LQ9 408 with 243 heads and nitrous

**Warren2129** · 12-26-2016, 01:30 PM

Hi guys, I just bought a LQ9 with 317 heads. My plans is to build a 408. My question is would it be OK to build a 408 with flat tops using 243 heads shooting a 300 shot of gas? Or would it be to much compression and better to keep the 317 heads? I plan on running a good forged bottom end with the MS3 cam.

**Warren2129** · 12-26-2016, 01:36 PM

I am building a street car that won't be a daily but it will be drove often. Want to run on pump gas. I may run a stand alone progress nitrous system.

***SMWS6TA*** · 12-27-2016, 07:23 AM

My 403 has forged internals, flat top pistons w/3cc relief cuts and currently a Polluter stg cam and stock 243 heads. Only work on the heads was new valve guides. BTR spirngs and trunion upgrade. When I'm ready for more a new cam and heads. I'll most likely be switching over to LS3 heads & intake with a custom grind cam.

I'm putting down 450/450 hp & tq to the rear. Rear is a 8.8 w/4.10 gears, M6 car.

All my internals can safely handle a 250 shot of Nitrous. 300 you'll want to get a cam, piston, and valves that can handle that much.

You'll also need to beef up the injectors and fuel pump and get at least a wideband to watch the A/F ratio for lean conditions otherwise you'll burn a piston. Plugs will also need to be taken in account as well.

***SMWS6TA*** · 12-27-2016, 07:26 AM

The 317's are good for boosted builds. The 243's flow better than the 317s and have a higher CR. You can use the 317's and have milled down to raise the CR and also have Port & Polish to raise the flow.

My 403's CR is 11.5-1. I use only 93 oct and it is practically a daily.

**Warren2129** · 12-27-2016, 07:49 AM

Originally Posted by SMWS6TA

The 317's are good for boosted builds. The 243's flow better than the 317s and have a higher CR. You can use the 317's and have milled down to raise the CR and also have Port & Polish to raise the flow.

My 403's CR is 11.5-1. I use only 93 oct and it is practically a daily.

Thanks for the info. I am still debating between 403 or 408, probably not much HP difference and with a 403 I still have some room incase something happens then I can bore it to a 408.

***SMWS6TA*** · 12-27-2016, 08:01 AM

That's kinda why I stayed with my bore 4.005". It started out as a stock bottom end rebuild. Had a rod bolt failure on test drive and that destroyed the crank. so new crank, new build.

Has a K1 forged crank.