Question about MAP readings, WOT, peak torque and relationships between all three

K

Kane

Member
I was wondering what folks thought about how some of this fits together.

The car runs quite well. No knock, no hesitation, etc. It just goes!

So, I am getting a reading from the MAP sensor of 99KPAs with key on. During WOT through the 5,400rpm shift point, it will drop a over time to a low point of 93kpa between 4,800rpm and 5,375rpm.

I am running OE exhaust minus the converter- yep, OEM Y-pipes and manifolds (to retain OE appearance). AIR is disabled for tuning.

Could the exhaust choke the engine down for the 5-6kpas?

More info... this is a crossfire based car. I have twin, bored out to 1.9" TBs on a Renegade intake with EBL. VE is good as is the injector duty cycle. Rings are good, valve seals are good, compression is good, plugs look good.
 

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yes the restrictive exhaust IS very likely the CAUSE, of the drop=off
back pressure in the exhaust reduces intake vacuum.
any time exhaust back pressure exceeds about 1 psi its starting to reduce the cylinder fill efficiency measurably that cuts the potential power, so swapping to a larger diameter 3" dual exhaust with an (X) pipe that reduces back pressure will help, WHY not even larger and less restrictive?
while little or no effective back pressure is good, you can use the mass of the fast moving exhaust gases and a properly designed exhaust header combined with proper cam timing and header design to time the exhaust pulses from the engine to have an even LOWER or NEGATIVE PRESSURE WAVE hit the exhaust port just as the next load of exhaust gases exits the cylinder, this is refered to as efficient exhaust scavenging and properly designed exhaust headers matched to a low restriction exhaust behined the headers can easily add 10% or more greater horse power than just a low restriction exhaust alone!
RELATED LINKED INFO WELL WORTH READING THROUGH



http://garage.grumpysperformance.com/index.php?threads/is-backpressure-hurting-your-combo.495/

http://garage.grumpysperformance.com/index.php?threads/x-or-h-pipe.1503/

http://garage.grumpysperformance.com/index.php?threads/calculating-required-exhaust-pipe-size.11552/

http://garage.grumpysperformance.com/index.php?threads/building-custom-headers.961/

http://garage.grumpysperformance.co...-guys-that-just-slap-on-factory-headers.3155/





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I suspected as much. Thanks Grumpy.


Thoughts on building an exhaust that looks stock- perhaps even reusing the 1982 OE manifolds?

I am wanting to keep the stock look under the hood. Sleeper effect... :)
 
For my education only, is the C.F. MAP used to measure Engine vacuum only??

I don't understand KPA's only PSI and IN/HG

Key on should be zero vacuum?? Idling up to approx. 20 in/hg?? WOT about zero but increasing to a few in/hg at redline??

Trying to do the conversion in my ancient, non metric head.:confused:

Thanks
Randy
 
Kane said:
Thoughts on building an exhaust that looks stock- perhaps even reusing the 1982 OE manifolds?
Click to expand...

What vehicle are we talking about? Because I have a set of 305HO L69 exhaust manifolds.

I also have my original 305 LG4 manifolds and SLP stainless steel Y pipe with 3" outlet.
I'm never going to use either again, so I would be willing to sell them.
That Y pipe gave the biggest increase in performance of any single part I bolted on.
It's 409 SS and turns brown in color from usage. Just looking at it, you would not think
it is anything other than stock, unless you closely inspected it.
 
Last edited:
Loves302Chevy said:
What vehicle are we talking about? Because I have a set of 305HO L69 exhaust manifolds.

I also have my original 305 LG4 manifolds and SLP stainless steel Y pipe with 3" outlet.
I'm never going to use either again, so I would be willing to sell them.
That Y pipe gave the biggest increase in performance of any single part I bolted on.
It's 409 SS and turns brown in color from usage. Just looking at it, you would not think
it is anything other than stock, unless you closely inspected it.
Click to expand...

I have a 1982 Vette. Unfortunately, I don't think the SLP Y pipe would fit. That's sad- because the one you have sounds really nice.
 
Loves302Chevy said:
What vehicle are we talking about? Because I have a set of 305HO L69 exhaust manifolds.
I also have my original 305 LG4 manifolds and SLP stainless steel Y pipe with 3" outlet.
.
Click to expand...

You probably already know this, but V8 S-10 guys like this style exhaust manifolds as they fit nicely.
That is the ones that don't run headers.

Thanks
Randy
 
GLHS60 said:
For my education only, is the C.F. MAP used to measure Engine vacuum only??

I don't understand KPA's only PSI and IN/HG

Key on should be zero vacuum?? Idling up to approx. 20 in/hg?? WOT about zero but increasing to a few in/hg at redline??

Trying to do the conversion in my ancient, non metric head.:confused:

Thanks
Randy
Click to expand...

kPa is exactly backwards. Read 0 in of mercury = 100 kPa so reading 99 kPa is correct for a engine that is off.
 
Appreciate the input, I just found an online calculator and 6 kpa is about 1.77 in/hg if I read it correctly.

Thanks
Randy
 
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