# Gutted cat and Thrush Glasspack equal worse performance..?



## Xyclonik (Feb 9, 2009)

Hello fellow Z owners,

I have an '86 z31 N/A, and recently I gutted the cat converter and replaced the stock muffler with the cheap Thrush Glasspack Hi-Flo (Cherrybomb).

For some reason, I seem to be getting worse performance and gas mileage than before... I thought the mods might have to opposite effect. The exhaust smells like it's running rich, like unburnt fuel. -Shrug- 

Any suggestions or input would be welcome.


P.S.- The light for my Oil Pressure gauge went out... looks like a lot of work to replace it.


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## Spongerider (Nov 18, 2002)

Xyclonik said:


> Hello fellow Z owners,
> 
> I have an '86 z31 N/A, and recently I gutted the cat converter and replaced the stock muffler with the cheap Thrush Glasspack Hi-Flo (Cherrybomb).
> 
> ...


Two things,

1st You should have turned in your cat for some decent money since cats contain platinum.

As Platinum Soars, the Catalytic Converter Gets Hot | Autopia | Wired.com

2nd. Do you know what the function of a catalytic convert does to your car? It's function is to catch unburnt fuel. By you deleting your cats you now have a smelly car. 

A NA (normally aspirated engine) car needs back press in order to have decent performance at low rpm driving such as the city. This is why you have a crappy gas mileage and worse performance then you had with a cat. 

I'm not trying to dog you but next time you mod your Z or any car do research so you don't waste your money and time.


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## Xyclonik (Feb 9, 2009)

Well, to be honest... the cat was already gutted when I bought the car. Also, my horrible performance and gas mileage are coming at higher RPM's, (3000+)

My 28 mile commute to work is mostly interstate speeds with few stops, and I first noticed the performance and MPG issues on my interstate trip to Florida, where I was driving mostly 85mph~3500rpm, and averaging 21 miles per gallon. With the cat gutted and the stock muffler, I was averaging about 26~27 miles per gallon on the highway. I wouldn't think the muffler would make such a difference, but yes... the back pressure is nearly non-existent now.


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## JonJam88 (Sep 14, 2009)

Did you replace the parts?


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## AZ-ZBum (Nov 21, 2004)

Spongerider said:


> A NA (normally aspirated engine) car needs back press in order to have decent performance at low rpm driving such as the city.


God I wish people would stop spreading this stupid piece of WRONG information.

BACK PRESSURE IS BAD FOR ANY ENGINE!!!


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## troyw (Oct 13, 2009)

AZ-ZBum said:


> God I wish people would stop spreading this stupid piece of WRONG information.
> 
> BACK PRESSURE IS BAD FOR ANY ENGINE!!!


Man, you are so right, and there's SO much info out there on this for both cars and motorcycles. Back pressure is bad, but also bad is if the VELOCITY of the exhaust is wrong. If you have a pipe that's too big, the velocity would be slowed, and thereby see a decrease in performance. You need to keep velocity up, and proper flow, and also proper fueling. I ran no baffle on a VTX 1800, fueled it right and lost nothing but weight off the bike. It ran awesome, and WAY better than without the restrictions of the cat and a restrictive header. Same with my cars. It could just be too-slow exhaust gas velocity, or the ECU can't compensate for the broader need for a new A/F ratio.


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## 87300zxT (Jan 27, 2010)

Spongerider said:


> A NA (normally aspirated engine) car needs back press in order to have decent performance at low rpm driving such as the city. This is why you have a crappy gas mileage and worse performance then you had with a cat.


no you've changed the exhaust and thus the breathing of the engine
a friend of mine puts it nicely, if you change the exhaust you change the world that the carb or injection is seeing, and you will have to make mods here to make it run good again


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