Exhaust Pressure Testing

[mangosmoothie]

https://forum.hptuners.com/showthread.php?68007-Ford-Speed-Density-Calculator

Ok this explained the concept a little better. Do you think it's something to worry about? There has to be other centri cars out there running dumps.....

Sponsored

 

[engineermike]

@mangosmoothie you're gettin’ it.

I do think you stand to lose at least 10 psi with those dumps because you’re losing the mufflers as well. I’m comparing where you are now (likely over 10 psi) to where you would be with dumps (likely near 0). Would you still have cats? If so, then it would be probably closer to 5 psi. So 15-30 hp gain due to pumping losses.

On the timing, you only get that power if it’s tuned for it. If the tune limits the timing then you won’t get that advantage without a tune revision. If it “rides the knock sensors”, (which I don’t recommend but most do it), then you will get it with no tune changes.

On the blowthrough, you are correct the logic is there but usually turned off and it wouldn’t work right anyway. Roush and some ecoboost motors have it turned on. I’m currently struggling to understand how it affects the wbo2 signal as well. 99.9% of the time it won’t matter because intake pressure is lower than exhaust. But I’m finally to a place where my boost (15 psi) is a fair bit higher than exhaust (<10 psi) so blowthrough is possible. I was commanding .75 lambda up top, which shouldn’t hurt power much but anecdotally it appears to have hurt it more than expected. I changed my command to .78 to test it but really need to test It on the dyno to know for sure. A couple of dyno pulls can answer this and get it straight.

To make matters more complicated, commanding a leaner a/f will make the pcm calculate an erroneously high cat temp which will throw it into cat-protect sooner and cause it go run even richer when in cat protect. Really the cat temp model could be tweaked to counteract this but making that accurate gets even more difficult.

All this makes you appreciate a factory matched and calibrated combination like the gt500 even more…

 

[Grimreaper]

hptuners is limiting in that it expects a fixed exhaust pressure vs dynamic for the SD model. even without blow through there is some decent variance depending on what the exhaust pressure is for the SD tuning. blow through just adds a big shift in the load/map. I am surprised that the way roush tuned it on my setup, its possible on cam angles that shouldnt allow it.

mike has the load or calculated boost pressure changed as you have reduced the exhaust pressure? or are you making changes as you progress in the tune? trims change at wot any?

 

[mangosmoothie]

@mangosmoothie you're gettin’ it.

I do think you stand to lose at least 10 psi with those dumps because you’re losing the mufflers as well. I’m comparing where you are now (likely over 10 psi) to where you would be with dumps (likely near 0). Would you still have cats? If so, then it would be probably closer to 5 psi. So 15-30 hp gain due to pumping losses.

Yeah, I have green cats. So it would dump after the cats obviously. So there IS a restriction but not much... I'm guessing my 9 lbs of boost would be greater than the exhaust pressure.

This is turning out to be fascinating..... There is quite a bit of power on the table here it seems like, but also a risk of damage I didn't consider. If I get our cam specs and had a WOT log of vct angles... It would theoretically be possible to see if/when overlap may be happening?

I can't fathom being the first and only blower car to run exhaust dumps like that.... And I can't imagine lots of overlap would be good at wot for a blower car. But this tuning portion is over my head to know if it's going to be safe to try... and I really don't think many tuners are going to know either....

 

[mangosmoothie]

someone hold my hand please lol

https://datazap.me/u/aaronc7/log-1638328243?log=0&data=15

Here is gen 1 cam specs

Intake Cam:
Advertised: 263
.050": 211
Lobe Lift: .235"
Centerline (Park / Max): 139 / 89

Exhaust Cam:
Advertised: 263
.050": 211
Lobe lift: .216
Centerline (Park / Max): 123 / 73

Valve events @ Park (no adv/ret) @.050":
IVO: 33.5 ATDC
IVC: 64.5 ABDC
EVO: 48.5 BBDC
EVC: 17.5 BTDC

Total overlap (Park) @.050: -51

Valve events @ Max @.050:
IVO: 16.5 BTDC
IVC: 14.5 ABDC
EVO: 1.5 ABDC
EVC: 32.5 ATDC

Total overlap (Max) @.050: 49


So I've found my WOT VCT angles. Now........ do we have cam specs on the gen 3 coyote for park and max? Also......... what is "max"? how many degrees and is it advance or retard?

 

[engineermike]

mike has the load or calculated boost pressure changed as you have reduced the exhaust pressure?

With a PD blower, the exhaust pressure can not change the air flow, so it can not change the load. The same is not true for NA, turbo Or centrif.

are you making changes as you progress in the tune? trims change at wot any?

I’m constantly optimizing my tune but little has to change with varying exhaust pressure since blowthrough is off and we have limited access to tune the exhaust pressure and temperature models. I have added 1/2 deg of timing iirc, though.

If you want to see something interesting, hptuners has defined some of the exhaust pressure model for the gt500 even though blowthrough logic is disabled. I’m guessing the exhaust pressure goes into the exhaust temp and torque models.

 

[cbrtrx]

someone hold my hand please lol

https://datazap.me/u/aaronc7/log-1638328243?log=0&data=15

Here is gen 1 cam specs

Intake Cam:
Advertised: 263
.050": 211
Lobe Lift: .235"
Centerline (Park / Max): 139 / 89

Exhaust Cam:
Advertised: 263
.050": 211
Lobe lift: .216
Centerline (Park / Max): 123 / 73

Valve events @ Park (no adv/ret) @.050":
IVO: 33.5 ATDC
IVC: 64.5 ABDC
EVO: 48.5 BBDC
EVC: 17.5 BTDC

Total overlap (Park) @.050: -51

Valve events @ Max @.050:
IVO: 16.5 BTDC
IVC: 14.5 ABDC
EVO: 1.5 ABDC
EVC: 32.5 ATDC

Total overlap (Max) @.050: 49


So I've found my WOT VCT angles. Now........ do we have cam specs on the gen 3 coyote for park and max? Also......... what is "max"? how many degrees and is it advance or retard?

Learning how things work is always good but you may be overthinking this just a bit. First off you mentioned you're running green cats, I've seen dozens fail with boost so I never recommend any high flow cat with boost. Then you want to basically pie cut in some cutouts. That will work fine but I can tell you again from experience that even when open about 25/30 percent of the exhaust will still flow through the mufflers so your overall backpressure between the cats and the cutouts will be greater then you think. Lastly I've run both full restrictive exhaust and true 3" dumped headers and there was no issue whatsoever at 15 psi and I wouldn't expect there to be unless you ran very aggressive cam timing that no mainstream tuner does anyhow. Just sharing from my experiences.

 

[engineermike]

This is turning out to be fascinating..... There is quite a bit of power on the table here it seems like, but also a risk of damage I didn't consider. If I get our cam specs and had a WOT log of vct angles... It would theoretically be possible to see if/when overlap may be happening?

I can't fathom being the first and only blower car to run exhaust dumps like that.... And I can't imagine lots of overlap would be good at wot for a blower car. But this tuning portion is over my head to know if it's going to be safe to try... and I really don't think many tuners are going to know either....

The risk of Lowering backpressure is on the safe side. The pcm will add whatever fuel it needs to make measured lambda the same as commanded lambda but actual lambda will be richer. You stand to lose power due to being overly rich, which cools the cats as well.

As for cam timing, I don’t have a copy of the procharger tune to know what cam timing you’re running, but I can tell you that almost every coyote/voodoo/predator tune i’ve seen runs the intake cam fully advanced (-20 deg) from about 2000 rpm to about 4000 rpm and the exhaust cam retarded 15-20 deg in the same range. This gives quite a bit of overlap and would chop like crazy at idle. Once you pass 5000 rpm, the intake cam retards anywhere from 10 to 30 deg. Past 7000 rpm, the exhaust cam might advance 5-10 deg. So it runs quite a lot of overlap 2000-4000 rpm, even stock. At high rpm, overlap reduces quite a bit (usually 25 deg less or so) but still way more overlap than idle.

Coyote cam timing control strategy at the different loads and speeds is a fascinating subject.

 

[engineermike]

@mangosmoothie i looked at your log and it looks like I guessed your cam swings pretty close.

Intake is -15 (15 advanced) up to 4200 rpm. It retards to +5 by 7000 rpm. Exhaust cam is locked at about 17 retarded the whole way. These numbers are very good and probably not much left on the table, but way more overlap than you might expect, especially below 4000 rpm.

If my math is right, the overlap at .050 with phasers neutral is -20 deg. Below 4000 rpm, your overlap at .050 is 12 deg. At that load/speed, it has the same overlap as a cam 224/224-106 lsa.

 

[mangosmoothie]

In the log I posted above my VCT angles are kind of doing you described. since I'm a cetri car, seems like I wouldn't run the risk of having intake pressure greater than exhaust until around that 5k region where it's less of a concern.

 

[Angrey]

Man, this thread has taken a turn for the complex.

Now you understand why a lot of tuners just eliminate the VCT on high HP applications. At some point, you're making more power than the chassis can handle anyway and it's just one less complexity and failure point to deal with.

Most guys aren't running the ragged edge of boost, so any challenges or losses with either the intake side or the exhaust side can be overcome with adding or subtracting some boost to get the desired output.

I'm all for optimization. Aside from turbo spool, I'm a fan of the "just size it like everyone else does" and tune/tweak the rest. I don't think it's going to move the needle all that much either way, even with a turbo/centri.

We need people exploring and rooting out these nuances, but for most people, just throw a good 3" system at it, tune away with fuel and spark and add some pully or wastegate if that's not enough.

I would think if you throw a 3" system on (and you're not worried about cats) then energy, focus and effort is much better served on the intake side of things (intake, TB, heads, etc).

 

[mangosmoothie]

Learning how things work is always good but you may be overthinking this just a bit. First off you mentioned you're running green cats, I've seen dozens fail with boost so I never recommend any high flow cat with boost. Then you want to basically pie cut in some cutouts. That will work fine but I can tell you again from experience that even when open about 25/30 percent of the exhaust will still flow through the mufflers so your overall backpressure between the cats and the cutouts will be greater then you think. Lastly I've run both full restrictive exhaust and true 3" dumped headers and there was no issue whatsoever at 15 psi and I wouldn't expect there to be unless you ran very aggressive cam timing that no mainstream tuner does anyhow. Just sharing from my experiences.

I think you're right and it'll work fine. That was my poor logic from the start - someone else at some point must have done this. I'm not smart enough to reinvent the wheel.

But it was a fun exercise in learning more about cam timing and what is happening with dual variable timing. The only car I've tuned myself just had VVT advance... It wasn't able to advance and retard, and couldn't do it on both. It also went open loop at WOT.

The coyote seems like a tricky animal to "understand" completely from a tuning aspect. Di + PI + advance/reatard intake exhaust, closed loop wot fueling, etc etc etc.

 

[mangosmoothie]

dang this thread has me re-thinking my entire exhaust choice now.

MBRP S7205AL 2018+ Mustang GT Street Cat Back - Aluminized - MBRP - Catbacks - Exhaust - 2018-2019 - Ford Mustang GT - Vehicle Selector - Shop Now (lethalperformance.com)
and
3" (Dual/Dual) SS304 Dual Thin Oval Muffler 17" OAL - Polished (stainlessbros.com)

seems like the cheapest way to get it done if you want a true 3" exhaust with a reso.

I think I'm going to get a dragy and try some 3rd gear WOT experiments with the entire catback removed, and 3" dumps straight off the collector. If it isn't measurably faster on the same day on the same road going the same direction with no catback then I'm not going to waste my time further. if it DOES make a difference, then I will entertain some dumps as described above.

I know a dyno would be more accurate to measure power gains but if those don't translate to real world performance... not sure I care enough to pursue.

 

[engineermike]

If I did mine over again, I believe I would start with a corsa sport catback with the rsc muffler. Looks like their muffler design has 3 different length helmholtz resonators built into it, which will attenuate low and mid frequencies. Then the stainlessbros 3” muffler can be added to reduce high frequencies if needed. All of them are straight-through designs that won’t impose any appreciable backpressure, and each hits a specific frequency range.

I went wrong with my 3 packed-core absorptive mufflers because it only really filters out high frequency so I’m adding bandaids to reduce the low frequencies now.

 

[andrewtac]

If I did mine over again, I believe I would start with a corsa sport catback with the rsc muffler. Looks like their muffler design has 3 different length helmholtz resonators built into it, which will attenuate low and mid frequencies. Then the stainlessbros 3” muffler can be added to reduce high frequencies if needed. All of them are straight-through designs that won’t impose any appreciable backpressure, and each hits a specific frequency range.

Been watching this thread, and this is what my plan I ended up with.

Sponsored