Okay. Theoretical Camshaft Question:

I understand the theory behind using dual pattern camshafts, but apparently not well enough.

1. I have been told that TPI engines are less sensitive to changes in exhaust duration (compared to intake duration) than other engines. Is this true, and if so why?

2. Under what situations (if any) is a single pattern camshaft actually advantageous? There seem to be a high number of them around, some of them with quite impressive torque curves. If using more exhaust duration to clear out the combustion chamber is helpful, then why choose a single pattern?

Thanks in advance,
Willie C.

 

It's simple, dual pattern camshafts are for when you're exhaust flow needs some help. You want your exhaust port to flow at lease 70% of your intake flow, if it doesn't you can use a dual pattern cam to help make up for this. This is even more important with power adders (super/turbo chargers and nitrous) because you have a lot more exhaust that needs to flow, and while the power adders help shove air into your cylinders, they're not nearly so helpful at shoving it out, especially nitrous. That's why the cams you see designed especially for power adders have dual patterns, and a lot have shifted centerlines and amounts of overlap.

A lot of people have had success with using 1.5 rockers on the intake and 1.6 on the exhaust, in effect making the cam dual pattern, or making an even larger variation in the pattern.

Nothing wrong with single pattern cams, the HOT cam is one I believe, just a matter of optimizing the cam to go with your engine combo. So basically if your intake and exhaust flow are already well balanced then a single pattern cam is the way to go, otherwise your probably pushing unburnt mixture out into the exhaust, overscavenging the cylinders.

Hope this helps.
John

 

Is it then harmful to use a dual pattern cam if you already have adequate exhaust flow? or only unecessary? I only ask this question because after looking at probably in excess of a thousand camshafts, I think i've finally settled on an intake lobe and possibly a separation angle that looks good to me for the LTR stroker I'm working on. My only question is how to select an exhaust lobe to compliment it.

While I'm on this subject, if anyone knows of any good book that deals exclusively with camshaft selection, I'd be eternally in your debt. All of the information I've found in automotive books amounts to only a section here or there on camshafts, which doesn't tell me anything I don't already know from looking at different motor combos and how they behave. I'd like to get as optimum a combo as possible here, so I'm interested in knowing a little bit more of the physics behind camshaft selection.

On the other hand, if the difference between one exhaust lobe and another is no more than 2 or 3 horsepower at the crank, I wont sweat it, I'll just find a lobe that I've seen work well in similar TPI engines. I plan on getting recommedations from TPIS, Comp Cams, and Crane before I order anything, but I'm interested in knowing enough beforehand to put whatever they tell me in some context. I honestly haven't seen a grind yet that I don't think could be improved by using less or more lift, or closing the lobe separation a little.

Four or five grand is a lot for a poverty-stricken college student to put into a motor, so I'm understandably ****. When it comes to my car, this IS rocket science

Thanks alot, ya'll
Willie C.

[Okay, when I went back and read reply a second time I think it helped me more than I originally thought. I'm going to go do some more shopping, but feel free to drop off your opinion on camshaft theory if you have one.]

 

Oops, did forget to mention that, the problem with camshaft selection is that they are so dependent on engine characteristics, that no one book will cover it all. Best bet is to call all the manufacturers, give them your engine AND tranny AND rear gear AND tire size AND intended usage info, they will then recomend a cam, get the cam numbers then punch the info into desktop dyno (well worth the $40 purchase price) and compare. In my experience they will all reccomend similar cams (provided you give them all the same info) and the biggest difference will be finding the one with the best price.
If you're going to stick with the LTR setup they'll probably suggest something with shorter duration and less overlap to build power below 5500 rpm. If that's the case you'll want to have a higher (lower numerically) rear end ratio or you'll have trouble putting power to the ground.
Good luck.
John