forged 2.4 pistons or cast?

[franky240z]

Trying to work out if I should go for forged or cast pistons.

 

Crank - lightened and balanced(dynamic balance too), PSI damper

 

cam - Intake Duration at 050 inch Lift 264

Exhaust Duration at 050 inch Lift 264

Duration at 050 inch Lift 264 int./264 exh.

Advertised Intake Duration 306

Advertised Exhaust Duration 306

Advertised Duration 306 int./306 exh.

Intake Valve Lift with Factory Rocker Arm Ratio 0.530 in.

Exhaust Valve Lift with Factory Rocker Arm Ratio 0.530 in.

 

carbs - triple 40's

 

Block - original 2.4(+1mm)

 

Ignition - mallory unilite dizzy and MSD ignition

 

head - e31 or e88 both will be big valve with light porting

 

Exhaust - 6 branch nimso long style

 

flywheel - standard lightened.

 

Compression 10.5:1 or upto 11.0:1(95 or 98 ron european fuel).

 

 

Basically i'm thinking with cast pistons i'm limited to about 6800rpm, if I went down the extra expense of forged what would it really gain me?

[NewZed]

Higher RPM.

[franky240z]

Yes but how much really, whats the safe limit for cast(balanced ones mind)?

 

So more rpm and another 10bhp?

[NewZed]

Sorry, you caught me goofing around.

 

I searched "cast pistons" and found a lot of reading, it's a fairly popular topic. Here's a good thread - http://forums.hybridz.org/index.php/topic/108206-broken-ring-land-lame/page__pid__1015399__st__20#entry1015399.

 

The extra power you'll get from more RPM would depend on the other components. The pistons just let you take advantage without breaking.

 

Piston decision should probably come after you know what the engine design is capable of.

Edited October 12, 2012 by NewZed

[ZIPPY]

That is a huge cam you have there, looks to me like it should peak right around the 8000rpm range. To really take advantage of that cam you will be revving well past the pratical limits of cast pistons. It sounds like you are putting some money into this engine, and a piston coming apart at 8000 rpm would utterly destroy a very nice build.

With forged pistons you will be to fully enjoy your high rpm horsepower with the peace of mind knowing that it won't scatter itself all over the place. Extra cost for pistons now, or extra cost for a new engine later? Or rev limit an 8000 rpm engine at 6800 rpm.

I took my high compression cast piston engine to nearly 8000rpm on the drag strip a handful of times, but I got lucky and the only thing that happened was my pistons started knocking their skirts off.

Also I would get as aggressive of a port job as you can afford. You have a very nice exhaust and intake system, and you are already putting big valves in the head... Don't settle for light porting.

[franky240z]

I might come down on the cam a little, I'd prefer a peak at 7500rpm. Which I guess is still outside the limits of cast? Its not a race car so won't be sat at that rpm all the time.

[PMC raceengines]

The carbs will kill the fun before the cam, the cam will make power from 4500 to 7500 with 45carbs and will go to 8 ,

i have that size cam in lots of engines , one with 48mm carbs that we rev to 9000 , so with your carbs id come back on the cam to about 280d and keep it cast piston make good torque and power at 6000 , if you want more go forged piston and run more cam and carbs

[franky240z]

I would have thought that 40's with big chokes would be a good choice for a 2.4, keeping gas speed up and a bit of smoothness at lower rpm?

[madkaw]

Frank,

i feel that I boxed myself in a little sticking with cast, so it will limit my cam selections if i want to upgrade from my stage 2 Schneider cam. With that said, I might contact the more L-6 engine friendly builders to profile my next cam to take full advantage of the cast limitations. I have a read many posts declaring 7k to be the limit, and I would want a cam that started tapering off right at that limit.

You will be pleasantly suprised of the torque of the little ole L24, especially if you take full advantage of it's power-even at the cast limits. I guess it depends on how deep your pockets are and whether you have to have new pistons or not.

The only plus for me with having cast is if I decide to pressurize, the cast will probably be a better choice.

Don't forget to notch those bores!!

[JCan]

For what it's worth.

 

Back in the day, my brother and I used cast pistons on our '72 240z(s), and I chose them for the rebuild I am doing right now with my son. My brother's car was very balanced. He raced through 8k RPMs all the time (street) and didn't have a problem the thing ran like a top! His engine was 11.5:1 2.8:block, flat top pistons, 0.327 lift and ~280 duration. Intake, tripple webbers, stock exhaust manifold mated to a 3" exhaust pipe (helps out at the high end I think). We used chromium rings (require more time to seat, but once you do they last a long time!). We used a 4 row radiator to keep engine cool.

 

On my car, I also used cast pistons in a 2.8L motor, turbocharged to 14PSI. I did break my pistons once when street racing a porche turbo. ( My car pre-detonated in the middle of the race (I ran out of water), after engine disassembly, I found vertical cracks along the side of 2 pistons which started at the top of the piston side and ran through the first and second ring (including cracking rings). I purchased a new set of pistons, checked the rods and crank (they were fine),and reassembled the engine). This is the only time I ever broke a piston. My car was a daily driver and I liked street racing.

 

One last thing - Back in the day, Forged pistons were noisy as they clacked against the cylinder walls. The majority of this noise went away once they warmed up. I hear today's forged pistons do not expand as much as a function of temperature. If you are not building a car designed for racing only I would save the money and use cast pistons. They are inexpensive, quiet, will take 8k RPMs as is my experience.

 

FYI - Jim

 

PS - Try cast, if you break a piston, no big deal, Right? This car is so easy to work on, 10 days and you'll be back on the road.!

 

PSS - it is fun to race next to guys driving V8s, once you start pulling away and continue revving through 8k RPM. In the heat of the race, some of your opponents will try to copy (let the engine run to high revvs) with sometimes disastrous results....

[johnc]

The carbs will kill the fun before the cam,

 

Exactly! You don't need to worry about cast or forged pistons, the carbs won't let you run a high enough rpm to matter.

I would have thought that 40's with big chokes would be a good choice for a 2.4, keeping gas speed up and a bit of smoothness at lower rpm?

 

You need to start over.

 

Carbs work via a venturi effect - a difference in bore diameter between the intake of the carb and the narrowest part (the venturi). A DCOE 40 with a 32mm venturi will be less tunable and less effective then a DCOE45 with a 32mm venturi.

 

Also, since you cam is designed to make power above 4,000 rpm why choke it off by purposely choosing a carb to enhance power below that rpm number?

[franky240z]

All Good points, I was coming back to this to say I've had a think about the cam and how I want the engine and have decided that 7000rpm ish will do, and using this cam which from all my research and info from here should do the job.

 

Part Number: 17036

Grind Number: 290F

Intake Duration (gross): 290

Exhaust Duration (gross): 290

Intake Duration (.050â€): 244

Exhaust Duration (.050â€): 244

Intake Valve Lift*: .495"

Exhaust Valve Lift*: .495"

Lobe Separation: 106

Intake Valve Lash: .008"

Exhaust Valve Lash: .010"

 

Should make good power/rev to 7k and work well with the carbs etc I've got?

 

ETA

 

John as a side note, how far should your subaru diff adapters stick out once fully home?

 

I guess the debate is, is it worth the extra $350 for forged pistons to future proof my engine? I guess they'll help the motor spin up a little quicker too.

Edited October 16, 2012 by franky240z

[duragg]

I went with ITM Cast and inclueded my Cam chart below. Dyno's 192 at 6900 (6700 is all now that I look back) rpm on DCOE40s.

 

Do I kick myself for not future-proofing the engine? Not really. 7000rpm is fairly lofty for street use and I think the lower RPMs would be even worse for what? 7500rpm?

If I redid anything I would put in a stroker crank with 240 rods and do the pistons then for the bigger bore anyways so it would have been a waste.

Then I would do the bigger DCOE or a GSXR ITB setup which is all just a whole new process of un-screwing myself with all new crap.

 

They are probably just fine for an NA motor.

2011 Dyno N240ZZ.pdf

Edited October 16, 2012 by duragg

[franky240z]

Yours is a 2.8 too, I'm sticking with a little 2.4

[madkaw]

That little L24 will surprise you done right- even as far as torque!

[ZIPPY]

I think the 290F cam looks a little better. Keep in mind that the advertised power band is still 3500-7500rpm. In your case with the L24 I imagine the powerband might be a little higher than the advertised. Unless your going to be living in the 7000rpm range is it worth having a cam that comes in at 3500+? The 270-80F grind makes some real power in the upper ranges (200hp+ with SU's) but is also relatively streetabel at normal driving rpm.

 

 

ITM flat tops. Hit 7900rpm fairly regularly. I think they might have very lightly kissed the head though. For cast pistons, I think they are very well made pistons.

 

 

[JCan]

Zippy. Nice pistons! how did you do it? did they really kiss the head? what was your piston to head clearance?

 

I am building a motor now, 2.8 with flat tops. Measured piston to head gap is 0.030 to 0.026. P90 head, This is the first time I have have designed the engine to have squish.

 

Franky. I contend that you should continue to use the cast pistons if you are going to use the car for street and if your cam does not have intake/exhaust valve overlap (a mild to close to medium cam).

 

Jim

[Leon]

Franky. I contend that you should continue to use the cast pistons if you are going to use the car for street and if your cam does not have intake/exhaust valve overlap (a mild to close to medium cam).

 

Jim

Every S30 cam will have some overlap.

 

Cast is fine for a street engine. I'm with ZIP, the cam mentioned looks better but will have a fairly high powerband on an L24. Call up Isky and talk with them, but make sure you have solidified your goals for the build. Otherwise you'll be wasting their and your own time.

[Gollum]

I'd go even further and say cast is fine for ANY NA engine that won't live on a track. I wouldn't hesitate to take cast pistons to the track as long as I knew it was built right and hasn't exhibited issues.

 

Brian Blake, aka 1FastZ, one of the best L engine builders of the NEXT generation (read, he's not an old guy who's been doing this forever, but a young up and coming), PREFERS hypereutectic pistons in many of his builds. I've owned cars with forged pistons and know plenty of people that do. They're not "fun" and most certainly this is true when you've built the bottom end to take 200+hp per liter.

For what it's worth, brian used cast pistons in his 3 liter that he revs to 8k on a regular basis. I really don't see RPM as a huge issue for cast pistons in a 2.4. As long as your bottom end is well balanced the only thing to break a cast piston in a L motor should be mean piston speed, or detonation. Detonation will be unlikely at 7k or above, unless something is severely wrong, and mean piston speed of a 2.4 would be pretty high when comparing to a 3 liter able to run to 8k.

[Tony D]

"Every S30 cam will have some overlap."

All I can say at this point is: "Muahahahahahahahahahaha!"