Well, my motor took a dump a while back... apparently I had some damage to my crankshaft and had some scoring on the cylinder walls... My oil pump may have also failed. (I'm replacing it, too)

The block is at the machine shop right now ready to be sleeved and haven't decided on pistons yet...

Before I knew I needed to replace the pistons, I ordered (and now have) the Cometic headgasket from Science of Speed which would put me at 9.8:1 CR.

At this point I don't know whether I should go with new stock compression pistons and the Cometic HG, or just go with the lower compression pistons and OEM gasket??

The pistons I am looking at are the CP 9:1 pistons, and then I'm not sure if I want to go with the 87mm, 87.5mm, or 88mm bore...

Any input would be greatly appreciated... Go easy on me, this is my first "engine build".

I am going with the following setup:

AEM EMS w/UEGO
3.2" (12psi) SOS Pulley, Belt
ARP Headstuds
New OEM crankshaft
OMNI 3Bar Map Sensor
RC 650 Injectors


I'm hoping for 370whp after the tune...


Am I going to notice a loss in power at 9:1 while not in boost?

Should I go with something like 10:1 (If I can even find some)?


Thanks a lot,
Matt

 

For a Comptech setup peaking at 12psi I'd not recommend too low a compression.

You can run 12psi on stock compression. 10:1 would be safer. 9:1 I think would be too low.

 

I called JE Pistons... They can "make" me a set of 10:1 pistons, but it will be pretty expensive...

He seemed to think 9:1 was overkill for 12psi of boost...

I'm starting to think that I might want to go with the stock compression pistons w/ cometic headgasket...

Is this retarded thinking?


Random Thought : Let's say I ever wanted to go back N/A at some point, would it be easier (cheaper) to do it by changing back to the stock headgasket vs. replacing the pistons?

 

Ummm, YEAH!!

Changing a head gasket requires pulling the head. Changing pistons involves a rebuild.

Added to that the fact that to use aftermarket pistons you have to bore and sleeve so the cost of the pistons is only part of the expense.

 

really? various individuals, as well as inlinepro, have been telling me that 10psi on stock compression is pushing it.

 

Well 12 psi is pushing it just that little bit further.

It's not a recommendation, just an example. But with the right tuning (and this involves a standalone ECU) it is possible.

 

then i guess once i get the EMS i'll shoot for 10

 

What type of fuel are you going to be running? 9:0:1 @ 10Psi Vs 10:0:1 @ 10psi will yield a overall different compression. As far as bore size, I went with 89mm bore. Then you Combustion chambers have to be matched to the piston. I think you will be happy with 87mm bore. Drop compression or run a higher octane fuel. To be honest for the little extra you are able to get I would tune the best you can now.


Note this is just a example, lots of factors come into play, fuel type, timing, and mre.

 

This car is purely a street car... I will be running 93 octane gas.


I talked to the guy at JE Pistons, and asked him if I could use stock compression JE pistons with the Cometic headgasket, and he said "No. Because the pistons were not meant to see 12psi."

What about the construction is different between their 11.5:1 vs a 9:1 piston... Isn't it just the dome height?

Now I'm even more confused than before.

 

So what does the chart value mean?