Wiseco recently released a shelf piston in 12.4:1 Compression Ratio for the K20A motors. This was previously sorely lacking in their 2008 and 2009 catalogs. A high compression piston for K20A motor was almost solely dominated by CP Pistons as they were very early adopters in this category.
I have always been a fan of Wiseco Pistons as I find them to be consistently superior in quality and value. Most of their offerings come in lightweight guises and also has the skirt coated. Many other companies tend to charge extra for that.
What we have here are all shelf pistons from the various companies. They are in the same bore size (86mm) and for stock K20A motors.
Left: CP Piston X-forging. Weighs 295 gram. Advertised 12.5:1 Compression Ratio. Skirt uncoated.
Middle: Wiseco Piston. Weighs 320 gram. Advertised 12.4:1 Compression Ratio. Skirt coated.
Right: Stock PRC Piston. Weighs 336 gram. Factory 11.5:1 Compression Ratio. Skirt coated.
All opinions are just based on visual inspection on these.
Firstly, lets take a look the obvious difference in the pistons.
- Wiseco looks to have the highest dome.
Although these have the highest dome, but only an advertised compression ratio of only 12.4:1. It could be that the dome volume is being negated by bigger valve pockets and thus sacrificing a little compression ratio.
- Wiseco and stock pistons has the skirt coated.
This would aid in reducing friction and making for a more efficient engine. This can even lead to improved fuel economy.
- Wiseco pistons offers anti-detonation grooves on top ring lands.
The stock piston also has the same anti-detonation grooves, but they are very shallow and finely cut and does not show up very well in the pictures. CP pistons however do not have any of the anti-detonation grooves present in their design.
- CP pistons weighs the least.
The mass on the CP piston is very evident. Just by holding different pistons in both hands, you can already tell that the CP piston is the lightest of the lot.
- Wiseco has the deepest valve reliefs.
Upon measuring with my trusty vernier caliper, I can safely deduce that the Wiseco piston will accept higher lift camshafts with significantly less problems compared to the stock piston. In comparison to the CP piston, it is marginally deeper, but at this stage where we are getting aftermarket camshafts bordering 14 mm of lift, every little bit of additional clearance adds up.
- CP piston has 2 sharp edges along top ring land
Both the CP and Wiseco piston are machined to accept the largest possible intake valve, and as such part of the top ring land is being machined into. Running my fingers along the edge, I realize that the machining process CP piston did for their edges does not have “finishing” work done to deburr it. I would sand it down a little bit more if I were to put it into any engines.
- CP piston under piston area more weight savings
There are more hollows in the forging that the CP piston uses; thus saving weight. It looks like CP piston used a special custom forging for this piston in order to achieve its superior weight.
- Wiseco has thicker top ring lands
The Wiseco piston measures a thicker ring land of 7.4mm vs the CP piston’s 5.2 mm. This thicker ring land should give little more insurance in abusive racing environments. The stock piston ring land measures 6.5 mm.
PS: In all pictures, Left-most piston is CP, Center is Wiseco and the Right-most is Stock PRC.
Now, lets compare between wrist pins, Left is CP and Right is Wiseco.
Both of the wrist pins weigh the same; 72 grams. The stock one is 90 grams. I do however realize that the Wiseco wrist pin has a taper on both outsides in its bid to keep the weight to the minimum.
Hope the data discussed here is of some use to anyone who are wanting to choose between these pistons.