lq4 bottom end build
 

I have a set of ls2 rods to replace the stock rods in my lq4 on a rebuild.

What is the compression height of pistons that is the more desirable. Is it 1.325?

These are the pistons Im looking to put in my short block.

http://www.summitracing.com/parts/slp-h1129ct75mm

Due to ring pack placement, the CH will vary from one piston to another and manufacturer to another. Those exact pistons are what I used in BigDaddy's mild LQ4 build. You will be close to zero deck depending on the block being used and whether you have it decked/or how much. With those pistons he was .0065 out of the hole (his block was decked something like .018) which with the LS3 gaskets (.040 thickness) and stock 317 heads (his were milled something around .017) should net somewhere in the neighborhood of 10.5:1 compression (he ended up at 10.78:1 IIRC). He was .030 over also. You can use those pistons on your LQ4 rods or the LS2 rods (machinist may have to bush or hone them depending). Are you over boring? We also used Mahle's OEM line rings which is ~$5X.XX from Summit and I file fit them because though they are pregapped, they are a bit tight for a mild/moderate build. They are PN PCR-41858CP-XX (XX denoting whether it is bored) which are a plasma top, cast second, and stainless oil setup. Should yield long life and have excellent ring seal/breakin properties.

Thanks for the GOOD information.

I'm planning a mild build.

Something that will get about 375 horse, maybe 400.

I'm using a standard 05 lq4.

Yes, it will be a 30 over bore for a 370.

Figured to upgrade the rods to Ls2, which I have.

Maybe have a porter work the heads some.

Light on the cam keeping it around 210 to 212.

Need to run 87 octane. Keeping compression around 10.2 to 1.

Lots of torque. Prefer 400 foot pounds from 2500 up to 4000.

Love to have a redline around 5200 to 5500 tops.

On a stock lq4, do the pistons stick out of the hole?

Here are your pistons and the rings I chose off Summit's site for a .030 LQ4. You can choose another ring pack from the one I chose to use but I was looking for an OEM quality replacement, standard tension ring and that was it, plus it was priced nice. You start looking at other ring sets and the price rises quickly and for the purpose you and I are talking, the performance gains you might see are negligible at best. Longevity and ring seal were my two main factors.

Pistons:
http://www.summitracing.com/parts/sl...make/chevrolet (the ones you had already chosen)
Rings:
http://www.summitracing.com/parts/pc...-030/overview/

As far as the compression, don't mill the heads...just a good valve job with good springs and viton seals will do what you need. I wouldn't even bother porting as ~80% of the improvement you might see with a cnc'd head will be with the valve job itself. Use a LS3 4.06 gasket (around .051 thickness) and with those pistons you should be very close to your goal. I would wait until machine and final assembly of the short block/heads is complete so that you can accurately calculate your static compression ratio but that will be damn close.

As far as the pistons being in the hole, the LQ4's were a dish to begin with so just he flat tops are going to net you a nice bump in compression. I didn't measure the stock short block but IIRC, yes, they are in the hole just a bit so if you have to mill a few thousands to straighten the deck (or to realize a zero deck with your piston choice) you will be fine. Just talk with your machinist and try and keep your quench at around .040 give or take a few thou. Good luck!

04 and up ls engines already have the good ls2 rods!

I though about that, but I figured he might have bought a bare block and rotating assembly. He didn't elaborate or else I missed it. I will say that last LQ4 I had was a '04 and it didn't have the floating rods. Depended on the build date...seems like about halfway through the '04 model run got the good rods. Mine I know for a fact was early production year.

I will have to double check.

Where are the casting numbers and suffix codes that give that kind of information?

The casting number I found is 12551364.

You are correct. If it was a 04 I would verify that it did in fact contain the ls2 rods. being you said its an 05 you should be fine. THe search button will be your friend on finding casting number information as well as other stuff.

Thanks, I appreciate the information.

Im baseing it being a 05 on what the information the yard gave me. Im not really sure.

I run those speed pro pistons in my 370 but the dish version. I run 243 heads with a dish piston for 10.5 to1 compression