tdrumm

iTrader: (18)

If you have two cams, A and B, with the same durations at .050, but cam A has larger advertised durations, in what way does this change valve events and how would this affect engine performance?

What about the opposite - A and B have identical advertised durations, but cam A has larger .050 durations?

Assume that all other variables are equal between A and B.

Thanks.

Adrenaline_Z

Good question, but you can't assume all the variables will be the same otherwise.

Durations and overlap regions must be different if the specs are not equal at
those points.

The valve lift at each crank degree will differ, along with the behaviour of the
air flow through the ports as pressure changes.

There is some good reading about overlap, duration, and the effects on the engine's performance within this forum.

J-Rod

.006 which is normally advertised (but some foilks use .004) is imprtant. But, it isn't as critical as say .050 to .200. The reason I say this is that the valve is open, but not that much.

But, one of the things it can help indicate is how agressive the lobe is.

Let me give an example. If you look in the camshaft discussion thread there are a few graphs which illustrate this.

https://ls1tech.com/forums/showpost....&postcount=311

Beast96Z

iTrader: (21)

In a nutshell, advertised - .050" will tell you the aggresiveness of the lobe. The smaller the number, the more aggrassive the lobe. A more aggressive lobe comes off the seat faster, meaning it has a larger .200" duration, which can drastically improve low-end power. More aggressive lobes wear on springs a bit harder, but tend to make great power. Using a slower lobe will increase spring life, at the cost of low-mid power. This is a very basic generalazation.

Rick@Synergy

iTrader: (5)

I find I stay away from aggressive lobes and stay conservative alot. The lobes I use are very easy on springs. Our cams still make great power and from this I also stay small on duration. I like the idea of more seat timing because it comes on and off the seat slower which also keep velocity higher. Its like a pin hole with pressure on one end. If you open slowly, then the speed is greatest. Hope that makes sence.

Rick

SStrokerAce

iTrader: (2)

Beast,

If you have a more aggressive cam it's going to limit your RPM capability of the cam. Doesn't always mean they are going to make more low end power either. The right combination of valve events and lobes can make you more TQ.

I agree with Rick, aggressive lobes have their place, but not on a lot of things.

Bret

DavidNJ

Unless you are only drag racing, there is more than power. An aggressive cam lets you have lots of opening without as great a sacrifice in drivablilty.

TTBOMK, all oval track cars use aggressive cams. Although I only buy the stuff for my engine, the Comp catalog lists cams as their most aggressive than talks about oval series such as Hooters, Trucks, etc.

Rick@Synergy

iTrader: (5)

Yes, alot of oval track guys love agressive lobes, but street guys wont. Everyone talks like they want all the power in the world, but once they find out how often their springs last, they change camshafts real quick.

SStrokerAce

iTrader: (2)

I'm talking about aggressive in different terms here.... the off the seat can still be quick, and the lobe can have a small advertised and .050 for the .200 duration but there is the right amount of lift there to make the lobe tame.

The lobes I use on SBC and LT1 stuff is much more tame than a XE-R lobe but most guys wouldn't think that by looking at the specs. Again specs don't count results do. (unless you can actually understand the specs fully)

Bret

DavidNJ

On a street engine, they would rarely run in the higher engine speeds compared to an oval car. In a typical weekend the car can be running at 4500-7000 for 45-60 minutes in continuous 3-10 minute intervals. Does a street car see that much time in that range in a year?

With a 28deg major intensity and 1.8 intake rockers, my springs last 6-10 weekends.

DavidNJ

Bret,

Could you elaborate?

Thanks,

David

Rick@Synergy

iTrader: (5)

What about guys that auto cross and open road race? Seems like they would be going through the same thing to me.

Rick

1989GTA

I would think that Comp Cams beehive valves springs would help out in the spring department and also create less stress on the valve train than say dual springs. I'm going to the Comp Cams XFI268HR-13 cam and I sure hope this is the case. Don't plan on running over 6200rpm at the strip. Allen

DavidNJ

Autocross does maybe 5 minutes per event.

Track days could be similar. I believe they have 2 hours per event. However, anyone who drives that hard in a track day should be doing some regular engine maintenance. They could be using a grind more suited for a 24-hour race though.

It depends on what they are trying to achieve. All these guys are going to 240 cams with rumpa-rumpa idles when for power they should be in the 250s or for street use in the 220s (or less). If they really want the power and maintain street use, an aggressive ramp provides a way to go.

It is the same deal with the hydraulic lifters. If your racing rules require it, fine. You have to go that way. However, if you value low maintenance and low noise over performance, those trade-offs need to be made everywhere.

SStrokerAce

iTrader: (2)

elaborate on the specs or why the lobes are not as aggressive?

All comes down to the relationship of advertised, .050, .200 durations relative to max lobe lift.

Bret

racer7088

Get the cams cam-doctored and then look at the acceleration and valve speeds you get with the actual rocker ratio and rpm you will be running. Like piston speed you have valve acceleration and speed and they both need to be controlled by the valvetrain to make any hig rpm power. The you can test them as some types of lobe shapes work better than others on the actual running engine.

Most cams you can look at advertised minus .050 and you will see the trends. A high rpm Comp Elim. lobe is pretty smooth compared to a medium rpm circle track lobe like the crazy TK lobe. One might be 36-40 between .020 and .050 (Comp 318-6) and the other like the TK lobes might be 28 degrees. The old Dash 30 and Dash 31s were used in Pro Stock and are "you guessed it!" They are 30 and 31 degrees between seat and .050.

DavidNJ

Flat tappets are faster. MAs are 27 deg, MH are 26. MH's are described as for classes having open valve train rules except for the stinking .842 flat tappet. Our rules also prohibit shaft rockers. With 1.8 intake rockers, I think the MAs are more aggressive than MHs with 1.7s. Next I would like to try 1.9s.

Where those gorgeous TKs shine is from .050. Where an MH might be 272-246-159, the TK is 274-246-172. They also have 18-30% more lift.

With solid rollers, shafts, big springs, valve spring oilers, Ti valves, retainers, keepers and maybe a 7000rpm or so limit, I wonder if they would last on the street.

MadBill

Flat tappets are velocity-limited by their diameter, rollers are acceleration-limited by acceptable pressure angles and the diameter of the grinding wheel, so as the duration increases, rollers start to shine for more than just lower friction.

SStrokerAce

iTrader: (2)

Guys the rules of the duration from .050"-.200" are pretty much usless without any information on the max lobe lift. The more lobe lift you have the less aggressive and more easily controled the valve is even if the .200" number is much higher.

The lobe lift in David NJ's examples is MH=.360 and TK=.425 HUGE difference there, all in all it only makes the TK lobe slightly more aggressive.

Bret