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

[paulviriyapan]

I am having Gerry install JE low compression pistons (9.6:1) in my engine rebuild, in the event I want to run a turbo or nitrous at a later time. My question is, how much h.p. will I lose compared to stock (when I'm running N/A)? Gerry and Ramon estimated about 5-10 h.p. if anything, but I was wondering if anyone has any experience with it. Thanks.

 

[4AU4LIFE]

Unless you are certain that you are going to be running FI later down the road I personally wouldn't change the compression.

If you decide later down the road to sell it for some reason that could be a major hurdle for someone to overlook unless they plan on going FI on it.

I recently had my engine rebuilt and was trying to get talked into going low compression but chose not to. I am very happy now that I didn't. But then, that is just me.

Sorry but I don't know the exact # of HP that you would lose.

Out of curiosity, why are you having your engine rebuilt?

 

[len3.8]

92, look at the So how many ..... Post.
He went into detail about what happened.

 

[4AU4LIFE]

92, look at the So how many ..... Post.
He went into detail about what happened.

Thanks for the tip. Glad you did it nicely rather than the way some people would've.

 

[Yellow Rose]

Glad you did it nicely rather than the way some people would've.

Awwwww, isn't that cute?

 

[4AU4LIFE]

C'mon Andy, you are slacking. You let almost 45min elapse before having a come back for me.

 

[sjs]

Back to the question, I think it is a given that all else being the same you will loose a little torque and HP. I've long forgotten the formula for calculating the approximate amount but I'm sure a web search will turn it up. Where it might be most noticeable is less torque at low RPM, but I don't think even there it will be much. To me your idea does not seem silly if you feel relatively confident that you will in fact add boost at a later date. If not and you sell the car it may limit the potential buyers but be seen as a plus to others, especially these days. So while it is apart, the relatively modes additional cost is worth considering.

 

[Black&Tan97Tnewportbeach]

Uhhhhhhh aren't you leaving out the part about what you'll gain being ported and balanced?

 

[sjs]

Uhhhhhhh aren't you leaving out the part about what you'll gain being ported and balanced?

He made no mention of any such work. I suspect that the stock NSX is very well balanced as-is, else it wouldn't appreciate reving to 8k as often as you like. Sure it could probably be better, but I doubt the difference would be greater than the repeatability of the measurement. Of course that's just my somewhat educated guess.

The term "porting" is often misused so you may need to be more specific, but any attempt to improve the flow of the heads is somewhat risky to do without a proper flow bench. Simple polishing of the intake and head ports may be worth doing, but even that has been known to do more harm than good in some cases. Port and gasket matching is generally a good idea if it needs any at all, so that‘s something to look at any time you have one apart. But the impact of all these things is likely fairly small on an otherwise stock engine, and slightly less so on one with less compression, but to your point, I suppose you might get that or more back if done correctly.

 

[paulviriyapan]

Well guys, thank you so much for the reply. Umm, yes Chris, I left out the part about the porting and balancing, so that should bring things up a bit.

The JE pistons are the same price for the lower compression pistons as for the regular. I was just wondering at the power situation with the lower comp pistons if I left the thing N/A. So the deal is that the pistons will be 9.6:1 as opposed to the stock 10.2:1 compression ratio, which would mean a loss of power. (Right?) But then the heads will be ported, AND the block will be bored out (the cylinders were galled due to the debris from my spark plug getting in between the pistons and cylinders). Of course the boring ain't that much, and is fairly negligible. (I think it was .025 of a millimeter or something like that).

Anyways, my entire thinking is that if I have to have my engine rebuilt, I want it BETTER than it was before, and it's better to shell out the additional $ for all the 'little things' since the engine is out. But, from my frame of mind, I am now worried that using lower compression pistons would be taking a 'step back' should I not be running F/I in the future. I mean it's like this: regular compression pistons with porting, balancing, and increased bore would mean even more horsepower than the lower comp. pistons. Right?

Even if I go with F/I in the form of nitrous or a turbocharger at a later date, the stock compression ratio should be OK for mild boosting as people seem to be running f/i on stock internals without any problems. And if that is the case, then what is the advantage of going to the lower compression pistons, when I'd be losing n/a power? Any opinions here? Thanks.

 

[KGP]

Since it's being bored, did you consider sleeving and taking it to 3.2, or even 3.5?

 

[paulviriyapan]

Yeah, I actually talked to Gerry about that. Gerry's feeling was that the hp made wouldn't justify the effort. Acura's 3.2 only makes 20 h.p. more than the 3.0 liter, and to do that I'd have to buy a 3.2 liter crank (cranks are $2k), whereas I already have a 3.0 crank. At 3.5 liters, there has been some reliability issues associated with boring it out that much. From what I hear, Comptech tried doing that but stopped. Supposedly it was causing too much stress on the con rods.

 

[RedlineRex]

would running this lower compression allow you use lower octane gas (when not boosting, of course)? that would be a good selling point that you would save more money at the pumps compared to other NSX drivers.

 

[KGP]

Yeah, I actually talked to Gerry about that. Gerry's feeling was that the hp made wouldn't justify the effort. Acura's 3.2 only makes 20 h.p. more than the 3.0 liter, and to do that I'd have to buy a 3.2 liter crank (cranks are $2k)

It might not be worth the effort, but I don't know why you would have to change the crank to accomidate a larger bore. Mine is being sleeved and is going from 3.0 to 3.2. Also, if you went to steel sleevs with the increased bore, you might be able to boost a bit more in the future should you decide to go the FI route. Just food for thought.

 

[paulviriyapan]

Can you bore out to 3.2 and still use the same crank? Well, I'm not an expert by any means, and if Gerry says I need a different crank, I'll take his word for it. What are the folks who are doing your rebuild saying about using the same 3.0 liter crank?

At this point, I don't know what sort of mods I should be making as I am a little gunshy about going the F/I route. First is the issue of $$ since the rebuild ain't cheap and if I want to go turbo it's gonna cost about $9k more (for Gerry's kit, which is the only one I would feel comfortable with on my car). Secondly, the NSX is my daily driver, and I am real, real scared about blowing the motor (again!). F/I can be made safe, but leaving things stock is safest without a doubt (of course what the hell do I know as I blew my motor stock!)

One possibility I'd look into for the future is going the nitrous route as it is significantly cheaper and it wouldn't be running constantly like a turbo or a supercharger would be. I mean I really don't need a whole lot more speed except for once in a blue moon.

Now if I go the nitrous route, would the steel sleeves, in addition to low comp pistons, be a good idea? I mean at this point my chief concern is safety (and not blowing up the motor). I know sleeves are a must on going high boost turbo or super applications, but what about nitrous? If I added the sleeves would I be safe in running crazy high shots of nitrous-150 shot? Just curious.

 

[sjs]

I wrongly assumed you were doing sleeves with the pistons. If your plan is to stick with stock sleeves, and bored ones at that, then I'd get the stock compression pistons since the cylinders will have defined your boosted power limit, which can be achieved at the stock compression for a better overall package IMO.

You can certainly do 3.2 without a crank. I think the stock 3.2 does it with a little bore and a little stroke, probably in part for the extra torque you get from a longer stroke, but I think the common approach of all bore for a 3.2 turbo is fine.

BTW, turbos are not “running all the time” in the sense of placing any load on your engine. They respond on demand and otherwise are essentially idling and passive. SCs of course spin, but at typical road speeds are also not doing any harm.

 

[4AU4LIFE]

that would be a good selling point that you would save more money at the pumps compared to other NSX drivers.

I'll take the stock compression and spend the extra $.10 per gallon at the pump. If you are that concerned with gas prices, maybe the NSX isn't the best choice.

 

[NA1 #2853]

The cranks are the SAME....

the only difference is in the bore.

BUT, the 3.2L crank has larger rod journal diameters, and i believe the pistons have larger wrist pins.

The only reasoning i can figure is to deal with the larger/heavier piston?

But if you use aftermarket forged pistons, often times they are lighter, and they come with ALOT stronger wrist pins.....

with the factory rods taking FI abuse in built low comp motors, i doubt a slighty heavier piston with bother it at all.

-Ray

 

[sjs]

The cranks are the SAME....

the only difference is in the bore.
...

-Ray

OK, I was misinformed awhile back when asking that exact question to someone who should know. That's what I get for not looking it up myself, but at least I said "I think..."

Forged pistons are not necessarily lighter and are often heavier (more dense) than cast. They could be lighter by making them thinner and with smaller skirts (not easy on the NSX) if that were a design goal, but it isn't always the case. (In fact, I personally haven't seen it in other engines I've worked on) They are much stronger of course, which is why the extra weight can be tolerated at high RPM.

 

[donwon]

I would do whatever Gerry says. Its like asking Da Vinci to paint something and limiting him to certain colors or telling him how to paint it. Just my narrow minded point of view. Hope it works out

 

[4AU4LIFE]

and they come with ALOT stronger wrist pins.....

I thought that you re-used your wrist pins unless they too were damaged but needed to get new rings?
Please educate me on this.

 

[sjs]

No relationship between the wristpins and rings. The stock pins are not likely to be used with non-stock pistons. It is likely that the small end of the rod will be bored to a larger size and possibly bushed and fitted with a full-floating pin. (the stock ones are press fit on the rod with only the piston floating.) Now, I don’t know if the stock rod has enough “meat” on it for both a larger size and a bushing for full-floating, so t may be just larger and stll press-fit, but I’m sure someone here can tell us.

 

[NA1 #2853]

exactly what SJS said

plus

also the wrist pin included with the aftermarket pistons are usually made of higher grade steel, that can take more abuse.

-Ray


Another question???

WHY, has no one tried after market RODS? Yes factory is titanium....blah blahm but its only partial titanium/alloy. But are there strength limitations? on a Extreme RPM engine or HIGH HIGH boost engine would an aftermarket be needed?

Look at the A&L, and AEM drag cars with NSX engines, they are all running aftermarket rods, and are running to 9000+, and i cant find where they say there are full titanium.... i think they are running 4340 billets.

Someone educate ME on this PLEASE!

 

[sjs]

As with most things, a lot of factors determine rod design and material, but for all but the most extreme cases cost is typically in the equation. In the case of nearly all street NSXs thus far the stock ones are adequate, so there is no need to spend huge money for new ones. However, if you want to turn the wick up on you FX500 to even higher power, they do offer alternate rods.

I honestly don't know what top import drag cars use, it's not my scene, but I suspect that given the "explosive" nature of those engines they don't use titanium due to costs. Less expensive materials have served very well for many years and are much less costly. The lighter titanium NSX rods save overall weight, reciprocating weight, and of course they sound sexy in the marketing campaign. Of those I think the reciprocating weight is the most important. That allows for higher and effortless revs with less stress and wear over the long life of a street engine. Calculate the G-forces and pressures that the various parts are subjected to when they change direction (from up to down & vise-versa) at 8000 RPM. It is extreme, and the less weight you have the better. More weight requires more strength requires bigger which means more weight which requires more strength... Just like power, suspension, brakes, etc... Titanium rods and light cast pistons allow high RPM and a smaller lighter crank. It all comes together in perfect balance. Then we screw it up. Oh well, all for fun.

 

[4AU4LIFE]

So let me understand this correctly then.
Since I was forced to use JE pistons in my rebuild, they came with new wrist pins already?