Anyone know approximately how much H.P. I'll lose with Low Comp Pistons?

[Gerry Johnson]

Here are the weights from a stock 3.0 piston & pin no rings 512gms, this is the weight of a 9.5:1 JE piston & pin with the clips no rings 498gms, the weight of a rod with a pin bushing, rotating is 356gms & the reciprocating is 104gms.
The JE piston has a much shorter skirt and less dome, if it was to have the same amount of skirt I am sure it would be heavier than the stock piston, by the way the skirt is 5/8" shorter.
Also they need to keep the weights close to allow for balancing, to heavy and you need to add weight to the crank shaft, to light and you need to lighten the crank shaft. On a inline motor such as a motorcycle the weight of the piston doesn't make a difference to the balancing. Just thought some of you would like to know, but correct me if i'm wrong.

 

[Arata]

We do rebalance the cranks, it just makes the engine feel smoother.
Thanks for the info, I would guess that the JE piston pins are heavier than the stock NSX pins, is that correct?

 

[Sig]

And by the way, I don't think there are many on the forum who think sjs is "completly out of touch."

Agreed!

 

[Gerry Johnson]

I didn't say you don't balance the crank, you just don't use a bob weight with a inline motor so piston weight is not a problem.

 

[sjs]

Here are the weights from a stock 3.0 piston & pin no rings 512gms, this is the weight of a 9.5:1 JE piston & pin with the clips no rings 498gms, the weight of a rod with a pin bushing, rotating is 356gms & the reciprocating is 104gms.
The JE piston has a much shorter skirt and less dome, if it was to have the same amount of skirt I am sure it would be heavier than the stock piston, by the way the skirt is 5/8" shorter...

Thanks Gerry, I figured someone would be able to give us some real numbers. That’s pretty much what I would expect considering that the forged piston is lower compression (rather than higher as with the domed pistons of normally aspirated race engines I mentioned above). I assume that the lower compression is achieved by slightly reducing the stock dome size, so that’s some savings there. But the skirt is 5/8” shorter? Wow, I didn’t expect that much given the already short 2 3/8” stock height. So they shortened it by over 26%, which combined with a smaller dome saved a total of only about 2.7% overall. Apparently if we compared pistons of the same external dimensions and compression ratio the forged would indeed be heavier, which was my point. Are the wrist pins part of that? Perhaps, but that’s besides the point since they come as a set.

As you say Gerry, with in-line engines such as I build the weight is not important in terms of balance, but of course it still counts plenty in other ways.

 

[sjs]

I do not assume it I know it for a fact.
The reason a forged piston is stronger is not oly because the structure is more dense, but also because the grain structure is aligned.
Metal has a grian structure, just like wood, and just like wood this grain can give it great strength. Try spiliting a piece of wood against the grain it has great strenght, with the grain little strenght, with a cast piston you have little to no grain structure, with a forged piston, that is the whole point, to have a structure.
If you think the piston manufactures do not take advantage of this, you are completly out of touch. You are using a 1969 Datsun piston as your example, try looking at some pistons made in the last 10 years, FOREGED PISTONS are LIGHTER.

So many things to respond to in such a short post. First off, as Gerry’s measurements showed, the miniscule reduction in weight is not from reducing material in areas where strength is typically an issue (pin boss and piston top) as you suggested earlier, but rather from reduced compression ratio (smaller dome for functional reasons) and by chopping the bottom 26% off the height of the piston. Although the latter makes me a bit nervous, I’ll assume that the experts know what they are doing and trust that they aren’t sacrificing longevity in a street engine to get weight back down to stock. But clearly that was the best way they could get there. So, if forged pistons are lighter, it’s because they are smaller, or the stock ones were pigs.

BTW, I am well familiar with the reasons why forged pistons are stronger, but that was not the point on which we disagreed. As I had clearly said, we agreed that forged is stronger. So while your analogy to wood grain may be informative to some people, it had no bearing whatsoever in this debate.

As for being “complet[e]ly out of touch“, I think you will find that I openly qualify many of my statements with the limitations of my own knowledge, including acknowledgement that much of what I “know” is significantly dated and potentially based on long-standing misconceptions. Given your tendency to speak in absolutes, you could probably learn from that.

But my age does give me some perspective, and your comments about the technology of 1969 only shows that you are out of touch with the history of your own subject of interest. Without going into too much boring and mostly irrelevant detail, let me explain that the L-series Datsun engine of that time was quite special. They came with a forged crank, five main bearings, aluminum head, overhead cam, and a 7200 RPM redline. Stock in a recycled tin can grocery getter. The L16 without counter weights was capable of 8k+ with stock rods and pistons. The L18 crank was semi-counterbalanced and the L20 fully counterbalanced, and again forged. I’d rate the stock pistons as exceptionally light and strong even by today’s standards allowing for changes in alloy technology, which is why even the forged Cosworths (you may have heard of them) were heavier. Even at the track, flat-top versions of the cast pistons held up surprisingly well at over 8500 RPM. Combine that with 4-wheel independent suspension, front disk brakes, and lots of potential and it is no fluke that the 510s immediately dominated their classes against pricey competition from Europe. So, insult me if you like, but watch what you say about my favorite car.

 

[DDozier]

Yah, what he said. ^

 

[sjs]

Well, a few seconds searching the Web turned up an interesting "paper" on just this topic. Now I have no idea what his credentials are, but Mr. Nixon seems to speak with authority. More importantly, he supports my position. Let’s just say that I’m more inclined to believe him than the claims of a manufacturer trying to sell their product.

http://www.motorcycleproject.com/motorcycle/text/cows-pistons.html

For those who prefer the short version, here is his summary, but the entire text is worth reading.

To sum up, the cast piston is light and very dimensionally stable. It is found in high-rpm mass-produced engines that are not subject to modification or prone to detonation. The piston is however fairly brittle, and the cost of its manufacture has limited its availablity outside the OEM sources and applications. On the other hand, the forged piston is inherently heavy and less dimensionally stable. It is a good choice for engines in which detonation is probable, and its wide availability has made it the choice of engine modifiers. The special demands of these end users has given the forged piston its own niche in the powersports market. The next time someone tells you how superior one piston type is over another, tell them the truth. Because, as Paul Harvey likes to say, "Now you know the rest of the story."

But hey, he may be hopelessly out of touch as well.

 

[NA1 #2853]

Gerry!!!!! Thank You!

That is awesome, i didnt even think of the weights of the piston/rod combo, and the counter weighting of the crank.

Also, would aftermarket 4340 rods like Crower/Carrillo/Pauter be in the same ballpark weight & strength wise as the stock titanium alloy rods? Looking at the weights of the rods i have used in the past on various honda 4cyls they are about the same weight.

In this next motor i am assembling, i am considering also putting in some good connecting rods..... i figure if its apart i'm going to do everything possible before it goes back together..... like molybendum coating the skirts, and cerimic coating the domes, and if a nice set of billet rods will be stronger, but approx the same weight, then i'll put them in. Reguardless if the stock rods will be good enough for my app, if i can put better ones in then i will. I dont want to skimp anywhere on this project.

-Ray

 

[Arata]

No insult was intented, 1969 pistons are not the same as 2004 pistons.
The information I gave was not just for you as others may be interested also. It is pretty clear you have your mind already made up, and I have no interest in trying to change it.
I would suggest people who are interested, take a look at what OEMs use in racing and there own production high performance engines. I already gave a number of examples.

AutoZine Technical

Let me sum this up:
"3. Forged components
Forging seems very old-fashion, but there is still no alternative way to obtain high-strength yet lightweight parts without it. From Honda Type R to all exotic supercars, FORGED pistons, crankshaft and con-rods are commonly used."
"2) Heavier pistons: while Ferrari uses lightweight FORGED pistons and titanium connecting rods, the W8 can only seek more cost-effective method to cut weight from pistons."

Omega Pistons

Let me sum it up. "The cost of a die-cast piston is considerably less than that of FORGES but their use is limited to road going vehicles, vintage car clubs and motorcycle clubs. Having said that, we have produced Mini pistons (Mini Cooper) for both road and race applications using this process with excellent results."

These guys not only make forged but metal matrix as well
Accralite Pistons

Mr. Nixon, may know more than everyone on this board, or he may not, his web site is devoted to VINTAGE Honda motorcycles, mostly from the 70's.

More of the same: Ferrari F40
The FORGED pistons were redesigned to improve combustion efficiency, and the IHI turbochargers were water-cooled although the Behr intercoolers remained air-to-air. The large tube-steel exhaust system was improved, most notably with a revised wastegate, and by the inclusion of catalytic converters. A horizontally mounted muffler exhausted gasses through three large tail pipes located centrally between the twin rear undertray diffusers.
F40

Honda S2000

Like other Type R, the all-alloy unit has hollow camshafts, FORGED pistons, con-rods as well as counter weight to reduce moment of inertia of moving parts. The VTEC is still a two-stage design, the second phase cut-in at about 5,000 rpm. Compression ratio of 11:1 is also unaltered. What separate it from other Type R powertrains are the bore / stroke aspect ratio and cam profile. Basically the 87mm bore is identical to Accord Type R’s, but the stroke is as much as 6.7mm shorter thus account for the reduced displacement. Measured at 87 x 84mm, we find it is obviously bias towards top end power and rev compare with Accord’s 87 x 90.7mm and Integra’s 81 x 87.2mm.

Honda Indy car
Pistons: Forged aluminum alloy


Ford SVT
The Cobra engine is built on a cast-iron block for strength. Other reinforcements include a forged-steel crankshaft, forged ¡°H-beam¡± connecting rods and FORGED pistons.

Viper
The engine's FORGED aluminum pistons are set in cast iron liners. The aluminum cylinder head features a conventional two valves per cylinder with higher-revving dual valve springs.

There are lots more out ther, Care to check, Lambos, Porsche, all other Ferraris?

 

[sjs]

...The information I gave was not just for you as others may be interested also. It is pretty clear you have your mind already made up, and I have no interest in trying to change it. ...

Oh my, you've been busy. No time to dig through all that now, but with a quick scan I saw a mix of common generalizations and no hard facts. But it isn't accurate to say that my mind is made up. I guess you don't know me yet.

Anyway, I'll have a closer look when time allows, but so far nothing concrete. One quick note, the Viper had forged pistons for just a couple years before going back to cast, and the fact that this car or that uses forged pistons is not particularly relevant. You can’t assume they are used for being lighter weight any more than you would accept me saying they do it just for strength. And things like magazine articles that list the cool parts and choose to lump them all together as being lighter also carry no weight with me (pun intended). Rarely if ever do those nimrods take the time to analyze the significance of each special part. You do seem to have dismissed my particular reference even though he is of a biker background as yourself. And that’s another thing, as I recall, all of your references were for motorcycles (not sure where you got those BTW) and it is no secret that reciprocating weight is hyper-critical in engine turnings very high RPM (like a motorcycle). So if those forged pistons have evolved to the point where they can be paper thin (and shorter?) and still be strong enough, then perhaps they can be lighter. But this is an automotive site and I still haven’t seen strong evidence that forged replacement pistons are typically lighter than the better cast ones. Not that my mind is made up.