Weak points? 3500 LX9 or LZ4

[nonhog]

Just thinking here before I get all worked up and try to source a 3500 for an upcoming
vintage car swap.
Any low points or concerns?
Thanks in advance!

[Mars]

Give them hell man they are TOUGH !! Even stock head bolts and gaskets can take a beating like you wouldn't believe. Valve float is always an issue with stock valve springs.

[1988GTU]

Rods are weak, but only if you're hammering on it and don't notice the boost gauge buried and rpm's are approaching 7k.

[Superdave]

350ish HP on a LX9, shifting between 6500 and 7k.. probably close to 100 1/4 mile runs, stock bottom end.


I did spin one rod bearing several years ago, not sure what was up with that. fixed it and haven't had a problem since.

[1988GTU]

Tending to wonder about the RPM limits creating the weak factor. 6000 rpm comes quick and it wants to go 7K, but it seems all my failures happen right around that lucky number of 7.... I'll probably see how high I can get the boost up too while staying under 6k and then compare how long this motor will last to the ones that failed at high rpm's.

[mickaz]

On a stock cam is valve float is a problem? ( with headders, 65mm throttle body and a tune?)

[CNCguy]

Although they both have the same displacement, the LZ4 has a shorter stroke, 76mm vs the LX9's 84mm, and may be better suited for higher RPM operation. However, it is expensive to swap due to the VVT and u-flow cooling setup. IMO, the LX9 is a much easier and economical swap.

[NateD4]

What oil pressure do you see when rod bearing failures happen.

[nonhog]

Although they both have the same displacement, the LZ4 has a shorter stroke, 76mm vs the LX9's 84mm, and may be better suited for higher RPM operation. However, it is expensive to swap due to the VVT and u-flow cooling setup. IMO, the LX9 is a much easier and economical swap.

Noticed your links, when is the site going to be up and complete? looks like some of the MG stuff would come close to working in my Volvo.

[NateD4]

Maybe I'm inserting my foot in my mouth but why stick with stock head bolts? They are only rated for 150 KSI vs 180-190 KSI for ARP studs.

As for valve float... Anyone run 400 lb springs?

[Mars]

Honestly though, I'm pretty much the only guy that really pushes these motors (3500) that I know of and they just RUN. even with stock stuff... (cept valve springs)

I had one fail and it's in my build thread, it may have been an assembly error or something though, the cam gear spun on the cam and blew the locating dowel pin right off and I lost the timing and wiped out a motor. I had another 3400 fail but it was high mile (200 000 miles) and just died from bearing issues. (it was more of a mock-up motor really)

Pistons, rods and botom end seem tough as hell.

[NateD4]

I think if the crank oil holes are properly blended they bottom end issues will go away with the right oil pump.

As for the cam... the design is a bit problematic as they start with a tube and hydraullicly swage the lobes onto them.

I'd think the bolts are the big problem in the rods above 7K RPM. Though I think they should be good for about 7500 or so with stock fasteners (I'd have to run the math).

[1988GTU]

High rpm (7k) and oil temps above 210F seem to shake up these motors. I've pushed some boost thru the 3500 and every failure I've had the oil temp was 220ish and I was spinning 6800+ for just about a 1/4 mile. Afr was on the rich side of 10-10.5 running 110 octane.

[NateD4]

What oil grade and type were you running?

The clearance on a stock engine might be a half thousandth tight for high rpm. Also the oil holes on the stock cranks I have done have any lead out on them. They need to be blended and polished to get better oil flow. GM recommends this in their old Chevy power book for gen II engines.

[1988GTU]

10w30 the green oil.