LSXRX7

Ok so I rebuilt my motor after getting it back from the tuner and getting some low hp numbers. He said check compression and do leak down test ect…. Compression test was at 180lbs all around he said it should be about 210to 220 with my setup... so rebuilt it with new pistins rings bearings ect…

Compression after 100 miles is at 125lbs????? am I missing something?? hear is all my mods the block is a LM4 aluminum 5.3 bored out to 3.908

Stock ls6 crank rods and heads (243) with new head gasket to .45 thicknesses

Mahle pistons flat top with valve reliefs
Patriot gold springs comp Chromalloy push rods stock length
Ms3 cam
Fast 90/90
42LB injectors, SLP oil pump, headers ….
Car runs fine decent power and no strange noises or back fires .Anyone with any suggestions that could explain low compression readings…..

hirdlej

iTrader: (31)

You should have never touched it in the first place. Are you sure you have your valves adjusted right and is your cam timing spot on?

LSXRX7

stock rockers no adjustment and cam is dot to dot... compression should be around 220 then 180 is way low IMO and now 120

LSXRX7

I also ran 1.85 rockers on the old setup for a few months then went back to stock... if i slightly bent my intake valves i know I will loose compression but would it back fire run bad???? just trying to find some answers thanks...

snake charmer

iTrader: (118)

was you running an aftermarket cam with the 1.85 rockers?

LSXRX7

yes the comp XER273HR-14
Valve Lift with Factory Rocker Arm Ratio: 0.581 int./0.588 exh. lift

LSXRX7

update.. did a compression test same thing went ahead and did a leak down test and sure enough air coming from intake side time to pull the heads and change the valves....
looking at valves im sure i will need to do a valve job if i put in 2.02 intake in stock seat and does any one by chance know the part number for stock ls6 valves intake exhaust ? thanx

PREDATOR-Z

LS6 valves are 2.00/1.55, and yes you'll need valve job for any valves you put in.

LSXRX7

Still have not pulled off my heads but was doing some research and found that the ls6 valves are 0.023622" longer. Cause of the ls6 cam base circle... I’m wondering if my thinner head gasket and base circle on ms3 cam 7.400 pushrods is holding my valves open slightly… any one think this is possible or had any problem with this before ??? my calculations make my pushrods .030622 to long

Sigforty

What is your lifter preload?

LSXRX7

stock lifters and rockers with 7.400 pushrods

PREDATOR-Z

How do you calculate that?

JBIRD02

iTrader: (4)

Well, assuming that your calculations are right, and it really is just a matter of your pushrods being .03 too long, you can get 7.375 pushrods from Thunder Racing.

Sigforty

That does no answer the preload question. Have you figured out how many turns it takes to go from a snug, can barely rotate the pushrod behind the rocker, to the 22 ft lbs you need to torque the rocker bolt.