Remember kiddos, when you only have 212 hp...
- Thread starter VuickB6
- Start date
This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.
The ol "back pressure" bull****. I knew it would show up.
Are you saying I am incorrect?
I spent years tuning engines mainly four cylinders and a few V6's and V8's. One thing that was obvious is if you wanted power then the exhaust gases need to be out of the combustion chamber ASAP. Tuning the exhaust manifold (Header) WITH THE CAMSHAFT PROFILE netted more dyno horsepower than any other exhaust mod.
You tune the manifold pipe size/length to scavenge cylinders, it is the way the manifold works that moves the power/torque figures further up or down the revolution range. Longer pipes brought the maximum torque figure lower in the rev range, but if you wanted higher horsepower (which would be way up the rev range) then the manifold pipes would be larger in bore and shorter, and in fact would produce more horsepower if they were not actually there at all. So anything further down the exhaust pipe away from the manifold does not have such a large impact, if you can remove any and all restrictions then you will net some gain, but on a stock engine the amount may or may not be 'felt'.
The OP may well have gained a couple of MPH, the lack of restriction would have allowed gases to leave the system more easily, but I would guess there was a more serious blockage in the first place? And if a vehicle is OBD compliant then the O2 sensors would have thrown a code and perhaps allow ECU fuelling to be reduced, thus lowering power output. This would definitely happen on modern OBD equipped vehicle (I work with OBD most days) but unsure if the 'limp home' mode is so aggressive in the old 80? I concede to lack of experience here, petrol 80's here in Spain are a very rare beast. The gases backing up theory (stacking) is a good one however, we found that as the gases move down the manifold and into the exhaust pipe, they are cooling very rapidly (losing energy), so in fact they are contracting, the result is the restriction is of less importance.
regards
Dave
You tune the manifold pipe size/length to scavenge cylinders, it is the way the manifold works that moves the power/torque figures further up or down the revolution range. Longer pipes brought the maximum torque figure lower in the rev range, but if you wanted higher horsepower (which would be way up the rev range) then the manifold pipes would be larger in bore and shorter, and in fact would produce more horsepower if they were not actually there at all. So anything further down the exhaust pipe away from the manifold does not have such a large impact, if you can remove any and all restrictions then you will net some gain, but on a stock engine the amount may or may not be 'felt'.
The OP may well have gained a couple of MPH, the lack of restriction would have allowed gases to leave the system more easily, but I would guess there was a more serious blockage in the first place? And if a vehicle is OBD compliant then the O2 sensors would have thrown a code and perhaps allow ECU fuelling to be reduced, thus lowering power output. This would definitely happen on modern OBD equipped vehicle (I work with OBD most days) but unsure if the 'limp home' mode is so aggressive in the old 80? I concede to lack of experience here, petrol 80's here in Spain are a very rare beast. The gases backing up theory (stacking) is a good one however, we found that as the gases move down the manifold and into the exhaust pipe, they are cooling very rapidly (losing energy), so in fact they are contracting, the result is the restriction is of less importance.
regards
Dave
Last edited:
Aside of the blown gasket as described by the OP, what are other ways to diagnose a clogged cat ?
Thanks !
Thanks !
Left out "burned valves" because no back pressure. Maybe a muffler does the same thing...
Not sure what this post is referring to? Burned valves can be caused by a number of different reasons. Running lean, ignition timing wrong, for example. Blocked cats normally cause a serious reduction in power, engine runs very hot, you can often hear system joints/muffler seams 'hissing' as exhaust gas is being forced out of any gap it can escape from.
regards
Dave
People always claim (used to anyway) (kind of a joke) free flowing exhaust causes burned valves-
Nevermind the fact that cats are required...I know I sound like a dip**** saying it, but you really don't gain much. I laugh at this often. There are gains to be had in a system without cats.....A SYSTEM!!! not oh lets punch a hole through here and see how she does.
Gotcha! I remember that train of thought, it was thought that without the manifold the cooler air/fuel charge was not drawn in and the hot exhaust gas gets drawn back in, it is a complete load of BS.
When someone tried to tell me this, I asked them if that is the case what happens when a turbo charger is fitted, the combustion chamber is always under pressure regardless of what valve is open..........they walk away slowly thinking up another piece of BS.
regards
Dave
- Thread starter
- #32
Cats may be required federally but there are no smog checks in my area so no one is going to know or care. And if I wasn't planning on swapping a V8 in I might have spent the money to do it right but this was just to get it driveable again on the cheap. System or not it runs better than it did before, and I also do not have a CEL so I'd say that's a win.
last night while I was under my lc draining my heater lines I tapped on the bottom of my front cat and can hear a chunk of something bouncing around in there. it sounded like a rock in a tin can. I do believe I have a melted cat. its the original in the front and a catco in the rear but most likely ill just replace both. probably going with the magnaflow system from the EM back. so my question is-is the magnaflow cats mounted like the stock ones? on its side or does it lay flat?
heres what Im looking at. https://www.amazon.com/MagnaFlow-23120-Stainless-Catalytic-Converter/dp/B004SH8CR4/?tag=ihco-20
hard to tell from the picture...
heres what Im looking at. https://www.amazon.com/MagnaFlow-23120-Stainless-Catalytic-Converter/dp/B004SH8CR4/?tag=ihco-20
hard to tell from the picture...
The front cat (the one that comes with the y-pipe) lays flat. The rear one mounts at an angle (about 30*-40*) presumably to help you get the bolts on. Do yourself a favor and trim the extra metal off the heat shield--the stuff hanging off past the welds. It will make it a lot easier to get wrench on there (or an impact for removal).
Cats may be required federally but there are no smog checks in my area so no one is going to know or care. And if I wasn't planning on swapping a V8 in I might have spent the money to do it right but this was just to get it driveable again on the cheap. System or not it runs better than it did before, and I also do not have a CEL so I'd say that's a win.
I think he was adding that for context so people reading this don't go punching out perfectly good cats looking for performance gains. Now if you ad a craptastic Airaid cold air intake, then all bests are off. Combined with punched out cats the gains could be an par with a supercharger.
With a Super Charger and an aftermarket cat as my only engine mods, I have hit 115 mph. Once. During an emergency. It was a little down hill. I was running Nitto Trail Grappler 37s at the time. They balanced out on a Road Force Machine with less than 2Oz per tire, so they musta been pretty round.
That was 2 Oz per tire
I normally put along at 60 mph on the way to work so I can get 13 mpg.
Round trip to work is 52 miles and when the price of gas is up it makes it pretty damn expensive. Haven't had to worry about it for a while because it's been as low as $1.35 and has crept up to $1.89 over the past few months. But yeah at those speeds I'm surely in the single digits but I was also running late for work. It's easy to do though since I'm on my way to work at 4 in the morning.
I hate you!
Premium is $3.19 and its a glorious day when I hit 10mpg
