Ford Muscle Cars Tech Forum banner

1 - 20 of 23 Posts

·
Registered
Joined
·
749 Posts
Discussion Starter #1
Can I get a laymans explanation on narrow and wide lobe seperation. What is the application for either. Why is one better then the other.

thanx
 

·
Registered
Joined
·
231 Posts
A wide lobe seperation is not always better than a narrow one, or vise versa. It depends on the combination of engine size, head flow and head flow balance, as well as a big one, exhaust flow capabilities. It also depends on what the vehicle will be used for and if economy and vacuum are important. If the heads have limited flow usually a narrower lobe seperation will be better. But, if the vehicle has cats or a full length 2 1/2 inch exhaust system, a wider lobe separation will almost always produce a much better torque curve. However, if you're trying to run as much cam in a street vehicle as is livable from an economy and vacuum standpoint, usually a wider lobe seperation will be better. It will allow a cam to be run that's big enough to make good power without ruining vacuum, off idle respone, and economy to the point of being undrivable.
 

·
Registered
Joined
·
7,470 Posts
In a nutshell...
Decreasing lobe seperation:
1. narrows the powerband
2. moves the powerband lower
3. makes the idle choppier
4. makes more power
5. pumps up the midrange

Increasing lobe seperation
1. widens powerband
2. makes the idle smoother
3. takes away from the midrange
4. produces less maximum power

Racers usually use tight lobe seperations. The only exceptions are Pro-Stock, where 500+ ci are revved to the moon with cams so HUGE, tight LSA's produce too much overlap, and nitrous/blower/turbo applications where overlap sends all that extra fuel mixture out the tailpipe!

Tight lobe seperations increase overlap and can help increase velocity in overly huge intake ports (4bbl 351C). This will pump up the midrange, and make the engine feel less "lazy". Some 351C drag race cams have lobe seperations as low as 102-104 degrees. Automatics generally need a tighter lobe seperation for extra midrange.

Small engines with big ports LOVE tight lobe seperations, large engines with small ports work better with wider angles between the lobes.

A GOOD, free flowing exhaust is required on engines with cams having tight lobe seperations. Without a good exhaust, the exhaust just doesn't flow out, creating a vacuum to pull the intake charge in like it should. The exhaust just backs up into the cylinder and makes the car run like crap. Extreme cases will even have the exhaust back up into the intake, which will turn the inside of the carb black with soot.

Tight lobe seperations can also be used on engines with restricted intakes and carbs. When used correctly, that extra overlap can help draw extra fuel/air mixture through the restricted intake ports and carbs.

A tighter lobe seperation also allows you to use a slightly larger cam in a lower rpm range. If you want maximum valve timing without revving the engine to the moon, a tighter lobe seperation can make it happen.

Here's an image that will help explain how WIDENING the lobe seperation DECREASES cranking cylinder pressure. You can see how spreading the lobes further apart puts the intake event further into the compression stoke. This causes the valve to be open longer into the compression stroke... bleeding off cylinder pressure.



Just be sure to use an extremely free flowing exhaust with extra tight lobe seperation angles. Their scavenging effect from their extra overlap NEEDS some free flowing tubes, or the exhaust will just get backed up into the cylinders... Really bad exhaust systems will actually make the carburetor get all black with carbon. Open er' up, and let er' fly! I really like the dual 3" woofers that occupy the underside of my car.

Good Luck!



_________________
Mike Burch, 66 mustang real street
302 4-speed 289 heads, 10.63 @ 129.3
http://www.mustangworks.com/cgi-bin/moi-display.cgi?220
http://www.fortunecity.com/silverstone/healey/367

<font size=-1>[ This Message was edited by: n2omike on 2/10/04 6:50am ]</font>
 

·
Registered
Joined
·
521 Posts
wow that is pretty detailed... it answered a few of my questions also
i have another though.... can you define "tight" and "wide" LSA??? i'm going to assume based on your one example that 102-104* LSA is a wide separation? and 112-114 is tigther? or is it the other way around.
THANKS
 

·
Registered
Joined
·
7,470 Posts
It sort of in the middle for a carbed application. For most of these street performance engines, 106 is really tight, 114 is really wide. Naturally aspirated racing engines will usually run between 104-108.

For an EFI setup (excess overlap tends to play havoc with the stock Ford computer) 110 is pretty tight. Most EFI cars come from the factory with 112-116.

Good Luck!
 

·
Registered
Joined
·
231 Posts
Small engines with big ports LOVE tight lobe seperations, large engines with small ports work better with wider angles between the lobes.
This is exactly the opposite of what is true. Better flowing heads or smaller cubic inches need wider lobe seperations and engines with poorer head flow or more cubes need a narrower lobe seperation angle. However, so you don't just have to take my work for it, there's a chart printed out in the March 2002 Muscle Mustangs and Fast Fords in the article dealing with Bennett Heads. Bennett reccomends the cam lobe seperation angles to use with thier heads on race engines of various sizes and compression ratios. I'll print it out as it's very informative.

CID C/R L/S
302 9:1 113
302 11:1 114
302 13:1 115
331 9:1 111
331 11:1 112
331 13:1 113
347 9:1 110
347 11:1 111
347 13:1 112
351 9:1 110
351 11:1 111
351 13:1 112
393 9:1 107
393 11:1 108
393 13:1 109
408 9:1 107
408 11:1 108
408 13:1 109

There's a fairly obvious pattern that develops in that chart and it's also pretty evident from that chart that higher head flow velocities generated by the extra cubic inches nessesitate a narrower lobe centerline angle and not a wider one.
These numbers were apparently generated by a million dollar computer program that's been proven on Pro-Stock and Formula One engines. It's pretty hard to argue with that or with Bennett. They really know thier stuff.
 

·
Registered
Joined
·
7,470 Posts
On 2002-02-25 17:55, Mach1Morgan wrote:
Small engines with big ports LOVE tight lobe seperations, large engines with small ports work better with wider angles between the lobes.
This is exactly the opposite of what is true.
Bennet is referring to heads (his) that are perfectly matched to the particular racing engine at hand. If a cylinder head is overly large for the particular engine, a tight lobe seperation (along with a free exhaust) will help pick up velocity. Looking at all the drag racing cam profiles for the 4bbl 351C Ford in various catalogs, one will notice that they have tighter lobe seperation angles than comparable Chevy grinds.

Engines with ports that are too small to handle the extra scavenging flow (through the intake and out the exhaust) that extra overlap creates are better off with wider lobe seperation angles. A buddy's hotrod 406 Chevy with factory heads worked great with a 114 lobe seperation.

Once a combination is optimized to a perfect balance and matched to a certain custom cylinder head (such as Bennet creates) a person can fine tune the angle. A certain Super Stocker I know runs and EXTREMELY tight lobe seperation... and that's with a 289, which is opposite of what you predict. And this LSA was determined after TONS of trial and error on the dyno. It all depends upon the combination.

What works for Bennet, on his custom heads and engines may or may not work for the rest of us.

Good Luck!
 

·
Registered
Joined
·
231 Posts
Once a combination is optimized to a perfect balance and matched to a certain custom cylinder head (such as Bennet creates) a person can fine tune the angle. A certain Super Stocker I know runs and EXTREMELY tight lobe seperation... and that's with a 289, which is opposite of what you predict. And this LSA was determined after TONS of trial and error on the dyno. It all depends upon the combination.

What works for Bennet, on his custom heads and engines may or may not work for the rest of us.
Mike. Super Stock rules force them to run with stock unported heads and only a 3 angle valve job is allowed. The engines are spun high and the port velocities are very high, because of the stock production port sizes and the high engine rpm, so it's reasonable that a narrow lobe seperation would be needed. It's exactly what I would predict.
It doesn't matter who builds the heads, or if they are custom or not. Bennett works within the same boundaries layed out by the laws of physics as the rest of us. What works for Bennett works for the rest of the world as well. Airflow and engine dynamics aren't affected by the name on the valve cover.
 

·
Registered
Joined
·
7,470 Posts
Mike. Super Stock rules force them to run with stock unported heads and only a 3 angle valve job is allowed.

It doesn't matter who builds the heads, or if they are custom or not. Bennett works within the same boundaries layed out by the laws of physics as the rest of us. What works for Bennett works for the rest of the world as well. Airflow and engine dynamics aren't affected by the name on the valve cover.
Porting has been allowed in super stock for years now. They are allowed 155cc with a 289 head (stock is 126). They are also allowed to use Motorsport blocks and aftermarket rods as well... but they must still use the stock compression ratio.

Extremely high compression ratios are normally used with immense 9000 rpm cams. To trap enough cylinder pressure to make a 9000 rpm cam work with a paltry 9:1 cr, the lobe seperation needs to be tightened to the max.

Bennett is using heads that are as close to ideal as is humanly possible. Most of what WE use is far from it. 4bbl 351C heads are too large for a 351, let alone a 302. When used on 351's and 302's, a velocity boost is required, and a tight lobe seperation teamed up with a freely flowing exhaust can help make that happen.

An engine with abnormally small ports works better with a wide lobe separation.

Let's discuss overlap. This is point at which the exhaust valve is closing and the intake is opening. The gases flowing out the exhaust port help draw the air/fuel mixture through the intake. Some of that extra intake flow will go right out the exhaust port. (This is why tight lobe seperations produce such stinky exhaust) A tight lobe seperation (extra overlap), along with a freely flowing exhaust can help create more of a "pull" on the intake charge. This helps with velocity problems associated with engines that have extremely large heads for their size, as well as engines with extra small intake ports. BUT, on engines with extra small intake ports... if the exhaust port is undersized as well, all bets are off. There won't be enough exhaust flow to make it all work. The extra overlap will simply pollute the incoming air/fuel charge with those backed up exhaust gases.

4bbl 351C's like tight lobe seperations to help pick up intake port velocity. 406 Chevys with stock heads and most 289/302 engines with stock unported heads like wider lobe seperation angles. Super Stock 289's trying to make decent cylinder pressure with huge cams and pathetic compression ratios like extremely tight lobe seperations. Their (now) ported exhaust ports and extremely efficient multi-step headers help make all that overlap pull extra air/fuel into the engine.

It is all simple physics, but the physics needs to be applied differently to each individual combination.

Good Luck!

_________________
Mike Burch, 66 mustang real street
302 4-speed 289 heads, 10.63 @ 129.3
http://www.geocities.com/carbedstangs/cmml_mburch.html
http://www.fortunecity.com/silverstone/healey/367

<font size=-1>[ This Message was edited by: n2omike on 2/26/02 7:48am ]</font>
 

·
Registered
Joined
·
46 Posts
I gotta admit that Mike is one of the most knowledgable engine theory guys I know, that does not work in the business.

Good explanation Mike.
 

·
Registered
Joined
·
3,700 Posts
Geez ... and nobody called anybody "Stupid"


Imagine that!

Sorry ... reference to another post ... Glad to see things can be debated in here ... Still
 

·
Registered
Joined
·
749 Posts
Discussion Starter #16
outstanding stuff


sooooo lets say I use my stage 1 TW heads from Panhandle Performance...

http://www.panhandleperformance.com/benchrace2.html

...on a 393 with say max compression for pump gas...or maybe the question is what is the max compression for pump gas(92 octane). Based on overlap it looks like octane requirements can be altered by lobe seperation? I know that what your trying to build in the end needs to be known. In my case its a 2900# 81 coupe street/strip car( see webpage for more details)...looking for max performance with the above mentioned cubes, heads, and of course pump gas.
 

·
Registered
Joined
·
231 Posts
Geez Mike. I'm not trying to be rude here or anything, but where do you get this stuff? I've got an NHRA rule book right in front of me and head porting is not allowed in SuperStock. They are allowed to port match the heads but can only go in one inch from the manifold face on the head and the 3 angle valve job has got to meet NHRA specs. If there's any evidence of porting in any of the runners past the once inch point, the heads are illegal for Super Stock racing.
If you look at the Bennett chart you'll see that they recomend a lobe seperation that 7 degrees wider on a 302 than they do on a 408 with the same heads. I'd tend to trust their judgement a little bit more than someone who had a buddy with a 406 Chevy that seemed to "work great with a 114 lobe seperation."
Contrary to what you're saying, if you have a engine with lots of cubes or small runners the port velocities will be higher on both the intake and exhaust runners. Up to the point where the exhaust port velocity begins to limit flow volume (about Mach 1) that extra velocity will create a greater pressure drop behind the initial exhaust blowdown. This low pressure area helps pull exhaust from the engine late in the exhaust cycle, when the cylinder is in the overlap postion. Opening the intake valve sooner exposes the intake tract to this pressure drop and initiats intake flow sooner. This is important because with a small port or big cubic inches, more time or duration of intake flow is needed to fill the cylinder as compared to a smaller engine. A cam with a narrower lobe seperation adds to this beneficial overlap period. However, on an engine with larger runners or less cubic inches the exhaust gas velocity won't be as high, the low pressure area behind the intial blowdown won't be as low and the positive effect is has on the intake charge will be less, so not as much overlap can be utilized. Also, with a big intake runner the flow velocity is low and reversion is created in the intake runner easily because of that. Opening the valve early while the piston is still rising without lots of intake or exhaust port velocity will do just that. It creates reversion. Reversion kills flow velocity and torque. The intake opening point is the second most important of the four camshaft timing points. The one that has the most effect on cam characteristics is the intake closing point. If running the intake lobe at a narrow 102 degrees made the engine feel less lazy, chances are it's because the engine was overcammed and the earlier closing point created by the 102 degree lobe center made the cam act smaller than it was and it picked up some midrange torque.

<font size=-1>[ This Message was edited by: mach1morgan on 2/26/02 3:22pm ]</font>
 

·
Registered
Joined
·
7,470 Posts
As for Super Stock engine rules, check out the link. Alex races a 9000 rpm 289 Super Stock mustang, and talks a little about his engine combination.

http://www.mustangsandmore.com/ubb/Forum12/HTML/002087.html

As for the smaller exhaust ports creating more of a low pressure area to draw in the intake charge... If the port is undersized, it becomes a restriction, and the exhaust flow gets backed up. It doesn't 'race' out the exhaust port, creating a low pressure area that helps pull in the intake charge. The exhaust simply backs up into the cylinder, diluting the air/fuel charge and reducing power. To take real advantage of overlap, a FREE flowing exhaust port (and system) is required. Yes, the higher the velocity, the greater the low pressure are. Bennet can create some seriously efficient exhaust ports for his heads. They have SERIOUS flow for their port area, and create a really strong low pressure area during overlap (as long as they are connected to a good exhaust system and headers).

Good Luck!
 

·
Registered
Joined
·
1 Posts
I was emailed about this thread and would like to add some some stuff too.

The SS heads issue
Unless superstock has changed rules in the past year, ported heads have been allowed since the mid 80's or so. For instance, look at the allowed port volumes on the 289 heads versus the as cast. I will have to get the rule book out and look at how it used to be written, but basically the valves have to be stock diameter, the valve position and angle had to be stock, and the valve job itself was limited to the angles shown in their chart. Other than that it was balls out port work (within the limits of the NHRA/IHRA port volume for the head). Take for example the C5AE-B head. This is a very nice SS casting. It comes factory as the typical 125-128 cc inlet port, but it is allowed a 155 CC volume limit. Other heads do similar. Generally the SS 289/302 motors end up (depending on volume limits) in the mid 140's for inlet port volume. Exhaust port volume is not so generous though. A typical 289/302 inlet port in SS will flow in the 215-230cfm range (28" H2O).

If S/S rules have changed in past year, then there would be lots of builders going thru the roof, because of years of development and the tossing out of huge amounts of money and time involved in the head flow studies, not to mention all the cams, intakes, and headers. I will check the current rules, but in the past 15 years or so porting was allowed. I have never even heard of them allowing a 'port matched' port that could only be blended inward an inch.

I am assuming the Alex being refferred to is denysenko. He is running pretty well. I think he has a record right now in L.

With regards to 'LSA' and being engine and usage specific, I disagree. That sounds like a magazine article or something. LSA is completely relative to the particular engine in question only. Comparisons of wide vs narrow are done, but it is really only an pointless observation. What can be wide for one motor can be narrow for another.

When you break down timing events and look at durations wth respect to time, the look begins to change. look at the timing events with respect to the overall flow capability and velocity potential and then look at the displaced cylinder volumes and the rpms being used. Then think of units of time thru which charge is drawn in or expelled. Remember flow is unit of volume per unit of time

The reason a pro stock(assume a truck motor) and a S/S cam look so different is mainly because of the flow potential of the inlet (and exhaust) with respect to the displaced to volume. Why can a 360CID truck motor have a 116-120 LSa, but a 360CID S/S motor uses a 104-108 LSA? (both are race motors!!) This gets into whether a motor is restricted or not. the argument about race motors needing a narrower LSA is incorrect. It has to do with flow and displaced cylinder volume.

The chart in the magazine is somewhat misleading, because what I don't think they said. That was that the chart was for an equal inlet capability (not a proportional inlet capability)and not just CID related.

So then the picture starts becoming that CID increases, then for a given inlet capability (and exh) that the port velocity will increase. And this even changes depending on the rpm band and how well the exh (as a system) flows. In general as the proportion of inlet capability drops with respect to the displaced volume, then the exhaust events should be retarded, or cylinder pressure will suffer.

But you still get a sense that its about 'LSA'. think of the truth being the timing events required are what they are. Then the LSA just becomes a calculated angle and is good for discussion, but really should not be the driving factor.

It's like torque and hp. peopl view them as different entities. hp is calculated from torque. just like LSA is calculated from valve events.

I put a lot of time into a write-up called 'cam truth'. Though it may not be interesting to
some enthusiasts, some of you may enjoy it. I also did a valve timing chart that demonstrates the valve events and how they change with respect to flow CID and rpm. It's what I have found to work and the end effect is that enables a single formula to be used to calculate valve events for any motor, based on flow capability, displaced volume, rpm, static CR.

take care

Buddy Rawls
http://www.wighat.com/fcr3

check out cam truth and the valve timing chart



<font size=-1>[ This Message was edited by: buddy rawls on 2/26/02 9:31pm ]</font>
 

·
Premium Member
Joined
·
14,758 Posts
I just wanted to thank Buddy Rawls for taking the time to come over and participate in this great discussion.

Buddy has been in the industry a long time and recently started his own business analyzing airflow demands and producing custom cams accordingly.

I strongly urge you anyone to take a look at his website for VERY detailed cam operation info

http://www.wighat.com/fcr3
 
1 - 20 of 23 Posts
Top