Exhaust systems 101

Someone asked these questions in another post after reading the Gene Berg Web site, and I thought the answers might be good info for some Newbies, so here goes:

What is a header vs an extractor?
Well, the HEADER is a tube of some sort that attaches to the head's exhaust port. On most factory engines it is made of aluminium or steel and is a casting. In those cases, the various exhaust pipes coming off the heads are of various lengths and often are run together or "merged" very close to the head - usually within a few inches. However, they are not especially efficient at coaxing the hot gasses out of the head if they are short runs AND make 90 degree turns within a few inches of the exhaust port. Doing that will cause "back-pressure" or a resistance in the exhaust gas escape route which means that some of the (pressurized) exhaust gasses remain in the cylinder after the exhaust valve closes, and that means a poorer incoming air/fuel mix and THAT means a loss of power.
The answer to this was to make the tubes larger diameter and longer with gentle, sweeping bends to reduce back-pressure and allow more of the gas to escape - they even found that tubes of certain lengths, or "tuned" tubes, would actually pull, or "extract" more gas from the head cylinder than other lengths (those were the guys who paid attention in their "thermo-dynamics" classes in college). In this case, all of the tubes from all cylinders should be approximately the same length (I say "approximately", because they should be slightly different on air cooled engines due to expected variations in head/cylinder temps, and the different tube lengths can be used to help even out the temp variations between cylinders). Overall, these new systems were more efficient because they allowed more of the spent exhaust gasses to escape than the original systems did. They are "tuned" to the proper length to "extract" the spent gasses to the efficiency level the engine designer wants.

What is a merged vs unmerged exhaust?
OK, so now you've got a bunch of tubes coming off your heads that are, say, 34" long, and you want to run them all into the same exhaust pipe or muffler. In that case you have to "merge" them into a common, larger pipe, commonly called a "collector". That's what it does - collect all of the header pipes into a common exhaust pipe, and provides "scavanging" or "extraction" of the spent gasses and a defined amount of backpressure (gas resistance) to the system. VW engines rely on exhaust backpressure to some degree. Without it, the engine's exhaust valves get too hot and burn out. In the early dune buggy days when some folks ran engines without exhaust systems, they called it "sucking the valves out" when they burned out their exhaust valves.
If you run an "unmerged" exhaust, then all four exhaust pipes come from the heads and run right into a muffler (which then becomes the collector) but the scavenging effect is usually less pronounced, because there is a larger area within the muffler to pressurize with smaller exhaust pipes out of the muffler and more overall back pressure. If you look at a lot of the original Porsche 356 and VW mufflers, that is how they are designed - unequal-length pipes coming from the heads and right into the muffler.

Continued in next post>

Berg states that any modified 1600 must have an extractor to benefit from larger valves, cam lift etc. - True?
Absolutely, because the larger intake and exhaust valves generally used allow more air to flow through the engine and if you've got more air going IN, you have to then get more air OUT, right? Think of that engine as a large air pump (that's all it really is, anyway). If you increase the size of the intake system with larger carbs and intake valves, then increase the size of the exhaust valves to match the intakes but stick with the same smaller exhaust pipes and muffler, then you can't get the gasses out as fast and you end up with massive backpressure. An extreme example of this is the old "Potatoe-in-the-taipipe" prank from Eddie Murphy's movie. If you cram a potato into a tail pipe, the exhaust pressure builds up until the intake system can't overcome it to get new fuel/air mix into the cylinders and the engine stalls. That happens to a lesser degree with a restrictive exhaust system, almost like having a choke on the engine.

I was all set to just go to the local perf. guys and have a big flowmaster jammed where my Bugpack is and hope that I get a deeper tone. Will this work?
Can't tell whether it'll be deeper or not unless you measure the length of the header tubes - they act like a pipe organ...longer tubes, deeper tone, shorter tubers, higher tone. And before you ask, mine are about 38" from the head flange to the collector flange, and another 12" or so from the collector to the muffler for a nice, bassy note.

GB states that one of the most common reasons for over heating is incorrect air flow. Too much and too little are both bad.
Absolutley - re-read my stuff above. If you have too much exhaust flow, you get burnt valves, and if you don't have enough, the spent, hot gasses stay in there too long raising the head temps. The trick is to get it balanced for YOUR engine - remember, it's an air pump and it should ba balanced all the way through from the carb venturiis to the tail pipe

Also he states that correct jetting of carbs is critical. I am running Del 40s which came off my old 2110 - can I use them on my new 1776?
I have Dell 40's as well and I believe they are well suited to a 2,110, although I would probably run 35's or 38's on a 1776 to match the flow. I am running 161.5mm main jets, and I believe that my idle jets are 60mm's. That combination makes it run a tad rich: 14.7 at idle, dropping to 12 off-idle to about 3300, then swinging to 13.5 from 3300 up to about 6500 (that's as far as I'll take it) - all this taken from a CB performance air/fuel meter.
If I had the same carbs on a 1776, I would probably start with 140's for mains, and 40 - 45mm for the idle jets and go from there, playing with jets until I got it to run right.

My car pulls really strong from 3k to 4300 and then just bogs. I mean it will wind to 5200 rpms but it feels like someone hooked a trailor to it during those last 1200rpms. What's going on?
Well, could be a lot of things (in order of what I would look at): exhaust system too restrictive, Main jets too small and leaning out at the top end, weak valve springs (you running heavy duty or dual springs?), Carb venturiis too big (or small)? Idle jet not matched to the venturiis and head flow, inadequate ignition advance at the top end (should be a max of 30 degrees BTDC at 4000 rpm. Lastly, 4300 should be somewhere around 85mph, so you're reaching the top of the power curve on most stock street cams. If you have something like a 490 duration and 275+ lift cam (which it doesn't sound like) you can maybe make more power on the top end, but you'll be sacrificing mid-range and overall drivability, as some of the more radical cams become more "binary", meaning that the power is either on or off with little middle ground, making driving it a chore.

Note that "big" cams are very drivable with the correct engine build; they like displacement and higher compression ratios. My 2,387 has a 320 degree advertised duration cam with about .550" lift at the valve (1.4 ratio rocker arms) and about 9.5 compression. It idles smoothly, pulls like a tractor at low RPM, and comes alive at between 3,500 and 4,000 RPM.

Del 40's are not well suited to a 2,110; they are too small. Most 2,110 xtreet performance engines are built using 42mm intake valves, 37.5mm exhaust valves, and some variety of ported heads with matched intake manifolds, a performance cam, and a 1 5/8" merged performance exhaust. Either 44mm Weber IDF's or 45mm Dels with 40mm venturies would be a better choice for this type of 2,110 (been there, done that).

Burned exhaust valves have little to do with exhaust flow that is too high; it occurs when exhaust valve temperatures are too high (lean mixture) and/or seat contact does not allow proper heat transfer from the valve head to the seat/head and valve stem to the valve guide/head.

Note that a high RPM "bog" can be caused by jetting that is too rich.

George, you mentioned in a previous post that you wished your old 2110 was set up with 9:1 cr instead of 8.5 - what kind of increased performance could one expect form a higher cr in your old engine?

Thanks

David, slightly "crisper" performance...

Thanks Gordon & George! I appreciate the education that you provide up "Newbies". When my car finally arrives up here in the big AK, you'll have to come visit and help me get it right. George, I'm waiting to get a quote back from CB, I've requested the performance suggestions you gave me. Thanks again guys for the help!

,Pat

Pat:

Ok.....I'll come up and visit and bring John Hallstrand along with his WasserBoxer powered Speedy - John was stationed up there in the Service (I think) and I'm just nuts enough to drive the Al-Can in a plastic car. Besides....he'll be able to show me how to straighten up after riding for several days.

Where the heck IS Chugiak up there, anyway????? Do we have to be airlifted in or something?? It took me two whole (long) days to drive diagonally across Texas, once.......what'll it take to get up there, a week to get to you from Seattle?? And if we do THAT, we might as well try to find a way to get over to Yellow Knife in the Yukon Territory (that address sounds SO cool!) to visit Brian Puskas during his 12-1/2 days of Summer or whatever he gets over there. Then it'll be a race down to the Lower 48 before the snow drifts set in in September - at least it'll be light all the time so we can drive 24 X 7 if we have to.

John: You'll be a "Dead Ass" veteran by then......you up for this??

Gordon
One of the "weirder-than-average Speedstah Guys" from Rhode Island

P.S; it takes something like 48 hours to drive I-95 from the top of Maine all the way down to Key West, Florida......40 MINUTES of that is in Rhode Island, diagonally, across the entire state.

Gordon & Dead Ass,
John has so many names/hats I haven't got a clue what to call him. If you guys did drive up, dead ass would definitely be warranted. I'm still trying to get down to Knott

Gordon,
If you go with Georges theory that Dell 40's are too small for 2110's, and go with bigger Webbers or something else, Sell the Dellortos to me, I love those things.
I don't build my engines very extreem and only look for around 5000 redline. I like torque! Like cruise along at 2000 rpm in too high a gear,then stick your foot in it and have it pull like it's gonna break the crank! You know, real world driving situations.
Dellorto 40's work!
Greg B

Gordon, I've been musing about doing Alaska on two wheels. I've been baited by recurring theme articles on this subject in the American Motorcyclist magazine. I've not mentioned this flight of fancy to sweet bride yet; I want my coffee in my cup and not in my lap, thank you.
DA

Hoss,

Go for it, marriage will understand, after you get back.!!I drove up to Alaska in 01 and it topped off my 50 state oddesey. I went in August and weather was deluxe except for a little fog near Fairbanks. I camped a few nights at some of the wonderful B.C. run campsites. Toured Alaska and then took the Alaska Marine Hwy ferry boat back down to Bellingham, Wn. The ferry boat has room for your bike and auto/camper. I met a couple from Detroit that were touring with their
BMW bike and trailer. They had hit several BMW bike outings on their way up and a big one on the way back in the south, Texas maybe. He was in his 60's and she in her 50's. He had never ridden a bike before embarking on this journey. He had sold his business in Detroit and decided to fullfill his fantasy. I got an e mail from them after they had gotten home and they made it OK except someone broke into their trailer in Texas at the meet and stolen a few things.
I'll talk to you more at Knotts about the trip!

Bruce

Bruce, thanks. I'm sure sweet bride would understand, but her comment over breakfast was negative on two wheels and positive on four. Gordon, hold that thought. Were you serious? Our cars would probably need a paint job when we returned. But wow, what an adventure!
John as in Hoss

Way back in the late 60's, my brother had a few friends living up there who convinced him that he could get a good construction job if he were to go there. So he drove his 1964 Austin Mini 850 from Massachusetts to Anchorage, found that he couldn't quickly find work of his liking, and then turned around and drove home - total elapsed time, three weeks!!

I remember he called home from Anchorage to ask for money to make the return trip - that was early Sunday morning, Anchorage time. The following Friday, around 4pm, he pulled in to the driveway at home in Central Massachusetts after driving almost straight-through (he was stopped by the police somewhere in Ohio for not starting when a stoplight turned green, because he fell asleep at the light - after he told them where he had come from (and showed them his gas receipts), they took him to a motel and even paid for a nights stay for him. He slept three hours and went back out on the road to continue on.)
When he returned home, he had HUNDREDS of stone dings in the front sheetmetal and the leading edge of the roof, and the windshield had 30+ chips and cracks (we all counted them). He had fitted the headlights with little plastic domes at a truck stop somewhere in Alaska, which deflected the stones from preceeding vehicles and protected the headlights.
He later told us that he found the top speed of the Mini 850 across the planes was 92 - 96 mph, and that it would hold that speed for hours if you put something heavy on the gas pedal.....it was also getting somewhere north of 45 miles per gallon.

That car lasted a couple more years of commuting before he sold it (and bought a 1275 Cooper "S" to replace it with).

I wonder if the roads up there have improved much?? I've never gone in that direction. Instead, I drove down to Honduras - Ray liked the cold (still does, in fact), and I like the heat......

Gordon

Greg:

Nope, thanks anyway, but I'll just be keeping my Dells.

Just to go into a little technical depth (to respond to earlier posts about 40's being too small for a 2,110), my 2110 has a 294 degree duration cam, with .435 lift (stock 1.25 rocker arms) and a supposed 8.5-ish compression. I'm running MOFOCO heads with 37.5 exhaust and 40mm intake valves, yes, slightly smaller than "usual". The intake manifolds were half-fast match ported to the heads, but the technician (me) didn't take his time and needs, someday, to clean them up.

My Dells have 40mm venturis which, according to my calcs, should give me max/ideal flow at just about 7,000 rpm on a 2,110. That might explain why I can pull to 6,500 in third. If I had 44's in there, the max flow would increase to somewhere around 7,750 rpm, but I wouldn't get any noticeable change in low-to-mid-range "grunt" (40 mm intake vales, remember?), my gas mileage would certainly drop and I would bet that my idle would worsen, although that might be corrected by playing with the idle circuit (a lot). If I were to go to 48's, the flow MIGHT increase a bit more, but now I'm beginning to see the effects of those 40mm intake valves so I doubt there would be much, if any, improvement - I'm really beginning to see intake restriction even with the 44's. So, the first reaction might be; "install bigger intake valves", right?.

Well, it idles smoothly, forever, at 950 rpm, pulls like a tractor from below 2000 rpm on up (although, by habit, I don't drop below 2K rpm unless there are moms and small children in the road), and really starts to breathe around 3,800 (you can feel the change). I can keep up with substantial American V-8's, I get a solid 25 miles per gallon, and since I've installed the horizontal discharge tubes in the Dells, I've never had a clogged idle jet (which, BTW, can be removed, cleaned and replaced in under a minute, anyway). 99.9% of my driving is on roads posted at under 60 mph, so 99.9% of my time I'm running between 2,000 and 4,500 rpm - right where my 40 mm Dells LOVE to be.

Do I believe that 44's or 48's would help my engine? NO. I would have to drive in the world above 5,500 a LOT to see the benefit, and I'm simply getting too old (and too cheap) to afford the difference.

Gordon
A "40 mm Dell Guy" and proud of it

An Austin Mini doing 92-94 mph....he was dreaming, or delirious, or both. I've owned a 1967 Mini 850 and a 1969 Mini Cooper. The only way to get an 850 going that fast would be to drop it off a cliff.

The ALCAN Highway is paved the whole way now, suppose to be considerably better than it was just 5 years ago when I drove part of it............ You used to have to book hotels or campsites months in advance, now it isn't a problem from what I hear.

,Pat

Ron O! Long time no see!

Before my Bro and his friends got into Mini's they were all 356 freaks (guess that's why I got mine). They got the Minis as "Winter Cars" and they all started with 850's, but none of them were especially stock (they used to ice race them, too). Later they all had 1,275 Coopers, some of which got raced.

I have personally been right-seating (Ray's was a left hand drive) on I-90 going out to Lime Rock Raceway at 90 mph or better in that same Mini 850. It wouldn't hold it on an uphill, but it wouldn't drop much below 88, either. I also remember it doing over 50 in 2'nd and 75 in third.....The Coopers, on the other hand, were incredible - great acceleration, unbelievable handling (like a breadbox mounted to a Go-Kart) and a top end well over 100.....but the cornering is what you fell in love with......never underestimate an original Mini.

By the way Gordon... if you ride (2) wheels to Anchorage, Alaska let me know,,, I am the Honda, Ducati and Aprilia motorcycle dealer in Anchorage.

Tony

Tony:

You'll never see me up there on 2-wheels anymore, but my older brother is just about nuts enough to do that. Last bike he had was a Yankee, 6-day bike - (2) Ossa 250cc's grafted together at the crank for a 500cc off-road bike. That was back around the late 70's, I think. He never worried about trees smaller than 3 inch diameter (it would take them out) or bumps lower than 2 feet (it would just go over them) and I swear you could cruise through the woods at close to 60 on it. You could start from stopped in 2'nd gear and accelerate to about 50 mph before you shifted. First gear was for climbing Oaks.

Come to think of it, an Alaska trip is something my son might do on his Honda.

Anyway, if we ever get up that way on a Coastal Cruise, we'll look you up!

Gordon

And What's an Aprilia???? One of those Siberian bikes??

Gordon,

If you do take that cruise stop by; I'll teach you how to drink scotch..... and we'll take a cruise in my speedster.

Tony

P.S. an Aprilia is an Italian exotic sport bike......!

Tony,

Dad's right. I very might well consider a trip up there with my VFR and the Givis bolted to it. If I do make it up there, I _definitely_ want to check out an Aprilla. :)

Chris

Stop by and see us.... If you are too young I'll teach you how to drink Pepsi!!!

Tony