Compression Ratio for Supercharged w/ Vortech
 

Hey Guys,

I'm currently in the middle of my 2nd engine tear down, and this time i'm rebuilding with forged internals. I'm boosted with Vortech, and looking to stay with either the stock 3.48" pulley or go down to the 3.25" pulley (9psi or 11psi max)

Anyways, I plan on running E85 for all performance related endeavors, and possibly stay with 91 in town. I DO have the option of running E85 all the time, as it's pretty available in here in Fargo, ND.

The question is - what compression ratio should I get for this build? My initial thought was 10:1, but since talking a lot of people recommend staying with 12.5:1 since I'll be running E85 most of the time.

The Argument for NOT going to 10:1 seems to be that because it's S/C'd, and I likely won't be pushing more than 11 or 12 PSI, i'll just be wasting useable power.

11:1 is also a option I feel, if I can get something in that size.

Lastly, has anyone tried lowering the CR a little with a thicker headgasket? Would you run into problems putting the timing cover back on?

Thanks!

Also - As a secondary item -

I got the report back on my heads which are currently being looked at - It looks like two of my exhaust valve guides are looser than they should be.

have people have been replacing them with aftermarket (GSC) or using new OEM?
GSC is $50 for 8, OEM is $19 EACH.

Thanks!

I would recommend staying at 12.5. If you don't plan on running crazy high boost pressure and you've got e85 readily available I don't see why you should go lower cr. Usually you only go with a lower cr if you want to run a lot of boost and you only have access to low octane fuel in order to avoid knock.

I just posed the same question to my Dyno shop and he said for low boost levels and e-85 like I'm doing with me Edelbrock E-Force supercharger I would be best to stay at the stock 12.5. So doing a search for 12.5 forged pistons CP are the only ones I have been able to find. Any comments on CP Pistons?

Keep your compression high.

CSG offers custom pistons, built to order based on your needs.

That's what Shawn Church said also.

Thanks for the input everyone. I'll stick with the 12.5 CR pistons. @CSG Mike - Pricing on those custom pistons vs CP?

Anyone ever have Valve guide issues? I wasn't able to find much when I searched.

Pretty close. Unless you check off a lot of options that OTS CP pistons don't have.

PM me with what you want.

I've had excellent experiences with CP Pistons. Current motor in my FXT has 90k on it, and has CP OTS 8.2:1 +.25 over pistons. She could do with some rings, but I want more revs. No indication of any issue with the pistons themselves.

Edit: It's not quiet driving around town or commuting, but a good solid pull, or at the autox, it's nice and quiet!

What do you mean it's not quiet? What do the pistons have to do with this?

The piston fitment and design can effect perceived piston slap.

Some pistons have different skirt designs that help keep this sound down a bit more, but depending on the design work better or worse. The CP stuff seems to forego a lot of the sneaky tricky design for just what works really well. As such, they don't have a fancy skirt to keep the noise down, but I make great power. When I get lots of heat in the motor, the pistons heat up a bit and quiet right down.

Hope that made sense!

Some types of piston design can make your car sound like a diesel after you put the engine together.

Yes, that made sense, and thanks for your reply. Is this noise enough to hear when every day street driving?