Jc803

iTrader: (2)

I have a LQ 408 with about 15k miles on it. Current compression ratio is 11.1:1 but soon I'm wanting to drop the compression to 9:1 for my turbo build. It has an Eagle rotating assembly so Ill be reusing crank and rods.

My questions are,

Will I need to hone the cylinders? I had the heads off recently and could still see the crosshatching in the cylinder walls from when the block was bored .030". But I've heard that not all pistons are the exactly the same bore so a machinist should match it or something like that.

If I reuse the crank and rods with new pistons should I have everything balanced or do pistons come balance and I could just pop them in.

Is it possible to go from 11:1 to 9:1 by buying heads with larger combustion chamber, or are new pistons a requirement. Current heads are AFR 230s with 70cc chambers.

Can I replace crank and rod bearings by just throwing new ones on or is machining required.

Thanks.

KCS

iTrader: (20)

If you got pistons with the same rings and reuse your old ones, then you could get away without honing it. Your machinist would probably recommend honing it to make sure piston-wall clearance is set correctly though.

If the new pistons don't weigh the same as your old ones, you will need to rebalance the crank.

You can replace the bearings without machining so long as the housing bores and crankshaft journals are within spec.

BlackDuk98

iTrader: (34)

I'm collecting info and planning on doing the same thing as the OP since my na 408 is no where near where I want it to be.

I am unsure what compression ratio would be good, I have read from 9-10.5, even higher on e85. I also looked into changing compression with larger chamber heads, 72cc is the largest I have come across. I am still just reading.

I was messing around with some online CR calculators, and my only problem is that I don't know my deck height on my iron 6.0. I have heard that stock untouched blocks can vary by .010, and I know that there was .005 removed from mine to true it up, so on the calculators I changed the deck height like .020 was removed with the 72cc heads and I think I came up with around 10.5 CR.

I think it's easier to just tear the motor down completely and change pistons, get it all balanced etc.

I also had to guess on the calculator with my head combustion chamber size. Mine were 64cc, but the machinist scratched them accidently and removed .002, so I guessed that they were 62cc now. I am a little higher CR than you with my close guesses, numbers got lost over the years.

Plus I.m not sure if my crank and rods are good enough. Callies Compstars, but I have read that they are good to 1000hp, and also good to 750hp, but have been pushed to 1100hp. I wish I could start from scratch.

Jc803

iTrader: (2)

I know what you mean by wanting to start from scratch. I'm debating putting my 408 in my wifes project car and building another 408 for myself. That way I'm only building one engine instead of 1 1/2

BLOODMOTORSPORTS

I actually used LS PISTONS

G Atsma

As opposed to??

91 Z28

iTrader: (3)

Hey that's great, so did I

G Atsma

Gotta love the occasional BOT..... LOL

BLOODMOTORSPORTS

the l8T build

G Atsma

More details, please.....

stockA4

Late to the party and way out of left field but hear me out here man. That's a hell of a lot of work to do to an already good running engine.

If it were me, I'd go find a used 5.3 to blow up while I sort out the turbo setup and I can admire that 408 on a stand without hurting it.

There's nothing wrong with 11 to 1 and a turbo. Absolutely nothing at all. Don't let anyone on here. Tell you you need less compression I would run more personally.

The only things you really need to do are check the ring gaps are loose enough (somebody else can tell you what they need to be) and If you are in there you might as well go with a turbo friendly camshaft grind Just install it without any advance or even back it off a couple degrees from straight up if you are worried about cylinder pressure. That's what you're worried about right cylinder pressure?

​​​​​​The smaller more efficient combustion chamber will always require less boost pressure psi atmospheres whatever to make the same power as a larger one, especially a nice, smooth aftermarket one that's even less prone to detonation than the OEM stuff 😎

Basically by reducing your compression, your increasing the pressure in the pipes outside the engine ( The boost ) over what you would need for a more efficient, higher compression naturally aspirated engine to make the same output.

What is your goal here?

If you're in this big of a hurry to throw your money away, send it to me for my advice because you'll make the most power this way Just optimize what you have with the right ring gap and camshaft timing.

G Atsma

Jc803 hasn't been here since 2016.

vette4fl

Not a bad idea.

Anytime you change the bearings all the caps should be torqued down and the bearings measured for size and consistency. New pistons call for balancing, that is if you want a balanced assembly.