you know these engines better than anyone. Thanks for the info.
I can't compare the knock to GTS knock as I have no GTS experience to relate to. I notice knocking the most at slow speeds. It lasts several seconds, reoccuring often. It pings a bit as I accelerate but it gets harder to hear as rpm climb.
I have no idea what the knock sensors and ECU are doing to the timing. That is one of the things I asked to shop to look into. No word back on that yet.
As for cam specs, I will have to ask Derrek for that.
"And if you use copper, you will have leaks....it is a truth. "
Marc@DEVEK
"The copper gaskets will leak when cold sometimes"
Derrek@Huntley
Thanks guys. This is exactly the sort of feedback I was looking for.
Derrek, I will get in touch re. Full Metal Jacket Gaskets & dyno tuning. What material are the gaskets ?
Marc, I will get in touch re. heads and cams.
I guess I have that conception because on dynos I am used to seeing full throttle runs to high rpm, which I considered a bad thing to do prior to a proper break in. That would be bad on the street or a dyno. So yeah there's nothing inherent about a dyno that is hard on an engine, it's all in how the engine is operated. I was led to believe the break in period was longer than you are saying. But you know these engines better than anyone. Thanks for the info.
I can't compare the knock to GTS knock as I have no GTS experience to relate to. I notice knocking the most at slow speeds. It lasts several seconds, reoccuring often. It pings a bit as I accelerate but it gets harder to hear as rpm climb.
I have no idea what the knock sensors and ECU are doing to the timing. That is one of the things I asked to shop to look into. No word back on that yet.
As for cam specs, I will have to ask Derrek for that.
"And if you use copper, you will have leaks....it is a truth. "
Marc@DEVEK
"The copper gaskets will leak when cold sometimes"
Derrek@Huntley
Thanks guys. This is exactly the sort of feedback I was looking for.
Derrek, I will get in touch re. Full Metal Jacket Gaskets & dyno tuning. What material are the gaskets ?
Marc, I will get in touch re. heads and cams.
08-14-2003, 09:43 PM
#29
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Hi Chris,
I just wanted to address your original question regarding head gaskets. I don't run copper head gaskets, but I have read bits on them here and there over the years.
Copper head gaskets have several good and bad points.
good:
They don't blow out
They are reusable
Bad:
They don't seal well on imperfect surfaces (typically leaking oil)
They are expensive
The quality of copper may vary
For a high compression application O-ringing is the way to go, but if you dropping your compression then it's probably not the most useful thing to do with your money.
A thicker gasket will alter the timing index of the camshaft vs. the crankshaft, however if you heads have been surfaced with a significant amount of material removal, this effect may be minimized. To correct the shift, you'll need to degree the cams and reindex the camshaft gear. On the other hand theshift may be relatively minor, and/or perhaps even beneficial to your power making goals
An old trick that the great turbo gods (Gale Banks, et al) would do would be to double up the head gaskets. This increaced the number of failure surfaces, but you might find it to be more fogiving than copper gaskets. You'll need to check you head bolts/nuts a few times in the first few thousand miles of running the engine, and make sure you use GOOD gaskets. Remember that gasket thickness with stock type gaskets are measured by the crush thickness, not the uninstalled thickness.
As for ECU replacement, it's probably not fully a function of the ECU itself. A lot of factors will determine how well high compression is handled by your OEM engine management. Fuel and air quality and temperature, combustion chamber shape, etc... With the fuel we have in this day and age, only a handful of cars have a better than 10:1 compression... and their cylinder heads were engineered to have such high compressions. I would dare to guess that if you could knock your compression down to 11:1 and have your combustion chambers and piston tops smoothed out, you might well be fine, but only with some really good fuel which is not always practical. There are plenty of "tricks" you can do to get 11:1 working on typical fuel, but you will lose significant power in each case.
Good luck,
Abdul
I just wanted to address your original question regarding head gaskets. I don't run copper head gaskets, but I have read bits on them here and there over the years.
Copper head gaskets have several good and bad points.
good:
They don't blow out
They are reusable
Bad:
They don't seal well on imperfect surfaces (typically leaking oil)
They are expensive
The quality of copper may vary
For a high compression application O-ringing is the way to go, but if you dropping your compression then it's probably not the most useful thing to do with your money.
A thicker gasket will alter the timing index of the camshaft vs. the crankshaft, however if you heads have been surfaced with a significant amount of material removal, this effect may be minimized. To correct the shift, you'll need to degree the cams and reindex the camshaft gear. On the other hand theshift may be relatively minor, and/or perhaps even beneficial to your power making goals
An old trick that the great turbo gods (Gale Banks, et al) would do would be to double up the head gaskets. This increaced the number of failure surfaces, but you might find it to be more fogiving than copper gaskets. You'll need to check you head bolts/nuts a few times in the first few thousand miles of running the engine, and make sure you use GOOD gaskets. Remember that gasket thickness with stock type gaskets are measured by the crush thickness, not the uninstalled thickness.
As for ECU replacement, it's probably not fully a function of the ECU itself. A lot of factors will determine how well high compression is handled by your OEM engine management. Fuel and air quality and temperature, combustion chamber shape, etc... With the fuel we have in this day and age, only a handful of cars have a better than 10:1 compression... and their cylinder heads were engineered to have such high compressions. I would dare to guess that if you could knock your compression down to 11:1 and have your combustion chambers and piston tops smoothed out, you might well be fine, but only with some really good fuel which is not always practical. There are plenty of "tricks" you can do to get 11:1 working on typical fuel, but you will lose significant power in each case.
Good luck,
Abdul
08-14-2003, 10:33 PM
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As I said before I really don't see an issue running the compression that high. I built a 7.0 ltr 928 running on the street with 12:1 compression and a very aggressive injection map and he has no issues on 93 OCT. Here in California the 91 Oct stuff is horrible and the same car would ping like nuts! But with the injection we could pull the timing map back and run a less aggressive fuel map to help the motor down low. That car has a full EFI system which is a bit much for a milder application such as yours Chris.