Topline 2G Pistons on 1G Rods, Whats the Compression Ratio?

[Mike1992]

I just picked up a set of used 2G Topline pistons mounted on machined 1G rods. After researching I've come to wonder if these have the factory compression ratio or different. The guy I bought them from said he bought them from importperformance and was told they were indeed the stock 8,5:1 Cr, however Slowboyracing also sell Topline pistons and told me all topline pistons 1g and 2g have a Cr of 8,0:1. Ive talked to both companies and both have something different to say. Anyone know the real deal?

 

[Big Woo]

I have seen this first hand, I could take some pics tonight and post them tomarrow.
Basicily the Topline pistons (at least the ones I have seen) have the same dish volume for both the 1G and 2G pistons. The only real difference is the wrist pin diameter, and the part number.

I have compared the dish size dimensionally and found it to be almost identical to a stock 1G piston.

I have not had any problems with the pistons, and have used several sets over the years on various cars, no returns or piston related problems.

As an alternative to the topline pistons, that does have the correct dish volume for the 2G compression ratio, and if you don't want to use the Mitsu piston, try the "ITM" brand.

 

[Mike1992]

Well then I guess they are 8,0:1, which defeats the purpose of me even going with 2g pistons Since I got a deal on the setup Im gonna use them in the rebuild anyways though. Sucks but oh well Thanks

 

[RiceKiller_TSi]

As an alternative to the topline pistons, that does have the correct dish volume for the 2G compression ratio, and if you don't want to use the Mitsu piston, try the "ITM" brand.

Big Woo is right and the ITM brand wont cost you much if you want to slap those other ones on eBay or what not. Check this out. ==>http://www.store.partsdinosaur.com/product92.html

These are the ones I'm installing for my rebuild.

 

[Mike1992]

Well considering I have to buy new rings, I might just do that as that would only be about $50-$70 more.

 

[Deez]

Why are you using used pistons? Anytime a rebuild is required, it is recommended to use new pistons and rings, since more often than not, you'll need to overbore the block to accommodate a new cross-hatch pattern to allow the rings to seal (and remove any abnormalities).

On the other hand, the topline pistons I've used in 'a' rebuild, were 20 over and supposedly 2G compression. After breaking in the motor and doing a compression test, the pressure in all cylinders were at a healthy 175 psi. Seems to me that they weren't lying. YMMV.

 

[RiceKiller_TSi]

After breaking in the motor and doing a compression test, the pressure in all cylinders were at a healthy 175 psi. Seems to me that they weren't lying. YMMV.

Did you use the Motoman break in method?

 

[Quasimondo]

Did you use the Motoman break in method?

What's that?

 

[Mike1992]

They are .020 overbore topline pistons, they supposeably have about 10k on them. The guy I bought the piston/rod combo from said hes going SBR stroker so was selling a few things. I paid $215 shipped for this setup, not bad if you ask me. I will be boring my block .020 over, I thought long and hard about it and coming acorss the rod/piston deal was the deciding factor. My cylinder walls have a nice fine cross hatch leading from the top pretty far down, so my cylinders look awesome but they're getting bored anyways. My engine has 91,000 miles on it and the compression really sucked, about 30 shy from perfect in each cylinder.

 

[RiceKiller_TSi]

What's that?

http://www.mototuneusa.com/break_in_secrets.htm

 

[Mike1992]

http://www.mototuneusa.com/break_in_secrets.htm

Ive never rebuilt an engine before, but that just seems wrong to fully load an engine in the first moment of startup.

 

[RiceKiller_TSi]

Ive never rebuilt an engine before, but that just seems wrong to fully load an engine in the first moment of startup.

I read the entire article twice and it makes logical sense to me. I'm going to break in my new engine using this method. I'm sure others will agree that this method has worked for them.

 

[Mike1992]

I just cleaned these pistons/rods up and noticed theres no 63DT on top, like the stock pistons. These ones have a very small 020 and an arrow on the top. THe arrow is normal, and I guess .020 means .020 over. But why dont these pistons say anything on top like a stock piston?
Examples 1 2 3 Also while cleaning them I used a s.o.s. pad and it took some coating off the piston (around the sides), is this a concern?

 

[Mike1992]

My Pistons/Rods Plz compare to the links, one post above and explain.....

 

[diambo4life]

Ive never rebuilt an engine before, but that just seems wrong to fully load an engine in the first moment of startup.

That's the correct way to seat the rings. I see and hear alot of people baby their motors for 2K miles by never going above 3,000rpm and they wonder why they have crappy compression and the rings never seated well. *suckers*

I don't know why you are upset about the 8.0:1 C.R. That's actually really good. You can run more boost on pump gas especially with the shitty 91 that seems to be rampant around here nowadays. It's a good deal for the pistons/rods but at the expense of boring out cylinders that are in good shape? I don't know. It's your choice I guess.

 

[Mike1992]

Im not an engine builder or anywhere near a tuner, but wouldnt that little extra kick in compression make me the same power with less boost? I also figure since my engine has 90,000 miles cylinder walls will probably be out of round and need a boring anyways. On another note I also have no access to a torque plate either, so I guess Im not gonna be usiing one. I know you know your stuff mike, and im going full on into this praying it works out for me. If you want I'd love to talk it over with you sometime and get some better idea's.

 

[diambo4life]

Mike, you will gain more HP from airflow (ie. higher boost) than a compression jump. Higher compression will make the car more responsive/peppy off boost but you will be limited on how much boost you can run with the high C.R. pistons which will effectively cap your airflow/HP. Also, your margin or window of tuning error is limited with "high" compression pistons like the 9:1 compared to 8.0:1 pistons. Did I mention that without a decent engine management tuning device (eg. DSMlink, AEM, Haltek etc) where you can independently alter timing, it will be a nightmare tuning high comp. pistons on pump gas ESPECIALLY on 1G's with their super aggressive timing maps? It's your choice.

 

[TurboDSMer]

I have a question how is the strength of the topline pistons compared to the mitsu? I know they use the same casting method but is there a difference in durability?

 

[Mike1992]

I ended up selling that combo I had and got my money right back. My blocks in the machine shop getting bored .020" over, decked and cleaned up. They wanted my pistons and rings to get the proper clearences for the honing process or whatever, I didnt quite understand that part. I purchased the "ITM" 2g pistons and will have my rods machined to fit when I take my crank in to get checked out and have the assembly balanced.

Diambo - I'll be having the block surface decked along with the head, I told the machine shop I'll be using a metal head gasket, (mitsu 4-layer) so both should come back smoother than a baby's bottom and the mitsu 4-layer should drop the cr a tad. I was told my block needed to be bored, it was out of spec at the top where the rings ride the most according to the shop by about 4 thous. WTF

 

[BreathinHeavy]

i broke my motor in to something close to that .... reconmended by darren at ffwd my engine builder