Just looking at the Isspro Performax gauges and i came across the drive pressure gauge. What exactly does this measure and when would it be useful??

Probably not the best description, but drive pressure can roughly be described as the resistance to exhaust flow or back pressure. It is the force of the exhaust gasses exerting against the turbine wheel in the turbocharger. As the exhaust gasses turn the turbine, the compressor wheel turns and compresses air that you read as boost... as this boost goes up, it is increasingly harder to turn the tubine and creates the back pressure or Drive Pressure.

It is valuable in higher HP engines to measure this to find when a turbo is outside its MAP or efficiency range. Mostly, when turbos are operating properly there is a 1:1 ratio of boost to drive pressure. When a turbo is outside the map, DP goes way up in relation to the boost provided. Too much DP can pop a head gasket.

that makes sense, thanks!

so how the opie kit helps keep you from lifting the head or just the freexe plugs. i know its taped into the water system but how does that stop dp or does it not?

No, Opies kit does not have anything to do with DP. It helps cool between 5&6
and relieve the pressure there to prevent the blowing out of the freeze plugs.

No, Opies kit does not have anything to do with DP. It helps cool between 5&6
and relieve the pressure there to prevent the blowing out of the freeze plugs.

BINGO!!!!!!!!!!!

In simple terms:

Drive Pressure = Exhaust Pressure

I see the answer has pretty much been given but i enjoyed the ease of reading the newest issue of Diesel Power. They explained every part of a diesel and how it works. Im pretty sure drive pressure was in there. So go pick an issue up [coffee]

When a turbo is outside the map, DP goes way up in relation to the boost provided. Too much DP can pop a head gasket.

Darn soulzoo, who stole your user name and posted this rubbish. I know you know better than that! [laugh]


A turbo's map has little to do with DP, it can be higher or lower when the compressor is in or out of its map. DP is a function of the hot side housing and size not the compressor. A large turbine turbo can be out of its efficieny map and still have relatively small DP in relation to boost.

Oh, one other thing, I really don't think a HG that is capable of containing 1500-2000 psi of cylinder combustion pressure is gonna be bothered by 60, 70, even 80 psi of drive pressure. Considering DP is not ever working directly on any part of the HG it simply amazes me people think it can effect it. Popping an HG is heat or combustion pressure not boost or DP. :)

The ratio of drive pressure to boost is however, important.

Oh, one other thing, I really don't think a HG that is capable of containing 1500-2000 psi of cylinder combustion pressure is gonna be bothered by 60, 70, even 80 psi of drive pressure. Considering DP is not ever working directly on any part of the HG it simply amazes me people think it can effect it. Popping an HG is heat or combustion pressure not boost or DP. :)

What your missing is not the drive pressure but the actions it creates.
Excessive drive pressure ( TIP = turbine inlet pressure) creates timing and the timing is what lifts the head.
So in a nut shell excessive TIP (DP) will wipe out a head gasket.

What your missing is not the drive pressure but the actions it creates.
Excessive drive pressure ( TIP = turbine inlet pressure) creates timing and the timing is what lifts the head.
So in a nut shell excessive TIP (DP) will wipe out a head gasket.

Creates timing??? :confused::confused:

Timing lifts the head???? :confused::confused:


Not sure what you are trying to say here but there is no timing involved in boost and drive pressure. Cylinder pressure causes HG failure, generally from too much air and fuel and/or the generated heat in-cylinder.

Once the compressor is out of its efficiency range boost curve starts falling while DP can go higher due to restrictions but that pressure is out of the cylinder and any backflow will actually drop cylinder pressures. One can pop an HG on 1st gen and I guarantee DP is not an issues there. ;)

Looking at boost and DP is too simplistic as rpm's, fueling, and inj timing play major roles. Just saying DP will cause HG failure is way too broad and misleading.

Advanced timing raises cylinder pressure.

Creates timing??? :confused::confused:

Timing lifts the head???? :confused::confused:


Not sure what you are trying to say here but there is no timing involved in boost and drive pressure. Cylinder pressure causes HG failure, generally from too much air and fuel and/or the generated heat in-cylinder.

Once the compressor is out of its efficiency range boost curve starts falling while DP can go higher due to restrictions but that pressure is out of the cylinder and any backflow will actually drop cylinder pressures. One can pop an HG on 1st gen and I guarantee DP is not an issues there. ;)

Looking at boost and DP is too simplistic as rpm's, fueling, and inj timing play major roles. Just saying DP will cause HG failure is way too broad and misleading.
What I said was TIP causes timing that pops the gasket!

Turbine inlet pressure once over 1-1 ratio will create timing, and yes timing will lift a head gasket.
Each cycle with a excessively high TIP causes a incomplete scavenging of the cyl, next stroke you pour same fuel but less air in (already some left over hot air in there) and this preheats the mix and that cyl now fires sooner. Read timing.
This action compounds over cycles creating excessive EGTs and more TIP and will result in lifting the head.

Hope your not so confused now.;)

Reduced O2 from an incomplete exhaust cycle (due to high back pressure) may increase cylinder heat but will reduce peak cylinder pressure due to incomplete burn of the fuel charge. More fuel does not always mean more pressure.

Advanced timing raises cylinder pressure.

Absolutely [guitar]

Darn soulzoo, who stole your user name and posted this rubbish. I know you know better than that! [laugh]


A turbo's map has little to do with DP, it can be higher or lower when the compressor is in or out of its map. DP is a function of the hot side housing and size not the compressor. A large turbine turbo can be out of its efficieny map and still have relatively small DP in relation to boost.

Oh, one other thing, I really don't think a HG that is capable of containing 1500-2000 psi of cylinder combustion pressure is gonna be bothered by 60, 70, even 80 psi of drive pressure. Considering DP is not ever working directly on any part of the HG it simply amazes me people think it can effect it. Popping an HG is heat or combustion pressure not boost or DP. :)

Well, we agree/disagree at the same time... while I understand it, I also understand that my ability to convey that into words doesn't always work right. And I was trying to keep it simple. HG popping is a cause/effect thing. Absolutely it is too much cyl pressure that causes this. But then you you have to examine how this happened. There is more than one way to do this for sure. And it is my firm belief that boost/DP ratio can contribute to this. Why can some (stock headbolts and HG) engines survive multiple passes at 55-60 psi boost and the HG is fine while others pop at 45 psi?

I did oversimplify the use of the term MAP... but I am sure you understand this is why II offers the same turbo with three different turbines for example.

Anyway, I'll respectfully hold my position on this one even if I didn't state it correctly...

Just saying DP will cause HG failure is way too broad and misleading.
Unless you actually understand how it works.

Darn soulzoo, who stole your user name and posted this rubbish. I know you know better than that! [laugh]


A turbo's map has little to do with DP, it can be higher or lower when the compressor is in or out of its map. DP is a function of the hot side housing and size not the compressor. A large turbine turbo can be out of its efficieny map and still have relatively small DP in relation to boost.

Oh, one other thing, I really don't think a HG that is capable of containing 1500-2000 psi of cylinder combustion pressure is gonna be bothered by 60, 70, even 80 psi of drive pressure. Considering DP is not ever working directly on any part of the HG it simply amazes me people think it can effect it. Popping an HG is heat or combustion pressure not boost or DP. :)

Well, we agree/disagree at the same time... while I understand it, I also understand that my ability to convey that into words doesn't always work right. And I was trying to keep it simple for the simple question of the OP that didn't need an encyclopaedic response . HG popping is a cause/effect thing. Absolutely it is too much cyl pressure that causes this. But then you you have to examine how this happened. There is more than one way to do this for sure. And it is my firm belief that boost/DP ratio can contribute to this. Why can some (stock headbolts and HG) engines survive multiple passes at 55-60 psi boost and the HG is fine while others pop easily at 45 psi? And these numbers can vary greatly dependent on mods... for instance, with my first hx35/40 over 3b set up I could hit 60 psi boost with about 75 psi DP. That setup is only flowing about high 80's in lbs/min of air. My next setup was flowing 113 lbs/min from the primary at as high as 85psi boost with DP a little over 100psi (studs and stock HG) this would support your position except there are those that have had HG failure at a lot lower boost and DP levels. Now with a cam and porting, my primary flows in the low/mid 130's in lbs/min, my HP is up but I only see 60psi boost and 60psi DP... The point of that is.. a lot more air + a lot more fuel = a lot more power which typically is from more force on the pistons from more cylinder pressure. I have to say I have seen more HG failure from folks trying to get 45-50 psi from their stock turbo than I have from folks seeing 55 psi from a silver bullet. So you have to ask why?

I did oversimplify the use of the term MAP for the purpose of the OP... but I am sure you understand this is why II offers the same turbo with three different turbines for example. On the compressor side it doesn't change, but a turbine (and EH) changes the way the exhaust (well the amount anyway) flows and for the purposes of the user, how that turbo responds.

Anyway, I'll respectfully hold my position on this one even if I didn't state it correctly... I always reserve the right to be wrong... heck, Nathan Wright's always having to run around and clean up (correct) behind me. It wouldn't be the first time here either. [laugh]

Reduced O2 from an incomplete exhaust cycle (due to high back pressure) may increase cylinder heat but will reduce peak cylinder pressure due to incomplete burn of the fuel charge.




Correct, if the light stayed static but that heat will light it sooner and increase expansion time raising pressure.



More fuel does not always mean more pressure.


I believe this.

As far as MAP goes...I know when I am out of the MAP of my turbo...at 45-48psi I see a 1:1 ratio...but at 50psi I see a 1.3:1 ratio. The MAP for the Super B Special is around 45-47psi, I can see it plain as day when I exceed that with boost, my DP starts to rise dramatically.

So I agree with soulezoo...ratio of DP/Boost is determined on the MAP of the turbo.

Turbine inlet pressure once over 1-1 ratio will create timing, and yes timing will lift a head gasket.
Each cycle with a excessively high TIP causes a incomplete scavenging of the cyl, next stroke you pour same fuel but less air in (already some left over hot air in there) and this preheats the mix and that cyl now fires sooner. Read timing.

Ah, now I see where your going. Down the wrong road but the direction is clear. [coffee]

Not gonna happen, mixing combusted exhaust with the incoming charge is a design to reduce cylinder pressures not increase them. Less oxygen in the charge results in less burn, less pressure, and less exhaust flow that lowers DP.

Again, excessive drive pressure CANNOT and WILL NOT increase cylinder pressures. It also CANNOT increase timing. If anything it has the opposite effect.

This scenario cannot happen in a DIRECT INJECTED fuel system. Until you have fuel you have no combustion. Combustion happens when the injection happens. It cannot happen any sooner so there is no ttiming advance by raising charge temps.

Diesel combustion is purely stoichiometric, you only get as much burn as you have oxygen. Once the oxidizer is gone combustuion temps cool and peak pressures drop. The more combusted charge you mix the less the temps and pressures and the less power you get, DP included.

Literally, the more drive pressure you have the LESS likely you are to pop an HG due to the lessed oxygen content in the air charge. ;)

Again, at this point EGT's are much more of an issue the DP ever will be. :cool:

As far as MAP goes...I know when I am out of the MAP of my turbo...at 45-48psi I see a 1:1 ratio...but at 50psi I see a 1.3:1 ratio. The MAP for the Super B Special is around 45-47psi, I can see it plain as day when I exceed that with boost, my DP starts to rise dramatically.


As I pointed out previously, once you are out of the compressor map the boost to drive pressure will diverge. Doesn't mean the map defines the DP it just means the turbine size and design at its limits there.

Did you mean boost at 1 and DP at 1.3 or opposite? If boost is exceeding DP then you are not neccessarily out of the turbo's compressore map.

Anyway, I'll respectfully hold my position on this one even if I didn't state it correctly... I always reserve the right to be wrong... heck, Nathan Wright's always having to run around and clean up (correct) behind me. It wouldn't be the first time here either. [laugh]

I was just yanking your chain a bit. [laugh] I knew where you were going but the original post just did not read right without further explanation. ;)

I don't disagree with your explanantion and questions except on one small point, excessive drive pressure is a sympton not a cause.

Your statement about why some pop at 45 psi and other will hold 50 psi is dead on. I would hazard a guess if circumstances were examined closely fueling and reultant EGT's would be the largest contribution to failures. As to why do stock chargers see more failures is telling also, quality of the air supplied by a single cannot compete with even a small set of twins at the same boost. Less oxygen is gonna drive cylinder temps higher and probbaly reduce pressures but heat has its own dangers also.

The fact that one can stress a HG on VE engine at a lot less psi and drive pressure than a CR tells me there are other factors here that will contribute. Notably cylinder pressures and cylinder temps. :)

Ah, now I see where your going. Down the wrong road but the direction is clear. [coffee]

Not gonna happen, mixing combusted exhaust with the incoming charge is a design to reduce cylinder pressures not increase them. Less oxygen in the charge results in less burn, less pressure, and less exhaust flow that lowers DP.

Again, excessive drive pressure CANNOT and WILL NOT increase cylinder pressures. It also CANNOT increase timing. If anything it has the opposite effect.

This scenario cannot happen in a DIRECT INJECTED fuel system. Until you have fuel you have no combustion. Combustion happens when the injection happens. It cannot happen any sooner so there is no ttiming advance by raising charge temps.


Been down the wrong road before....that's how I find new places LOL

Are you than saying the ignition doesn't take place at the injector tip and it still does it in the bowl of the piston in an super heated cyl?

Been down the wrong road before....that's how I find new places LOL

For sure, the road not taken can lead to some interesting places. ;)


Are you than saying the ignition doesn't take place at the injector tip and it still does it in the bowl of the piston in an super heated cyl?

If it did, how long do you think the injector tips would last?

What do you define as a super heated cylinder?


To understand the questions a little better let me throw some numbers and ideas out there for consideration.

1. Cylinder temps pre-combustion can easily reach 1200-1500 degrees BEFORE injection depending on load and fueling.

2. Max cylinder temps in the center of a combustion event can easily exceed 5000 degrees depending on fuel and air available.

3. What we are reading on EGT gauges is only 1/4 to 1/3 of actual cylinder temps.

4. Combustion events must occur in air without the fuel stream hitting any part of the engine and far enough away from the engine components to not super heat them.

The last one is extremely important to the question about where combustion starts. I guarantee if you put 3000 degrees to a tip it will disintergrate. Having combustion start that close to a tip is gonna cause a big BOOM! I think everyone has seen pictures of torched heads and pistons when spray patterns are not optimal and routed onto engine surfaces.

It doesn't happen that way for a couple of reasons. Diesel fuel needs to mix with ait to combust efficiently. Diesel will combust at the same rate irregardless of the charge temp as that is its nature. Given the fuel is leaving the nozzle at 23k psi and it has to spread a bit to mix with oxygen molecules combustion will be fairly far away from the tip irregardless of charge temp. We have all seen what happens when a cumbustible gas is shot form an orfice under pressure. Combustion does not start at the orfice but rather farther along the stream when enough oxygen has mixed to support ignition. Gas well fires are a good example.

How much preheating will you really get by mixing combusted gases with a cooler denser charge? This is a normal route to reduce cylinder temps and pressures in emissions engines so its really nothing new to an engine designed to run that way.

Since the exhaust gases are oxygen depleted and mixed with the fresh charge isn't more realistic that will retard combustion and limit the burn length rather than the other way around?

No 6 oh no - Agree with everything but stoichiometric combustion of diesel results in a little bit lower peak temp of 4100 degree F (2550 Kelvin).

Good overall comments on the subject though.

No 6 oh no - Agree with everything but stoichiometric combustion of diesel results in a little bit lower peak temp of 4100 degree F (2550 Kelvin).

Yep, it will. That plus multiple events and in-cylinder EGR are the solution Cummins uses to control high cylinder temps and pressures for emissions purposes.

However, its pretty clear that pushing into higher drive pressure areas it is gonna be a fuel rich environment not stoichiometric. [coffee] Hence my comment temps could run to 5000 degrees depending.

For sure, the road not taken can lead to some interesting places. ;)




I'm with you on much of the stuff but heres were I go down the other road.


It doesn't happen that way for a couple of reasons. Diesel fuel needs to mix with ait to combust efficiently. Diesel will combust at the same rate irregardless of the charge temp as that is its nature. Given the fuel is leaving the nozzle at 23k psi and it has to spread a bit to mix with oxygen molecules combustion will be fairly far away from the tip irregardless of charge temp. We have all seen what happens when a cumbustible gas is shot form an orfice under pressure. Combustion does not start at the orfice but rather farther along the stream when enough oxygen has mixed to support ignition. Gas well fires are a good example.


The air is already in the cyl so I dont understand the point?
Nothing can combust until heated to a gaseous state there for higher temps will result in sooner combustion. Agreed?
We are not mixing the fuel with the air as much as heating it to a gas with-in a controlled temperature for its ignition. Agreed?
The O2 in a cyl transfers the heat to the fuel there for higher temps will transfer the required heat faster. Agreed
Did I loose you or are still following where I'm going LOL
I may have oversimplified my statement that the TIP will in itself blow a HG.
I believe it is a huge factor in how much heat is left in the cyl.
My personal experiences are limited to just 5 of my own HG failures between the three trucks (P-pump , VP and CR) but there has always been the same common denominator, excessive DP and EGTs. I have never had high EGTs without high DP but I have yet to have a DP gauge hooked up to my CR so....
I have seen a CR go many pulls over 650hp with its DP and EGTs with-in check without spitting a HG yet watched countless stock turbo 12vs chunk a HG at less than 350hp but screaming DP and EGTS

The air is already in the cyl so I dont understand the point?
Nothing can combust until heated to a gaseous state there for higher temps will result in sooner combustion. Agreed?
We are not mixing the fuel with the air as much as heating it to a gas with-in a controlled temperature for its ignition. Agreed?
The O2 in a cyl transfers the heat to the fuel there for higher temps will transfer the required heat faster. Agreed


The air is present but the fuel is in a solid stream on injection. Fuel cannot combust unless an oxidizer is present. If the fuel molecules are too close together there are not enough oxygen molecules to support combustion. There is a time lag as the fuel spray disburses and mixes with oxygen molecules to support continued combustion.

It really doesn't matter how hot the charge is. If there is insufficient oxygen to support combustion it doesn't happen. Diesel fuel does not vaporize like gasoline so you really don't convert it to gas for combustion but force the state change of the fuel and oxygen into something else. Combustion actually starts on the edges and proceeds thru the denser center as a flame front with the center having the highest temps.

My personal experiences are limited to just 5 of my own HG failures between the three trucks (P-pump , VP and CR) but there has always been the same common denominator, excessive DP and EGTs. I have never had high EGTs without high DP but I have yet to have a DP gauge hooked up to my CR so....
I have seen a CR go many pulls over 650hp with its DP and EGTs with-in check without spitting a HG yet watched countless stock turbo 12vs chunk a HG at less than 350hp but screaming DP and EGTS

Not disagreeing with any of this. Just saying DP is more a symptom of other problems, chiefly EGT's, than a direct cause for he HG going. Cylinder pressures and temps are the direct cause, high DP is the result of too small turbine design and quite likely a lot of fuel being exhausted still expanding in the exhaust track.

It really doesn't matter how hot the charge is. If there is insufficient oxygen to support combustion it doesn't happen. Diesel fuel does not vaporize like gasoline so you really don't convert it to gas for combustion but force the state change of the fuel and oxygen into something else. Combustion actually starts on the edges and proceeds thru the denser center as a flame front with the center having the highest temps.
Not trying to argue, just on a road trip.[coffee]
I don't understand what you mean since nothing solid or liquid combusts, only its vapors but I am reading that you say it doesn't vaporize?



Not disagreeing with any of this. Just saying DP is more a symptom of other problems, chiefly EGT's, than a direct cause for he HG going. Cylinder pressures and temps are the direct cause, high DP is the result of too small turbine design and quite likely a lot of fuel being exhausted still expanding in the exhaust track.

I do agree EGTs are the grim reaper but would you have the EGTs without excessive DP?

To the OP sorry about hijackin your thread [redface]

I don't understand what you mean since nothing solid or liquid combusts, only its vapors but I am reading that you say it doesn't vaporize?


Wood is solid and it combusts, coal is solid and it combusts, so solids can and do combust. Heavy petroleum products fall into the same category. They don't vaporize easily so they need to be atomized to smaller particles and mixed with oxygen to promote efficient combustion. The bonds on diesel molecules are pretty strong so it takes quite a bit of energy to break them. High pressure and relatively high temperatures to start the combustion process. The upside is once you break the bonds the energy released on the conversion is quite high.


I do agree EGTs are the grim reaper but would you have the EGTs without excessive DP?


Sure, I garantee a 21 cm housing is not having problems with DP but you push over 40 psi and enough timing a gen 1 would eat a gasket. Been there done that. :D

Wood is solid and it combusts, coal is solid and it combusts, so solids can and do combust. Heavy petroleum products fall into the same category. They don't vaporize easily so they need to be atomized to smaller particles and mixed with oxygen to promote efficient combustion. The bonds on diesel molecules are pretty strong so it takes quite a bit of energy to break them. High pressure and relatively high temperatures to start the combustion process. The upside is once you break the bonds the energy released on the conversion is quite high.[QUOTE/]


Come-on, you yankin my chain?
For the most part nothing combusts in its liquid or solid state! It must be first heated until its physical state converts to gas.
Atomised liquid fuel changes to a gaseous state known as ' vaporisation ' and its molecules attach to the air oxygen molecules, 'volatilisation '. Liquid fuels cannot burn until they are volatilised.
So if the volatilisation reaction is generated from heat (be it from temp,pressure,chemical reaction,etc) is it not logical to assume if the reaction took place with-in a higher temperature it would happen sooner (IE hotter cyl)? Are you saying it's a matter of its physical properties only allowing it to convert at X rate?





[QUOTE]
Sure, I garantee a 21 cm housing is not having problems with DP but you push over 40 psi and enough timing a gen 1 would eat a gasket. Been there done that. :D
I can build a few set-ups guaranteed to fail also but in the majority of failures I have seen are stock turbo trucks. I have not run a DP gauge on all these rig but do know in my personal cases DP and EGTs go hand in hand.

Come-on, you yankin my chain?
For the most part nothing combusts in its liquid or solid state! It must be first heated until its physical state converts to gas.
Atomised liquid fuel changes to a gaseous state known as ' vaporisation ' and its molecules attach to the air oxygen molecules, 'volatilisation '. Liquid fuels cannot burn until they are volatilised.
So if the volatilisation reaction is generated from heat (be it from temp,pressure,chemical reaction,etc) is it not logical to assume if the reaction took place with-in a higher temperature it would happen sooner (IE hotter cyl)? Are you saying it's a matter of its physical properties only allowing it to convert at X rate?

In as much as you described the process it is correct. I cut a few corners rather use the correct language and descriptions which is REALLY off track for this thread. [duhhh] Consider what is actually happening though. Combustion is the naked apes poor attempt to describe the physical manifestation of unknown phenomena. [laugh] In reality it is a chemical reaction driven by and producing energy in the form of heat.

Does it happen faster the higher the temperatures? Is it measurable? How much is the difference? A more likely description is the "combustion" is speeded up somewhat by the presence of more energy to drive the process, but, I don't think initiation is going to happen any faster at 400 degrees as opposed to 1400 degrees. Given the presence of adequate oxygen in the form of high boost you could very well see faster spikes in pressure that may emulate what advanced timing would do.

The common factor I do see in most failures is excessive heat and EGT's. Given we can run 70-80 psi of DP as long as the compressor is capable of delivering the same volume of "good" air and not have the same problems for the most part I surmise that the DP is a symptom not a cause. Really though, anybody running 650-700 hp is o-ringed, fire ringed, studded, and a really expensive head gasket. Not quite the same thing as relying on a stock head gasket and stock turbo.


Thats my s-s-s-story and I'm stickin' to it. [coffee]

(or until I can understand the physics and chemistry better) ;)

Come-on, you yankin my chain?
Really, ya think? [laugh]

At least we can both agree we derailed this thread LOL [coffee]

Here is a monkey wrench in the whole thing. Since combustion (oxidation or exothermic reaction) requires O2, if there is a lack of O2 there will be no fire no matter what the pressure or temperature is in the cylinder.

So if there is excessive residual exhaust in the cylinder that has depleted O2, then even if it is hot it will delay or inhibit the combustion.

Here is a monkey wrench in the whole thing. Since combustion (oxidation or exothermic reaction) requires O2, if there is a lack of O2 there will be no fire no matter what the pressure or temperature is in the cylinder.
Understood and accepted.


So if there is excessive residual exhaust in the cylinder that has depleted O2, then even if it is hot it will delay or inhibit the combustion.
Unlike a naturally aspirated motor, a turbos charge is a pretty constant pack.
In the beginning stages of TIP being to high I doubt the residual is excessive but still present enough to limit the next charge somewhat. It does seem reasonable that it would compound with successive cycles, no?
This residual charge however contains unspent fuel that gets the next ride up with an now over abundance (new charge) of O2 and heat. To me, this is the Monkey wrench.

My head hurts [duhhh] need sleep

Forest for the trees, forest for the trees....

Here is a monkey wrench in the whole thing. Since combustion (oxidation or exothermic reaction) requires O2, if there is a lack of O2 there will be no fire no matter what the pressure or temperature is in the cylinder.

So if there is excessive residual exhaust in the cylinder that has depleted O2, then even if it is hot it will delay or inhibit the combustion.


It won't delay it but it will inhibit total combustion. This is normal for a 600 series engine and is what allows them to run more DP and still function adequately. It is a problem though when you push beyond a very narrow operational limit. Not a lot of leeway to add fuel and not cause problems.


In the beginning stages of TIP being to high I doubt the residual is excessive but still present enough to limit the next charge somewhat. It does seem reasonable that it would compound with successive cycles, no?
This residual charge however contains unspent fuel that gets the next ride up with an now over abundance (new charge) of O2 and heat. To me, this is the Monkey wrench.

You are just not going to get a lot of compounding even with high DP. Most of the combusted charge is going to be evacuated by the piston coming up on the exhaust stroke. You will get some mix on the intake stroke with the valve overlap but the incoming charge is much heavier and denser than the hot combusted air and will displace a lighter hotter charge at even higher pressures. Since the exhaust flow has to stop and reverse to back fill the cylider that also will tend to minimize the mixing. The overlap is not that large you will get large amounts of mixing rathe rjust enough to provide control in a stock situation.

I doubt you will see any measurable amounts of uncombusted fuel back filling the cylinder though. This would mean the fire has essentially gone out on the previous event and you are already loosing exhaust temp and flow thru the turbo. When that happens DP drops, boost drops, power drops, and things just get really hot and smoky.

More likely you get some mostly combusted fuel with no oxygen left mixing with the fresh charge. Net effect is probably still going to be a negative effect on total combustion temp and pressures.

Boy what a can of worms the OP started with such a simple little question [laugh]