Harold,

Say I have an engine that runs great with a 110 LSA cam. I then install the exact same cam with a tighter LSA, 106 for example, could I potentially have a problem with too high of a cylinder pressure? ie: detonation

This next one is dependant on your answer to the above question. Would it be beneficial when building the motor to leave some 'tuning room', such as swapping cams with different LSA's, with the compression ratio? I am referring to a pump gas street/strip type motor. I'm guessing that if the motor was on race gas you could swap cams to your hearts content.

Rod

I guess in Team Chevelle speak this is all about DCR. I'm kind of wanting to hear what some of your experience is in using this as another way to tune an engine combo.

Rmchevelle,

The simple answer is, yes, tighter LSAs raise the cylinder pressure and the DCR, among other things. If you intend to try different cams in a moderately low-CR engine using pump gas, it might be a prudent thing to sacrifice a 1/4 to 1/2 point of CR for peace of mind. This is particularly true if you want to investigate tight LSAs, like 106 to 104. Going wider in LSAs, from 110 to 112 to 114, lowers DCR, as well as giving flatter torque curves.
He is an example you wouldn't have thought of: On large(420-434-441-454) SBCs using alcohol and very high(14-15:1) CRs, a short duration cam with a tight LSA, say 257/265 at .050, 104 to 106 LSA, may have SO MUCH DCR that the ignition can't fire the charge when around peak torque, which is the point of peak cylinder pressure. The CR has to be dropped to under 13:1, or else a much bigger cam run to kill off the cylinder pressure.
I defer to Pat Kelly on points of DCR, he is the expert there.

UDHarold

Harold,

Why would D2K indicate that a little-bitty solid cam made like 250/210, 104/(installed at) 108, .440"/.440" gross lift will make a very flat torque curve and hold it from off idle to almost 4500 rpm in my 9.75 to 1 CR. 283" motor BUT several of the guy's have said the 283" won't like it???

I'm after a solid cam that does just this in my 283..

pdq67

pdq67,

The cam you're talking about is a 108 LSA, installed 4° advanced to 104 int CL, right?
Look at the R/S ratios. The 283 with the 5.7" rod and 3" stroke has a 1.9:1 R/S ratio. A 350 with the same rod length and 3.48 stroke is 1.638:1.
This means a 108° LSA will act like a big LSA in a 283, and a tighter LSA in a 350.
Big LSAs=flat torque curves.
A 210 at .050 solid lifter cam, even with tight lash, will probably be all done by 4800-5000, because of lack of sufficient duration.
Wide LSAs only give more top-end horsepower if the cam is big enough to support it. Small cams on wide LSAs just have flat torque curves.....

UDHarold

Harold,

Thanks for your response.

Originally posted by UDHarold:
I defer to Pat Kelly on points of DCR, he is the expert there.

UDHarold Sorry to use the term DCR but I knew that someone might respond using that term. I know that your knowledge is based more in terms of cylinder pressure. With your extensive cam designing and grinding experience you've probably had many 'higher end' customers (Professional and Sportsman racers) over the years attempt to tune their engines this way such as the example you gave above.

Rod

Originally posted by UDHarold:

Look at the R/S ratios. The 283 with the 5.7" rod and 3" stroke has a 1.9:1 R/S ratio. A 350 with the same rod length and 3.48 stroke is 1.638:1.
This means a 108° LSA will act like a big LSA in a 283, and a tighter LSA in a 350.


UDHarold Care to elaborate on this? How will an lsa act wider with a longer r/s ratio?

Harold,

No, 250/210 and ground 104/104, then installed on 108!

That's what has me puzzled, why the 104 and then retarding it to 108??

Please explain and I/we really appreciate the lesson..

AND how would your old 267/267--239/239(at .050" lift), 110/?, /510"/.510" gross lift solid cam work in my 9.75 to 1 CR., 283?

I was going write you a letter with a SASE in it asking...

Is it too aggressive for a daily driver compared to the little old -097 cam??

Thanks again.. pdq67

pdq67 & Travis G,

pdq67----Hold onto your horses! I've got a new 256F at .016" that is 223 at .050 and .468" valve lift, .014" valve lash. It should be made in the next 2 months, along with my other new cams.
As far as D2D, I never argue with it, because it does not ALWAYS represent the real world, but instead how it was programmed, and sometimes it will lead people astray, depending upon the inputs and the programming. I suspect it may be looking at a very long power stroke and an early intake valve closing, just because of the 250 duration. I suppose the timing you are using for a symmetrical cam is 17-53-45-25. 53 is still a very short intake closing point, and 45 is a very late exhaust opening point, therefore a very long power stroke.

Travis G---An engine's R/S ratio describes show the piston will pull away for TDC, as well as the point of max piston velocity, and a few other things, such as Pat's DCR. Longer R/S ratios pull away from TDC slower, causing lower port velocities, but then they also hit peak piston velocity later, which helps build high port velocities from that point on.
To compare our 283 and the typical 350, both using the 5.7" rod, we find the 3" stroke 283 has a R/S ratio of 1.9:1, and the 3.48" stroke 350 has a R/S ratio of 1.638:1. The 283 would have to use a rod of 4.914" length for the 350's R/S ratio, and the 350 would have to use a rod length of 6.612" to equal the 283's R/S ratio.
Connected to this is the port volume. If you were to try the same cylinder heads and cam in both a 283 and 350, you would find that the larger ratio of port volume to cylinder volume, as well as the bigger R/S ratio, made the 283 engine a high-rpm screamer, and the 350 a much-torquier engine.
This is true of all engine families.
This is where you depend upon your cam company's expertise, or else you could buy a number of different piston/rod combinations, to see which one worked best with your cam.

UDHarold

So dwell time at tdc is the major factor that influences what amount of overlap a particular rod/stroke likes. The larger rod/stroke ratio the less dwell and the less overlap needed so the cam would be ground with a wider lsa. ?? Exhaust system also plays a major role in how the overlap works from what I hear.

Baddbob71,

Exhaust systems, particularly the exhaust cam, play most of the effect during valve overlap.
If I could tell the amount of exhaust gasses left in the cylinder in that degree before the intake valve opened, and their back-pressure, I could predict how the intake would fill the cylinder. Any engine-builder worth anything has at some time tried the same cam grind, but on 2 different LSAs, in one engine. Let's say you pick a 106 LSA and a 108 LSA. When dynoing, put the intake lobe centerline at the same place for both cams. This gives the intake side the exact same lift curve, for both 106 and 108 LSA. But the exhaust centerline is in 2 different locations.
The engine will have 2 different power /torque curves! The same intake lobe in the same intake center line filled the cylinder differently, according to where the exhaust lobe was placed. At each RPM, there was a different exhaust volume and backpressure right before the intake valve opened, and this gave different velocities in the intake port, and different filling rates.
As far as dwell times vrs LSAs, it all depends on what you want the cam to do, torque curve-wise. I had several SS/LA-record setters in the early 1980s using 102 LSA and about 98° of overlap. One of my popular 540-598 BBC cams has 97° of overlap with its' .772"/.765" valve lift, and 114° LSA.

UDHarold

Thanks Harold,

I see what you mean by the long power stroke!!

pdq67