|The MGA With An Attitude
Considering a CAM UPGRADE and/or SUPERCHARGER - PP-203
At 08:57 AM 12/3/04 -0700, Joshua Juel wrote:
>"I would like to upgrade my camshaft next summer but still want good drivability and something that wouldn't need to be tossed if I wanted to pick up a Moss supercharger kit in 4 or 5 years."
Oops. Slow down there. These are significantly opposing needs when it comes to cams. For a naturally aspirated engine you can increase thermal efficiency a bit with higher compression. That can give a tad more power and/or a bit better fuel economy, as long as you don't mind the increased cost of premium fuel (to prevent detonation).
For increased peak power (at high speed), you can change cam timing to have more valve overlap, allowing more time for both the intake and exhaust strokes. At lower engine speed this is detrimental to torque and power and efficiency. So there is a drivability and efficiency tradeoff for increased peak power output. The more power you shoot for the harder it is to drive around at low to moderate speed on the street.
Increasing valve lift and/or enlarging the ports can also increase air flow and power at high speed, but larger ports make for slower air flow at low engine speed, which can create some additional problems with drivability.
Increased valve timing overlap for more power at high speed also shortens the effective stroke of the engine at low speed. Lots of valve overlap means both intake and exhaust valves are open at the same time during the top part of the stroke between exhaust and intake strokes. At low speed this allows undesirable exchange of gases between the exhaust and intake, either blowing exhaust into the intake port or wasting fresh fuel and air out the exhaust port. It also makes for lost air and low compression at low speed when the intake valve is open far past bottom dead center. For this reason it is common for a performance cam to want much higher compression ratio. The fast street cam comes with a recommendation of about 10:1 for compression ratio. A full race cam might like 12:1 compression ratio.
On the flip side, for a supercharged engine there is no need to use a radical cam. In fact the supercharger works better with a stock cam or only a very mild upgrade cam where you won't lose so much of the pressure charge to valve overlap between strokes. And the supercharged engine can be made to produce more power with lower compression ratio. There is a practical upper limit on combustion pressure before you run into detonation. Using lower compression ratio you can spin the supercharger faster and cram more air into the engine before you reach that critical point of detonation.
So while a naturally aspirated engine likes high compression and more valve timing overlap for peak power at high speed, the supercharged engine likes low compression and much less radical valve timing to make peak torque at low to moderate speed. If you want something like 140 HP, a supercharged engine is very friendly and easy to drive on the street, vs. a full race engine that is unruly and only performs well at high speed. The demands for a camshaft are totally different. So if you have in mind to install a supercharger in the near future, you don't want to go very radical with the camshaft.
"I realize the Kent cams are regrinds whereas the Crane ones are newly manufactured, but do you have any critique between the two?"
Not much. I personally prefer the new cam vs. a regrind, mostly because I have heard too many horror stories about reground cams failing early from worn cam lobes (probably a fault of improper surface hardening). That's not to say that all reground cams are bad, just more likely to have such problems. The new forging for the Crane can also allows for more lift without reducing the base circle. That also makes for a less sharp peak on the cam lobe, which can only be better for reduced wear.
"Also, what other parts should I be looking at if I go to the lengths of switching out camshafts in your opinion?"
Any new cam requires new tappets. When you are buying new tappets, it is a golden opportunity to switch to the shorter lighter tappets of the later 18V engine (and matching longer pushrods) to reduce the mass of the valve train. This allows higher revving without valve float, and reduces the requirement for extra strong valve springs to accommodate higher valve lift or engine higher speed.
Also be aware that many uprated cams require a change of cam timing. This does not necessarily require an expensive adjustable cam sprocket. If you know where you need to go with the cam timing you can usually get there with some combination by skipping the timing chain over one tooth and/or installing an offset key for the cam sprocket (a $6 part).
"I don't plan to tear down the whole engine just now but at the same time don't want to overlook anything while the engine is out of the car."
So what is your intent? Are you building a race car, or tying to maintain a mild mannered street machine? The best combination to have both at the same time would be a supercharger with stock camshaft and low compression.