How do engineers decide what redline will be?

[twitchy]

When they build an engine, what do they do, or take into consideration to say the redline will be XXXX RPM's??

For example, and I know they arent really comparable, my friend has a Z24 Cavalier :down: with a DOHC 2.4 liter and 150 horses like mine and it redlines at like 5 grand....i dont get it, aside from cam setups what else is there?

[Shin_Kudo]

I think maybe they test the engine and figure out what RPM the engine can redline at and still be reliable. Just guessing, though.

Your friend's Cavalier probably runs a higher compression. I'll bet it has less torque than a 250.

[twitchy]

weird....anyone know for sure? i think hers redlines early to prevent the engine from exploding..its her third engine

[kandyflip445]

Bore+Stroke. Weight of the materials used. Those are two I know of.

[citizen]

simple equation to figure out piston speed and geforces for all the moving components. not sure quite how they figure but it probably something like "with the design and materials we are using factoring in x amount of safety the components can safely handle this amount of geforces/piston speed. i calculated piston speed and geforce for various bore/strokes at diferent interval of rpm, dunno where it went too though, maybe ill go look for it.

[90RS13]

piston speed, and what the valvetrain can handle are factors, but a main factory is as simple as cam selection. If you get a big cam it doesn't idle as well, isn't as fuel effecient and raises the peak power to a higher RPM as well as shifting the whole power band higher, cause the engine can now breath better at higher
rpm's. Factory engines for economy cars especially need to be fuel effecient and Idle good. Therefore smaller cam. Also making the powerband at a lower rpm and so on. Look at the cars dyno when it's stock. Power will start to fall off before or around redline. For a good reason. What's the point in an engine revving to 10 grand when it stops making power at 5. See what I mean?

[citizen]

they also look into factors such as harmonics and lubrication.if i remember correctly mean piston speed for the ka at 7,000rpm should be around 12m/s and i *think* max piston speed was around 27m/s, maybe ill run through the calcs again later because ilost my old ones.

[anthony240]

From Ask Sarah, NissanPerformanceMag.com:

What is involved in stroking an engine? I want to increase my 1991 2.4L KA24DE to a 2.8L. How would I go about doing that?

Confucius say "There's no replacement for displacement." OK, so it wasn't him, and that doesn't include forced induction which can give any sized engine a "boost" in power. But everything else equal (such as efficiency), larger capacity engines will produce more power because they can burn more air and fuel. Displacement is a factor of bore (size of the piston) and stroke (distance the piston travels up and down). You can increase displacement by increasing the bore with a larger piston, increasing the stroke by moving the pin further out on the crank, or both. The more reliable and efficient way is to increase the bore and stroke in equal proportions. Increasing the stroke gives the engine more leverage to rotate the crankshaft, upping torque for greater low rpm power. However, you can only stroke so much before you blow. Piston speed is a product of both stroke and rod. The highest velocity is when the crank to the rod is at 90 degrees. If the rod is longer, 90 degrees is reached further down the bore so it gives more dwell time at TDC (it will be going away from TDC at a slower speed). Plot velocity at crankshaft rotation and it's much less pointy at TDC. Since the piston accelerates from TDC slower, it gives cylinder more time to fill, which means better breathing at higher rpm. By the way, increasing rod length will not increase displacement but it will allow you to accommodate a larger piston that might hit the cylinder head. Peak piston speed is determined by stroke and rpm. Piston seeds increase as rpm rise. With the 96mm stroke on the KA24DE, at redline, peak piston speed is approaching 3890ft/min. You would increase displacement about 100cc for every 4mm increase in stroke length. That 2.8L goal would require a 108mm stroke! To maintain a safe peak piston speed of about 3500-4000ft/min, a 100mm stroke is about as large as you'd want to go. A 108mm stroke would result in a redline of 5900rpm to maintain safe peak piston speeds. Too high a piston speed can lead to ring seal and lubrication issues and a shorter piston life. A longer stroke would need a lower redline. A low rod to stroke length is good for the low end torque but it doesn't breathe as well top end. With a longer rod, the redline could be higher but you'd lose low end power. A larger stroke would also affect rod angularity. Anything under 18% is considered a good number. To calculate rod angle, convert the lengths to inches (165 mm = 6.496" and 96 mm = 3.78") and divide the stroke by 2 and then divide again by the rod sin (6.496 sin). You get a 16.71% rod angle. With a 108 mm stroke, the rod angularity would hover at 18.8%. A higher rod angle leads to excessive wear to the pistons major thrust area and also can cause ring failure and excessive cylinder wall wear. This can also lead to higher oil consumption. You can achieve your higher displacement goal if you overbore to about 91mm, reweld and offset grind the crank, for about 2.7L. Get forged racing pistons such as those from CP or JE Pistons, which can accommodate the OEM piston rings and should be used with Carrillo steel connecting rods with 3/8" SPS bolts rated at 220,000 psi. Otherwise, start saving your money for the rebuild! The engine won't rev well or last very long but it will have a large displacement.

Will an SR20 crank work in the KA24? What about the FJ20 crank? My goal is to build a 9000 rpm turbo KA.

Sarah: You'll need to do a lot more than to destroke a KA24 to rev that high. Using a long stroke (hehe), the block would break in half around 8000 rpm due to engine harmonics. A 72mm stroke could produce low peak piston speeds that may get your motor to around 9000 rpm, but you'd also need custom valve springs, thicker rods, forged pistons, cams, and keeping your fingers crossed. No KA24 cams on the market right now could handle that type of high rpm running. While the bore centers between the SR20 and KA24 are close and the shorter 86mm stroke could produce an extra 1000 rpm of racing happiness, getting the SR20 crank into the KA24 would still require some machining and modifying. Since the KA24 is based on the L-series engine, the L and Z series cranks would take less machining to fit in the KA24 and come in many different strokes. A L20-B head has an 86mm stroke and is fully counterweighted but doesn't flow much better than a KA24 and probably wouldn't last above 8000 rpm. The Z20 head has the same bore and stroke as the L20-B (85 x 86) and the Z22 would allow you to keep a higher displacement of 2289cc than the other 2.0 crank choices with only 2140cc, but it has a longer 92mm stroke limiting you to about 7000 rpm. Although the JDM FJ20 has a smaller 80 mm stroke, the crank's bore center is too far off to consider. Also, finding one would be like looking for a hooker at a 5 star resort- rare and not cheap. There is a KA20 made in Japan with 86 mm stroke that would bolt in the KA24 producing a 2140cc engine with lower peak piston speeds, allowing you to rev to about 7500 rpm, but still nowhere near your "goal."

That's more than you need to answer your question, and any other you have, it did for me.