As we said last month in Part 1 of this Ask The Experts series, there were so many good, solid questions we received for Isky Racing Cams Chief Operations Officer, Nolan Jamora, we had to expand it to two parts. In fact, there were still some questions we weren’t able to include — simply due to space and time constraints (if you need a tech question answered, you can always check out Isky’s tech page, or give their tech line a ring). In this final installment of the Isky Cams Ask The Expert series, we tackle lobe separation, valvesprings, valve opening speed and much more. Get ready to absorb some knowledge and read on.
1. I’m wondering why the newest generation of engines, like the LS platform, tends to run such wide lobe separation angles like 118 degrees. The remarkable thing is they maintain high power output, good torque, etc. Following this example, one might think a natural progression would be to use such a grind in a well-built Gen-I motor to achieve the desirable effects like good idle and vacuum while still making power, but I don’t see this trend happening. – Ben W.
The LS engine series, some 40-50 years post-SBC, takes full advantage of modern advances in engine design. First and foremost, electronic fuel injection (EFI) guarantees equal fuel distribution to all cylinders — an advantage that has proven to be unattainable throughout a carbureted engine’s RPM range. Couple that advantage with the LS platform’s modern cylinder heads — which have been extensively researched for port-flow and valve-angle optimization — and you have an unbeatable advantage over the old small-block platform.
This combination helps the engine achieve the high power output and good torque that you mentioned. However, in order for the EFI sensors to properly calibrate the air/fuel mixture, a strong vacuum signal necessitates reduced valve overlap, hence the wider lobe separation angles (LSA) like 116- to 118-degrees on longer duration cams.
Now to be honest, I’m certain that if you could get the engine to run properly (maintain air/fuel mixture) at higher valve overlap, you would likely see an even greater mid-range torque and overall performance increase by reducing the LSA down to 108- or 110-degrees. But, this would mean either running a carburetor or waiting for a future advancement in EFI air/fuel calibration to come along. As for the old small block catching up to the LS, EFI units are available and if someone were to make an LS like head retrofit, who knows what might happen?
Revving at least 10-percent, or even 500 rpm above the power peak before shifting ensures that when you drop into the next gear you will be closer to the torque peak and primed for faster acceleration to the next shift point. – Nolan Jamora, Isky Racing Cams
2. My question is for those of us who are currently running flat tappet hydraulic cams in our hot rods and looking to change to a hydraulic roller setup. When spec’ing a replacement roller cam and staying relatively close to our current duration numbers (at .050-inch), how much of a power jump can we expect, and in what part of the RPM band, just by going to a roller? – Pete T.
A good rule of thumb is to look at comparing the hydraulic-roller cams to a similar flat-tappet hydraulic cam in this way: Within the confines of the same duration at .050-inch lift, the hydraulic roller will allow you shorter seat-to-seat or running duration numbers. This will buy you some better off-idle throttle response and higher engine vacuum.
Alternatively, you can achieve higher power output if you maintain the seat duration numbers and gain area under the lift curve with more peak lift and greater duration at not only .050-inch, but all the other higher-lift area checkpoints (.100-, .200-, and .300-inch lobe lift) as well.
3. Is there a relationship between peak piston-speed and peak cam-lift for the intake and/or exhaust? I expect you would have to get into some clever up and down ramp pairings that are different on the intake and exhaust. It looks more complex for two-valve, poly, or wedge engines where there is a dramatic difference between intake and exhaust camshaft events. – Bob S.
Although there has always been a curiosity about a connection between peak piston-speed and peak cam-lift, the magic number you’re looking for, unfortunately, does not exist. It helps to relate it to spark advance timing. Just because the hemispherical combustion chamber requires less spark advance due to the efficiency of its design, than say the wedge chamber (maybe 10- to 15-degrees less in some cases), it doesn’t mean the higher or lower number is “better.”
The only thing that matters is to find the number the engine likes. Leading the spark a certain number of degrees merely coordinates the maximum expansion rate of the burning fuel/air mixture with maximum leverage advantage in transmitting this force through the piston/rod to the crankshaft. That number varies from engine to engine. In some respects, advancing or retarding the cam timing mirrors the spark advance quest.
4. The internet abounds with experts talking about how the intake valve can be opened too fast (apparently as opposed to too soon) and lose “the proper depression in the chamber for continued filling after BDC.” Hearing “the proper amount” of anything makes me twitch. What are your thoughts on that theory? – Shawn W.
There is a lot of good information on the internet that makes you think. Unfortunately, it often is incomplete and leaves you hanging out on a limb. For example, if you were to find a blurb about say, brain surgery. What good would it be to know the surgeon’s thoughts on how to navigate your incision to avoid damaging delicate brain tissue, if you had no idea how to proceed once you were in there?
From experience, I can tell you that the phenomenon you referred to seems to have merit. However, theoretically pinning any of this down to an absolute value is an extremely difficult task. It all has to be sorted out via trial and error.
However, I will relate a composite of numerous customer-input stories from through the years, which seem to validate the general theory. In oval track racing, the very best drivers are so “dialed in” to their cars and engines that without ever having been informed, they can actually sense a lag in throttle response after the crew makes no other change other than to swap out a set of 1.6:1 rocker arms for the standard 1.5’s. True stories, many times over and you be the judge!
5. What are your views on using mechanical roller lifters in an engine that will see 100-percent street use? What are the pros and cons of mechanical roller lifters? I have heard the needle-bearing lifters can fail after less than 5,000 miles of use and cause a catastrophic failure of the engine. – Brian D.
Mechanical Roller Lifters can be utilized in street-only, high-performance vehicles, but in the vast majority of cases, only when they incorporate needle-free lifter bearings – as with our EZ-Roll lifters. In order to understand why needle bearings can (and do) fail in as little as 5,000 miles or less, it is necessary to understand the needle bearing’s limitations and what makes the needle-free EZ-Roll Bearing a superior choice.
Mechanical needle-roller lifters are particularly vulnerable in a street engine not merely as a result of all the low-speed operation, but because of the stop/start “skid” action of the needles every time the lash is taken up. So in four-cycle engines making 10,000 crankshaft revolutions this cycle (known in engineering circles as needle roller SKEWING) is repeated 5,000 times. As it repeats again and again, the needles and their corresponding bearing races gradually erode. This erosion produces an increase in diametrical clearance in the bearings.
Once this diametrical-clearance increase goes past .001-inch and approaches .0015-inch, the bearings are prime candidates for catastrophic failure (fracturing of the outer bearing wheel with needles and fractured races scattered like a frag grenade). The higher the valvespring loading and more aggressive the camshaft action, the sooner this will happen.
You don’t find this phenomenon occurring with hydraulic roller lifters for two reasons: A) They generally run lower valvespring loads. B) Hydraulic Roller lifters are pre-loaded to their respective cam lobes. In other words, unlike mechanical rollers which go from an unloaded to loaded condition instantaneously, hydraulic rollers ramp up from a pre-loaded to a fully loaded condition, and never experience needle roller skewing when adjusted properly.
So the bottom line would be on anything except a very mild mechanical roller camshaft and valvetrain, run the life-extending needle-free EZ-Roll lifters. They will give you the peace of mind you can’t achieve with needle roller bearings on the street.
6. I have read you should spec a cam that makes power 500 rpm past your target peak RPM. Why is that? – Arlene B.
The advice to spec a camshaft that makes power 500 rpm past your target peak RPM is a variation of the advice-of-old to run the engine 10- to 15-percent past the power peak. The latter is the proper advice for this reason: If you only ran the engine up to the power peak before shifting, you would drop too far down on the torque curve (below the torque peak), and therefore, would have a more difficult time accelerating up through that gear.
Revving at least 10-percent (or even 500 rpm) above the power peak before shifting ensures that when you drop into the next gear you will be closer to the torque peak and primed for faster acceleration to the next shift point.
I don’t necessarily agree with what you read, because it can easily lead to over- camming the engine. Unless of course it’s designed for future allowances to run a lower rearend ratio and eventually running the engine RPM higher.
7. Is there any advantage to be had from running lighter valvesprings on lighter (titanium) exhaust valves? – Graham B.
If by lighter, you are referring to the spring loads (pressure numbers), then the same advice applies to lighter exhaust valves as with lighter intake valves: You can run the same spring loading to gain an RPM advantage. Or, you can lower the valvespring loads — to merely account for the lower valve mass — to run at about the same RPM level as previously. Just make sure that those lightweight, titanium exhaust valves can “take the heat.”
8. How much twist is in a V8 camshaft from the front drive to the back of the cam, where several lifters are pushing against rotation during lift, and in the direction of rotation during closure as it rotates at high-RPM? Is the twist a variable with RPM or random? Is it enough that it may affect piston-to-valve clearance in tight engine setups? – Robert S.
Four factors can affect how much a V8 camshaft can torque-up or “twist” from front to back during operation: The body or core diameter, the valve-open spring force, the length of the camshaft, and the quality of your crankshaft’s torsional vibration dampener.
So, it’s actually not possible to answer this question unless you know all four of these variables. However, if your question is in the context of “in an extreme case” (for example a SBC V8 camshaft with a small diameter camshaft base circle for rod clearance in a stroked engine and/or an extremely high lift cam lobe), the answer is approximately 2-degrees in either direction.
To answer the latter part of your question, yes, in tight piston-to-valve-clearance situations, this can definitely be enough to get you into trouble (all the more reason not to cut things too closely). This twisting effect is actually the worst in oval track racing where the engine can experience multiple up/down episodes per lap. That is where you really need a high-caliber crankshaft harmonic dampener!