Things we learned on Dyno & 03 555 HO

[rbattelle]

Just put a 6 inch tip on the stock exhaust, looks big, runs stock!

Something just doesn't seem "right" about doing this.



-Ryan

 

[Devan Manis]

Just put a 6 inch tip on the stock exhaust, looks big, runs stock!

i know what doesn't seem right. Thats exactly the same kind of thing ricers do ie- a fart can muffler with nothing else... sounds cool (NO) so lets do it.

 

[TCluff]

Yea but dam does a big pipe look good. :-laf



#ad

bigger isnt always better, but it sure as hell looks cooler

 

[DHerzog]

Thanks, DLENO and everyone else... this is the best $35 I spent on my truck!!



I think, based on what I have learned here, that I'll leave my exhaust stock... I have no mods and don't want any loud noise. I will put on a big 5" stainless tip, as I think they look great!!!



If and when I opt to do any mods, I will do the exhaust at that time... . works for me... thanks again for all the great info.

 

[DLeno]

Thanks, DLENO and everyone else... this is the best $35 I spent on my truck!!



I think, based on what I have learned here, that I'll leave my exhaust stock... I have no mods and don't want any loud noise. I will put on a big 5" stainless tip, as I think they look great!!!



If and when I opt to do any mods, I will do the exhaust at that time... . works for me... thanks again for all the great info.

yes i agree, leave it alone or add the tip for kicks. you already have 4" pipes turbo to tip anyway and I know thats good for 500 HP on a no-CAT 555 engine. So even with your CAT (which you cannot remove legally) your stock exhaust will go a long way. For your truck I wouldn't be in a hurry to do exaust. maybe just the muff.

Well i staright piped my truck (the factory 3. 5 exhaust is more than adequate, just the muffler is the big restriction) and I noticed a definite drop in power pulling from low rpms. And now finally we have the answer why

correct; the muff is the restriction. Changing that also changes the peak HP behavor. My theory is that the stock ECM is calibrated for a certain drive pressure-to-boost pressure behavior. when you reduce exhaust restriction, slightly less drive pressure is required to acheive the same boost pressure, so the ECM backs off when it sees boost develop sooner



BTW, if you have ever seen the turbine pinwheel on this little turbocharger (see TDR issue 48) you might wonder if 3. 5" pipes aren't too big!

So does a fueling box that does boost fooling solve the de-fueling behavior?

Correct. When you boost fool, that problem goes away. I've measured the difference in drive pressure. Ted and others have measured very small HP output differences. I think Ted was the one who originally brought this to the attention of the forum.

if i put 3. 5 inch ex on my truck i will gain power, probably not, it would spit the head gasket out from extremely high drive pressure.

Head gaskets are ok with high drive pressures, which can approach the 60psi region with an exhaust brake. Its the combustion pressures they don't like, which are considerably higher than drive pressures due to the compression ratio

 

[TCluff]

yes i agree, leave it alone or add the tip for kicks. you already have 4" pipes turbo to tip anyway and I know thats good for 500 HP on a no-CAT 555 engine. So even with your CAT (which you cannot remove legally) your stock exhaust will go a long way. For your truck I wouldn't be in a hurry to do exaust. maybe just the muff.



correct; the muff is the restriction. Changing that also changes the peak HP behavor. My theory is that the stock ECM is calibrated for a certain drive pressure-to-boost pressure behavior. when you reduce exhaust restriction, slightly less drive pressure is required to acheive the same boost pressure, so the ECM backs off when it sees boost develop sooner



BTW, if you have ever seen the turbine pinwheel on this little turbocharger (see TDR issue 48) you might wonder if 3. 5" pipes aren't too big!





Correct. When you boost fool, that problem goes away. I've measured the difference in drive pressure. Ted and others have measured very small HP output differences. I think Ted was the one who originally brought this to the attention of the forum.





Head gaskets are ok with high drive pressures, which can approach the 60psi region with an exhaust brake. Its the combustion pressures they don't like, which are considerably higher than drive pressures due to the compression ratio

higher drive pressure=higher combustion pressure=stretched head bolts=blown head gasket

 

[rbattelle]

higher drive pressure=higher combustion pressure=stretched head bolts=blown head gasket

Why would higher drive pressure result in higher combustion pressure? That would seem to imply mean effective pressure could be increased by increasing exhaust back pressure? If that were so, output could be improved by installing a more restrictive exhaust.



Maybe I'm having trouble making the relationship you're suggesting. Once the exhaust valve opens the cylinder vents to atmosphere. High drive pressure would cause more pressure in the cylinder during the exhaust stroke, but not anywhere near as much as there would be during a combustion event.



I think I'm missing something from your equation. Can you explain further?



-Ryan

 

[TCluff]

Why would higher drive pressure result in higher combustion pressure? That would seem to imply mean effective pressure could be increased by increasing exhaust back pressure? If that were so, output could be improved by installing a more restrictive exhaust.



Maybe I'm having trouble making the relationship you're suggesting. Once the exhaust valve opens the cylinder vents to atmosphere. High drive pressure would cause more pressure in the cylinder during the exhaust stroke, but not anywhere near as much as there would be during a combustion event.



I think I'm missing something from your equation. Can you explain further?



-Ryan

drive pressure is the exhaust pressure between the head and turbo, drive pressure is only good for one thing, and that is to drive the turbo, there is no effective way to build more power with restriction, there is no more fuel in the exhaust on an efficient engine, the higher drive pressure will stay in the combustion chamber cause it cant escape, due to back pressure, so when the valve closes, there is already some ex pressure in the combustion chamber, thus taking up valuable space that more air and fuel could be injected, but when force fed by a turbo, it packs in tighter causing higher combustion pressures, that probably hurt performance, and also drive the pyro straight up, an ideal settup would have a lower drive pressure than the boost pressure, to keep air fuel and ex flowing smoothly and to keep from superheating the combustion chamber, keeping the egt lower

 

[rbattelle]

Okay, I understand what you mean. Thanks!



-Ryan

 

[JOblenes]

What blows head gaskets?

OK now my curiosity is getting the best of me. So to complicate the discussion on drive pressure=higher combustion pressure=stretched head bolts=blown gasket question, can we do a few mental exercises, for example if I were to take a normally aspirated (non-turbo) diesel engine and hook up my shop air compressor to supply intake air at say 70psi to simulate boost, would the head gasket blow? Or how about if I replace the compressor wheel on my turbo with an electrical generator and power the lights in my garage, If I connect too many light bulbs will I blow the head gasket? Just trying to stimulate the discussion.



In the examples above I mean all other engine behaviour being the same, of course (i. e. fueling levels, etc. )



BTW does anyone know what type of cylinder pressures are generated in our engines during a combustion cycle?

 

[DWestfall]

Loss of Power & MPG

So that explains my loss of power and poor MPG ever since I put 4" exhaust on. It falls flat on its face just before shifting into 2nd (defueling?), underWOT, then pulls pretty good. I may just put my old exhaust back with some sort of performance muffler. Any suggestions? My MPG's have dropped 1-3 MPG ever since. Normal driving.

 

[JHardwick]

So that explains my loss of power and poor MPG ever since I put 4" exhaust on. It falls flat on its face just before shifting into 2nd (defueling?), underWOT, then pulls pretty good. I may just put my old exhaust back with some sort of performance muffler. Any suggestions? My MPG's have dropped 1-3 MPG ever since. Normal driving.

This sounds like it pertains to unchipped motors with bigger exhaust, you have another problem it sounds like to me.

 

[Floor It]

Higher drive pressure does not equal more cylinder pressure. Yes back pressure will cause exhaust gases to stay into the cylinder and there is boosted air trying to rush into the cylinder, but just because the exhaust gases are still in the cylinder doesent mean the cylinder will now be packed tighter hence more cylinder

pressure. What happens is the cylinder will only fill so much (unless you change your combonation) due to camshaft opening and closing there is only so much time boosted air will have to compact the cylinder full, whether there is exhaust gases still in the cylinder or not. If there is exhaust gases still in the cylinder it will not compact tighter, it will only be what we call a dirty cylinder and will not make complete power as if it was a clean filled cylinder.



John

 

[DLeno]

... the higher drive pressure will stay in the combustion chamber cause it cant escape, due to back pressure , so when the valve closes, there is already some ex pressure in the combustion chamber, thus taking up valuable space that more air and fuel could be injected,

Sure. so there is a very small volume of hot exhaust gasses trapped near TDC when the exhaust valve closes.

but when force fed by a turbo, it packs in tighter causing higher combustion pressures,

When the intake valve opens, the cylinder volume expands and the intake is force fed by the turbo as the piston decends, mixing the left over exhaust gas with incoming cool air. During this time, the large amount of cool air and the small amount of left over exhaust gasses combine and face the turbocharger. Thus, the intake air pressure at this time can not be greater than what the turbocharger can deliver.



Think of it this way: if the intake side was able to see drive pressure, then boost pressure and drive pressure would always be the same.

 

[hasselbach]

drive pressure is the exhaust pressure between the head and turbo, drive pressure is only good for one thing, and that is to drive the turbo, there is no effective way to build more power with restriction, there is no more fuel in the exhaust on an efficient engine, the higher drive pressure will stay in the combustion chamber cause it cant escape, due to back pressure, so when the valve closes, there is already some ex pressure in the combustion chamber, thus taking up valuable space that more air and fuel could be injected, but when force fed by a turbo, it packs in tighter causing higher combustion pressures, that probably hurt performance, and also drive the pyro straight up, an ideal settup would have a lower drive pressure than the boost pressure, to keep air fuel and ex flowing smoothly and to keep from superheating the combustion chamber, keeping the egt lower

This is so incorrect I'm not sure where to start.



If you have residual exhaust in the cylinder, and you are packing say 40 psi on the intake charge, you are only going to get 40 psi of charge in the cylinder (assuming 100% volumemetric efficiency) Just because there is residual pressure of exhaust doesn't mean you are going to pack more intake charge during the intake cycle.



Now, assume you actually have more residual exhaust in the cylinder due to a restricted exhaust system or turbo. Exhaust, which is an inert gas cannot burn again due to lack of oxygen (similar to EGR principle). Since it cannot burn again, it actually absorbs the heat from combustion, (again similar to what the EGR function is to do to reduce peak combustion temps which form NOX), and in effect lower peak pressures during cylinder and exhaust cycles.



And, say for instance you actually had something like 100 psi of residual exhaust pressure when the intake opened. You are going to see massive amounts of reversion where in fact intake charge will be pushed out of the port until the exhaust pressure lowers (dropping piston) whereas intake can then assume to fill the cylinder.



Higher exhaust pressure has minimal effect on cylinder pressure (if not the opposite as proved above), and will not effect head bolts in anyway.



Some of you need to understand the actual camshaft timing of a motor, and to understand that the exhaust valve closes after TDC (anywhere from 20 to 40 degrees depending on the camshaft used). This is called overlap where at both the intake and exhaust valves are open together, the principle is to allow the mass of the intake charge to push out the exhaust charge (or in a naturally aspirated motor, the exhaust tuning to actually suck the intake charge in the cylinder)

 

[hasselbach]

When the intake valve opens, the cylinder volume expands and the intake is force fed by the turbo as the piston decends, mixing the left over exhaust gas with incoming cool air. During this time, the large amount of cool air and the small amount of left over exhaust gasses combine and face the turbocharger. Thus, the intake air pressure at this time can not be greater than what the turbocharger can deliver.

.

This is not exactly correct, the intake valve will open prior to TDC, prior to any suction occuring within the cylinder. Again, anywhere from 20 to 40 degrees BTC the intake will open. Whether by pulse tuning in a aspirated motor or positive pressure of a turbo motor, the intake charge will fill the cylinder even as the pistion is still going up on the exhaust event.

 

[DLeno]

you're right i did not include overlap in my discussion. Thanks for the additional detail!

 

[rbattelle]

Ah HA! DLeno and HasselBach make good sense. So then I was right originally... TCluff's equation implies increasing backpressure would increase mean effective pressure, which means you'd make more power with a more restrictive exhaust?



[The fact that cylinder pressure equalizes to boost pressure is so obvious it never occurred to me. ]



-Ryan

 

[DLeno]

Well, the answer to that question is not very profound because the TCluff equation contained a number of contradictions

 

[hasselbach]

Well, the answer to that question is not very profound because the TCluff equation contained a number of contradictions

Contradictions?



you mean like statements such as 'honest politician' or 'cheap divorce'?