Looking to build my motor and concerned that if I get 8.5:1 compression pistons that it may introduce to much lag. I'm not after top end hp numbers but a really nice solid drive on the street. The HKS low comps are 9:1 and the cosworth pistons are 8.8:1.

I'm thinking of getting the cosworths with tomei rods. Anyone used the cosworth pistons or have anything good/bad to say about them. I did search but everthing I found is based on other manufactures.

http://www.cosworth.com/shop_item.php?productid=403

My goals on my HKS ST is to run only about 10psi which should put me in range of around 440hp at the wheels from what I have seen from other installations with my lit.

If the low comps will introduce lag will a set of cams resolve this and if so which duration/lift would you recommend.

Michael

 

besides the cosworth and HKS pistons, don't forget to look at:

Mahle
Wiseco
JE
Arias
CP
Ross
Diamond

those are just the shelf stock pistons that come to find.

of course, you can have custom pistons pistons by any of these manufacturers.

 

Most of the above off the shelf are 8.5:1 and hence why I listed only the CW and HKS. I know CP's make custom pistons but was trying to get away of getting a set made if required.

 

You will come hardpressed to notice a difference between 8.5 and 9.1. The size of the turbo is going to have more to do with lag then the CR of the engine. I believe the Wiseco's are 8.8 and the Arias extreme duty are 9.1.

 

i think the mahle's are 8.8 also.

 

arias extreme duty...9:1 .....sharif sells em cheap...990 shipped....hehe

 

+1. I really dont think there will be much percievable difference between the two. Just go with the highest quality pistons you can get on your budget. I haven't heard of any customer feedback on the Cosworth pistons.

 

to the OP.....for a street motor, you should likely choose a piston that keeps the oil ring above the piston pin area.

 

Michael
I used the Arias 9.1 extreme duty in Andre's rebuild & custom 10.1 Arias ED's in Murrays motor. Both motors had no excesive noise when cold & represent excellent value at under a grand US. The Cosworth pistons come friction coated & will be excelent but expensive given that the rings are an extra cost.
IMHO the tomei rods are too expensive. The Eagle ARP2000 combo are great value and to date with 00's sold I have not heard of 1 complaint or failure in a VQ

Peter

 

I thought the Arias are 8.6 but after checking forged internals website I can see that they get there own spec'd pistons made. The Arias site also state they can come pre-coated skirts which is good as I want to reduce the hassle to get it done later. My only concern is the required "Piston to cylinder block clearance" which may introduce slap/noise with Arias.

 

For tight piston to wall clearance and an 8.8:1 ratio, go with the Mahle pistons. The forging uses a 4032 alloy which does not expand as much as 2618.

The skirts are coated to reduce friction, and the rest of the piston is also coated in some other material. It is a beautiful forging; I was carefully examining them at PRI this past weekend.

remember that most of the "extreme" pistons on the market have pushed the rings down so much that the oil ring is in the pin area. This will almost certainly cause the engine to burn more oil than a piston that keeps the oil ring above the pin area.

Here's a couple pictures of the Mahles that one of their engineerings emailed to me.

 

I have read in the past that for a motor that you want more then factory amounts of HP to not use 4032 alloy. I could be wrong, but I thought that it expanded more then the 2618. I know the 2618 is stronger and still will not have piston slap is installed properly. I have CP pistons, which are 2618, and have no noise on cold starts and they perform awesome.

 

I likethe CP pistons alot.

the 4032 alloy is a better street alloy due to the much lower expansion rate. Also, it's strength properties are more stable over time.

2618 has higher overall strength, but it's service life will be shorter than 4032.

 

got an email from cosworth for the fun of it and their piston to bore recomendation is 2.5 thou for their pistons

 

Sounds like they are using the 4032 alloy for their pistons.

manufacturers of 2618 alloy pistons for the VQ usually recommend .0035 to .004"