I know that upgrading my exhaust to, at least, 2 inches will help the performance of the fresh, carbed B-20F in the PV544, but if I were to hook it up to the almost new, OEM 1.5 inch till my budget outlook improved, would I be doing damage to the engine? It does have a P1800, two-into-one, 2 inch down pipe on it.
Bob Silvius

Hi Bob,

As JohnMc says, it won't hurt anything, but you won't get much high end out of it. That's fine for break-in purposes, so no rush.

With the dual head pipes, I'd recommend going to a 2-1/4" system. Best performance and quiet will come from locating the muffler as far aft as possible, prefereably behind the axle. Routing the pipe under the axle is also a cheap performance enhancer (instead of making the flow go around a 90 - 180 - 90 degree bend combination).

I like MagnaFlow brand mufflers at the moment...

--Phil S.

To the best of -my- knowledge it won't cause any damage. However, the engine will be quite strangled above 4000 rpms or so. I did that for a while on my PV (original style 1 1.4 inch ID exhaust). The engine was just missing that 'on cam' feel it should have gotten above 3500 rpm, and the pwoer just trailed off slowly. When I finally got around to putting a header and 2.25 inch exhaust I found all the power that had been hiding.

I bought the header used for $125. And the ehxaust was done up very nicely by a muffler shop with stock tubing and 2 18 inch glasspacks for another $180. I should have gotten at least one real muffler, though. It's pretty loud with just the glasspacks, at least with the throttle open.

John, did you put one of those glasspacks in the under-the-passenger's- feet location where the old low aspect oval muffler went? I've been thinking of doing something like that, but am concerned about heat. Not much height to play with there. Thanks for the input -- you too, Phil.

Not there - I've had trouble in the past with my low slung 1800E with a muffler in that position. So I have the first one right in front of the rear axle, and the second one behind the rear axle. With the stock OE muffler on the 1800 speed bumps and whatnot always seemed to want to scrape it, and after having it patched several times I just had it replaced with a section of pipe.

My car has a 2.2 liter 'big bore' B20F engine in it. I really need some sort of muffler on it instead of that 2nd glass pack. But the loud deep bellow it makes is quite appealing on a juvenile level.

Apologies for showing a non-PV on this part of the board, but here's an illustration. This is a 3" system. The lone MagnaFlow muffler can be seen peeking out. It's surprisingly quiet -- quieter than my old 2-1/2" system with the muffler ahead of the axle. There's a similar location for muffler fitment on a 544.

Aw shucks, Phil, that's awright, us PV-ites, being the pioneers, so to speak, take pride in them new-fangled vehicles that followed in our tracks, and we like to look at their pretty pitchers. But seriously, I'm impressed that this one muffler would do the job for you. It looks like it's attached to one of those under the axel pipes you mentioned earlier. Thanks for the help.

Bob

Bob, the further back you put the muffler, the quieter it'll be, and will also impede the flow the least. I wouldn't say mine is exactly quiet with a 3" pipe, but it's not offensively loud; just sorta deep sounding. Smaller pipes are quieter yet.

Brief discussion on pipe diameter, which is subject to a fair amount of mythology:

Restriction (a.k.a. backpressure) is always detrimental to performance and economy, but flow velocity in the exhaust tends to increase efficiency / power. The column of gas flowing out the pipe hs mass and momentum -- it will leave a low-pressure condition behind between exhaust cycles, and that helps the intake air get moving. The first of the intake air helps flush out the last of the exhaust. This is why cams have overlap -- the exhaust valve is still open for a bit while the intake is opening up. The more overlap, the higher the RPM at which this is efficient.

If we use a header that's perfectly matched to the cam and everything else, that header contributes greatly to the efficiency of that process, and the engine runs best with no exhaust pipe at all. That's why I'm using a huge pipe: to get as little effect on the header as possible. With the stock manifold and downpipe(s), or with just any old header, the exhaust pull / intake flush is not going to be optimised without bringing the rest of the exhaust system into play.

So the trade-off becomes apparent:

Small pipe = high velocity at low RPM (good low-end torque) = restriction at high RPM (poor peak horsepower -- you basically hit a maximum velocity in the pipe at which faster pumping rates don't increase the flow).

Large pipe = low velocity at low RPM (poor low-end torque) = good flow at high RPM (good peak HP).

With the double-downpipe manifold, a 2-1/4" pipe seems to be the best compromise. If low end is more important than power above 4000 RPM, a 2" pipe might be better. The more free-flowing the muffler is, the better, in all circumstances.

Phil:

Thanks for the excellent information on exhaust pipe sizing. However, I don't quite understand the advantage of using a smaller pipe under _any_ circumstances. Any comments on the fallacy of the following reasoning would be appreciated:

At any given engine speed and load a fixed volume of gas must be displaced from the engine cylinder. It is true that a given volume of gas will have greater velocity in a smaller pipe. However, the rate of flow will be the same in a larger pipe as a smaller pipe with the exception of friction losses.

There will be a greater resistance to flow as velocity is increased. The gas travelling along the walls of the pipe experiences a certain amount of friction based on the roughness of the wall. Changes in direction also contribute to resistance to flow.

The higher velocity in a smaller pipe might somehow be beneficial for creating negative pressure on the discharge side exhaust valve when the valve is closed. What am I missing here?

Thanks much for all the help you provide.

Joe Menacker

Joe,

Your reasoning is exactly correct -- friction is greater in the smaller pipe, restricting the overall flow. But flow isn't continuous; it's pulsed -- and we'd like low pressure behind one departing pulse to assist the following cylinder's flow. As long as pressure down the pipe can be evacuated between pulses, the fast momentary flow near the headpipe(s) leaves lower pressure behind it than slow flow would -- and the flow velocity at that point is determined by the diameter of the pipe.

In other words, the small pipe is better at low overall flow volume, but builds pressure at higher flow volumes that kills the benefits (and then some). The large pipe doesn't provide much benefit at low flow volume, but hits its balance between velocity and pressure at higher flow volume.

If we have a header that's well tuned to do all that work, we don't want the rest of the system to mess with it.

Does that clarify at all?

Phil:

Thank you very much for the clarification. I had not taken into account that the exhaust system is a pulsed flow. I suspected that harmonics were involved but hadn't applied them into the picture in my mind.

As you stated earlier, the header does all the work of coordinating the exhaust pulses from the head discharge and from there you want to minimize any further restrictions to the exhaust flow out of the system.

Best regards,

Joe