What is the 4.0s breaking point?

[CaptainBanana]

Hey guys,

Whats considered the 4.0s weak point?

I'm currently in final stages of machining for my build which I'll detail a little below.

It's not a stroker (sorry, I'm here because it's a tech based forum not for stroker specific info).

The combo is as follows:
NVH Block 30 though over.
Block decked.
Forged pistons.
Eagle rods./ARP Bolts
Factory Crank.
Fully balanced rotating assembly.
Crank tunnel aligned.

TUPY head professionally ported.
LS1 valves and springs.
ARP Headstuds.
MLS gasket.

Clegg stage 2 cam.
Duration 269 Intake | 273 Exhaust
Duration @ 0.050 Valve Lift 215 Intake | 219 Exhaust
Valve Lift 0.484 Intake | 0.480 Exhaust
Lobe Seperation Angle 114 Degrees
Double row timing set.

PSR GT3582 roller bearing turbo with 1.01 exhaust AR
GFB 44mm waste gate.
Aeroflow air to water intercooler.

I haven't bought the roller rockers yet, I'm on the fence now wether I go 1.6 or step up to 1.7 I need to speak to my engine machinist about the combo and seek their opinion.

The car has supporting mods for fuel, engine management etc. I originally built the turbo set up on the stock internals but have now gone bigger on the turbo, intercooler and of course internal airflow potential.

While I have no idea what it will make powerwise I am unable to find any suggestions as to what the 4.0 weak point is? Where does it show? I know Keith Newcomer lifted a head at 1000hp and Rick Mudge says they have that timing chain harmonic but that's about it. FYI I started building this motor due to timing chain failure

What's the consensus on safe power limits for these? Not for one dyno pass or a season of racing but as a weekend cruiser that can take the occasional burst?

Vid below of the original build.

https://youtu.be/c0a-dz1GB20?si=03fRxcYwK3sLInaC

[Randy Bobandi]

Whats considered the 4.0s weak point?

It's not a stroker
LS1 valves and springs.

Duration @ 0.050 Valve Lift 215 Intake | 219 Exhaust
Valve Lift 0.484 Intake | 0.480 Exhaust

PSR GT3582 roller bearing turbo with 1.01 exhaust AR
I haven't bought the roller rockers yet, I'm on the fence now wether I go 1.6 or step up to 1.7

What's the consensus on safe power limits for these?

We don't know the limit of the OE castings. Nobody has split a block, nobody has truly tested the platform to it's maximum power potential. We can only make an educated guess based on similar engine designs and general theory.

Do you have a power goal for your motor? Peak power will be around 5000-5500 RPM with that camshaft. I would opt for the 1.7 rockers if I was stuck with that cam to get as much RPM as possible. You would have to make sure the valve springs take the added lift into account. The comp 26981 spring would be a good option installed at 1.700". That cam also has negative overlap. Not ideal for max output but at least you wont have to worry about blowing exhaust out of the blowoff valve.

With the 61-62 turbo, I would expect a 1:1 pressure ratio early, around 15 PSI. So this looks like an easy 500 HP combo at 15 PSI. Could surely push it to 600 HP on E85. Head gasket and distributor gear is what I would focus on. It would be smart to get at least an .060" head gasket from Cometic, maybe an .080" as long as it doesn't tank your compression. Keep an eye on the distributor gear as routine maintenance. Install an exhaust backpressure sensor as well as a coolant pressure sensor. You won't have to worry too much about lifting the head if you cut boost when coolant pressure reaches around 30 PSI.

What transmission are you running?

[CaptainBanana]

Whats considered the 4.0s weak point?

It's not a stroker
LS1 valves and springs.

Duration @ 0.050 Valve Lift 215 Intake | 219 Exhaust
Valve Lift 0.484 Intake | 0.480 Exhaust

PSR GT3582 roller bearing turbo with 1.01 exhaust AR
I haven't bought the roller rockers yet, I'm on the fence now wether I go 1.6 or step up to 1.7

What's the consensus on safe power limits for these?

We don't know the limit of the OE castings. Nobody has split a block, nobody has truly tested the platform to it's maximum power potential. We can only make an educated guess based on similar engine designs and general theory.

Do you have a power goal for your motor? Peak power will be around 5000-5500 RPM with that camshaft. I would opt for the 1.7 rockers if I was stuck with that cam to get as much RPM as possible. You would have to make sure the valve springs take the added lift into account. The comp 26981 spring would be a good option installed at 1.700". That cam also has negative overlap. Not ideal for max output but at least you wont have to worry about blowing exhaust out of the blowoff valve.

With the 61-62 turbo, I would expect a 1:1 pressure ratio early, around 15 PSI. So this looks like an easy 500 HP combo at 15 PSI. Could surely push it to 600 HP on E85. Head gasket and distributor gear is what I would focus on. It would be smart to get at least an .060" head gasket from Cometic, maybe an .080" as long as it doesn't tank your compression. Keep an eye on the distributor gear as routine maintenance. Install an exhaust backpressure sensor as well as a coolant pressure sensor. You won't have to worry too much about lifting the head if you cut boost when coolant pressure reaches around 30 PSI.

What transmission are you running?

Thanks for the response Randy,

I went with the 1.7 rockers to give me the .515 lift which will help a little. The heads have been set up specifically for this build and the springs are for the LS1 valve combo with almost factory seat height and diameter and required spring rates.

I'm running 98 octane (95 octane US) and don't want to run E85 due to very limited availability outside of metropolitan Melbourne.

The cam was chosen for the overlap/lobe sep to make sure it still drives nice on the street. I have had the rings gapped for 15psi on that particular turbo so that's the goal boost level. Backpressure should be OK at that level, in actual fact with the 1.01ar exhaust housing I've seen much bigger power pushed from these turbo on other 4lt 6's.

What are your concerns with the distributor gear, are you worried about oil pump loads on the OPDA gear or something else?

500hp would be the happily accepted HP, more would be nice but purely for the sake of achieving more as the buid is limited by the clutch and 6 Speed gearbox not the engine.