Subaru Impreza GC8 & RS Forum & Community banner

1 - 11 of 11 Posts

·
Registered
1998 RS, RBP
Joined
·
106 Posts
Discussion Starter #1
Hello everyone;
Long time. Has anyone ever built a 2.5L NA engine using 100.5mm replacement pistons?

The question comes as SCCA Improved Touring now allows the 98-01 Impreza RS in the ITS category and out of the few things that are allowed one of them is pistons up to 0.040"(1mm) over bore. That would make it 100.5mm and even when we can find them we'd never done it nor know anyone that has.

Thanks,
 

·
Premium Member
overdeveloped beater
Joined
·
8,335 Posts
Cu$tom order a set and have a block bored to fit them. It's an $800 test.
 

·
Registered
1998 RS, RBP
Joined
·
106 Posts
Discussion Starter #3
There is a company DNJ that offere them from STD to 0.040" over and the reason I ask is to know if anyone has used them and what sort of results they'd had.

If they are worth it for a NA engine perhaps could be used for a track setup as we all know they isn't any replacement like displacement.

std/99.5mm = 2456cc
0.20"/100mm = 2481cc
0.40"/100.5mm = 2505cc
 

·
Registered
2001 Impreza RS - Black
Joined
·
227 Posts
I don't believe any one (or hardly anyone) has gone that large is because I believe the factory service limit is +0.20" at 100mm. Will another +0.20" cause the cylinder walls to be too weak or some other issue? I have no idea, but for how much it costs to try, and the cost of it going wrong, I'm not surprised not many have done it.
 

·
Registered
1998 RS, RBP
Joined
·
106 Posts
Discussion Starter #5
I don't believe any one (or hardly anyone) has gone that large is because I believe the factory service limit is +0.20" at 100mm. Will another +0.20" cause the cylinder walls to be too weak or some other issue? I have no idea, but for how much it costs to try, and the cost of it going wrong, I'm not surprised not many have done it.

My thoughts exactly, I have a shop and been working and racing Subarus for some time. My question comes from the fact that I saw the pistons and thought perhaps that someone went against any logical and just wanted to findout their results.

Thanks,
 

·
Banned
Joined
·
4,438 Posts
if you can feel the net result from a 24cc net piston only change on this engine, you would be an aberation of human kind ;)

Wossner makes them btw...in a low compression though (8:1) for forced induction
 

·
Registered
1998 RS, RBP
Joined
·
106 Posts
Discussion Starter #7
if you can feel the net result from a 24cc net piston only change on this engine, you would be an aberation of human kind ;)

Wossner makes them btw...in a low compression though (8:1) for forced induction
I agree with you on the minimum results of the additional 24cc, but remember that in a very restricted rules of SCCA Improved Touring any gain is good gain. My mejor concern is relaibility for an NA engine with factory everything.

Thanks,
 

·
Premium Member
overdeveloped beater
Joined
·
8,335 Posts
Keep in mind that next season you may need to hone the bores....if you go max now, you'll be tossing that block when it needs minor refreshing.
 

·
Registered
2001 Coupe
Joined
·
1,261 Posts
In other sanctioning bodies you sometimes see oversized allowances go up in time as old race mills need more and more refreshes, especially if it's on a block that's now fairly old.

Out of curiosity, does scca allow you to keep the "free" compression you'd get from that big of a piston or do you have to bring it back down?

Just curious. Never read an scca book.
 

·
Registered
1998 RS, RBP
Joined
·
106 Posts
Discussion Starter #11
In other sanctioning bodies you sometimes see oversized allowances go up in time as old race mills need more and more refreshes, especially if it's on a block that's now fairly old.

Out of curiosity, does scca allow you to keep the "free" compression you'd get from that big of a piston or do you have to bring it back down?

Just curious. Never read an scca book.
j. 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.

k. Balancing and “blueprinting” of the engine assembly are
permitted. Lightening of parts beyond the minimum material
removal necessary to balance is prohibited.

l. Manifold and cylinder head port matching is permitted. No
material may be removed further than one (1) inch in from
the manifold to cylinder head mounting face(s). Carburetor
mounting surface(s) shall not be modified, and external dimensions
of the cylinder head or intake manifold may not be
reduced to facilitate internal porting. Two piece manifolds are
not intended to be port matched at their intermediate point.
Valve guide material is unrestricted.
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.
 
1 - 11 of 11 Posts
Top