Cam Lobe Theory

[W8Dart421]

Hey Speedtalk (Don) This Ryan Johnson, I'm the guy that ported Don Gentry's W5 heads and buddy of Scott Brooks. I also ported the Big Block Cylinder heads for the moparts.com PHR Enginemasters entry. Scott told me I could get ahold of you over here to pick your brain about cam theory.

I guess my BIGGEST question is regarding splitting the intake/exhaust lobe patterns. I've been talking to a bunch of guys, including varoious cam grinders about this subject recently, especially after Don's car slowed down with the new big Comp Cam with the MM series lobes. I wanted to know WHY that cam did'nt work well, and I kind of wanted to prove to myself that it was'nt so much the non split pattern. Just as a reminder, here are the specs on that cam:

Comp Cams "MM" series #6589 lobes I and E, .420" lobe lift, 271 @ .050", 189 @ .200", Ground on a 108 LSA installed at 104. With Don's 1.56:1 Erson Rockers, lift at the valve was ~ .637"

That was the new cam. His Previous cam was a Comp with the SQ .875" Lobes, .386"/.390" Lobe lift, 260/266 @ .050" 175/181 @ .200", Ground on a 108 installed at 104. With 1.56 rocker, ~.582/.588" at the valve after lash.

Engine is 416", 4" stroke, 4.070" bore. 11.90:1 Compression, Large plenum Single plane, 1025 Race Demon, 1 7/8" primary tubes, 3" collector, 5000 converter, 4.30 gear, 28.5" tire, 3411# race weight.

The heads flowed 302 @ .500", 315 @ .600", 320 @ .650" Intake and 219 @ .500", 232 @ .600". 239 @ .650" Exhaust with NO tube. With a tube 257 @ .650" Across the board, the exhaust was ~6-7% higher with a tube.

Exhuast to intake flow percentage at .650" is 80.3% with the tube.

(oh and BTW, I have had Scott Brooks' Diamond heads on my bench after they were on your SF110 and my bench is lower than yours, so you can take that into account when thinking about the raw #'s)

Now, am I wrong in feeling that the exhaust lobe should be designed taking into account the exhaust flow PERCENTAGE rather then just looking at raw flow #'s? I've been trying to research this the best I can, which leads me here.

Scott Brown at Straightline Performance designed a new cam for Don's engine, it's a 262/274 @ .050" and Lift is somewhere around .630 at the valve. on a 112 LSA, installed at 108. I was baffled when I saw the 12* split pattern. Scott's reasoning was something about "look at Pro Stock exhaust flow. They have VG Exhaust ports and run alot of split. I BELIEVE he mentioned 18*-20* split. Well frankly from what I have found, the Pro Stock heads will have ~ 60-65% of flow on the exhaust ports. So I can understand a big split pattern on them to make up for the exhaust port that just can't keep up with the intakes. But what about on a head that has 80% E to I flow? I'm not a cam "guru" so that's why I'm asking. I looked at the specs on a Comp. Craftsman Truck Cam I have that came out of a W-8 motor, and ran the head flow #'s on it, and the cam had a 12* split and the heads were 65% flow. I see Comp put no split recently on the W-9 motor in the Oct. 2003 Mopar Muscle magazine buildup. Ran the Flow #'s and got just over 80% with no tube, and guessing around 83% with a tube. Talked to the cam grinder who is doing the cams for our Enginemasters buildup, and he did'nt want our flow #'s, he just wnated to know the flow percentage for him to design a couple cams. On those heads I ended up with 75% flow with tube, I'll just say our cams have 4* split. I can't really say any more than that until the contest is over.

So I'm just curious as to how a cam should properly be selected for I/E split in your opinion? Do you take the collector size/exhaust size into considereation? I assume so..... Like on Don's car I'm not thrilled with the 3" non changeable collectors, with those big heads on a 400+ inch motor.... I'd like to see a 3 1/2". He really does'nt ever race with exhaust on, so the collector Dia. is the only real issue I see there.

The engine LOST about 35 HP according to the MPH with the Big MM Comp cam....... The spring pressure is dead on to what Comp Recommended for that profile, so I ruled out valvetrain harmonics for now. I think it was a mistake to go with the MM lobe, I think there was just too much duration "under the curve" for his collectors, compression, and stall speed. If I had to design another one it would probably be somewhere around 266/268 with a slower .875" GM lobe, .630"/.640" at the valve, and on a 108 in at 106...........

Anyhow, just would like to hear your thoughts..................... Thanks, Ryan J.

OH and BTW, the car gained no MPH, by putting the Straightline cam in last week.......

[speedtalk]

Hi Ryan, Thanks for posting. Don't change your thinking on cams based on this car. The problem is not the cam! Assuming I had to pick a flat tappet cam, my choice wouldn't be but 5hp from yours. Yes, I look at exh% to pick the duration. The owner of this car is very nice, but I've seen it a hundred times - he's listening to too many people. Like I've always said, there's a thousand ways to make X horsepower, PICK ONE. Thanks again for joining the board, Don Terrill

[W8Dart421]

Don: THANKS for the reply...........

[speedtalk]

I had ground something 20 degrees smaller and we picked up 60HP and 80#/ft of torque and flattened the torque curve by 800 rpm.

I just got off the phone with French Grimes about his upcoming interview and he made it clear he thinks overcamming is a big problem. In fact, just about everyone that's been on the show and commented on cams has said the same thing. I think it shows missing the duration numbers on the high side is a lot more harmful than missing them on the low side. Reminds me of shift points.

[speedtalk]

If you have any tech questions you'd like me to ask during his interview, post them here:
viewtopic.php?t=151

[J-Rod]

Cstraub sent me a link to this site. I am also very interested in many of the technical aspects here.

Its funny the topic of cam design/lobe selection should come up. That is one of the key things we have been going round and round about in the LS1/LS6 world. The world of cam design there is much more simplified. The various "Tuners" look in the Comp or Lunati catalog for a lobe, they call up Comp or Lunati and tell them I want Lobe X on the intake and lobe Y on the exhaust on a 112 or a 114LSA. 112 if you want to make power. 114 if you want it to idle better. You ask one of them about VE's and you can hear the crickets chirping. The answer to making power thus far has been to just throw more lobe at it.

You have the two camps of reverse split vs standard split. The standard split guys are making big power with some very large camshafts. The reverse split guys are making good HP, just not near as much torque.

The I/E% on an LS1/LS6 are in the 80% range with the intake in place. The main issue is that the intake runners on this engine are about 15" long, so its much like a dual plane. This does help the motor make torque, but often times folks tend to have it in there mind they need to cam an engine like this like they would if they had a SBC with a Victor Jr. on it.

Anyhow, this cam thread on Ls1tech.com might give you some insight on how different folks are doing things and wha tthe results are.

http://www.ls1tech.com/forums/showthread.php?t=101100