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  • 1 Post By matto20v
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Old 29-11-2018, 01:21 PM
Scoobie Noobie
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Default Boost limits of stock v8 jdm sti ej207 valvetrain

I am currently rebuilding my heads and I could use the collective experience of the gurus out there..

The OEM V8 JDM STI SAVCS heads, being shimmless buckets are good for 8200k rpm right out of the box.

That is with the following OEM spring rates..

49lbs @36mm SET

130 lbs @ 24.65 LOAD

But at what boost levels ?

I have heard of people cramming up to 30 psi into the stock heads with no ill effect..

Furthermore there are some mathematical gurus who have calculated that given that 31 psi boost is the upper limit and after that this will overcome the OEM spring pressure of 49lbs and open the valve.

My heads, have done 150k,and came out of a stock GDB from Japan.

The machine shop tells me have been boost floating- evidenced by the back of the exhaust valve becoming concave and the valve spring seats showing wear as well.

This is true.. but the valve tips don't look bashed, and there is very little wear on the intake valves, which being heavier should boost float first right?

Also we measured the spring tensions.. It was within the factory spec of 49+-3lbs.

Also this was a ex japan GDB with a stock turbo and ECU.. so over revving couldnt have been an issue.

I asked them if something else could be causing this.. he tells me that with his 30 years of experience that the valves have been boost floating..

So im a bit conflicted ..

Got it rex runs 30 psi wit these heads using the OEM valve springs..
Plenty of others also attest to this.
But are they getting boost float and don't realise it?

If we tear down the engines will we see the same "subclinical' valve seat wear as my heads?

Bottom line is the following..

Should i get new OEM valve springs or should I upgrade..

I am going to use new stock oem valves and looking to push the engine to around 28-30 psi..

thanks in advance?

Last edited by ssgoulash; 29-11-2018 at 01:24 PM. Reason: spelling
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Old 30-11-2018, 07:58 PM
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I'm no expert but the way I see it is, if you're going through all this effort, why skimp on valve springs? Upgrade them while the engine is getting the attention.
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Old 01-12-2018, 01:14 PM
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Bram knows their stuff

Valve spring upgrades are a must at higher boost. Subaru valve is 36mm / 1.417" diameter. So it has a surface area of a bit over 1.5 square inch. (π R^2)

Hypothetically, If you put 30psi of boost against the back of the valve, thats going to be 45 pounds of force trying to open the valve. (30 x 1.5) Thats 45 pounds of force the spring has to overcome before it even does anything to controlling the valve during the 4T cycle.

What starts to happen is the valve spring has a hard time controlling the valve, and it no longer follows the cam ramp profile smoothly. And it begins to hammer on the seat and wear or crack the valve.

I dont know how accurate your specs are for OEM spring, but you say 49 pounds force at seat pressure. Thats not very much left over when you take 45pounds of force from the 30psi of intake air pushing against it.

The valve springs I have going into my engine have about 80 pound at seat, and like 220 pound or something at 10 or 11mm lift. Whats important with valves is not so much the poundage, but the rate at which it increases as the spring is compressed. But most valve spring options for Subaru will not be classified this way. Instead they will typically give some force values at seat and a open levels of lift.

And Subaru valves are small, imagine the valve springs needed in a blown V8 with 1.5inch diameter valves...
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Last edited by Bram; 01-12-2018 at 01:23 PM.
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boost, ej207, jdm, limits, sti, stock, v8, valvetrain

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