HI all,

My motor blew up and I am looking to build a 408 motor. Looking for all the low hanging fruit and not really a max effort motor. Mid RPM HP is important as the vehicle will tow a couple times a year.


2004 LQ4 iron block
The engine builder is using all SCAT rotating parts. Dont know model #s.
4" crank
Target of 10.5:1 SCR.
I have PRC 5.3L St 2.5 heads I plan on recycling.
I didn't know what cam to tell him so I just said 228R Texas Speed.
LS3 TB
FAST 102 LSXRT intake
Kooks 1 3/4 Headers


My questions are the 5.3L heads are out of the suggested CID ranges put out by Precision. Is this going to choke the motor. Moving to LS6 PRC heads is going to pull another 2K dollars out of my pocket. The heads outflow any stock head and the flow chart for the LS6 heads is very close. I know where is more to heads than flow charts though.

Cam: I am aware some guys are running high 230s for their cams on 408 but I am worried about drivability. It stalling all the time or surging. But is a cam this small going to choke the motor or screw it up?

Any suggestions for changes? My builder is still doing machine work on the block so I have plenty of time to change stuff.

 

Yes the heads will work but they are far from optimal and will definitely hold your set up back. Same for the 228r. It has great drivability in a 346. It will be non existent in a 408. Talk to some cam grinders about a cam more suited for a 408 that fits your needs.

 

Most of the things you do to improve low end power will hurt/choke the top end. For example, small ports, small cams, and long runners all help low end power but at the expense of top end power. If low end power is what you want, you have to be realistic and accept that the peak horsepower numbers won't be setting the world on fire.

I think you have a good list of parts so far. You may want to get with someone about spec'ing a cam for you. Lots of variables go into picking the right valve events, such as your converter and rear gear ratio, neither of which were mentioned.

 

Agreed. I'd say talk to a cam grinder instead of an off the shelf cam. Same on the heads. They are not the best but not the worst. Seen some people make good power with them. One other thing I recommend is maybe bumping the compression up a bit from 10.5:1. Get closer to 11:1 maybe. More power everywhere.

 

Why not move to an LS3 top end? You could sell your intake and heads to help fund it.

 

Alright I got an email into Pat G for the Camshaft. Time to grab a beer and wait for the results on the cam...

I just priced the LS3 CNC heads from GMPP and the LS3 intake and it is actually not too bad pricewise. Wouldn't the FAST LSXRT make more power though?

 

Out of box ls3 manifolds perform pretty damn well.. i had a brand new fast and i sold it and kept my ls3 ... more than enough for what i was doing at that time

 

I am a really big fan of cathedral port heads, but on this I must agree that LS3 heads and intake sounds about perfect for what you are looking for. Then get a custom cam spec'ed for the 408/LS3 in combination with your Denali's stall/rear gear/tire size.