Hello!

I just re-read Dave Emanuel's book "Small-block Chevy Performance." He hits on a couple of topics when it comes to cam selection, one that piqued my intrest is the LSA!

He states that a mostly stock carbed sb with 9.5:1 compression responds to a cam with 108 degrees of LSA, by increasing the torque across the board by 30lbs, as apposed to the same engine running 110 LSA cam.

He states that this occurs because the overlap is shorter on a tighter seperation cam... and that the cylinder preasures are increased, thus causing the bang to be bigger.

My engine, is a 350 bored .030, with 76 cc heads, unshrouded valves... nothing done to theme except valve seals. My goal, is to put a set of headers on the engine... switch to an aluminum manifold similar to the stock dual plane... and tune it for throttle response and gas milage.

Its going into a 75 vette with a th400 and 3.36 (i'm pretty sure of that...) gears. Given that my rpm on the highway wont be as low as I'd like it, and that the bottom end will be stock, besides ARP bolts... I'm not going to push this beyond 6,000 rpm. I'd like to pick a camshaft that allows me to have the best mpg... instead of max power. The rest of the package does not support chasing more horsepower currently!

I am going to keep the back preasure low... and re-build the carb, tune the air fuel ratio, and adjust the timing once the car is back together, but again, i'm going for throttle response and gas milage!

I've heard about the extreme energy cams, but I'd like to keep the engine nice and reliable... no super steep ramps! I'm torn between a hydraulic lifter or solid lifter valvetrain, but currently I'm leaning towards hydraulic.

I've heard the 268h is a nice camshaft, but I suspect its to large for my current combo, given that its advertised as a 4 speed, or high stall auto !

Thanks guys... I'm looking for a cam!

 

Considering your combo and your desire for better fuel economy I think you'd be happier with a wider LSA. The narrower angles do increase torque once rpm is high enough to overcome the effects of increased overlap but make for a less efficient engine up to that point. Tighter angles increase overlap, not the other way around.

 

That Crossfire hating idiot has written a book??? What you have stated is bassakward. Think about it for a minute. As you spread the lobe centers (increase the LSA), with given lobes, the overlap DECREASES. Generally, a cam of a certain lift and duration will have less PEAK torque, and a broader, flatter torque curve (better drivability), with increasing the LSAs. The reduced overlap makes it have a better idle and is more smog friendly. Take a look at all the "computer friendly" cams offered. They all have a healthy LSA. And in case you aren't aware, the LSA is ground into the cam when it is made. It isn't something you can "adjust" on installation.

RACE ON!!!

 

"I just re-read Dave Emanuel's book "Small-block Chevy Performance." He hits on a couple of topics when it comes to cam selection, one that piqued my intrest is the LSA!

He states that a mostly stock carbed sb with 9.5:1 compression responds to a cam with 108 degrees of LSA, by increasing the torque across the board by 30lbs, as apposed to the same engine running 110 LSA cam.

He states that this occurs because the overlap is shorter on a tighter seperation cam... and that the cylinder preasures are increased, thus causing the bang to be bigger. "


Either you misquoted him or he is flat out wrong! For a given duration, the "tigher" the LSA the more overlap, but LSA cannot be considered alone. It has to be considered in the context of duration.


LSA is just a very gross and often misleading indication of overlap. For example two cams with the same LSA and different durations will have different EFFECTIVE overlap - with the longer duration cam having more.

Viewed another way, as duration is increased, LSA must be increased to keep effective overlap the same.

Overlap can harness exhaust wave dynamics to aid in exhaust scavenging and starting the inlet process at high revs, but you need headers and low exhaust backpressure, preferably open exhaust, to get the most benefit. With manifolds and even a good low restriction exhaust system there is a relatively low limit on overlap to achieve good low end torque, throttle response, and fuel efficiency.

Most aftermarket cams IMO have two much overlap, which is one reason why I like factory cams. As examples, the base SB cam has .050" lifter rise durations of 194/202 and a LSA of 112, but due to the short durations, effective overlap is very low. The L-79 cam has durations of 221 with a LSA of 114, and the L-46/82 also has a 114 LSA with 224 duration, so it has a bit more effective overlap. The LT-1 cam has durations of 229/237 with a 116 LSA and even though it appears to have a "wide" LSA it has more effectice overlap than the L-79 cam and effective overlap is at the limit of what a 350 can effectively handle with manifolds and OE exhaust.

The Comp Cams XE268 - a popular fitment - has durations of 224/230 with a LSA of 110, so it has considerably more effective overlap than the OE SHP cams and IMO is a poor choice unless you have headers and a very low restriction exhaust system. With manifolds the torque bandwidth does not match that of the OE SHP cams.

The Engine Analyser simulation program is very good at selecting cams because it computes actual EFFECTIVE overlap in square-inch-degrees, and 3.5 is about the limit to achieve good torque bandwidth on a street SB. Once effective overlap is established the relative location of the torque curve on the rev scale can be moved by adjusting the point that the inlet valve closes and a later closing inlet valve moves the torque curve up the rev scale, but doesnt harm idle quality or fuel efficiency as is the case with high overlap.

The point that the inlet valve closes is the primary influence on the top end power characteristics of the engine.

Duke

 

SWCDuke - That's why I have custom cams ground with steep ramps and 112 LSA. You end up with less overlap for big duration. I've even considered as wide as 114.

 

I dont think I mis-qouted...

I'm looking at a blurb on pages 80-81 called "Effects of Lobe seperation on power."

The part of the blurb I'm interested in, is test two, with two different dyno sheets... it shows that the same engine, with a 112 degree LSA, vs a 108 degree LSA which 30-40 more lbs of torque from 1750 to 5500 rpm (compared to same engine with 112 degree.)

Very cool reading in this thread.

I'm under the impression that dynamic compression ratio's go up with a tighter lobe, and that a long duration, wide lobe cam gives higher top end performance. Is this correct?

 

It appears he wasn't mis-quoted. You both say, that he said the same thing. It's just that these statements are WRONG!

RACE ON!!!

 

I pulled out smokey yunicks book "Power Secrets"

He agree's with yall...

Looks like I get to throw daves book on the back shelf!

Wider LSA makes MORE power at low RPM's with sane duration... Closer LSA makes more top end power, by shifting the engines optimum working rpm.

That being said...

I've heard about guys whiping lobes on the XE cams, so I'm thinking about staying with a regular style ramp. I'm pondering going with a dual pattern as well, seeing that I'm looking for efficiency.

The heads I'm using are 882's... they dont flow real well but matched with flat top pistons I'm thinking it will work out ok. I do not know, however, what the max lift I can run on these heads, nor how to find out by looking at them.

Once I get this all finished I'm going to write a book! "Hotrodding for dummies, like myself!"

 

Dynamic CR has NOTHING to do with overlap. It's purely a function of the total cylinder volume at the inlet valve closing point and the clearance volume.

Given this author's MAJOR error, I would put little faith in ANYTHING in that book, which can only have been poorly written and edited! For a given inlet and exhaust lobe, a cam with higher LCA will generally make more low end torque because of less exhaust gas dilution. Depending on exhaust configuration, peak power could be more or less. On an engine with a restrictive exhaust the the cam with less overlap - higher LCA - might make better power due to both less overlap and a later closing inlet valve.

LCA in and of itself has little meaning. You can vary LCA by varying the indexing the inlet lobe, exhaust lobe, or some combination of both.

I used this method to reindex the cams in my Cosworth Vega. As originally installled the 218 degrees duration at .050" lifter rise above the lash point cam centerlines were indexed at 102 ATDC on the inlet side and 110 BTDC on the exhaust side for a LCA of 106, which is very narrow. The engine had to idle at 1600 revs (OE spec) in order to achieve stability and have a steady vacuum signal (16") for the speed density EFI system to operate properly, and the engine would not pull at all from below 2000. I always expected the excess overlap was there to promote a high exhaust gas residual to control NOx.

Using a couple of engine simulation programs I tried every combination of advancing the exhaust cam and retarding the exhaust cam in two degree increments up to 20 degrees. The best was retarding the inlet cam eight degrees and advancing the exhaust the same amount, so the new centerlines and LCA are 110/118 and 114. It's probably no coincidence that the valve timing numbers are now essentially the same as the L-79 cam.

Fuel economy improved ten percent and the engine now idles butter smooth at 900/18". I can lug it down to 200 revs in first gear, then floor the throttle and it pulls away. Third gear will easily pull from 1000 and 1500 in fifth. and there is no transient detonation on a hot day like before. Torque bandwidth is broader and top end power is higher and doesn't roll off at all past 7000 like it did before, but stays level to 7500.

NOx emissions did double, but they are still only one half the limit.

When I system engineer an engine the last thing I work on is valve timing. At that point the displacement, CR, inlet system, head flow characteristics, and exhaust system configuration including a reasonable estimate of backpressure are established. The purpose of the cam is to get the most out of the available inlet and exhaust flow, not work against it. On a street high performance SB I usually start with a modest amount of overlap and a relatively early opening exhaust valve and then vary the inlet closing point to optimize the torque curve in the range of 2000 to 6500. Once the inlet valve closing is determined I juggle the overlap to see if there is a better combination and usually end up with a little less overlap than I started with. The last event to vary is the exhaust valve opening. Opening it later might increase low end torque slightly at a slight expense to top end power, but it's not dramatic. As a rule, opening the exhaust early will do no harm, but opening it too late will cost top end power.

As the exhaust opening point is advanced with a fixed closing point the LCA is widened because the centerline of the exhaust event is also advancing.

It turns out that the OE LT-1 cam is the closest to this "ideal cam" when using the OE type manifolds. For a vintage racing engine with headers and open exhaust the GM "140" off road cam is closest, but the 30-30 isn't that bad.

Duke

 

Here is a good article on cam intensity & other info..
http://www.harveycrane.com/Secrets.htm

 

Very cool! Thanks for sharing the info!

I have a question...

If the exhaust valve closes, right after top dead center on the exhaust stroke, and the intake closes right as the piston starts descending the cylinder... is that not the perfect timing for the intake to open? Is the perfect time for it to close right at the bottom of the stroke? So the compression stroke compresses a completely filled cylinder? My understanding, is that the time that the intake and exhaust valves are open at the same time causes some of the intake charge to exit with the exhaust... at what point does that have an impact on gasmilage?

If the exhaust valve is open at the start of the pistons rise in the cylinder... is that not the perfect time for the exhaust to be open? Once the piston is at tdc is that the right time for the valve to close?

So... where does overlap come into play? Is overlap the LSA?

Thanks again guys

 

Oh...

Do you guys subscribe to the notion that you can run the valve lash super sloppy once the motor is broken in... to see if the engine picks up power, needing more exhaust lift?

I'm forgetting what this is called. Do you need a dyno?

 

Opening and closing the valves exactly at TDC or BDC won't work. You need to take inertia of the fuel air mixture into account. The vapors take on a "spring effect" when the flow constantly stops & starts (like when it meets an intake valve). This is even more of an issue because of the vacuum that draws it all in.

Overlap is basicly how long both the intake & exhaust valve are open at the same time. Overlap is affeced by the LSA & duration. Look at it this way, 90% of Comp Cams blanks are on 110 deg lobe seperation to start with, then they are ground to a given duration & lift.

 

Running to tight is bad & too loose is worse. The cam manfs usually will give you a range of adjustment. Too tight will burn valves & too loose will damage the camshaft.

Edit: This if for solid cams..Hyds just are set per directions

 

David Vizard preaches much the same as D. Emanual - 108 LSA makes optimum torgue in the 350" with 2.02" vlvs. But its related to vlv size (2.02") and c.i. where 106 LSA is optimum for 383" and 104* for 400" mtrs. The 2 Davids preach much the same idea with different wordage. And lets not confuse LSA with LCA here. I'm not hear to argue or reinvent the wheel. If u are interested try D. Vizard Chevy sb camshafts & vlv trains. Too much to type here.
But u say u want best mileage and throttle response. The wider LSA's will get u better fuel economy, idle, more rpm and carb, ignition, trans & rear gearing will get the other. And if ur worried about a softer bottem end then that Chevy LT1 #3896962 cam sounds more like what u want.
And no, that vlv adjustment doesn't change for a hyd lifter and to run valve lash super sloppy could allow that vlv train to eat itself.
JMHO is that if this ur first cam swap don't focus on the max pwr possible. Try something middle road that u know will work. BTW i have a stock LM1 Goodwrench "Hecho en Mexico" bottom end and installed a single pattern Crane Energizer 266* with 110 LSA - $88 with lifters from Summit. Not the biggest cam i can stuff in it but it can cover for most any other mistakes i made - carb, heads, intake, gearing, etc.
Good luck there blue dude. cardo0

 

Isnt this fun???


Say Duke...I'm a little confused by this statement.....

"Viewed another way, as duration is increased, LSA must be reduced to keep effective overlap the same"


When you increase duration and the lobe gets "bigger" the overlap increases naturally unless you INCREASE the LSA. Right? If I reduce the LSA from say 112 to 108, I will increase overlap even more.

Many folks get confused here with all the terms and conclusions drawn from the published tests. I think he's doing the right thing by analyzing the actual strokes and what occurs in each. That's how it will all start to make sense.

For one thing, overlap has nothing to do with DCR or cranking compression. What happens is that cams with increased overlap "usually" also have larger lobes and the later intake closing event makes for low DCR and the need for more static compression ratio. The cylinder pressure change comes from the different closing event primarily. Now as RPM rises, the added overlap can help get cylinders full to increase power at thise high rpms.

But as was noted, it is easy to have a cam with vastly different characteristics with similar sounding numbers. You can have "small" lobes with tighter overlap that will usually advance the intake lobe. This will build cylinder pressure helping low end power AND add a little overlap to help top midrange stuff. But the issue will be that the advanced small lobe will likely starve the engine as rpm climbs. What is really bad is when folks advance the cam another 4* and really kill top end.

Dukes description of issues with too much overlap is dead on. If there is not a serious freebreathing exhaust system, higher backpressure will cause reversion issues that will actually kill power.

I've done a lot of looking at current cam designs and it's interesting to see that even the Pro Stock teams are really moving to less and less overlap compared to what was common a few years ago. The duration numbers are also smaller, but the lobes are much more aggressive. Now that they have springs that can handle the ramp speeds they can design cams to work much better without as many compromises.


Continue to do some studying and really read what is written. There are often lots of typos or just flat wrong discriptions in books because what a person is thinking sometimes doesn't come out the fingers to the keyboard. And the guy proofreading it doesn't know any better either.


JIM

 

going to read

 

427 Hotrod, that clicked!

... The lower the LSA the longer the "hang time" is between opening and closing, making more overlap! The bigger the LSA number, the farther apart the cam lobes are, causing less overlap... Lobes right beside each other have lots of overlap... but a cam with a lobe 180 the other would have none. The farther you move to 180, the less overlap you get. The closer you move the lobes to being in-line the more overlap you have!



Sorry about the valve remarks... I'm trying to remember what the technique was called. Basically you do a dyno run, with normal valve lash... then you do a dyno run with .040 lash on the exhaust. If the power goes up... the motor wants more duration. Has anyone heard of doing this? I think I read about it on a VW air cooled site, but I'm not positive on that either

I'd like to stay with light valve spring preasures, because I've already had the heads re-done... springs checked out, valves checked out... just had some valve seals installed. Probably should have port matched my intake to the heads... but I have no intentions of keeping the 882's long term!



Thanks guys!

 

Yup, I had a brain fart! I went back and changed "reduced" to increased.

To clarify the "reversion" you refer to it is not the inlet reversion that can occur with a late closing inlet valve. At less than WOT and even at WOT with exhaust back pressure, exhaust pressure is higher than inlet pressure, so exhaust gas can migrate into the inlet port during the overlap period causing considerable exhaust gas dilution. This is what causes the "lumpy idle" of high overlap cams, and it also reduces fuel economy, increases HC emissions (but reduces NOx since it's basically a full time EGR system). The exhaust configuration - headers or manifolds and the amount of exhaust backpressure determines the overlap that an engine can effecitively use before it begins to do harm. Also, the more efficient the ports, especially at low lift, the less overlap (and duration) the engine needs. This may be why some modern racing cams have less duration. Also, the higher the flank acceleration, the less duration and overlap you need to achieve maximum power in the upper third of the rev range.

Someone mentioned that most Comp Cams are ground on a 110 LCA. My take is that this is an "economy" measure - they grind a half dozen or more durations on the same blank rather than really DESIGNING a cam for a specific application, so the user ends up having to guess what will work. All OE cams are ground on unique blanks to meet the individual application.

Someone mentioned the "LT-1 cam" as 3896962. This is not correct. The "962" hyraulic cam was used on the L-46 and L-82 from '69 to '79. It has three degrees more duration (224) than the L-79 cam and is effectively four degrees retarded relative to the L-79 cam with lobe centers of 114/114 relative to 110/118 for L-79.

The LT-1 mechanical lifter cam is service part number 3972178.

Duke

 

One of the problems with opening and closing the valves at TDC and BDC, is that you can't close (or open) the valves, instantly. An intake valve that would close at BDC (180* ATDC), wouldn't flow diddly squat at 170*. The same is true if the intake opens at TDC. The flow at 10* ATDC would be just as slight. The effective "flow duration" would be ridiculously short. An offshoot of short duration, is the inability to develop good valve lift. Of course the other reason for longer than what would appear to be, "ideal" opening and closing points, is inertia. It takes time to get the gasses moving. Hence an earlier opening. The same is true at closing. Once the gasses ARE flowing, their inertia wants to keep cramming the cylinder with mixture against the upward movement of the piston. Of course engine speed is what makes the difference. The faster the gasses are moving, the more energy (and inertia) they have and the greater the benefits of longer duration. That only scratches the surface, but it gives you an idea why the valves open early and close, late.

A note on overlap. When both valves are open during the overlap period, maximum advantage is taken of the aforementioned, inertia, at high rpms. At low rpms, with less inertia, the piston can push the air fuel mixture, and exhaust gasses, back into the intake. This is the major cause of the rumpity rump idle, some are seeking. It is the "sound of power". But only at high rpms. For reasons that should, now, be apparent, long overlap KILLS bottom end torque (power).

Altering the lash, on a solid lifter cam only, is a good way to determine if the correct cam has been selected. But you have it backward. Increasing the lash, REDUCES the duration (and lift). If the engine makes more power the more lash, the cam is too BIG. This exercise is most valuable if performed at the track. That way you get real world performance numbers rather than dyno numbers, that may not transfer, directly, to performance. Remember, we race cars, not dynos.

RACE ON!!!