mtx28

well? i know basically what the rest of the cam's different characteristics do... but besides idle, what does a shorter LSA do for the engine?

mtx28

any ideas?

MRZ28HO

A shorter LSA will decrease effective compression (dynamic, not static CR). Plus in a naturally aspirated engine (turbo/blown engines are a totally different issue), it helps to clean the combustion chamber for the "new" incoming fresh air/fuel mixture. Good for emissions. But, there is a flip side to shorter LSAs, less vacuum. This is due to the fact that both valves are open at the same time for a longer period of time. Therefore your "sucking" power is not that good (reminds me of an ex. ). This is why OEMs have relatively large LSAs (114-117), to promote vacuum, which the computer uses to function properly. Shorter LSAs also do not allow high rpms because they spend the "new" mix to fast at upper rpms to effectively make power. Longer LSAs help promote higher rpm power because more of the "new" mix is retained and used to make power. And, unlike shorter LSAs, they increase your compression, which could be fatal (denotation ). I haven't seen it happen, only heard of such cases. I could keep going, but I think this is becoming a handfull for ya.

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George P. Lara
1994 Z28 LT1 T56
1984 Z28 High Output
Member: SCCA, SCFB, SC3GFB

[This message has been edited by MRZ28HO (edited November 13, 2000).]

Apeiron

You mean cylinder pressure, not compression, don't you?

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1984 Silver Z28, 383 cid 4 bolt, ARP Fasteners, 2 1/2" Cowl Induction Hood, 600 cfm Performer Carburetor, Torker II Intake, Performer RPM Heads, XE268H Cam, Magnum Roller Tip Rockers, MSD6AL Ignition, Blaster2 Coil, Recurved HEI Distributor, 700R4, TransGo SK, 2.73:1 Limited Slip Dana 44, Terminator Headers, Custom 3" TIG Welded Stainless Exhaust, Flo-Pro Twister Muffler, 4 Wheel Disc Brakes, Oil Cooler

Roy

Let me see if I can simplify that somehow. Tight lobe seperations tend to cause power to build early & quickly in the power range of a given cam,and then trail off. Wider lobe seperations cause the torque curve to be spread out more evenly and higher given the same camshaft.
Roy

jcb999

One thing I would add is that engines produce more torque with tight LSAs (look at the LSA angles of racing cams), but the key is making sure the engine has sufficient vacuum and (if it is an ecm motor) can go into closed loop.

[This message has been edited by jcb999 (edited November 14, 2000).]

MRZ28HO

For simplicity, yes.

AlkyIROC

Roy has it exactly!

A typical street cam has a wide LSA 114-116. The engine operates over a wider rpm range and the wider LSA also allows a smoother idle. Most race cams are 106-110 LSA. The operate in a narrower rpm range and have rough idles. Going one step further into ProStock cams. These have enormous lift and duration but have gone back to 114 LSA to help the engines idle.

When playing with cam specs for my new big block I tryed many different combinations. Because of the car weight, tire size, gearing, converter stall etc, a tight LSA wasn't going to work well with my combination. They would all work but not to the best ability with the rest of the combination. The cam I'm going to pick is actually a blower grind with a 114 LSA. It's more suited for my race car than an actual race cam.

Another factor to watch in cam grinds is valve overlap. Too small an overlap can have the same effect as a tight LSA. I found a great Crower cam that I wanted to use until I found the overlap was only 27 degrees. I needed something closer to 60-70 degrees to work well.

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Stephen's racing page

87 IROC-Z Pro ET Bracket Race Car and knocking on the SuperPro ET class
383 stroker (carbed) with double hump cast iron heads and pump gas
454 Big Block buildup now in progress for the 2001 racing season

Best results before the 383 blew up
Best ET on a time slip: 11.857
Best corrected ET: 11.163
Best MPH on a time slip: 117.87
Best corrected MPH: 126.10
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

87 IROC bracket car
91 454SS daily driver
95 Homebuilt Harley

ChrisFormula355

Not sure how this fits into the argument really..but my Comp cam extreme 262 in my firebird has a short LSA...110, and it has assloads of torque and makes power up to 6,100, I shift at around 6,000 though to play it safe.

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Nice car: 1988 Formula 355, mildly ported/polished Vortec heads, RPM intake, Comp Cams Extreme 268 cam, 650 Holley double pumper, 5-speed/3.73 Richmond gears, and N02 on the way
Old 1/4mile: 13.5@105mph

Evil Car: 78 Pontiac Grand Prix, 500inch caddy motor, edlebrock intake,HP Holley 750 DP race prepped carb, B@M TH350 tranny, 16inch Iroc-Z rims, blacked out chrome, soon to run 10's...

There's no replacement for displacement!

ChrisFormula355

Oh and yes the idle is really choppy and kinda rough but thats exactly what I wanted! Sounded mean as hell at idle and it revs up super quick.