Let's talk Volumetric Effiency, carbs, RPMs, and MPG for a minute

[rreed]

The purpose of this exercise is to learn a bit about how and/or why small carbs work on "big" engines (say, ours) and to what extent. Maybe a bit why a 450 CFM carb might be better in some cirucumstances than a 600 CFM. I need to dig up that chart that shows the break down of the MC2100 sizes to CFM because I'm sure a lot of people are still running their stock carbs. For the comparison of 450 to 600-ish CFM carbs here, we might as well say 4V carbs. Anyways, for the sake of this conversation let's pretend that our ignition system and timing are all perfect.

Of course gearing is a major consideration since it does have an effect on RPMs at a given speed. Since mine appears to be 4.10 geared and I'm running the stock T-18 manual, my comforable top speed is 60 mph @ 2,500 RPMs. Some day when I win the lottery maybe I'll swap in an NV4500 so I can go a little faster at a little lower RPM. I know the low gearing is going to hurt mileage but Ristow's example of a similar truck getting what I would consider impressive mileage w/ a small carb had me really thinking about this and wanting to learn.

Questions about different types of intakes are also a good point, I was assuming either stock 4V intake or the common Edelbrock Performer intake I think people run here (again, we're getting into 4V carbs). Or really for that matter the stock 2V intake w/ whichever MC2100 it's supposed to be running. Surely the split vs. single plane would make a difference in air velocity?

Regarding mechanical vs. vacuum secondaries, I'm not sure I agree w/ all the reports that indicate mechanicals are good for x while vacuums are better for x. I hear people preach it as well as hot rod/performance magazine articles. I just don't know. My Edelchock is mechanical and I love knowing exactly when the secondaries are going to open, from an economy point of view as well as performance. I have complete and udder (whoops) control over it but I'm more of a control freak when it comes to operating a vehicle anyways. I hate automatics/love manual transmissions, find myself preferring a manual choke, will run twin stick t-cases in all my rigs eventually, blah-blah-blah. In my mind the vacuum secondaries might start to open when I'm climbing a slight grade when I still have plenty of grunt left in the motor--I don't want or need secondaries opening all the time. Will they preform better under part-throttle operation (w/ a light spring)? Probably so, and I'm sure it's arguable that might be better than me having to put my foot way into it and get the motor wound up a bit before I get into the secondaries. Perhaps I could swap out the spring (in a Holley) so they'll come in real late, I guess. I believe the older Q-jet on my CJ-5 is also mechanical; just like my truck I can feel the tip-in in the pedal at around 50%-60% throttle (the Edelflood I think is 2/3-ish throttle), it suddenly gets a little firmer and you can hear the "vwoooom" under the hood. Point being is that it strikes me as a matter of taste which is better for your application. I'm happy and prefer my mechanical secondaries but admittedly haven't run a vacuum secondary carb yet. It's all theory and I might be a little weird (or dumb). The way things are going, I'm starting to snoop around on fleaBay for a Holley 450 CFM and learn about tuning Holleys.

Also good questions on exhaust. People typically indicate bigger pipes for high end horsepower or single smaller pipe for low end torque. Some folks say you need headers on an AMC to open them up and let them breath. I also recall seeing a lot of posts where people where sick of their headers and were wanting to swap back to stock exhaust manifolds. I never read into why. My truck has stock manifolds (no stove choke thing) w/ true dual pipes, I think maybe 2"? I think it's whatever size the "collector" of the stock manifold is. I'll have to look. Is it best for my setup and purpose? I dunno, I didn't put them there. I'd like to swap it to single larger pipe (2 1/2" for 360 c.i.d.? but there we go running numbers again) but that's way down the road. Maybe at least install an H-pipe to balance the two out. I understand that's supposed to be "better." Or whatever. I'll start another pointless exhaust thread some other time. Haha

[Ristow]

very random thoughts typed out over a couple hours once again.....


the dual plane is more tolerant of a bigger carburetor,as the engine sees only half of it.

barring overlap of the intake cycles among the cylinders,the cylinders are pulling air in one at a time. a single plane intake like a torker lets that cylinder draw form the whole carburetor. velocity is lower because the air is being pulled through both primary barrels rather than one.

the dual plane,or split plenum like an offy equa-flow or street dominator,lets the respective cylinders draw from half the carburetor. velocity is higher now,as the air is being drawn through one primary barrel (or 2) of the carb.

intake runner length is also a factor. notice the long runners on the performer intake. look a performer rpm,they're shorter. look a torker and victor jr. even shorter,and straighter.

did you know air is actually heavy? and once moving has great momentum? that force is what is being exploited in the varying runner length on the intakes. on the perfomer,the air is pulled thru those long runners,into the intake port and then the combustion chamber. there is a certain range of velocity and valve timing where everytime the valve closes,the air charging thru the runners crashes into the intake port,and the pressure wave of the air behind it compresses the air even more into the port,just as the valve opens again,allowing that built up momentum to spill into the cylinder.

or to put it another way. several thousand Ristows outside the Little Debbie plant on free little debbies day. they open the doors and the ristows race in 2 by 2 at full jiggle. i mean speed. they shut the doors and the ristows crash into the doors,and begin building up pressure against them as they all pile into each other against the doors. just before the doors buckle they open them again and all the fat guys spill into the lobby.

very beneficial for torque and low end power. but as you rev out those long runners start costing power. the air can't get to the intake ports fast enough,the ram effect diminishes and the intake is less effective. that's where shorter straighter runner lengths come into play. the performer rpm has shorter,straighter runners,bit retains the dual plane feature. it is rated for higher rpm due to the shorter runners. that ram effect still happens,but at a higher rpm. the torker and victor intakes take it a step highr in the power band. they also give each cylinder the whole carb. great for 7000rpm power with a double pumper and a 4 speed,lousy for pulling the trailer to the dump thru town in the truck with a TH400. veocity is low due to the whole carb feeding each individual cylinder,and no lower rpm ram effect into the cylinders due to the short runners,not long enough to allow the air to build momentum in the applied rpm range.


on exhaust. exhaust is a very important factor in making power. headers,with all their hassles,are king. period. when talking max VE,max power, tube length is a huge factor. equal length,tuned,shorties,full length,etc.....all factors. crossover placement is crucial. pipe diameter. tail pipe diameter after the mufflers. all factor in.

far more complicated to perfect than any of us can do in the garage or at the pipe shop.

there is so much hype and BS when it comes to exhaust it'll float your head. a lot of it from people who have never experimented with it.

when they start talking back pressure just smile and hit the mental delete button. you don't want back pressure. ever.

you do want a pressure wave in the pipe tho. because behind the exiting pressure wave is a vacuum. that trailing vacuum wave will pull the exhaust wave coming up behind it. it's called scavenging. and aside from better flow,it is what headers do best.

the crossover,as i understand it,is there to alleviate the disruption of the waves that happens when 2 cylinders on the same bank fire,which happens twice on most v8. 5/7 and 8/4.

good velocity in the pipe makes a good pressure wave. and headers set the individual waves of the cylinders up beautifully with the long tubes and the collector.

pipe diameter matters too. too big and the pressure wave is diminished. HOWEVER....you still have an exhaust system that flows well. you don't have that pressure wave in a restricted exhaust either. you have a system that the motor has to push the exhaust through.

which would you rather have? i'll take the too big.


a common myth seen on the web and even here is headers don't work until revving to 5 grand or whatever high rpm number they want to insert,and cast iron manifolds make more torque down low.

of course,they can never produce a dyno sheet to back that up.

you will see very mild dips in torque in the very low rpms on some dyno sheets. you will also see strong gains in the mid range. 2500rpm and up. 20% in some cases. i've had guys point to the slight 5-10 lb dip at 1500 rpm and declare the stock setup superior,all the while ignoring the 50+ lbs gained at 3200 rpm.

yup....if you never rev above 3000 rpm you don't need headers.


or dual exhaust.


or a 4 barrel.


or even a V8.

for the rest of us that want roll on power,headers and big single or dual exhaust is the ticket. got a short window to pass an 18 wheeler? gee...i wonder which setup they'd prefer now.....

since none of us are able to calculate specific tube length,precise crossover placement,tailpipe diameter and length,we just go with a free flowing exhaust. it doesn't really matter a whole lot because,as mentioned earlier,we really aren't even shooting for a big VE number.

very basically,a free flowing exhaust of any variety beats a stock restrictive one every time.


mechanical secondaries. great for a light car that revs very quick and dwells in the higher rpms. too quick for a vacuum/automatic 4 barrel to keep up. most cars don't need one. even most fast ones. when the throttle is opened,full cfm is exposed,whether the motor can use it or not. if not,airflow dips very low,fuel shear suffers,and power is lost,even though it may feel like it has a lot as it crashes then picks back up. a car needing a double pumper would have it sitting atop a victor jr. or similar intake. see how the two go together?

i don't know what Ghinmi is running on his Wagoneer,but i would be surprised if it's a mechanical secondary unit. i would guess a VS Holley with quick reacting secondaries. although if he says he is faster with a mechanical carb i surely would not doubt him.

My Edelchock is mechanical and I love knowing exactly when the secondaries are going to open, from an economy point of view as well as performance.

the secondary butterflies open mechanically. the air door above is automatic. same with a q-jet. it is NOT a fuel friendly arrangement. it does nothing to account for whether or not the airflow through the primary side has been optimized before it starts dumping fuel into the mix. the edelbrock will start supplying through the secondaries anytime the secondaries are opened. even at idle.

Holley's setup is tuneable,and it does not open until airflow through the primaries is optimized. a much better arrangment,although you can't amaze your friends with it by cracking the throttle open while looking under the hood,and see the secondaries jump a bit. they won't on a holley,because they're not needed at that point.

In my mind the vacuum secondaries might start to open when I'm climbing a slight grade when I still have plenty of grunt left in the motor--I don't want or need secondaries opening all the time.

you are absolutely backwards. the carb you have now is the one that will start feeding in the secondaries whether you need them or not,based SOLELY on throttle position.

[rreed]

Glad I started this thread, I did want to learn. Yes I forgot the weighted door thingy on the Edeljunk and the Q-jet does have a similar door thingy on its secondaries as well. I undertsand they're supposed to "temper" the feeding the secondaries are giving the engine based on load or air velocity (someone will clarify I'm sure) but whether or not they do a good job I agree is debatable. I'm also not trying to start another Edelbrock vs. Holley fight here. Makes sense that mechanicals are indeed better for higher revving engines--I bust myself there because as I indicated, mine don't open until I'm way into the pedal and getting the engine wound up to make use of them. I do certainly like the idea of Holley's secondaries being tunable. I think in my mind I keep comparing how my Edelbarf is setup and works/behaves to how I would setup a Holley, were I to get one. I'm thinking in my head since I don't "need" my primary's enrichment to kick in until around 4" that I wouldn't "allow" a Holley's secondaries to open up until around 4" or even 3" but I'm sure I'll be corrected on that too. Two completely different things. Haha We might be getting a little off course again, but the same general realm of air velocity and what should be feeding it and how.

Thanks for the lesson on vacuum secondaries vs. mechanical Ristow and thank you for being patient w/ my idiotic questions and musings. On my list of Holley homework is find out what a double pumper is. I've heard that name tossed around a bit from old muscle car guys.

Great stuff on dual vs. single plane intakes and what sort of use the engine makes of them and the carb along w/ them.

On exhaust, my CJ-5 had those long fender well exit headers w/ side pipes on it when I got it. Mufflers I think were those common purple hornies though they were rust brown. Sounded like a classic stock car, LOVED the sound and the power that thing cranked out (Chevy 350 w/ 66-67 Corvette intake, the Edelplop 1405 that is now on my J-20). But it was to be a trail rig and those side pipes had to go to make room for Sanderson block hugger headers and single 3" pipe w/ a Flowmaster (sorry, it was the fad of the week at the time) Delta flow 40 series. Immediately noticeable loss of power. Very surprising really. I only went w/ 3" pipe all the way back because the block hugger (read: very short) collectors were 3" and I didn't know any better at the time. I've wondered ever since if I should have run 2 1/2" pipe (again, going w/ the general belief that ~350 c.i.d. "requires" 2 1/2" pipe, ~400 c.i.d. requires 3," 258 needs 2 1/4" etc. based soley on size) but I also wonder if going from the long headers to shorties also had a detrimental effect on scavenging, etc. This one also generally doesn't see above 3,000 RPMs. It's 4.56 geared w/ 33" tires which is pretty low in general but still, even in second gear and roaring under the over pass for kicks and grins I can't tell the difference b/t 3K and 4K on the tach. It makes a loud noise and doesn't get very far very fast at either RPM. I just know that w/ the old long headers and side pipes it strained my neck a lot more than has ever done after the shorties and single large pipe. I'd so love to correct whatever it was I screwed up. My guess is w/ the large pipe I've lost my exhaust inertia at <=3K RPMs so there's not much scavenging at the short headers. I dunno, I'll get my notepad out for the response on that one.

Do I/we need a V8? Yes. They're fun, they're powerful, and they darn well get the job done a heck of a lot better than any I6 could ever hope to. I just close my eyes and imagine someone appealing at the gas pump. I've pulled my 4K lb. camper w/ my I6 ZJ. Around town it's perfectly fine. Out on the highway I hope to heck the little old blue hair peering through the steering wheel doesn't run my a** over. The J-20 is happy to have something to do w/ it hitched up.

[Ristow]

the edelbrock,Q-Jet,motorcraft 43xx series....all are automatic 4 barrels. the secondaries are mechanically opened,but the airflow is controlled by the air flap above.

they are NOT considered a mechanical secondary carb.

On my list of Holley homework is find out what a double pumper is. I've heard that name tossed around a bit from old muscle car guys.

the double pumper refers to the mechanical secondary holley. the throttle opens both primary and secondaries directly. no air door,no vacuum pot. floor it and all 4 bores are wide open.

they are called double pumper due to the fact there is a accelerator pump on both the primary and secondary bores. if there were not there would be a bog as the secondaries rolled open.

you will frequently see people incorrectly refer to the dual inlet center hung bowls found on many holleys as double pumpers.


the secondary circuit on a vacuum holley is not dependent on vacuum in the manifold. it uses airflow in the primary circuit flowing past an orifice in the venturii to create a vacuum that manipulates the diaphragm in the vacuum pot and opens the secondaries. it is airflow derived. a better system.

[Ghinmi]

there is so much hype and BS when it comes to exhaust it'll float your head. a lot of it from people who have never experimented with it.

when they start talking back pressure just smile and hit the mental delete button. you don't want back pressure. ever.

you do want a pressure wave in the pipe tho. because behind the exiting pressure wave is a vacuum. that trailing vacuum wave will pull the exhaust wave coming up behind it. it's called scavenging. and aside from better flow,it is what headers do best.

the crossover,as i understand it,is there to alleviate the disruption of the waves that happens when 2 cylinders on the same bank fire,which happens twice on most v8. 5/7 and 8/4.

good velocity in the pipe makes a good pressure wave. and headers set the individual waves of the cylinders up beautifully with the long tubes and the collector.

pipe diameter matters too. too big and the pressure wave is diminished. HOWEVER....you still have an exhaust system that flows well. you don't have that pressure wave in a restricted exhaust either. you have a system that the motor has to push the exhaust through.

which would you rather have? i'll take the too big.

^^^ should be a required read before anyone starts talking about pipe sizing and backpressure. Another thing to add is that as exhaust gasses travel away from the engine they cool down. Your exhaust gas velocity drops like a rock. This is why it's a good idea to put the mufflers as far back in the system as possible. Also why you can run a smaller diameter tailpipe without creating backpressure.

i don't know what Ghinmi is running on his Wagoneer,but i would be surprised if it's a mechanical secondary unit. i would guess a VS Holley with quick reacting secondaries. although if he says he is faster with a mechanical carb i surely would not doubt him.

It's actually got a hp750 mechanical carb. I originally had a 670 vacuum secondary unit but made the switch to mechanical secondaries when I started getting into nitrous. Back then it had a pretty mild 400 hp 401 with a dual plane and I saw absolutely zero difference in track times. It's hard to beat a well-tuned vacuum secondary carb.

[Ristow]

great point on the cooling gasses and dropping velocity in the pipe.


ah...forgot about the nitrous. i can understand that changing a lot of things,as quick as that power comes on.

[Ghinmi]

Yeah, the nitrous changes things a lot. The engine last year wasn't an ideal combo but the instant low end torque from nitrous hid a lot of sins. The converter flashed at 3200 rpm without it and 4600 with it. Fun ride.

[ScottsMojo]

Back then it had a pretty mild 400 hp 401 with a dual plane and I saw absolutely zero difference in track times. It's hard to beat a well-tuned vacuum secondary carb.

Yea mild, only 400 hp...

[Hewes11]

wow. just starting to use this forum and I have tons of questions. I have a 1974 J10, 1978 J10 and many parts laying around from a 1982 J10. I am doing a frame off on the 78 which had the TH 400 full time with full time front end axle. I am going to pull the 4 speed and transfer case from the 74 and put in the 78 with lock out hubs. both have the 360 v8. One of my questions is: The 360 in the 78 did not run, but the 74 did so I am going to start tearing that one down first. How do I want to build up the 360 to get the best mileage commuting 40 miles per day to work? I have bought headers and plan to have dual exhausts. The 74 has axle ratio of 4.09, the 78 has 3.54 and the 78 has 3.31. I am open to all ideas - this is a blast so far. I know there are many trucks out their that get better mpg for commuting but its a midlife crisis to once again have something like the new 77 J10 Honcho I bought back when I got out of the Navy Seabees.

[Rich88]

...How do I want to build up the 360 to get the best mileage commuting 40 miles per day to work? I have bought headers and plan to have dual exhausts. ...

Not what you wanted to hear, but you need to realize that FSJs are heavy non-aero-dynamic bricks. And the cost of squeezing out add'l mpgs is the same or exceeds the cost of gas if you do nothing.

The intended purpose of headers to increase available horsepower at high rpms. You will not notice any difference for commuting purposes. However dual exhaust may kick mpg up mpg a few decimal points.

You can spend a few thousand $ and much time messing with cams, carbs, intakes, etc. to tease out another mpg or two, and maybe break even (and certainly not get ahead) in 3-4 years. Or you can take the same money (and no add'l time) and spend it at the gas pump like most of us do.

The wisest place to spend money is to ensure what you do have is working and tuned properly. You should be able to achieve 12 mpg on the highway with stock. If not, something's wrong, old, worn or de-tuned and I would work on that first.

But let's do the math. Assuming 40 mi/day commute, lets compare 12 mpg vs. an increase to 14 mpg...assuming you actually achieve it:

For 12 mpg:

40mi X 250 working days/yr = 10,000 mi/yr

10K mi/12mpg = 833 gal

833 gal X $4/gal = $3332

For 14 mpg:

10K mi/14 mpg = 714 gal

714 gal X $4/gal = $2856

Gas savings is $3332 - $2856 = $476/yr.

Next add up the cost of all cam, carb, intake, tires, etc. mods (vs. stock) you'd make to achieve an add'l 2 mpg and divide by $476, and that's how many years to break even. The next question would be, how many more useful years for that FSJ remain to get ahead?

Obviously the variables are the cost of gas and how many miles are actually driven. But you get the idea.

[Ghinmi]

Not what you wanted to hear, but you need to realize that FSJs are heavy non-aero-dynamic bricks. And the cost of squeezing out add'l mpgs is the same or exceeds the cost of gas if you do nothing.

The intended purpose of headers to increase available horsepower at high rpms. You will not notice any difference for commuting purposes. However dual exhaust may kick mpg up mpg a few decimal points.

You can spend a few thousand $ and much time messing with cams, carbs, intakes, etc. to tease out another mpg or two, and maybe break even (and certainly not get ahead) in 3-4 years. Or you can take the same money (and no add'l time) and spend it at the gas pump like most of us do.

The wisest place to spend money is to ensure what you do have is working and tuned properly. You should be able to achieve 12 mpg on the highway with stock. If not, something's wrong, old, worn or de-tuned and I would work on that first.

But let's do the math. Assuming 40 mi/day commute, lets compare 12 mpg vs. an increase to 14 mpg...assuming you actually achieve it:

For 12 mpg:

40mi X 250 working days/yr = 10,000 mi/yr

10K mi/12mpg = 833 gal

833 gal X $4/gal = $3332

For 14 mpg:

10K mi/14 mpg = 714 gal

714 gal X $4/gal = $2856

Gas savings is $3332 - $2856 = $476/yr.

Next add up the cost of all cam, carb, intake, tires, etc. mods (vs. stock) you'd make to achieve an add'l 2 mpg and divide by $476, and that's how many years to break even. The next question would be, how many more useful years for that FSJ remain to get ahead?

Obviously the variables are the cost of gas and how many miles are actually driven. But you get the idea.

I think you're reading his post wrong. He's got a variety of parts and is trying to put together the best combo for his application. The only aftermarket parts he mentioned are headers, which WILL pick up power all the way across the board and help fuel mileage. Prove me wrong.

Hewes11-
Both 360s that you have are essentially the same, pick whichever is in the best shape and run with it. You are on the right track with the T18a and Dana 20 combo. I would shy away from the 4.09 gears, I don't think you will see much difference between the 3.31 and 3.54 gears.

Thank you for your service! My 1983 Cherokee spent time in Italy with a member of the Seabees. I found a patch tucked under the dash, if I can find it again and you want it - it's yours.

[Ristow]

I think you're reading his post wrong. He's got a variety of parts and is trying to put together the best combo for his application. The only aftermarket parts he mentioned are headers, which WILL pick up power all the way across the board and help fuel mileage. Prove me wrong.

absolutely correct. i have headers. i drive over 100 miles a day. they work. period.

[Rich88]

absolutely correct. i have headers. i drive over 100 miles a day. they work. period.

My experience is admittedly limited to headers on my '76 Cherokee 360/4350 a few decades ago. And I made no other changes. Power improved a bit as expected when passing or charging up long highway grades at 75. But as for overall mpg improvement combined driving, if there was any, it was too small to measure. And there certainly was no ROI for what the conversion cost, except for the cool factor of passing Broncos uphill. So in that sense, yes, they work.

Having said that, it was be useful and interesting if several people with more experience than I could raise their hand and claim exactly how much more mpg was derived from headers alone. But if one is putting together a package that included cam/intake/carb etc., as often happens, then there's no way to say what contributed to mpg.

[Brown Bear]

More HP/lb-ft, same MPG = WIN

[Ristow]

More HP/lb-ft, same MPG = WIN

yup.