SpeedTalk Store - Opinion Columns

Lobe seperation question

General engine tech -- Drag Racing to Circle Track

Moderator: Team

Lobe seperation question

Postby chevy_power » Sat Mar 22, 2008 2:11 pm

Lobe separation angle effects (from chevy hi performance mag)

NARROW LSA
Increased overlap
Increased low-rpm torque
Narrower powerband
Reduced idle quality
Increased cranking compression
Decreased piston-to-valve clearance

[/b]WIDE LSA
Reduced overlap
Improved top-end power
Wider powerband
Improved idle quality
Reduced cranking compression
Increased piston-to-valve clearance

My questions are, if running a narrow lsa, the increased overlap should help top-end horsepower and decrease low-rpm torque/cranking compression because it would bleed off cylinder pressure at lower rpms right?
If running a wider lsa, you would have less overlap and therefore more low-rpm torque. But they say a wider lsa produces more top end power?

I'm sure thats the way it is but I'm just confused on how it works.

Any comments are greatly appreciated.

Speed kills, save lives, drive a honda
chevy_power
Member
Member
 
Posts: 140
Joined: Mon Mar 17, 2008 6:29 pm
Location: Kansas

Postby Stan Weiss » Sat Mar 22, 2008 2:35 pm

When LSA or overlap are changed they have an effect on intake valve closing. This effects DCR which is linked very closely with cranking compression
Bore = 4.0" Stroke = 3.35" Rod Length = 5.7" CR = 13.594 Wrist Pin Offset = 0.0"
DCR vs Intake Valve Close ABDC
Image
User avatar
Stan Weiss
Guru
Guru
 
Posts: 2791
Joined: Tue Feb 20, 2007 1:31 pm
Location: Philadelphia, PA

Postby Metrobilly » Sat Mar 22, 2008 3:01 pm

LSA cannot really be considered by itself without also considering a lot of other camshaft, air flow, and tuning variables. What you got from the magazine is an over simplification. If some of the geniuses on this site discuss this in detail I'm sure I'll learn something.
Metrobilly
Member
Member
 
Posts: 116
Joined: Thu May 18, 2006 8:07 pm
Location: Maryland

lobe separation

Postby bigjoe1 » Sat Mar 22, 2008 3:07 pm

Tighter centers work better when the compression ratio is not real high( high being 14 or 15 to one) At 11 to one or lower , a 108 will be better everywhere than a 112. We are not talking about 50 HP here, maybe 10 to 15 would be a reasonable differance. The RPM range has a lot to do with this to. If you want 4500 to 7000, closer centers ( like 108) will be good, If you want 7000 to 9000, then 114 centers would be good. These examples are foe 380 to 420 inch small blocks. There are exceptions to this, but in general terms, this is the way it works for me.

JOE SHERMAN RACING ENGINES
bigjoe1
Show Guest
Show Guest
 
Posts: 5234
Joined: Thu Aug 24, 2006 7:16 pm
Location: santa ana calif-92703

Re: Lobe seperation question

Postby Motor Daddy » Sat Mar 22, 2008 3:32 pm

chevy_power wrote:Lobe separation angle effects (from chevy hi performance mag)

NARROW LSA
Increased overlap
Increased low-rpm torque
Narrower powerband
Reduced idle quality
Increased cranking compression
Decreased piston-to-valve clearance

[/b]WIDE LSA
Reduced overlap
Improved top-end power
Wider powerband
Improved idle quality
Reduced cranking compression
Increased piston-to-valve clearance

My questions are, if running a narrow lsa, the increased overlap should help top-end horsepower and decrease low-rpm torque/cranking compression because it would bleed off cylinder pressure at lower rpms right?
If running a wider lsa, you would have less overlap and therefore more low-rpm torque. But they say a wider lsa produces more top end power?

I'm sure thats the way it is but I'm just confused on how it works.

Any comments are greatly appreciated.



Using the same lobes, and "assuming" the cams are being installed straight up (for an apples to apples comparison), reducing the LSA means a greater overlap, but along with that comes a lower ICL and a lower ECL, which means a earlier intake closing point and a later exhaust closing point. The earlier the intake closes, the more cylinder pressure is built. The engine will have a higher DCR.

Basically, advancing the intake lobe (which happens with a tighter LSA cam installed straight up) means shifting the power to a lower RPM.
User avatar
Motor Daddy
Pro
Pro
 
Posts: 458
Joined: Wed Jul 11, 2007 7:31 am
Location: Henderson, NC

Postby panic » Sun Mar 23, 2008 9:03 am

The low speed stuff is a bit confusing, in part because there are 2 big things changing at once.
1. high OL (narrow LSA) means (other variables excluded) that the volume and percentage of exhaust gas in the chamber is higher, which reduces vacuum and makes for rougher running. The engine must turn faster before you have positive flow-through In>Ex and complete chamber flush - which is a critical event of the torque curve.
2. narrow LSA also means (other variables excluded) early IVC, which increases DCR and effective stroke length - adding power by increased cylinder pressure and volume. It also rises to full capture (no intake reversion - the other critical event of the torque curve) at a lower RPM. However, a wide LSA has better inertia fill ABDC due to late IVC when the engine is turning fast, and the added VE will more than balance the reduction in DCR.

These 2 critical events will be present at the leading edge of the torque peak, but need not occur at the same RPM, since the effects will vary between engines with the same cam data due to displacement, static CR, rod ratio, in/ex flow balance, etc.
User avatar
panic
Guru
Guru
 
Posts: 1576
Joined: Sat Dec 11, 2004 12:04 pm
Location: Ecbatana

Postby quickd100 » Sun Mar 23, 2008 10:33 am

Interesting discusion, do different cylinder head types favor different lobe centers? Is there much difference? Case in point, my 528ci Hemi, I run an old, old, MP grind on 104 centers installed at 102. The Hemi is known for over scavaging at times and blowing part the intake charge right out the exhaust. Would a wider LSA be benifical, can I make more power with a wider LSA cam? Dave
Image
quickd100
Pro
Pro
 
Posts: 247
Joined: Sun Jul 18, 2004 6:34 am
Location: Nielsville, Mn.

Postby MadBill » Sun Mar 23, 2008 11:14 am

As has been mentioned elsewhere by bigjoe, compression ratio is a factor, but Hemis ports do tend to have high relative flow and the valves are conveniently disposed for flow-through, so I'd say yes, especially if your CR is perhaps 12:1 or more.
Wouldn't it be nice to test 2 or 3 cams identical to your current grind except for 2° incremental reductions in IVO/EVC ?
You could get a feel for the potential by looking at V.E. and B.S.A.C. If they both grow suspiciously large at high RPM... (Of course exhaust extractor efficiency is also a factor.)
"There's no product that can't be made cheaper and worse."
MadBill
Guru
Guru
 
Posts: 8658
Joined: Tue Nov 15, 2005 10:41 am
Location: Northeast of Toronto, Ontario Canada

Postby panic » Sun Mar 23, 2008 11:14 am

"do different cylinder head types favor different lobe centers"

Definitely, a 4 valve cannot live with a low number that would be gold in a wedge engine due to much better cross-flow on OL.

In general (with exceptions), best 2 valve LSA value is inverse to the included valve angle total; viz. hemi < BBC/385/Cleveland < wedge < Heron.
Last edited by panic on Sun Mar 23, 2008 11:24 am, edited 1 time in total.
User avatar
panic
Guru
Guru
 
Posts: 1576
Joined: Sat Dec 11, 2004 12:04 pm
Location: Ecbatana

Postby panic » Sun Mar 23, 2008 11:21 am

"Would a wider LSA be benifical, can I make more power with a wider LSA cam?"

This is a more complex question, because just increasing the LSA (but not the event dims) does much more than just reduce flow-through, especially later IVC (= lower DCR, better inertia fill) and later EVO (more work done, but higher pumping loss).
There are also very opaque effects, such as how much intake curtain is exposed at what level of vacuum ATDC on intake, where small changes may affect total VE.
User avatar
panic
Guru
Guru
 
Posts: 1576
Joined: Sat Dec 11, 2004 12:04 pm
Location: Ecbatana

lobe centers

Postby bigjoe1 » Sun Mar 23, 2008 11:32 am

This is a gereralization, but, the more HP per cubic inch, the more you want wider lobe centers. Real high output engines can run goood with 114 or 116, maybe(prostock-118) this means for most nice race engines, 108 or 110 is going to be pretty close to optinum.Your hemi would like a 108 centerline.

JOE SHERMAN RACING ENGINES
bigjoe1
Show Guest
Show Guest
 
Posts: 5234
Joined: Thu Aug 24, 2006 7:16 pm
Location: santa ana calif-92703

Postby chevy_power » Sun Mar 23, 2008 1:17 pm

Is this why gm has gone to wide lsa's for their ls engines? Or is it to perserve the idle quality. Another thing is I've heard that flat tappet cams actually lift the valve faster with durations less than about 224 @ .050, therefore having an advantage over roller cams with less duration. If this is the case why does gm goodwrench use mostly roller cams with small duration figures @ .050 in their crate motors. I would think a quick opening flat tappet cam would work better in these engines other than the friction they produce over their roller cousins.

Sorry for all the questions I'm just trying to understand cams a little better and the reasoning behind why things the way they are.
Speed kills, save lives, drive a honda
chevy_power
Member
Member
 
Posts: 140
Joined: Mon Mar 17, 2008 6:29 pm
Location: Kansas

Postby Dodge Freak » Sun Mar 23, 2008 1:42 pm

I take a guess, first cost, GM uses roller cams on all their motors, having 2 types cost more money. Second these new motor oils don't work too good with flat taper cams, better to use a roller cam that will last a long time.

Idle quality does play a big part why GM uses wide lobe centers, better to have a good strong idle than 10-15 more HP-how GM sees it.

I seen the same thing about flat taper cams working better than a roller cam for short duration. If thats true or not :?:
Dodge Freak
Guru
Guru
 
Posts: 1708
Joined: Sun Oct 21, 2007 5:56 pm

roller vs flat tappet

Postby bigjoe1 » Sun Mar 23, 2008 2:11 pm

It would be a rare instance when a flat tappet is better than a roller. In full race engines that MUST run flat tappets is about the only time where this might happen. It does NOT happen in street performance type grinds. As an example, I never thought that a hyd roller was any better than a flat tappet of the same duration ect. But, now I have changed my mind, since I started using the Comp xtreme energy hyd rollers, It appears that the hyd roller is about 20 HP better than the same duration flat tappet, There is also more mid range torque and hp too.When you get to custom hyd rollers, I can see over 40 HP than the same duration flat tappet.

JOE SHERMAN RACING ENGINES
bigjoe1
Show Guest
Show Guest
 
Posts: 5234
Joined: Thu Aug 24, 2006 7:16 pm
Location: santa ana calif-92703

Postby chevy_power » Sun Mar 23, 2008 2:30 pm

I know some of the horsepwer comes from the reduced friction. Plus roller cams have more area under the curve than a flat tappet.
Speed kills, save lives, drive a honda
chevy_power
Member
Member
 
Posts: 140
Joined: Mon Mar 17, 2008 6:29 pm
Location: Kansas

Next

Return to Engine Tech

Who is online

Users browsing this forum: BigEd36, drc767, goldmember63, Rallyace, Sprinter99, Yahoo [Bot] and 14 guests