Thread: Tdi 300 turbo pressure

Bit of an open ended question.

Till you pop the head gasket ?

Till the (stock) compressor becomes inefficient ?

Stock boost pressure is 1-1.1bar/14-15.5 psi.

The head gasket seems to be able to take up to around 17-18psi reliably, and IIRC the comp map I've seen, that's about the limit before the compressor becomes inefficient too (maybe 20psi if you pushed it) but you'll be popping the head gasket then unless you O ring the head, and then you'll probably need to use studs instead of bolts, and if you've spent that much you really need a better turbo (VNT from the UK) and by then you've spent so much you may as well have fitted a 4BD1T and done it properly.....

Funny that
I was going to write a new topic discussing "high boost reasonable EGT on a 300tdi"
Might just add it into here.


Background info:
Newly (200 hours) built engine from sump up. New Turners performance head, elring composite torqued to spec.
New hoses and radiator.
Reused the turbo from my old beater disco so it would have 400k on it now. No play in thespindle, blades intact, spins freely. When I first bolted her up I could only get 11psi boost (measured at manifold) before the wastegate would open. On JC advice rather than removing hairdryer adjusting wastegate rod replacing... I put a Dawes valve in the pressure sense line. Then set it up so that the wastegate would bleed off at 18psi max.


So now to the question!
We just got back from a quick trip up north.
While barrelling along the PDR I was noticing high boost especially when coming off the noise at 90 , rolling into the smaller dips and undulations and then getting back onto it. By high boost I mean up to 23psi. Sustained boost was in the range of 18-20psi. Obviously the adjustment on the Dawes has moved.

EGT never got above 550 degrees (pre turbo)
water temp never got above 94 degrees

I know that the experts (Bush65) have previosuly stated that the 300tdi garret is no longer efficient above 20psi... but just wondering what will happen if I keep running high boost low EGT scenario.

My assumption / internet wisdom is that the issue will be head gasket leakage
BUT riddle me this when mr atomised diesel is cruising around in the pot just after injection and then gets squished and then gets all funky and goes bang Id be guessing the pressure in the pot at TDC is in the realms of thousands of psi... if the air coming in is at 2psi or 20psi how can it compete with thousands at explosion????

Now if I was running 750 degrees EGT and 23psi I could see the writing on the wall BUT

low EGT and high boost - what will happen?

ps obvious things like induction hoses blowing off and IC damage not withstanding of course

Steve

Originally Posted by roverrescue

BUT riddle me this when mr atomised diesel is cruising around in the pot just after injection and then gets squished and then gets all funky and goes bang Id be guessing the pressure in the pot at TDC is in the realms of thousands of psi... if the air coming in is at 2psi or 20psi how can it compete with thousands at explosion????

Steve

My guess would be that the problem isn't simply the difference in pressure between 2 psi and 20 psi.

The extra air allows extra fuel to be burned, so it is the bigger bang that increases the pressure.

Leaving aside all the losses through inefficiency and other variables, if you push air in at 14 psi, in theory you have twice as much air in the cylinder as a normally aspirated engine. There is twice as much oxygen, so you can burn twice as much fuel, so you have twice the pressure.

Increasing from 14 to 20 psi doesn't give just a 6 psi increase in pressure when the fuel burns. It gives almost 50% more pressure when combustion occurs.

Things don't work quite that well in practice, but you get the idea.

My understanding is that when you take the compressor outside its efficiency range all that happens is the air gets beat up and creates lots of heat for less and less boost increase, and at the extreme end I'm guessing cavitation and compressor damage would happen.

EGT's would only be affected to the extent that the IC will only be able to cool the increasing hot boosted air so much.
If you're keeping fuel sane, EGT's will remain lower.

Under load, combustion pressures will still be high and remember that EGT's don't tell everything about combustion temperatures.

Quoting myself

I think if injection was more advanced it created higher piston temps, (and more NOx) but EGT's may remain sane, inject the fuel slightly later (relatively speaking) and EGT's may go through the roof but chamber and piston temps may stay OK.

From this thread. very high egt

So you can still potentially have high piston/chamber temps with relatively OK EGT's when you have more than enough air (boost)

[edit] This thread discusses EGT's and timing directly Timing and EGT's

As the other guys have already said. Boost alone won't break much. It's the combination of boost, fuel and injection timing.

The original 300tdi turbo has a 56mm garrett compressor that I don't have a compressor map for. I can get low 20's from a 52mm garrett T25 compressor and efficiency isn't terrible.

The extra air allows extra fuel to be burned, so it is the bigger bang that increases the pressure.
This will only be the case if you increase the fuel - my primary screw is still wired in OEM position.
(Aneroid is maxed though)

Have read those links Rick.
I have the timing set at 1.6mm plunger lift as per JC so quite likely my "reasonable" EGT are a function of the advanced timing...

Ill adjust the Dawes down but the issue is the problem only reared its head fully loaded flogging along the dirt ... my test roads around the shed dont really emulate the condition.

It still makes me wonder though, would running the higher boost but not increasing fuel actually change much load wise on the internals?

Dougal are you there?????


S

both my tdis are setup able to peg a 25 psi boost gauge, the fuelings not standard, the injection timing is (due to the wild variations in fuel quality that I make them run) and the wastegates are wired closed, EGT on the tdi300 wont push past 600 unloaded and will probably happily climb past the end of the EGT guage uphill with a trailer on.

the tdi300's got a watercooled turbo sucking air through a snorkle and dumps into a three inch exhuast. Its also getting a coolant/charge air cooler to sit between the turbo and the air/charge air intercooler.

Boost pressure and EGT is managed soley by the right boot thrust-o-meter

Although Dave seems to have proven you can go higher than 18psi without blowing head gasket, there is a clear difference between pressure at intake and pressure at combustion.

Logic tells me 3psi difference at intake multiplied by compression ratio of 19:1 = 57psi for a start. Plus I'm sure it's much more complicated than that. I also assume that there is a difference in pressure at full compression before combustion and the fraction just after combustion (energy from bang). That could also be multiplied exaggerating the additional boost...?