Camshafts with an odd engine
Camshafts with an odd engine
(OP)
Well, I've been working with a Nissan KA24DE recently. 2.4L DOHC 4v/cyl 89x96 Inline 4, 9.5:1 compression ratio. Now, the undersquare aspect is a bit odd considering I'm trying to extract high-RPM power. The reason why I'm trying is that the engine redlines at 6900rpm stock, yet the stock power peak is at 5600rpm, and the engine goes dead after 6000rpm or so. The torque peak is waaay down at 4200rpm, and drops pretty fast afterwards. The other obstacle is the intake manifold. It had a set of secondary butterfly valves (now gone) to assist low end torque, and the runners are also very long (about 18").
Obviously, the first thought would be to change the cams. There are a few aftermarket makers of cams for the engine, but neither set of cams gives very good results at all. They gain what you'd expect from just increasing the lift (5-7 horsepower, no real change in the locations of the power/torque peaks).
Now, as I understand it, the purpose of the longer runners is to increase low end torque by creating a supercharging effect by getting the intake charge moving at a higher velocity, but the problem is they may restrict it at higher RPM because they don't build velocity fast enough to get a good cylinder fill. However, wouldn't it stand to reason that having a really late intake closing point would vastly improve high-end power with this setup, since the intake velocities are higher due to the long stroke and long runners? Some people have tried replacing the manifold with a plenum-less ITB setup, and that didn't have an affect on power by itself. So I figure something has to be going on with the cams, and I'm guessing that the aftermarket cam makers are probably used to making cams for more conventional high-performance motors, hence the poor results.
With all the hearsay and misinformation on cams that you can find everywhere, I'm looking for a more professional angle on this problem. Hopefully I'll find one here.
Obviously, the first thought would be to change the cams. There are a few aftermarket makers of cams for the engine, but neither set of cams gives very good results at all. They gain what you'd expect from just increasing the lift (5-7 horsepower, no real change in the locations of the power/torque peaks).
Now, as I understand it, the purpose of the longer runners is to increase low end torque by creating a supercharging effect by getting the intake charge moving at a higher velocity, but the problem is they may restrict it at higher RPM because they don't build velocity fast enough to get a good cylinder fill. However, wouldn't it stand to reason that having a really late intake closing point would vastly improve high-end power with this setup, since the intake velocities are higher due to the long stroke and long runners? Some people have tried replacing the manifold with a plenum-less ITB setup, and that didn't have an affect on power by itself. So I figure something has to be going on with the cams, and I'm guessing that the aftermarket cam makers are probably used to making cams for more conventional high-performance motors, hence the poor results.
With all the hearsay and misinformation on cams that you can find everywhere, I'm looking for a more professional angle on this problem. Hopefully I'll find one here.





RE: Camshafts with an odd engine
RE: Camshafts with an odd engine
RE: Camshafts with an odd engine
To start, the intake runners would appear too long. Something more in the line of 14 inches from the bell of the intake to the center of the intake valve (remember, it moves) should get close. All depending on port shape, size, runner shape and size, etc. Lots of variables that you must determine on a flow bench. Of course, the exhaust side is critical to the end result, that should be a given.
Camshaft design, acc rate, lift, duration, etc.--- there are some very good companies that do OHC cam designs and given the right flow numbers and engine specifics can, no doubt, get pretty darn close. The great part of a DOHC design is that you can have extemely high acc rates unatainable in a pushrod design (yielding good mid AND top end) and, you can adjust the timing specs for both the intake and the exhaust independantly---makes it easy to experiment.
The specs for a Cosworth engine from the 1980's I have here in front of me are :
Intake 276 @ .050"OBC open 28 btdc close 68 abdc lift .451"
Exhaust 280 @ .o50"OBC open 70 bbdc close 30 atdc lift .434"
nominal installation is 110 degrees but I set these at
106 intake anf 109 exhaust for a power band that ranged from 5500 to 8500 with best performance 7000 to 8000.
We were able to get something like 0.12 hp/cc with very little effort on a proper high compression race engine.
Take this all just FYI. Do not assume that these specs will even be close for your engine. A trip to the library for some of Caroll Smith's books to start???
Rod
RE: Camshafts with an odd engine
RE: Camshafts with an odd engine
http://ka2
Keep in mind the engine redlines at 6900. That graph doesn't even go past 6000.
http://www.pdm-racing.com/products/DOHC_stock.html
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Those are the dynos from the cam install, as opposed to "stock", where stock is a modified intake system, a header and a lightened pulley.
RE: Camshafts with an odd engine
You might try "Design Products Racing" in SoCal (Orange Cnty)714-892-1513 and ask for Don. He raced a circle track 240SX (tube frame) car for several years as well as a 510 in SCCA and has a good grasp of performance data for Nissan engines.
Rod
RE: Camshafts with an odd engine
RE: Camshafts with an odd engine
Rod