I have been experimenting with many modifications to the 60* V6 engine exhaust I can safely say that the Fiero exhaust system has a few weak spots.
It is common knowledge that the exhaust manifold ports are not open to the diameter of the exhaust port tubing used to make them. Inside each manifold port tube of the exhaust manifold is a small slot instead of a rounded unrestricted tube. This slot restricts exhaust flow and robs power. It should be ground to the size of the manifold pipe for unrestricted flow. This can be done with a die grinder and a carbide bit.
The crossover pipe is also very restrictive. When the 1 3/4" diamter pipes meet at the Y junction the tubing necks down to about 1". This Y section can be replaced but it will take a bit of plumbing work and mig welding to correct it.
The last item which I believe is the most restrictive part of the system is the catalytic converter. This unfavorable restriction can be eliminated by hollowing it out. You remove the cat and use a hammer and a long large screwdriver or chisel to do the work. You chip away at the beehive material and it cracks. You then shake the case and it just falls out in pieces. The hollowed out cat will allow the engine to run cooler, and breath much better, This will free up horsepower and eliminate a super hot potential fire hazard. For ultimate HP gain, the mixture may have to be richened up a bit to compensate for the less restrictive exhaust, but an Accel adjustable fuel pressure regulator should do it. Boosting the pressure up about 5 psi should be sufficent. Of course this mod is for "off road use only" A hollowed out cat retains the politically correct look but if emissions testing is done in your area the car may not pass the test without some adjustments like retarding the timing a bit or adding fuelon power or alcohol additives to the gasoline.
As for the 10 HP figure; this is only a guess, but my cars all run remarkably better without the CAT than with it. You can actually feel the difference in power.

------------------
87GT 3.4 Turbo Best 0-60 5.2 seconds
http://turbofiero.fierojoe.com/turbo.htm

Removing or gutting the cat on a stock 2.8L will cause you to lose low end torque. Unless you have a modded V-6, the low end torque loss will be very noticable.

Sorry for being daft, but how would restriction ADD tourqe? I mean, it makes no sense at all.

I've heard that 1FST2M6 dynoed to see what happens. Do you really lose tourqe, and if so, is it replaced with HP?

------------------

^{The Black Beauty (85 GT) - Bustedato (86 SE, Parts car)
Don't sweat the petty things, and don't pet the sweaty things
AOL IM: KSSouter
MSN IM: I_R_Sootah@hotmail.com}

Backpressure is necessary for low end torque in a naturally aspirated motor, but I cant explain why.

Dennis, was this on your turbo or on another car? My vr4 loves open exhaust, I have 3" all the way with no cats, and I didnt feel a loss in low end, just the turbos spool much faster. But on non turbo motors I have heard that getting rid of your cat can cause a loss of low end torque but a gain in top end horse power. So if you drive around using low rpms all the time its probably not a good idea, but if you frequent the upper rpm's, you may like it.

------------------
Ryan S.
86 Fiero GT - twin turbo project
95 3000gt VR-4 - big turbos, big fuel system, all supporting mods and a 60 shot of NOS
71 Chevy Camaro - project

quote Originally posted by Sootah: Sorry for being daft, but how would restriction ADD tourqe? I mean, it makes no sense at all. I've heard that 1FST2M6 dynoed to see what happens. Do you really lose tourqe, and if so, is it replaced with HP?

As the burnt gasses leave the cylinder, The back-pressure allows some burnt gasses to remain in the cylinder. I am not sure of the exact scientific reason that this is a good thing, but it has to do with the burn rate of the next compression cycle. If you increase intake flow then the exhaust flow can be less restrictive. In Dennis' case he is using a turbo so the compressed A/F mixture creates more power.

When Travis tested the cat/no cat issue, I think there was a 4 lbs of torque lost at the low and 3hp gain at the upper RPM range. With the loss of torque at the low end your launch is less.

Scavanging, look it up.

------------------

Ben Cannon
88 Formula, T-top, Metalic Red. (2:13.138 at Sears Point) "Every Man Dies, not every man really Lives"
88 Formula, Northstar, Silver, In-Progreess. -Mel Gibson, "Braveheart"

quote Originally posted by artherd: Scavanging, look it up.

Correct. At lower engine speeds, some backpressure helps with scavanging and helps to up the torque. As engine speed increases, the dynamic changes and the backpressure becomes a restriction to flow volume.

Lower backpressure causes a loss of low end torque with a potential gain of upper rpm horsepower. The actual gain has to do with wether your engine can make use of the added flow at hi rpms. If it can't, you just loose torque with no gain.

This is why Ferrari used a 2-stage muffler system on the F355. The low-speed side was quieter and had more back pressure to help low rpm torque, but under heavy throttle and hi rpms, it switched to a hi-flow side for better top end horsepower. Ferrari doesn't do things for the gimmick.

quote Originally posted by Formula88: This is why Ferrari used a 2-stage muffler system on the F355. The low-speed side was quieter and had more back pressure to help low rpm torque, but under heavy throttle and hi rpms, it switched to a hi-flow side for better top end horsepower. Ferrari doesn't do things for the gimmick.

LOL I remember my topic about a variable back-pressure exaust around 6 months ago, and cited Ferrari as using it. Do a search for 'Variable Back-pressure exaust'.

People said to 'not waste my time'. Guess they didn't know I work in a performance exaust shop......

Rogue

okay, quick question here:
if you hollow out the cat, wouldn't the expansion and recompression of the exhaust gas goign through it create a restriction? if the diameter of the tube (the part where the pipe becomes the cat case) increases, the exhaust gas velocity decreases, then it's packed back into the normal pipe diameter and is compressed and its velocity must again increase.
a. wouldn't this cause a resonance pulse that would affect scavenging and
b. wouldn't trying to pack the exhaust gas back into a smaller diameter pipe after running it through a chamber more than 3 times the original pipe's diameter cause a huge restriction?

i would think the better approach would be to gut the cat then weld a pipe inside the original case to keep flow smooth while keeping stock appearance.

I did a really nice job removing the restrictions and rewelding my 88's exhuast. I lost a noticable amount of low end. I'm sorry I did it. The low end loss was worse than the top end gain.

------------------
Captain Midnight

Basic principle of Supertrapp exaust, tunable.

But remember, the Fiero transmission wasn't designed for all that increased power!

------------------

Man I don’t know where everyone keeps getting the idea that back pressure is good; its one of the most mis-understood misconceptions ever. No back pressure is ever good for any engine period. The reason why the low-end torque is reduced is because the increased flow reduces exhaust gas velocity at low rpm thereby reducing the amount of air the engine takes in. The exhaust system is a compromise and its diameter effects exhaust gas velocity.

The reduced exhaust gas velocity at the lower rpm means the engine takes in less air thereby making less power. Did you ever wonder why equal length long tube headers increase low-end torque significantly? They certainly don’t “increase back pressure”. They increase exhaust gas velocity at a specific rpm and help scavenge out spent exhaust gases thereby making more room for a fresh air and gas to come in. Short length headers do the same thing only at higher rpm.

------------------

[This message has been edited by 88formula (edited 08-10-2002).]

quote Originally posted by 88formula: Did you ever wonder why equal length long tube headers increase low-end torque significantly? They certainly don’t “increase back pressure”. They increase exhaust gas velocity at a specific rpm and help scavenge out spent exhaust gases thereby making more room for a fresh air and gas to come in. Short length headers do the same thing only at higher rpm.

you have that backwards. long primary tubes help upper rpm,s while short primaries help lower rpms. think about it: in order for the negative pressure pulse to spill over to the next primary at the right time at high RPM, it would have to be longer as the gases are moving farther in the tubes in the same amount of time.
the intake runner lengths work that way (the idea there being charge velocity and resonance tuning, not restriction), but not the exhaust. part of the reason these engines make so much low end power is because the primaries are very short- almost non-existent (being manifolds and all...) so the scavenging effect is much more efficient at lower rpms, since the exhaust gas isnt' moving nearly as fast as at high rpm.

if shorter length headers help higher rpms, then we're appraoching the FSAE car all wrong. the primaries on that car are about 2 feet in length, with our peak power just above 11,000 rpm. (and just to clear it up, the headers work VERY well on that engine...)

there's also tube diameter, but we don't need to go into that...

[This message has been edited by filthyscarecrow (edited 08-10-2002).]

Dennis,

Car Craft magazine did this about 15 years ago. I have the article...somewhere. From memory:

Porting the exhaust manifolds was good for 9 additional hp.

Low restriction Cat was good for 4 more hp

Remove the water separator was good for 6 hp!

Grand total - 19hp for next to nothing.

I could be off an hp or two, I'll try to find that article.

I removed the cat from my car (no worry about inspections as I am a licensed inspecter) and replaced it with a head pipe. I really didn't notice any difference in the seat of the pants feel. But I love the raspy snotty sound the exhast makes upon accell. If I lost any ftlbs of torque or any HP I didn't notice, but the exhaust note more than makes up for it.

------------------
A coward dies a thousand deaths..................A soldier dies but once.

quote Originally posted by Toddster: Dennis, Car Craft magazine did this about 15 years ago. I have the article...somewhere. From memory: Porting the exhaust manifolds was good for 9 additional hp. Low restriction Cat was good for 4 more hp Remove the water separator was good for 6 hp! Grand total - 19hp for next to nothing. I could be off an hp or two, I'll try to find that article.

According to an article by Herb Adams back in the late 80's or early 90's, removing the water separator did absolutely NOTHING. Seriously.

I have heard that porting the exhaust manifolds is supposed be 8-10 HP. I've not seen this verified on a dyno, but it wouldn't surprise me.

Dennis, do you pass NJ emissions with out a cat?

------------------

--Adam--
1987 Blue GT 5-speed
IM AOL: GTFiero

quote It is common knowledge that the exhaust manifold ports are not open to the diameter of the exhaust port tubing used to make them. Inside each manifold port tube of the exhaust manifold is a small slot instead of a rounded unrestricted tube. This slot restricts exhaust flow and robs power. It should be ground to the size of the manifold pipe for unrestricted flow.

You're right. It is common knowledge. And I would prefer to replace the cat with a low-restriction unit than gut out the old one. I agree with you on the crossover Y-pipe, though.

quote Backpressure is necessary for low end torque in a naturally aspirated motor, but I cant explain why.

Well, it's not just a question of backpressure, but rather the entire intake/exhaust flow picture. If you keep the intake system stock and open up the exhaust, then yes you will lose low-end torque. I'll explain below. But if you open up the intake to match the opened-up exhaust, the overall airflow will improve, thus increasing torque. But you CAN have too much of a good thing. Open up the intake and exhaust too much and your engine will run like crap at low RPM. This is due to 2 things:

1) even the stock camshaft has some valve overlap (i.e. time when both intake and exhaust valves are open)
2) since the valves can only flow so much, opening up the ports past a certain point will only serve to reduce flow velocity

These 2 things will not be beneficial to your engine at low RPM. During valve overlap, when both the intake and exhaust valves are open (at the end of the exhaust stroke and beginning of intake stroke), the exhaust gas leaving the cylinder helps to pull fresh intake in. As the piston starts its downward movement to pull the intake charge in, the exhaust valves are actually open for a short period of time. This short period of time is when the scavenging happens.

BUT, if your exhaust ports are too open, then the opposite happens. Instead of the exhaust gas leaving and pulling the intake gas in to replace it, the exhaust gas gets pulled back into the cylinder. You see, backpressure works both ways. It acts as a metering mechanism for the exhaust gas. Not only does backpressure control the rate of exhaust gas leaving the cylinder, but it also controls the amount of exhaust gas that gets pulled back in the cylinder. Big, open exhaust ports will result in little to no exhaust gas control, which will allow more exhaust gas to be pulled back into the cylinder. This results in less fuel/air mix in the cylinder, and therefore less power.

However, as RPM increases, so does the flow velocity. The faster the exhaust gas leaves the cylinder, the faster the intake gas is pulled in, and the more benefit from scavenging. You don't have to worry so much about exhaust gas being pulled back into the cylinder, due to the fact that it's leaving at such high speed. Plus you get the cool added bonus of the intake gas pre-pressurizing itself. The intake gas being scavenged into the cylinder will be moving at such a high speed that the intake's momentum will cause it to pack itself into cylinder. And since more fuel/air mix is being packed into the cylinder, there will be higher combustion pressure and thus more power. So the unrestricted flow is beneficial at higher RPM.

Wow, this ended up being a lot longer than I intended.

[This message has been edited by Blacktree (edited 08-10-2002).]

Dude , you wrote a lengthy post to tell us removing the cat makes HP... It's pretty common knowledge.

As for the loss of TQ , yes there is ,I have had mie out for 3 years now , 10Hp @ the wheels? Doubtful perhaps 5 , nothing I could feel on a low mile car. It seems to have moved the powerband upwards and removed the torque hump that is present in the 2.8's power band.

I would say if you felt a difference in power it was due to the fact that your cat was clogged.

The performance gain is marginal , george balzer ran .1 sec faster with his bone stock 86GT without a cat , he picked up 2 mph on average I believe , but there is no way in hell you will feel that.

JM

------------------
Jonathan McCreery

I We Todd Did , I Sofa king We Todd Did.

quote Originally posted by JD86GT350: But remember, the Fiero transmission wasn't designed for all that increased power!

And the Unitized Fiero Chassis would twist up like a pretzel.

quote Originally posted by filthyscarecrow: you have that backwards. long primary tubes help upper rpm,s while short primaries help lower rpms. think about it: in order for the negative pressure pulse to spill over to the next primary at the right time at high RPM, it would have to be longer as the gases are moving farther in the tubes in the same amount of time. the intake runner lengths work that way (the idea there being charge velocity and resonance tuning, not restriction), but not the exhaust. part of the reason these engines make so much low end power is because the primaries are very short- almost non-existent (being manifolds and all...) so the scavenging effect is much more efficient at lower rpms, since the exhaust gas isnt' moving nearly as fast as at high rpm. if shorter length headers help higher rpms, then we're appraoching the FSAE car all wrong. the primaries on that car are about 2 feet in length, with our peak power just above 11,000 rpm. (and just to clear it up, the headers work VERY well on that engine...) there's also tube diameter, but we don't need to go into that... [This message has been edited by filthyscarecrow (edited 08-10-2002).]

http://www.headersbyed.com/bildbetr.htm http://www.dalhems.com/engine/ssheader.html

Read these!

quote Originally posted by filthyscarecrow: you have that backwards. long primary tubes help upper rpm,s while short primaries help lower rpms. think about it: in order for the negative pressure pulse to spill over to the next primary at the right time at high RPM, it would have to be longer as the gases are moving farther in the tubes in the same amount of time. the intake runner lengths work that way (the idea there being charge velocity and resonance tuning, not restriction), but not the exhaust. part of the reason these engines make so much low end power is because the primaries are very short- almost non-existent (being manifolds and all...) so the scavenging effect is much more efficient at lower rpms, since the exhaust gas isnt' moving nearly as fast as at high rpm. if shorter length headers help higher rpms, then we're appraoching the FSAE car all wrong. the primaries on that car are about 2 feet in length, with our peak power just above 11,000 rpm. (and just to clear it up, the headers work VERY well on that engine...) there's also tube diameter, but we don't need to go into that...

Is it gas velocity or resonance? You can't pick and choose. The only things that differentiates exhaust flow from intake flow are temperature and density. Any resonant or flow momentum based effect that is useable on the exhaust is useable on the intake, and vice-versa.

The resonant effect is based on the speed of sound in the gas, not the speed of gas flow.

Study wave mechanics and you'll realise that the high pressure pulse released when the exhaust valve opens reflects as a low pressure pulse from the junction of the primary and collector. In a properly designed system, this low pressure pulse travels back up the primary tube and helps evacuate the last of the exhaust gases as the exhaust valve is closing. That's why knowing cam duration when designing headers is necessary. Figure the speed of sound in your exhaust pipes based on your EGT's, the open time of your valves at peak TORQUE RPM and I bet you'll find that the time it takes sound to travel twice the length of your header primaries is close to the same as the open time of your exhaust valve.

Now, if you can write out a good explanation of how to determine proper primary diameter, I'd love to hear it, as I've never run across a good one. Read any good books lately?

I did an engine dyno test at school over a year ago with an engine that based at 380 HP and 440 ft-lbs of torque naturally asperated. This engine was out of my F-body and had short tube headers and was tested with half my exhaust from my car on the dyno. I was using a cat-con from a 94 chevy truck with the 4.3L V6. This unit has a 3" in and 3" out pipe. With the cat on, I only lost about 3-4 ft-lbs of torque (at the flywheel) across the board compared to running the engine without the cat.

Granted, the stock cat-con's that come on the 2.8L fiero are not 3", but I don't think removing the cat and not replacing it with another (better flowing) unit is really worth the work involved. Besides, the good thing about having a cat (that works) is that your exhaust does not have as much of a tendancy to paint your rear bumper cover black above your exhaust tip(s).

------------------
1987 Pontiac Fiero Coupe #18,838
3800 Series II SFI; 4T60-E Trans w/ 3.33 final drive; Terminator exhaust; 4 wheel vented disc conversion; Walbro 307 fuel pump; W-body air box w/ K&N filter; Rear 32mm sway bar; 134a functioning A/C; GM CD player w/ factory location sub; much more and...stock GN's TURBO on the way!

1987 Pontiac GTA
-5.7L SuperRam MPFI
-4L60-E Trans
-3.73 SRD

http://dtcc.cz28.com

lose torque.. yep.. its VERY noticable as a daily driver... or light to light toy.. but if you keep the car tached up above 4200 all the time you'll be happy.

ditching the cat i lost even more torque.. yeah you'll gain 3 hp at 5300 but it's not worth the 7-11lbft loss off idle to 3000.

just my findings...

------------------

why does a chicken coop have 2 doors?? Cause if it had 4 door
it'd be a chicken sedan.

[This message has been edited by 1FST2M6 (edited 08-14-2002).]

quote Originally posted by Fierofreak00: I removed the cat from my car (no worry about inspections as I am a licensed inspecter) ...

Great. I'm impressed.

Gail Banks did an article years ago where he showed dyno graphs of a stock cat, no cat (just a tube), and high flow cat. He use sensors in the exhaust at different points to measure back pressure. He found that if using a high flow cat, versus the replacement tube, that back pressure at several points was very slightly increased. .5 PSI? (remember the .5, can't remember the unit of measure) Anyway, removing the cat caused a penalty for two reasons... one, it threw off the 02 sensor because it changed the operating temperature (so you would need a heated o2 with out a cat) and two, the cat retains a certain amount of heat for light off, and that this heat helped keep exhaust temperatures higher and also exhaust velocity up.

For what it's worth...

------------------

85 F-40 Replica (unfinished)
88 GT 5-speed
02 Millennium Yellow C5

[This message has been edited by godalex (edited 10-05-2002).]

quote Originally posted by Archie: And the Unitized Fiero Chassis would twist up like a pretzel.

OK Arch: I will finally answer one of your posts sine it was made about me and not a Fiero..
Yes I did make that comment but it was not to be taken literally. The comment was made to illustrate that you can't simply swap in a monsterous high torque/high horsepower engine and have reliability without doing anything else. I still maintain that all car components have design limits and if you don't stay within these limits, modifications need to be made.
You certainly must know that a radiator design can only dissipate so much heat, a tranny can only tolerate so much torque and yes the cradle and frame can only take so much power short of reinforcement. Perhaps the frame/cradle is a bit stronger than I once believed it to be but it still has design limits. Read the posts here. Don't you read about instances of blown GETRAGs and axels. At the Zinns meet I met a guy who snapped one of your axels at the spleens (you did replace it though) and there was a reported case of a broken cradle just this week from a guy with a V6. If you wish to respond, please feel free to do so, but how about keeping the discussion about Fieros and not people on the forum?

------------------
87GT 3.4 Turbo Best 0-60 5.2 seconds
http://turbofiero.fierojoe.com/turbo.htm

I run without a cat. and I lose about about 2psi of boost. Of course, that's good since I have less restriction and better flow. For my application the cat. only adds unwanted heat to my engine compartment.