With the absolute minimal overlap on stock cam, I find that saying exhaust pressure is priority 1 a little hard to swallow...
I've ran pretty lean on occasion - but with E85, it is much more forgiving.
I'll bet anyone who wants to try running that kind of boost at that AFR they will be replacing parts in the near future.


-Rogue

 

So if I am running a 3" with a cat would I be able to run higher boost if I took it off? and if so about how much more boost? Also what would be the est. difference from a 3" to a 4" exhaust system?

(My wallet )

 

well how applicable is this to a supercharged application where there is no turbine adding restriction to the exhaust. assuming just a stock NA exhaust, with the factory tube headers, a higher-flowing cat, and a stock muffler, would 10-12psi boost intercooled on 9.5 cr in southern california climate pose an issue on straight 91? i am playing with the tunerpro and will be buying an ostrich to do my own tune, but right now i am thinking i will use the factory timing curve, just pulled back uniformly on the WOT map. part throttle will be almost identical to factory, just with a little less timing as well. my SC only works on full throttle (vacuum bypass valve)

 

We are not saying 14.1 is a good a/f under boost. We like mid to upper 11's.

We probably would of been replacing parts if not for the exhaust pressure being well under control.

That 14.1 was done on several dyno pulls back to back. We were trying to richen, but couldn't that day as we maxed out that particular fuel pump set-up. Our comment was to point out that if you have the correct turbine, hot housing and exhaust for your application, you can properly control the back pressure and get away with things (improper tuning, boost spikes, inedaquate fuel, etc...) you can't running high exhaust pressure. The stock cam has enough overlap to cause intake problems when you have higher exhaust pressures.

 

"So if I am running a 3" with a cat would I be able to run higher boost if I took it off? and if so about how much more boost? Also what would be the est. difference from a 3" to a 4" exhaust system?'

You drop about 5 psi of exhaust pressure in your cross over by going from a 3 to a 4". Don't have
a solid number on a cat, but I would think 3-5 and maybe more. Again, don't know of that one
as we don't run cats here on our cars.

We sell an exhaust pressure kit. You can turn up the boost until you see your exhaust pressure
start to spike, and then you will know what your safe max boost is for your particular set-up. No other
way to tell. For example, we had a customer in Canada that built a 3.0L running a K27/6. He had
gasket problems all the time while running 12 psi of boost. We explained his turbo was WAY to
small for that car, or even a 2.5L running much boost. When he installed the kit on his car, he reported
he had 28 psi of exhaust pressure at 12 psi of boost. We sold him a turbo that would control
his exhaust pressure, and he ran a whole season with no problems with gaskets. It's important, especially
on track cars or cars running for more then a couple gears and doing it repeatedly.

 

well how applicable is this to a supercharged application where there is no turbine adding restriction to the exhaust. assuming just a stock NA exhaust, with the factory tube headers, a higher-flowing cat, and a stock muffler, would 10-12psi boost intercooled on 9.5 cr in southern california climate pose an issue on straight 91? i am playing with the tunerpro and will be buying an ostrich to do my own tune, but right now i am thinking i will use the factory timing curve, just pulled back uniformly on the WOT map. part throttle will be almost identical to factory, just with a little less timing as well. my SC only works on full throttle (vacuum bypass valve)

Same as above comment from us would be applicable on a SC'd car in terms of finding the max boost
for the configuration of the engine and exhaust. So yes, you could have to much boost for your
exhaust system. If you can't get out what you pump in, you have issues.

 

Is it the exhaust system after the turbo which is most important here, or is the headers and cross over as important?

 

The turbine side of the turbo is the single most important aspect of exhaust back pressure, IMO.


-Rogue

 

Wow, not something I would expect you guys to make public. I know you have posted dynos also of 20+ psi runs on pump as well.

Wouldn't the reduction in exhaust pressure directly lower EGT???

Also, how much does the WG dump to atm elevate any exhaust pressure?

 

Which would bring us back to bigger hot side or more efficient turbo that allows less pressure yet still not super laggy like a #10 hotside.

 

isnt the 951 muffler a higher flowing unit than an NA? and 951 piping is like 2.5" isnt it?

 

When Lindsey Racing was running that high of a boost they were probably running 4" exhaust. They have a lot of their dyno runs on their web sight that you can look at. One thing that I did notice was how much the 4" exhaust was used by them and how much higher of a boost pressure they could run. It makes sense that the HG would fail if the exhaust side boost pressure was too high.
I was always so concerned about the intake boost pressure and did not even think about the exhaust side pressure.

 

im thinking i could get away with a used 951 system on my 944 for a little more flow for a lot less boost lol

 

Yep - I think that would be the perfect, inexpensive upgrade for NA owners...


-Rogue

 

What's a good way to measure exhaust back-pressure? I have a bung on my exhaust right next to the O2 sensor and I always wondered if I could put the car on a lift and start it, plug a pressure guage into the bung and read the pressure without it melting the gauge or something like that. Anyone ever actually taken a measurement of this? Maybe a manometer instead of a regular dial gauge? Then I'd need another bung on the other side of the cat...

Just curious what the actual resistance to flow the cat is.