How much PSI is safe with stock head bolts?

Thanks for all the contributions to this thread. I am working on a similar build right now, and the search got me here.

robnitro said:

Ok. Yeah, so why is it ok to use ARP studs which clamp much harder, but the stock studs people act like they are made of glass?

Its marketing and VW picks specs based on the 155 ft lbs stock configuration.

Clamping the head down a bit more is not a problem.
The volvo study also looked at other factory bolts, showing that they arent even near their stretch point using stock specs.

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I can say on the Volvo Si6 engines when I do warranty pistons for oil consumption issues, they have increased the head torque substantially, now its 4 stage, 44 nm, then angle torque is 90 then another 180...

Ok, so they increased head torque, but it would still probably be less clamping force if you got the ARPs. (I don't think the stock bolts can come near to that, but I do think the stock bolts are under rated in the spec, like the Volvos were)

So, how does that make it bad to run more on the stock bolts when the ARP is pushing even harder?

Or are you saying that these stock bolts are not holding?
Any idea why they are using more oil? Could it be also related to running much more power?
I'm just confused as to what exactly is getting messed up.

robnitro said:

Ok, so they increased head torque, but it would still probably be less clamping force if you got the ARPs. (I don't think the stock bolts can come near to that, but I do think the stock bolts are under rated in the spec, like the Volvos were)

So, how does that make it bad to run more on the stock bolts when the ARP is pushing even harder?

Or are you saying that these stock bolts are not holding?
Any idea why they are using more oil? Could it be also related to running much more power?
I'm just confused as to what exactly is getting messed up.

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The piston oil control rings are flawed, they redesigned the rings ...volvo extended the warranty on these engines...I was just stating that volvo increased the torqing of their heads

studs pull on the cylinders differently than bolts. My buddy who is a machinist was saying something about that and that he does something to the blocks/cylinders when switching from bolts to studs on the race engines he builds ( not TDI stuff)
But I never got more into that discussion,,,I have run 30 psi on vnt 17 ( not on purpose)on stock head bolts no issue. When you start pushing 32+ on upgraded turbo I think is where you have issues with the head lifting with a big turbo..you have to remember, its volume of air pushed , not psi....example,,,a small turbo at 10 psi will flow substantially less than a larger turbo at same psi...so boost is relative to which size turbo ...

But back to the question, I do not know the yield limits of the stock head bolts,,,I do know from experience and Jeff from RC will attest, there are always one ur two new head bolts that will not yield and just keep spinning and won't get to the 110-120 lbs ft usually noted on the final angle drive... I always have extra in case lol

I don't quite follow how 10psi on a large turbo is any different than 10 psi on a smaller one. Pressure is pressure. What would make a difference is the chamber surface area. 10psi in a 1/4" hose is nothing. 10psi in a 80gal compressor tank is a bomb. In terms of head bolts, you have the surface area of the head at the top of the cylinder being pushed upwards at the 500+psi of compression our engines make. The idea of any bolt is to maintain preload. the bolts must always maintain preload when you combine all the cylinder pressure, coolant pressure, etc that's acting on the bottom of the head.

The difference between bolts and studs is that you waste energy twisting the shank of a bolt where a stud is much closer to pure tension. You can get the same preload with lower torque using a stud.

kapps said:

I don't quite follow how 10psi on a large turbo is any different than 10 psi on a smaller one. Pressure is pressure. What would make a difference is the chamber surface area. 10psi in a 1/4" hose is nothing. 10psi in a 80gal compressor tank is a bomb. In terms of head bolts, you have the surface area of the head at the top of the cylinder being pushed upwards at the 500+psi of compression our engines make. The idea of any bolt is to maintain preload. the bolts must always maintain preload when you combine all the cylinder pressure, coolant pressure, etc that's acting on the bottom of the head.

The difference between bolts and studs is that you waste energy twisting the shank of a bolt where a stud is much closer to pure tension. You can get the same preload with lower torque using a stud.

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It is VOLUME of air being pushed..I will use my RX7 for example,,,the twin turbos stock run 10 psi...I switched to a large and I mean Large single turbo, same 10 psi, sane piping, but that 10 psi made 75 whp more...because I was pushing much more volume of air and obviously more fuel ...the compressor wheel and map at 10 psi was much more efficient.

You can see my cars at www.pfsupercars.com
Under projects " Greg Smith supra and RX7...the turbo on my RX7 as you can see in the pic is HUGE...it made 75 more whp over the factory twins at just 10 psi and more fuel... when we were done...it was doing 610 whp ...over the factory rated 255 flywheel hp,,,at max boost around 30 psi

Ok I see what you're saying. I'm thinking more steady state. Although technically, the smaller turbo wasn't able to maintain 10psi as the valve opened and air started flowing into the cylinder. The larger turbo was able to maintain more pressure. If there was a constant 10psi source, power would be the same.

Assuming that Intake manifold pressure is the same with a small or a larger turbo at 3000 rpm (for example) then the difference in performance can be due to the exhaust manifold pressure required to make the same intake manifold pressure with either turbo.The intake and exhaust temps would also need to be factored in.

So yes psi is psi regardless of the turbo at the SAME engine rpm and no other changes other than turbo.

That would be my take on this.Not sure if I'm correct ?

andy2 said:

Assuming that Intake manifold pressure is the same with a small or a larger turbo at 3000 rpm (for example) then the difference in performance can be due to the exhaust manifold pressure required to make the same intake manifold pressure with either turbo.The intake and exhaust temps would also need to be factored in.

So yes psi is psi regardless of the turbo at the SAME engine rpm and no other changes other than turbo.

That would be my take on this.Not sure if I'm correct ?

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Yes psi is psi regardless, but the volume will be more or less depending on turbo compressor size/efficiency. You are flowing more volume of air at the same psi, simple

I run 34 psi on mine for the last 6 months or so...

But to answer your question, 24 psi is right about the limit. There is no certain answer. Some people would replace studs on any turbo other than stock and some people run 26 psi no problem. Depends on a lot of factors, peak cylinder pressures being the big one.

52172 said:

Just as the title says. I don't want to push anything too hard, but want a big boost in power. I'm gonna be running a 1949vc hybrid that can substain 24 psi. Is that reasonable with the stock head bolts and a great custom malone stage 4 tune? My mechanic and I are gonna do real time logs to dial the tune. Thanks!

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Are you running a 1749VC or a 1949VC?

blitzed said:

Yes psi is psi regardless, but the volume will be more or less depending on turbo compressor size/efficiency. You are flowing more volume of air at the same psi, simple

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If psi is psi at the same engine rpm then the only difference can be intake air temp.If IAT's are the same with both a larger compressor and smaller one due to charge air cooling bringing IAT's down to ambient air temp (for example) then the volume should be the same I would think ?

Pressure is created by resistance to flow.
if both turbos can generate enough flow to create 10psi then they are both flowing the same volume of air.
the larger turbo is capable of flowing more air more efficiently. thus cooler IAT'S
If you are adding more fuel to the larger turbo because of lower IAT'S you will make more power.
if both turbos generate enough flow to create 10psi in same intake system and same IAT'S then they will both produce the same amount of power. Provided the rpms are the same as well. flow is irrelevant. Your orifice that you are forcing the flow through is unchanged.

You have a compressor housing with a inlet, compressor wheel, and outlet. The smaller the wheel the faster it has to turn to pull in a specific volume of air. The larger the wheel the slower it has to turn to bring in the same volume of air. Now when the wheels are close in size you can make up differences with compressor wheel design, fin angle, ect....but lets leave that mess out of it. The wheel takes in the air and forces it against the housing wall to compress it, the faster it turns, and the smaller it is, the faster the velocity, the hotter the air gets. As air gets hotter it tries to expand and makes it harder to compress making it harder for the wheel to intake more fresh air (cfm). The larger turbo can take in the same amount of air into the intake side of the housing, compress it in the housing without generating as much heat, and allowing it to take in more air easier because the air isnt trying to expand as much because its not as hot. A larger turbo will move more air but will have less of a mechanical advantage since the air opposes with greater leverage (due to fin surface area further away from the shaft center). So while a large turbo has a RPM/heat advantage it has to work for it
Garrett airflow

·*Wa = Airflowactual (lb/min)
·*MAP = Manifold Absolute Pressure (psia) =35.1 psia
·*R = Gas Constant = 639.6
·*Tm = Intake Manifold Temperature (degrees F) =130
·*VE = Volumetric Efficiency = 0.98
·*N = Engine speed (RPM) = 5000rpm
·*Vd = engine displacement (Cubic Inches, convert from liters to CI by multiplying by 61, ex. 2.0 liters * 61 = 122 CI)
Actual Flow = Map (ambient+boost) x Volumetric efficiency x RPM/2 x displacement
all divided by gas flow and intake temps.

So using above If I have 10lbs boost on large and small turbo assuming intake temps are the same for both ,and yes this is possible if the larger turbo was not intercooled. Just bare with me..lol Also assume that both setups have the same amount of exhaust backpressure (ignore reality for a second will ya ;o)...)

.....................small turbo.......... large turbo*

Boost.................10PSI................10PSI

flow....................less...................more

RPM...................fixed.................fixed

Displ..................fixed..................fixe d

intake temp........fixed..................fixed

exhst bck press...fixed..................fixed

gas constant.......fixed..................fixed

VE......................less...................more

Conclusion:*If intake temps, backpressure, boost and RPM are held constant for both setups the larger turbo will make more power because volumetric efficiency has increased due to the greater flow of the larger compressor wheel at 10PSI.

DECREASED INTAKE TEMPS and BACKPRESSURE ARE NOT THE ONLY REASONS FOR PERFORMANCE GAIN.

But the above conclusion is of course incomplete in the real world as larger turbos do decrease backpressure (further stimulating volumetric efficiency, and a larger turbo running within it 73% efficiency island or there abouts will produce a more dense intake charge assuming that the same intercooler is used for both setups.

The end result of a larger turbo operating in it's efficiency range is more power, but it comes from increased flow, as well as reduced exhaust backpressure and lower intake temps.

Comparing let's say smaller twins to a larger single where

P= NRT1/V = 10 lbs
P= NRT2/V = 10 lbs

temperature goes down the N (number of molecules) has to go up in order to maintain the same pressure. More oxygen molecules in combustion chamber = more power.


Single turbo:
Pressure at the manifold = 10 psi

Twins:
Pressure at the manifold = 10 psi

CFM of single > than CFM of twins

Using a single, the end result is alot more oxygen molecules at the COMBUSTION chamber due to a higher CFM. In the combustion chamber: Pressure using a single does not = the same pressure as using the twins.*Its all about CFM. PV=NRT directly applied at the manifold, where the pressure is measured, does NOT depent on how much air is moving through it. If two differently sized turbos were rotating at the same speed, while the LARGER turbo would compress more AIR (ie volume), BECAUSE of it's greater volume (and that area is in the denominator in the pressure equation), the psi it would be running at would be LOWER. Ie, the SMALLER turbo would indeed be compressing a SMALLER volume of air, yet it's PRESSURE (psi) would be HIGHER.*

Intake flow if VE of engine is unchanged (same emp same rpm) is the same for 10psi (if same iat) big vs small turbo.

But big turbo can do that boost with less exhaust manifold backpressure, the exhaust flow is better, less exhaust left in the cyl so the cylinder can take more air at the same iat,pressure.

Dont forget advance inj timing and how much fuel its that combo that brings pcp high. Boost only dont do much

Psi

I was running a stock vnt1749v with RC3 i would daily spike 30+ psi until the head finally lifted and it pressurized the coolant system so bad that when driving it would never come to temperature and would just be boiling the whole time. so as a last resort i overtorqued the stock head bolts another 90* and reduced the combustion leaking by 80% and it was hardly noticeable and i had working heat. then i got a little stupid and put in .216 nozzles and added the envry mod on top, that lasted about 2 months then last Thursday i had the smoke switch on and spiked 37 psi with the stock turbo and i exploded the turbine shaft or compressor wheel. o well, i was experimenting on a 270,000 mile turbo /engine lol.

now i just ordered a vnt1856 and im putting head studs in. and removing that stupid switch.

Farfromovin said:

I run 34 psi on mine for the last 6 months or so...

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Someone got themselves a new hairdryer...