Compression Ratio increase

P

PDoane

New member
The rules say

Where a factory specification for original cylinder head thickness
can be proven, a tolerance of .025 inch less than the
service limit will be permitted. Under no circumstances may
the compression ratio be increased by more than one-half
(.5) point. An offset key may be used to return cam timing
to the factory specifications. On engines with dual overhead
camshafts, this key shall be installed on the crankshaft only.

Since it says that right after the line about head thickness and before the line about offset key, I would assume you gain the CR by milling the head, but it doesn't say you have to. Milling the block or CR increasing pistons are other options.

Would a CR increasing piston violate the piston rule?

Engines may be bored to a maximum of .040 inch over standard
bore size. Factory replacement pistons or their equivalent
with the exception of diameter shall be used. Cast or forged
equivalent pistons shall provide the same dome/dish/valve relief
configuration, ring groove width and spacing, pin height relationship,
weigh no less than the factory standard bore pistons.
Piston rings are unrestricted.

Thanks for any info/help

Peter
 
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Seems like it would be hard to meet the "factory replacement or equivalent" piston limitations and increase the compression...

K
 
The original intent of the compression (and overbore) rule was not as an allowed method to increase performance, but as a method to allow engines to be economically serviced/rebuilt. As such, it was expected that all parts will remain within service limits (e.g., blocks and heads will be milled within FSM service limits.) However, absent the compression ratio limitation you could machine all parts to within service limits yet stack up those limits to a significantly higher compression ratio. Thus, the "Under no circumstances" compression ratio limit.

However, being racers that we are, we intentionally choose to use that serviceability allowance to increase performance (Roffe Corollary).

So, to answer your question: no pistons, keep the block and head milling to within factory service limits, up to an additional 1/2-pt compression ratio.

GA

P.S. The ultimate irony, of course, is that rules that were originally intended to allow greater flexibility in service repair are now being used in such a way as to invalid their original purpose...but that's racing.
 
Agreed with Greg and Kirk. Have to use stock pistons, and can only get the advantage of the compression bump with milling the head.
 
This type of read on the rules is common - and should be a lesson to the ITAC and CRB when writing rules and class intentions.

It's very clear to me that the rule ONLY allows CR increases through the milling of the head and then caps it at a half a point regardless of your service parameters.

Peter, help us through the thought process here, why would you think you could raise the CR any other way?
 
Keep in mind that an exact OEM equilavent yet overbored piston will increase compression. In my D16z6, the calc say so by .1 cr for every .5mm (.020) overbore. Ironically, * I heard* factory oem overbore pistons have their the pin height adjusted to make CR the same as the original bore piston. Just an FYI and I felt the need to post unless info. :)
 
mossaidis said:
Ironically, * I heard* factory oem overbore pistons have their the pin height adjusted to make CR the same as the original bore piston.
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That is completely correct; aftermarket pistons do the exactly same thing. It's how they're intentionally designed engineered to maintain the same CR.

I've always wondered what would happen if someone protested the pin height on an overbore piston, how that would end up...ain't gonna find a standard overbore piston that's NOT changed on the pin height... - GA
 
I think that shaving the block should also be allowed. I'm pretty sure them newfangled aluminium blocks were not very common when this rule was written.
 
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I mis-typed my original post. Other than milling the head, you can also mill the block or use CR increasing pistons (which I assume are just pin height differences). Thanks for letting me know about the O/S psitons changing the pin height to retain OEM CR. I hadn't heard that before.

I felt the inferrence of the CR statement right after the head height statment is that CR increase is by head milling, but it is not definitively written that way. Warpage of the head or block can be fixed by milling the item. I think Greg got it right in that the intent was to allow for that kind of correction to a high mileage motor. I also thought the CR increase rule might be intended to allow for carbon build-up on an old motor.
 
Flyinglizard said:
Milling the block is allowed.
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for Peter, not all manufacturers adjust pin height for CR, in fact I would say most do not. You cannot have custom pistons with a different pin height to achieve a differen CR. however if you bore your motor out, and leaving the pin height in the stock location causes (will cause CR to increase due to the change in volume) then that is legal.

mike, As for milling the block I see no where in the ITCS that allows milling the block. I know that in the past you used the 'no spec' factory tolerances as your reasoning for phasing the cam at whatever degree you wanted http://www.improvedtouring.com/forums/showthread.php?t=29454&highlight=timing. That same loose argument could be made here as well, but I believe that it is against the intent of the rule IMHO.

I believe block decking should be legal, in the days when the rules were written the norm was aluminum head and iron block. Now days with wet sleeve aluminum engines, they are not as durable as the older iron block motors. I also think that you should be able to run whatever headgasket you like to acheive the +0.5 CR. this way if you have a good block and head, you do not have to shave your good head to get the extra half point of compression allowed. However, both are currently illegal. So save the money and buy a bad a slightly warped head, mill it flat and go.
 
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Flyinglizard said:
Milling the block is allowed.
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only when the FSM says it is. and only to those specs. the head is allowed to be machined PAST the factory specs or to a dimension set by the club when their is not an indicated service limit.

if your FSM doesn't include a tolerance on deck height or another indicator of same, the block cannot be milled. I think that the rule should read like the head, for those cars that dont have a tolerance in the manual.
 
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Chip42 said:
only when the FSM says it is. and only to those specs.
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...and in a casual glance at the regs, I can't find that rule any more! Maybe I'm just missing it, but "back in the day" the regs specifically used to say that all work to stock parts must be done to FSM specs, which is how we were allowed to machine the block and head(s). If that's missing, that's a biggie.

Anyone else find that somewhere?
 
Greg Amy said:
...and in a casual glance at the regs, I can't find that rule any more! Maybe I'm just missing it, but "back in the day" the regs specifically used to say that all work to stock parts must be done to FSM specs, which is how we were allowed to machine the block and head(s). If that's missing, that's a biggie.

Anyone else find that somewhere?
Click to expand...

In my mind, it's covered by - 9.1.3.A. Purpose:

They will be prepared to manufacturer’s specifications except for modifications permitted by these rules.​
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I can't imagine "...manufacturer's specifications" not including the FSM called out in 9.1.3.C.
 
The standard block prep for any engine includes the deck.
The rules says .5 compression increase to address the fact that many FSM dont address a rebuild at all. Just replacement.
Many FSM dont even have a block deck spec any where in them. If they did , why? to address the rebuild specs of milling the block..
"All" old bolcks will have the bolt holes puled up a bit over time. If you cant deck the block , than you have to find a newer block??
Dont kid your self. Deck the block , cc the assembly.
PTW sizes, ring sizes, head bolt torque, etc. Usually fall outside of the FSM specs .
Worry about total compression, cams and cam timing. Stuff that makes power. IMHO. MM
 
Flyinglizard said:
Many FSM dont even have a block deck spec any where in them. If they did , why? to address the rebuild specs of milling the block..
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Because you can "stack the specs" on an engine with FSM specs (which were standard practice back in the 80's) and end up with more than .5 CR increase. That's why the verbiage "Under no circumstances may the compression ratio be increased..." instead of an allowable "you can do this" kinda thing. Increases in compression ration were not seen as an allowed modification but rather as a expected consequence of a normal rebuild within FSM specs.

Gary, I concur. Don't remember seeing that ever get changed, but that works. - GA
 
PDoane said:
Other than milling the head, you can also mill the block or use CR increasing pistons (which I assume are just pin height differences)
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Another illegal way to easily and cheaply increase compression is to use a piston, for instance, that has a flat-top instead of a dish. That is the case with the SR20's we're running now. I have a spare motor waiting to be built for next year that has UK-spec flat top pistons in it. They raise the compression from 9.5-1 to 10-1, but I'll have to toss them for the new rebuild and get the head CC'd and milled to get that extra .5 point. I'm assuming most motors have pistons available with a dome or such that would be similar...
 
Brettt said:
Another illegal way to easily and cheaply increase compression is to use a piston, for instance, that has a flat-top instead of a dish.
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Not IT-compliant, gotta use stock pistons or 'exact equivalent'...want me to delete your post...? ;)

On edit...just re-read the post and see he knows that...see my highlight. Not a smart move posting that...just sayin'...
 
Yes, I was in no way advocating doing something illegal in the build of a car; I was simply trying to point out another way other than pin height that pistons can raise compression. After reading my post again, I can see that it was fairly vague and didn't clearly state that I wouldn't be using the flat-top pistons in the new motor. What I meant was that we'll have to discard the non-IT spec pistons in order to use the stock dish-top pistons and have the combustion chambers CC'd and the head milled accordingly if we want to legally gain the extra .5 point of compression. In all honesty, we're thinking of not even bothering trying to get that last .5 point, since the machine work required for the HP returned isn't really something that is likely to be in our budget at this time.

I apologize for any confusion my prior post may have caused. I won't build my car with any illegal parts, nor will I advise anyone else to do so. What's the fun of winning if you know you're cheating?
 
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Here's another illegal freebie for you... An easy way to up the compression on a 4-valve KA is to use stock US-spec 3-valve pistons on the 4-valve. instantly bumps you from 9.5:1 to 10.5:1.

and umm yeah.. obviously not legal in IT, but works for STU and EP if you want to run at the back of the back with the rest of us half-built cars. :)
 
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