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Tight Lobe Separation angles

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Tight Lobe Separation angles

Postby la360 » Sat Apr 02, 2005 10:06 am

Tightest I have ever really dealt with is , is 106. I have come across a few grinds in some local and US Cam Grinders catologues that list camshafts with the lobe sep in the 100-104 ranges. As there are some very experienced engine builders on this board, I was hoping some of you could give me some examples on where tight lobes like this would be used.
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Postby bill jones » Sat Apr 02, 2005 11:58 am

-we ran a 255x261 with 104 spread and the intake CL at 98 for 6 years in a 350 SBC oval track Chevelle on asphalt, where the rules limited the porting to where you weren't allowed to see any obvious porting effort looking down the intake port, but the short turn, the rest of the chambers and the exhaust ports could be ported.
-We ran the engine to about 6500rpm and we were the only people who had a cam with tighter than 106 lobe spread.
-This car was capable of winning any race, qualifying #1 or #2 and starting from the rear of the field because we ran fully inverted fields, for the full six year period altho the owner only raced it about 6 times a year (at the best paying races) he won every race he entered except for two where he was involved in some fender banging and he finished 2nd in both those races.
-That's the only experience I have with a 104 cam.
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Postby maxracesoftware » Sat Apr 02, 2005 1:25 pm

I have come across a few grinds in some local and US Cam Grinders catologues that list camshafts with the lobe sep in the 100-104 ranges. As there are some very experienced engine builders on this board, I was hoping some of you could give me some examples on where tight lobes like this would be used.


100-104-108 LCA can be used in NHRA SuperStock Classes that require
using stock valve OD sizes

even with Ported SS Heads , those SS Engine Combos can tolerate
100-104 LCA because relatively smaller OD valve diameters/Flow during OverLap Period won't be such a overscavenging problem as with much larger valves...and to keep RPM range 5000 to 8500 with great Torque Curve for relatively heavier weight SS-Cars
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Postby Cheapstreetduster » Sat Apr 02, 2005 2:26 pm

larry what is considerd small?
1.88" ?
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Postby maxracesoftware » Sat Apr 02, 2005 5:50 pm

larry what is considerd small?
1.88" ?


Chevy SS 283/305/327/350 = 1.720/1.500 , 1.840/1.500, 1.940/1.500
size combos w/ 45 deg angles

a SS 283 w/automatic has made pretty good TQ/HP Curve with 1.720/1.500 valves and Cams on 100-102-104 LCA and tried out on
as little as 95 to 98 Int CenterLines..usually right at 102-103-104 Int CL on larger CID sizes like 305/327/350 automatic trans 3000 to 3800 Lbs
9.5:1 to 11.5:1 CR, OEM or aftermarket steel Rods

on the opposite end of Scale like ProStock 500 cid or very large CID Engines w/large Bores/Valve Size ODs
116 to 120+ Centers and the use of 50/55/58 deg seat angles
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Postby la360 » Mon Apr 04, 2005 2:48 am

It's basically what I thought on the subject. The main reason I asked the question, is because a few weeks ago, I was setting up a Cam Doctor for a friend of mine, the camshaft we measured was a Cam Motion Roller for a small block chev, the duration @ 0.050" numbers were in the 280's ground on a 104 lobe separation. This is a circuit engine, which I don't know alot about. It was a 360 cube engine, with a Bowtie raised Port 18 degree head, individual runner EFI, and it runs on 100 octane fuel. I have always been of the opinion that engines that are under headed generally benefit from a tighter lobe, but something like this wouldn't be so in my opinion.
Any thoughts????
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Postby buddy rawls » Mon Apr 04, 2005 10:18 am

You cant group specific LSA to applications. LSA is a calculation from the valve events. the valve events and ramp rates are what drive the end product of durations and lobe separation angle. Typically the greater the inlet capability, relative to the displaced cylinder, the earlier the exhaust event will need to be (wider LSA). (within a range of E/I ratio).

So in general, the more restrictive the inlet path, the later the exhaust needs to be (narrower LSA). this is why you will see narrow LSAs on SS and stock eliminator set-ups, yet wide on the more flow capable set-ups.

with a restrictive inlet and very nice flowing and large exhaust cross-section, it is entirely possible to have LSA way on down into the 98-102 degree range. this is what some the higher rpm foriegn stuff (nat aspirated) is using when they are showing E/I ratios in the upper 90% region.

ITs all relative to the specifc set-up in question.

If any assumptions or guidelines are being looked for, they need to be looked at from the perspective of restricted flow or unrestricted flow , relative to the displacement and rpm. an unrestricted motor could become restricted at higher rpms. Everything is relative. this is the part that is left out of most every cam book and article I have seen. They always group duration and LSA into usage and rpm, and it really does not work that way.
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Postby Motorman 407 » Mon Apr 04, 2005 7:29 pm

Wider lobe separation angles will allow for the torq to stay in peak longer and at higher rpm if it's the same duration using narrower lsa. A pro stock motor ex. need short ,relative, intake duration for acceleration of the line and at the gear changes but that will kill topend so they use a wide lsa to pull trough on the other end. Wider lsa also makes the motor idle smoother that's why oem uses wide lsa on low rpm motors like the production V8
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Postby la360 » Tue Apr 05, 2005 3:05 am

I think alot of camshaft articles are written so the average guy can get a basic understanding of what goes on. Obviously, because of this, alot of blanket statements are made, and alot of rules of thumb are suggested. This isn't much good for someone like myself , with a fairly good basic unstanding of what goes on, but I haven't come across any articles that take it a step further, hence why I asked the question I did. Hopefully some of you might shed some light on why the engine builder in the example I mentioned in a previous post.
Why would you decide that a tighter lobe separation is best in a given combination? The SS engines that Larry mentioned are obviously one example, but I am curious as too why someone would run a fairly large camshaft with a tight lobe in a relatively well flowing induction set up .
I am just using this Bowtie headed Chev as an example, I am basically trying to further my understanding of camshaft timing and it's effects.
Thanks for the replies
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Postby John Wallace » Tue Apr 05, 2005 9:59 am

One possibility is that the tight lobe centers make the engine think it has a higher compression with the intake closing sooner and the exhaust opening later.
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Postby buddy rawls » Tue Apr 05, 2005 3:56 pm

Without knowing all the specific parameters of the motor and vehicle set-up, its hard call. However, it is entirely possible that the valve evnts are not correct for the motor. Thats does not mean the motor will not perform and make excellent power.

Tuning a motor via adjustments in valve events are basically a trial and error process of honing the desired response over the desired range. In reality what is happening is the process of tailoring the cylinder fill to the desired range of vehicle response. Its not about adjusting the LSA or dropping/adding duration. Its about working the valve event response into the set-up.

there could be sound rationale for the cam, or it could be a design that they have used as a fit-all in the past.

With respect to the PS cams, a motor so well tuned to provide well over 100% VE, does not get there by removing/adding some duration for a stronger launch, or bringing the exhaust in earlier to increase or broaden out the rpms. The valve events are like they are because the motor and vehicle parameters. A very high inlet capability simply does not need as much time of valve opening time, and super cylinder filling requires an earlier exhaust especially as the efficiency of the incoming flow versus the cylinders volumetric flow increases. The baseline valve event (or rather the trends) look like they do because of the way the motor utilizes and captures flow potential. Small in-the-field changes hone the set-up, but the trends will always be to the flow parameters of the motor.

On the narrow LSA question, to really analyze the cam, all the specifics would have to be looked at. For instance with a very high E/I capability, even a strong inlet flow, relative to the cylinder flow, could utilize the later exhaust events of the narrower LSA.

I have worked with some road race motors that had well over 90% E/I in the low and mid lifts, and this dictates a substantial amount of the valve event trends. Even though the inlet capability was high, relative to the displaced cylinder volume at xxx rpm, the high exhaust flow could utilize the later exh valve events to a sound advantage, producing far more torque in the midrange. this could be the similar case of your particular motor.

Its all relative!
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Postby Motorman 407 » Wed Apr 06, 2005 8:25 am

quote
A very high inlet capability simply does not need as much time of valve opening time, and super cylinder filling requires an earlier exhaust especially as the efficiency of the incoming flow versus the cylinders volumetric flow increases. The baseline valve event (or rather the trends) look like they do because of the way the motor utilizes and captures flow potential. Small in-the-field changes hone the set-up, but the trends will always be to the flow parameters of the motor.

So you are saying the more flow the less duration. Why would a PS guy give away filltime in the cylinders just because the heads flow good? I seen heads with less airflow run faster than higher flowing heads because they have higher velocity. Dragracing is about acceleration and thats why cams have gone down in seat to seat duration the last few years and with the help of spintron testing and valvespring quality they are able to make much more .200 lift duration today. Belive it or not a short stroke 500 PS motor needs help at the gear changes for recovery. The same "trend" is smaller header tubes, smaller oval intake runners, smaller exhaust valves etc.
This is found out at the race tracks and not by teoretical ideas.
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Postby buddy rawls » Wed Apr 06, 2005 9:53 am

Its every bit about velocity and the inertial fill and expell of the cylinder. Thats the whole idea. For a given rpm band, and increase flow cross-section requires less valve activity for equal cylinder fill. BUT, that may not be advantageous for power, within that rpm band.

the key is in the matching up of the port velocity and piston movement without sacrificing any unneeded/unusable flow potential which can get contaminated or slow the overall process down. no dead areas, all useful/quality flow at correct port velocities.

Flow does not equal power. flow equals power potential, but even that is hazy. Yes, a smaller effective cross-section with a quality flow can definitely outpace a larger crossection with equal flow. atleast within equal rpm range considerations, and depending on the situation, possibly all around. the reason is velocity and the associated inertia is what is filling the cylinder. flow potential by itself is not much help. take the port away and just the intake or exhaust valve act like a venturi, and you will not have much.

pro stock racer is not giving away filltime by decreasing duration because of high inlet flow capability. Its not about the numbers (duration/LSA) its about the optimizing the charge exchange within the rpm range that will provide (in the case of an all out effort set-up) the greatest cylinder filling potential (match-up of cylinder volumetric flow, port flow and velocity, static compression) to provide the most response in the rpm range.

When the match-up is made, the engine's revving and acceleration will be stronger, the power to the ground is higher per unit of time.

Its all relative. the specific numbers (durations, lobe separation angles, etc) are not where the trends lie. For instance the old PS truck motors. 114 LSA would be narrow, yet a 360 SS motor would die with a 114 LSA because the exhaust is way too early. yet the 106 LSA on the truck motor would have died.

But its really more than this, because even this is focusing on the numbers. ITs all comes from the timing events. The durations and LSA come from those numbers. So the subject is not about decreasing the intake duration or widening/narrowing the LSA. Its about where the changes are occuring, are the openings staying the same, but the closing is shortened......... ITs in these numbers that the changes to the motor's flow are occuring. the end effect is seen as a duration or LSA change, but its deeper than that. Its happening at the opening and closing points and the ramp rates.

the argument of the pro-stock scenario is not that the shorter intake timing and earlier exhaust are chosen to get the car down the track (they are), but rather the engine paramters (flow/x-xection/port velocities/displaced cylinder flow) dictate those events. And those can be calculated, then finely honed on the dyno and track. but the trends are basically going to follow what the motor parameters are requiring, and not deviate that much. Basically nothing happens in a motor that is not supposed to. It may not can be readily calculated, but its all there. Its an engineering model.

In actuality, I think we are looking at the same aspect of valve evnts, but from two sides. You are looking at it from the vantage point of what is working, I am looking at from why it is working.
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Postby Jay Allen » Fri Apr 15, 2005 10:10 am

You get the intake events where they need to be. Then get the exhaust events where they want to be. The result (or by-product) is LSA.

I really oversimplified that. By this what I am trying to say:

2 + 2 = 4.

4 is the RESULT of 2 +2. If you just look at 4, there is an infinate number of ways to get there.

LSA is the RESULT of proper timing events. If you focus solely on LSA, you'll get confused. Very confused.
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Postby ibmorjamn » Thu Jul 23, 2009 6:05 pm

another interesting thread !
I have a short distance to go and even shorter time to get there !

My single is 108mm x 70.6mm 114.47mm rod or 4.251" x 2.77" rod 4.5" also makes 45 ft. lbs @ 6800 rpm
37mm int. 32 mm exh.
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