TIP for turbo gurus
 

I'm just trying to understand this.......
I read a post about head gasket failures with single vs stacked turbos and a single has higher TIPs than a stacked set up at the same boost,and that I can understand but the part I cant grasp is how TIP levels can huff a gasket? The same gasket holds back the combustion pressure and that must be thousands of times higher. I'm sure theres a simple thing I'm missing here [eyecrazy] Hope someone can enlighten me on this.
Thanks

Good question
 

Think I understand it but found myself having trouble putting it into words....so I clearly don't fully understand it.

Simplest answer I can formulate has to do with this...The higher the TIP, the higher the pumping loss on the exhaust stroke. The higher the pumping loss on the exhaust stroke...the higher the Cylinder Pressures on the combustion stroke. Higher cylinder pressures obviously create more stress on the head/gasket.

RJ

Re: Good question
 

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Thats kinda where I get lost ......
If the TIP is higher I figured it would cause for a incomplete evacuation on exaust and result in less allowable intake charge netting less cyl pressure [eyecrazy] This has been bugging since I know I'm missing something simple. It's been my experience when a exaust system pressure exceeds the intake pressure the result is less HP which in turn equates back to lower cyl charges from duration, timing,etc. I obviously havent got a grasp on this one yet. Sure hope somone can shed some light on this for me..... Do ya think it's darker in here?[eyecrazy] [eyecrazy] [laugh]

I not convinced "incomplete evacuation on exhaust" results in "less allowable intake charge". If pressurized intake air is no less...there is more "Stuff", and hotter stuff, in cylinder to compress ...resulting in HIGHER cylinder pressures.

Make sense? Where is Don M. when we need him?

RJ

I guess thats the part I dont understand.
I thought if you had a intake charge of say 45psi and you would always net a 45psi cyl charge, netting say 5% spent charge and 95% freash.So what your saying is you would actually get 45psi plus any spent charge left over?
Sorry for the dumb questions just trying to grasp this one.

don't forget the HEAT associated w/ driving a single to high boost pressures... heat kills head gaskets too...

I asked about this on TDR, and the consensus was that it was a combination of the higher pressures and the heat.

Forrest

Ratsun
 

Remember it is an easily compressed gas coming in to the cylinder under pressure. Seems to me possible to slam full 45 psi cool air charge on top of a residual exhaust charge, but it will be hotter as the piston starts up...resulting in seriously high cylinder pressures at TDC.
Result of the higher pressures has to be accompanied by higher Temps..as Forrest mentioned. RJ

Re: TIP for turbo gurus
 

High TIP (or drive pressure as it's sometimes called) can huff a gasket just as easily as boost can. The cylinder doesn't care so much whether the high pressure is coming through the intake or backflowing through the exhaust. Nature always tries to relieve pressure imbalances (which is why we have wind).

Don't think that "the gasket holds combustion, so it should hold high backpressure". They are NOT two different things.

What you want to ask is "How does high backpressure (TIP) affect the amount of peak cylinder combustion pressure and when it occurs?"

THIS is what's important. It's not TIP-- it the effect TIP has upon peak cylinder pressure.

Same thing with timing. Excessively advanced timing can create HUGE spikes in peak cyl pressure with little changes in boost or TIP.

Anyway, back to TIP. What happens when we have high TIP, specifically when it's greater than boost pressure?

As any have noted, when TIP is higher than boost pressure, the cylinder will see a reversionary effect at overlap where the higher pressure of the exhaust tries to backflow up the intake. Thus, the engine can't really get fresh air until the overlap period is over, and only the intake valve is open. So, the engine is breathing less effectively because it has less time (in terms of crankshaft degrees) to breathe.

This reduced breathing causes there to be less oxygen in the cylinder, and EGT will increase significantly within that individual cylinder. This reduces HG life.

Hope this helps. If it doesn't, ask more questions:)

Justin
 

Think we both tried to say same thing. Oversimplified, Back pressure from High TIP's results in more pressure inside the cylinder as it starts to compress...giving higher than normal Cylinder pressures. Agreed?

I followed you up to your last sentence...than you lost me.

RJ

As any have noted, when TIP is higher than boost pressure, the cylinder will see a reversionary effect at overlap where the higher pressure of the exhaust tries to backflow up the intake. Thus, the engine can't really get fresh air until the overlap period is over, and only the intake valve is open. So, the engine is breathing less effectively because it has less time (in terms of crankshaft degrees) to breathe.

This reduced breathing causes there to be less oxygen in the cylinder, and EGT will increase significantly within that individual cylinder. This reduces HG life.

Hope this helps. If it doesn't, ask more questions
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Ok thats the part I missed (I think[eyecrazy] The cyls charge is actually less air but still the same fuel causing a richer burn that is in effect addvancing timming causing the higher cyl pressures?

Yes, this is true. Reversion is higher with higher TIP.

When the cylinder is left with old combustion gasses...the temp rises faster during the compression stroke. It also tends to burn the fuel at a faster rate near TDC. This will raise the spike of high pressure near TDC. "PMax" is the correct term. The maximum cylinder pressure spikes or PMax. The injection system is designed to discharge the fuel at a pre determined rate and give a predetermined pressure spike. The faster you can burn fuel near TDC the higher the PMax. Generally speaking anyway.

If the cylinder is hotter the fuel will burn more quickly. The pressure will spike higher and the flange will experience more stress. The reverse is in winter when its harder to burn the fuel at idle....the cylinders are colder, the engine will expel more unburned parcels.

The argument of a single needing head studs more than a twin system is mostly a box that not every truck fits in. Many twin systems run higher TIP than the singles. LOL.

I have seen twins over 130 PSI in TIP with 65 PSI in the manifold. Depending on fueling, wastegates, etc.

Not all headstuds are created equally. I have seen improperly installed good studs fail just as fast as stock bolts. There is an optimum way, a wrong way and then the others are somewhere in the middle.


Don~

Thanks Don
 

Great information!

The more I learn, the more impressed I am with the importance of measuring the Turbine Inlet Pressure. Very glad I took the time to measure mine instead of just assuming they were high...because I had EGT problems.

Don, I am installing an external wastegate, as a winter project. Been told edge of MAP for my turbo is at 45 psi boost (I'm at 1:1 TIP at 45 psi). Would you set it there or go for slightly less boost and a little negative on the TIP ratio?

RJ

I dont know. The lower the TIP the better, but the engine reallly is great at dealing with the 50/50 TIP. The cylinder head, the valve lift and the cam all deal with high TIP very well.

The intake lift is a paltry .343 at the valve. The exhaust is .430 at the valve. I find most of the intake valves are set deeper than the exhaust. This helps with reversion. The lower lift on the intake helps too. The cam opening and closing points are also designed for TIP and low pumping loss.

I cant wait to see your results.

Don~

Thanks again, Don
 

Seems like every answer you give always leads to more questions! But enough bugging you for now. Guess I'll have to try it both ways and see. Besides the R&D is half the fun!

My goal has always been reducing EGT's so I can play with the power I have...and maybe create room for a little more. External wastegate may or may not help but it feels like the smart thing to do.

RJ

When things slow down here I want to send you up some different injectors. They are providing more power, less EGT and less smoke.

Don~