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NO!!! In our application, more compression is better unless it’s at the expense of cam timing. Increasing bore to legal limits does increase the CR and this is a very, very good thing.
Free Tip: If you have more than one 885 cylinder heading laying around, CC them before you select which one to use. All 885 castings are not created equal and there is a small variance in combustion chamber size. Smaller is better; I’m just saying…
[quote=“Foglght” post=66840][quote=“Ranger” post=66823]
What does piston-cylinder wall gap and compression #'s have to do with a thicker HG or valves hitting pistons?
Area available on the head? Area?[/quote]
Part of the function of compression ratio is bore size. Increase the bore size and you increase compression. You’d only need to account for that if you had to bore for oversize pistons rather than hone and re-ring. If you increase compression due to overbore, you would want to think about a thicker head gasket.
However, I am interested in the actual BMW specs for compression tolerance, as well as chamber size.[/quote]
You are trying to avoid increasing compression? Why?
NO!!! In our application, more compression is better unless it’s at the expense of cam timing. Increasing bore to legal limits does increase the CR and this is a very, very good thing.
Free Tip: If you have more than one 885 cylinder heading laying around, CC them before you select which one to use. All 885 castings are not created equal and there is a small variance in combustion chamber size. Smaller is better; I’m just saying…[/quote]
Oh no, you misunderstand. I agree that more compression is generally better. A few things come to mind when doing this:
There are factory compression limits. There is an overbore allowed for rebuild. However, increasing compression is not in the rules and is not expressly called out. If you were going to do that, I think a few people would call that illegal, thus the comment about thicker gaskets.
Increased CR is good for power, however when building a race motor that only allows for pump gas are you going to heat soak the motor and get knock sooner?
I completely agree that the smaller chamber size is better. Again, you will increase compression. The question I still have is about chamber size limits, and factory compression limits. I bet those guys at Metric Mechanic know what those limits are, or Turner, or any decent BMW shop.
Yup, its neat and a good part of building rocket race motors. Unfortunately, I’ve been down that road. I don’t build them myself, but I do know what to ask for. If this is supposed to be the cheap spec class, there are quite a few simple rules that can negate most of this medium style building and separate everyone from those very few who will go far above and beyond just to have a couple extra hp’s.
Sorry for being a negative nancy. Don’t hate me.
NO!!! In our application, more compression is better unless it’s at the expense of cam timing. Increasing bore to legal limits does increase the CR and this is a very, very good thing.
Free Tip: If you have more than one 885 cylinder heading laying around, CC them before you select which one to use. All 885 castings are not created equal and there is a small variance in combustion chamber size. Smaller is better; I’m just saying…[/quote]
Oh no, you misunderstand. I agree that more compression is generally better. A few things come to mind when doing this:
There are factory compression limits. There is an overbore allowed for rebuild. However, increasing compression is not in the rules and is not expressly called out. If you were going to do that, I think a few people would call that illegal, thus the comment about thicker gaskets.
Increased CR is good for power, however when building a race motor that only allows for pump gas are you going to heat soak the motor and get knock sooner?
I completely agree that the smaller chamber size is better. Again, you will increase compression. The question I still have is about chamber size limits, and factory compression limits. I bet those guys at Metric Mechanic know what those limits are, or Turner, or any decent BMW shop.
Yup, its neat and a good part of building rocket race motors. Unfortunately, I’ve been down that road. I don’t build them myself, but I do know what to ask for. If this is supposed to be the cheap spec class, there are quite a few simple rules that can negate most of this medium style building and separate everyone from those very few who will go far above and beyond just to have a couple extra hp’s.
Sorry for being a negative nancy. Don’t hate me.[/quote]
“If you were going to do that” means do what? The only thing discussed re. increasing compression is cherry picking heads. It’d be real hard to take exception to that. It sounds like you’re trying to say that if a person bores 020 over they’re “obligated” to run a thicker HG. That’s goofy.
I’ve yet to hear a SpecE30 racer say his engine is knocking. Our engines are supposed to be able to run on 87. So run 91. Heatsoak? What precisely is heatsoak anyhow? If a person is concerned about high engine temps, there’s numerous ways to improve the cooling system, many of which are free.
[quote=“Ranger” post=66848]
“If you were going to do that” means do what? The only thing discussed re. increasing compression is cherry picking heads. It’d be real hard to take exception to that. It sounds like you’re trying to say that if a person bores 020 over their “obligated” to run a thicker HG. That’s goofy.
I’ve yet to hear a SpecE30 racer say his engine is knocking. Our engines are supposed to be able to run on 87. So run 91. Heatsoak? What precisely is heatsoak anyhow? If a person is concerned about high engine temps, there’s numerous ways to improve the cooling system, many of which are free.[/quote]
Do the stackup for CR with cherry picking your favorite head, using a thin head gasket, using 20 over pistons, and maybe taking .015 off the head for “flatness.” I’m going to bet you aren’t anywhere near the factory tolerances anymore. If so, its not legal. If the community IS going to deem that legal, which would be a hell of a leap that’s fine too.
If I remember reading the original owners manual it recommends 89 as a minimum, but performs better on 91. If anyone has an original owners manual they can confirm. We can debate the possibility of knock on 93 octane with increased compression at another time. I just got back from Korea and I’m tired as hell.
I have at least 3 original owners manuals and can’t be bothered to go find one. I’m quite sure they say 87. Maybe they say 89 or 91 RON which is still not as good as 87. My motorcycle said 98 RON which is equivilent to 91 RON/MON. Nowhere in the rules does it limit compression. The rules are item by item.
[quote=“Foglght” post=66850]
Do the stackup for CR with cherry picking your favorite head, using a thin head gasket, using 20 over pistons, and maybe taking .015 off the head for “flatness.” I’m going to bet you aren’t anywhere near the factory tolerances anymore. If so, its not legal. If the community IS going to deem that legal, which would be a hell of a leap that’s fine too.
If I remember reading the original owners manual it recommends 89 as a minimum, but performs better on 91. If anyone has an original owners manual they can confirm. We can debate the possibility of knock on 93 octane with increased compression at another time. I just got back from Korea and I’m tired as hell.[/quote]
Dude, WTF? Since.020 pistons and decking the head to the limit (4.909"?) are legal, the resulting compression is legal. The HG would need to be OEM thickness tho, I’m not sure what you meant by “thin”. Creating restrictions that are not specified in the CCR just gets folks riled up. Go race more and quit obsessing over the rules, they just aren’t that complicated.
Re. octane. P5 of the manual says 87 AKI, 91 RON. http://www.gress.org/Home/Cars/TrackTales/DIY/e30_Owners_manual.pdf
+1
Overbore and shaving the head with your favorite flavor of stock (or thicker) HG is explicitly allowed by the rules. Thererfore, the resulting CR is legal…
+1
Overbore and shaving the head with your favorite flavor of stock (or thicker) HG is explicitly allowed by the rules. Thererfore, the resulting CR is legal…[/quote]
Resulting compression is legal, if it falls within factory limits, as stated by the very rule you are referencing.
8.8:1 is stated as factory.
Sorry you don’t want to discuss rules in a rules thread with the very title of “rule nerds question.”
8.8 is nominal and somewhat irrelevant. By your logic, .020" pistons aren’t legal because they increase displacement by 20 CC over “factory”.
Besides our rules are not exactly aligned with “factory” specs. Our rules permit shaving the head and .020" over pistons and therefore permit the consequences of these changes.
[quote=“FishMan” post=66859]8.8 is nominal and somewhat irrelevant. By your logic, .020" pistons aren’t legal because they increase displacement by 20 CC over “factory”.
Besides our rules are not exactly aligned with “factory” specs. Our rules permit shaving the head and .020" over pistons and therefore permit the consequences of these changes.[/quote]
My logic is this:
9.3.1.2.6. No engine component may be modified in any manner not specifically permitted
or authorized by the Factory Service Manual, the Bentley E30 Manual, or Factory
Technical Bulletins.
9.3.1.5.1. Compression ratio may be changed only within the tolerances affected by
resurfacing for trueness and within factory tolerances or as allowed by these
regulations.
I don’t have the specs from BMW, but it looks like as long as you stayed within factory compression specs one would be fine. If the manual doesn’t state a compression spec, you’d have to fall back on chamber size, deck limits and cyl head limits instead of an actual compression number.
IIRC the bentley manual calls out a PSI spec for compression. Is that not valid then in your eyes?
This really isn’t me bitching, I’m just discussing. I’ve got at least another year before I can build a motor.
Does anyone have dyno data on the engine that is “just put together” versus the one with the “trued” block surface and the head at 4.910 as well as a thin head gasket?
I do.
The piston(s) hit the head. You have to install the thicker .30mm/ .012" headgasket. Remove head reinstall,try again, try again, try again. You go from psi numbers of 170/175 to 180/185. Great, go to the dyno.
No appreciable difference in the hp or torque.
Fish and I have talked about this. You need a chip to take advantage of the compression and higher octane fuel. Chips aren’t allowed.
RP
(Your series co-director for the SE and trying to watch out for the whole spec e30 community.) Thanks to Ffish and Ranger for sharing their MM experiences and to Chuck Badder for teaching us all a thing or two. Also, Levie knows a thing or two. All of these raceers are on the up-and-up trying to keep the rules stable by sharing their knowledge.
[quote=“Foglght” post=66866]
My logic is this:
9.3.1.2.6. No engine component may be modified in any manner not specifically permitted
or authorized by the Factory Service Manual, the Bentley E30 Manual, or Factory
Technical Bulletins.[/quote]
The work being discussed is specifically permitted and authorized.
The work being discussed is within factory tolerances and also allowed by the CCR.
Analyze the variables associated with compression, create statistical models to define standard deviation, and then develop a test that will work in the paddock. Knock yourself out. I’ll be the one drinking beer, trading one-liners and flirting with everyone’s girlfriend.
Imagination not valid. Bentley discusses relative compression #'s but not absolute #'s. Compression test #'s by there very nature are never going to be absolute enough for direct comparison. All sorts of things affect how fast a starter turns an engine, and that’s just one variable among many.