More Exhaust Duration

[FirstEliminator]

   Hey guys,

    I ponder a lot about the engines I'm going to be putting together----someday. I've been thinking about extra exhaust duration in comparison to the intake. I remember reading that extra exhaust duration tends to reduce low-end torque. What circumstances would benefit from extra exhaust duration? Has adding 6-10 degrees just become a trend or tradition? Any basics on determining if extra exhaust duration is appropriate?

  thanks,
     Mark

[blykins]

Saying that adding exhaust duration reduces torque would probably be incorrect.   

Anytime you introduce more exhaust duration, you increase the amount of overlap that you have.  Overlap can do a lot of great things for you:

1.  Helps make that jiggety-jiggety sound that we all like.
2.  Helps a poorly performing exhaust port, or an exhaust port that is poor performing relative to the intake port.
3.  Helps scavenge the intake side on a low-velocity intake port.

Those are the pluses.  The minuses, again, would be based on overlap.  Overlap can hurt you sometimes as well:

1.  Reduces vacuum necessary for power brakes, accessories.
2.  "Clouds" up low-rpm operation.  Remember that anytime you have both valves open at the same time, the more chance for reversion. 

Most of the off-the-shelf cams that you see have 6° split in them.  That's a universal design and will probably fit the bill for a lot of engines.....but it's by no means optimal for a lot of engines. 

All of my camshafts are custom ground for me.  I don't use anything that's off-the-shelf....for various reasons.  But having a cam custom ground allows me to change how much split I have between the intake and exhaust, which helps me custom tailor how "clean" an engine is at low rpm operation, how much vacuum it has, how it sounds, how it would "hang on" at high rpms while crossing the stripe, etc.  The majority of my cams for engines with modern cylinder heads (BBM, Survival Motorsports, Pond, etc.) have about 0-6 degrees split in them.   

You will see me stretch that split out for different reasons:  a poor flowing exhaust port, a BIG intake port that's slow and needs rpm or help from the exhaust to make it work, etc.  As an example, my Boss 302 camshafts will have 8-10 degrees of split and my Tunnel Port cams will usually have around 10 degrees as well.  The reason for that is because both of those cylinder heads will have the exhaust flow about 60% of the intake flow.  They also have huge intake ports that are very low velocity at rpms that we see on the street and they need help.

It would be very easy to write a book here but for sake of time and the condition of my typing fingers, what I have outlined would be a good summary for the "basics". 

My advice would be to get a good camshaft event calculator and start playing with it.  The DCR calculator that I use by Pat Kelley will allow you to enter in advertised and .050" camshaft events to see how much overlap is involved between the intake/exhaust lobes.  That's a great place to start playing around. 

[Drew Pojedinec]

Brent,
have you ever had a camshaft made with differing intake events?
Lets say on a dual plane, you have two high cfm ports and two lower ones for each bank.  Have you tried increasing the duration/lift for the slower ports to help even them up?

[blykins]

Your idea is a good one and has actually been around for a couple of decades.  Warren Johnson always said to treat each cylinder like a single little mini engine.   

Comp Cams came out with their "four pattern" camshafts a few years back that's supposed to have lobe designs that vary, based on the runner lengths of the intake for the cylinder that's being fed.  They're more of a "universal" four pattern camshaft though.

It's certainly worthwhile to have a head guy try and balance the flow/velocity on the intake.  Modifying the camshaft to cater to each cylinder can also be worthwhile, it's usually a max-effort type deal and you really need to focus on intake charge pulses as well in order to go at it with an educated approach....it's not a "pick a cam out of a catalog" scenario.

[Drew Pojedinec]

Sure enough.  Was just something I'd thought about.... thanks for giving it a name "four pattern cam"  now i've got something to google and read about.

Just engine theory for me anyway, I lack the resources or motivation to build an engine worthy of such things.  Now I need to figure out why one bank of intake ports on a twin turbo cat c32 are square and one bank has oval intake ports on the same engine.....

[machoneman]

Long ago (and far away? LOL) folks like Traco who did a massive number of mainly road racing racing SBC's and BBC's (think: McLaren Can-Am cars) looked into similar altered camshaft profiles for the 497 CID all-aluminum BBC's. The problem was and remains that the paired intakes did not flow the same. Note that the ram tubes on all BBC's then (and now) are staggered in height in an effort to equalize flow with Lucas- type metered fuel injection. Hence cam profiles were similarly altered to even-out so to speak the cylinders.

Pics and dyno test of a typical road-racing BBC of the era:

https://www.youtube.com/watch?v=v6dMJ5L6G2c

[FirstEliminator]

Jiggety-Jiggety sound is nice, though not what I have in mind for my wagon. Smooth and low.  With the desire to use as much duration as I can without losing idle quality motivates me to stay up past my bedtime reading.
   Here is a reference I've found that gives an idea on recommended overlap on usage. It leaves out variables.
   
"These are the valve timing overlap ranges that are most likely to work correctly (thanks to David Vizard for providing this):
                       trucks/good mileage towing 10-35 degs overlap
                       daily driven low rpm performance 30-55degs overlap
                       hot street performance 50-75 degs overlap
                       bracket/oval track racing 70-95degs overlap
                       dragster/comp eliminator engines 90-115 degs overlap "

  Another chart is:
http://www.grumpysperformance.com/CamUsageChart01.jpg

   What I'm looking at for cam is probably to keep a single pattern in attempt to keep the highest vacuum. Lunati's Voo-Doo series has a nice looking lobe that is 268 advertised with 227 @.050. If the cam were 227/227 with 110 lobe separation it would have 48 degrees overlap. With a 112 lobe sep it'd have 44 degrees. Without testing the waters and buying multiple cams to really see, it doesn't seem like there is much difference between 44 and 48 of overlap. The 112 would improve the idle a bit. But, a noticeable amount? Or would 112 flatten the peak torque a noticeable amount? Perhaps compromising at 111 is the way to go.
  The engine in question is a 482 with compression low enough to run on pump gas. Performer RPM, CJ heads or C1 heads, cast long headers.
   Going by these charts and listening to experienced people it seems like overlap in the mid 40's with this duration and this size engine seems like it's give a good idle-----good as in smooth.
    One thing is, I had a guy tell me a Comp 280 would idle like a taxi cab in this engine. I've never heard a Comp 280 idle smooth in any engine. Keeping in mind I consider idle speed to be about 600.  I like a low idle and envy the Packard strait 8's that had an idle speed of 350 rpms.

  thanks,
    Mark 

[My427stang]

I think you are severely undercamming for the head you want.  Here is a link to my 445 with a 282S but centers spread to 112, and on 106.  It idles pretty smooth in the cab, and theoretically you could go even more if you needed less overlap and go 114 and still stay on that centerline, plus you have 40 more cubes

https://www.youtube.com/watch?v=Uf3D5c9ZZfs

FYI, with a 268 lobe on 106, you'd need 9.5:1 to run that cam max, and to do that you'd need a 30 cc dish with a 76 cc head, or a 34cc dish with a 72cc head.  Can be done, but the pistons likely start to lose any significant quench pad.

[blykins]

I'm not a Vizard fan and I'll leave it at that.   You'll end up with a 102 LSA camshaft if you pay attention to his stuff. 

A 224-ish duration @ .050" would be super gutsy at low rpm.  I built a 482 inch FE for a forum member that tows.  The camshaft is a 224/230 @ .050" hydraulic roller, 112 LSA, and will idle around the yard with 10000 lbs behind it in a '65-66 F-series pickup. 

Voodoo series camshafts are pretty aggressive lobes.   They would be noisy. 

EDITED to backpedal on harshness. 

[My427stang]

Take whatever David Vizard says and flush it.

LOL and I agree!

[FirstEliminator]

I'm not a Vizard fan and I'll leave it at that.   You'll end up with a 102 LSA camshaft if you pay attention to his stuff. 

A 224-ish duration @ .050" would be super gutsy at low rpm.  I built a 482 inch FE for a forum member that tows.  The camshaft is a 224/230 @ .050" hydraulic roller, 112 LSA, and will idle around the yard with 10000 lbs behind it in a '65-66 F-series pickup. 

Voodoo series camshafts are pretty aggressive lobes.   They would be noisy. 

EDITED to backpedal on harshness. 

    Yeah, I saw that formula about dividing the individual cylinder volume by the intake valve diameter to determine LSA and tried it. 102 would have been optimistically trying to sway that equation back to reality.   

    VooDoo lobes are noisy? Gosh, maybe that is what I hear with my 351w. Been wanting to kill that engine since the 3rd time I've put it together. The Voodoo lobe numbers between advertised and .050 look beautiful---like cake available for eating. However, is the longevity compromised? Is it too aggressive of a lobe for a small cam? The cam in my 351w is a 207/213 --112. Idles smooooooth and has good pep.

    The truck sounds nice, but has too much rumpity-rump for what I'd want with the wagon.

   Brent, why a roller cam in that towing engine?

     Looking at Lunati's cams, I've noticed the flat hydraulic VooDoo lobes appear to be faster ramps than the hydraulic roller VooDoo lobes. Ya think it would be the opposite.

     I've got a little while to decide on the cam. At the rate I'm going, my deadline is going to be here before I know it.

[blykins]

Voodoo lobes are noisy, as well as a lot of Comp Cams lobes.   

Most of my street engines get hydraulic rollers.   I'll only do flat tappets in specific situations or if it's a rule race engine.   For you, the risk wouldn't be that bad considering your spring pressures would be fairly low, but the risk is still there.    There is zero risk of a cam going flat with a hydraulic roller.

[FirstEliminator]

I lost one lobe. It was a bad day. Ruined the oil pump and bearings. The piston that was inline with the failed lobe had metallic was loaded up between the piston pin and it's bore.  I did make a nice piston pin press which was about the only fun part of the situation.

[My427stang]

Mark,

If the budget allows I would look at a hyd roller or potentially a slower ramp hyd flat tappet than what you are thinking.

The combination of more advertised duration and a wider LSA will keep things mild, but allow you more flexibility with fuel tolerance and be quieter.  I think a 224 duration cam is a bit mellow with a short advertise, but as Brent showed, his HR profiles, especially if you add a little exhaust duration will hang in there a bit more partially due to the entire lift curve, not just past .050.

I wouldn't be afraid to spread centers though, even if mine is a bit too lumpy for your taste, despite the dyno's showing more WOT torque with a tight LSA, part throttle a clean wide LSA cam runs real well.

[blykins]

With a low duration camshaft, you could potentially close the LSA up and make more torque without sacrificing vacuum or anything else, since the overlap would still be very low.  As Ross pointed out, a tighter LSA will make more torque. 

It's also interesting to see the effects of head flow on camshaft selection.  I did two 445's, both similar except for camshaft and cylinder heads.   One had Barry's as-cast heads with a 235/241 @ .050" hydraulic roller and the other had a set of out of the box Edelbrocks and a 243/249 @ .050" hydraulic roller.  Both peaked at the same rpm, but the one with the SM heads made more horsepower with less camshaft...