Static compression and boost*

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lexcrob
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Static compression and boost*

Postby lexcrob » Sun Feb 12, 2012 12:47 pm

As i prepare to start my build thread i wanted to get some input on some topics.

Ill start with this great read that i found: http://www.xcceleration.com/cr-boost%20101.htm

After reading that i believe the answer to why supras can make 500hp without forged pistons. There are of course many variables to that but thats an elementary assumption. It also leads to the question big question: are forged pistons really needed :ohno: Im starting to believe they are sort of a gimmik in a way. Of course they are stronger and take more "detonation" but what if low compression cast pistons were available? Could say 8:1 static compression create a powerhouse? With that said i've found another example: being ground down crankshafts have almost no relation to their real world strength. In other words "beefier rods and arp humbug" are reserved for MAXIMUM output situations that in theory would never be needed in say a street car. There are obvious limits such as induction hardened cranks have a surface hardness of roughly .030.

All that being said now i believe the dyno queen 400hp ka bone stock secret is obvious. Detonation Prevention* not bottom end strength that is the popular belief!


And this week ill find out my piston deck height finally had an idea how to check it.

Lastly anybody got any opinions on qench area as it seems it is yet another grey area to the ka series motor and most import engines.<honda all motor guys are taking advantage of it but most are unaware from my research. I now see why the brightest opinions are not on forums as their opinions make them less money since they would normally be in the automotive field if they have that educated of an opinionon something.

Cheers


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Re: Static compression and boost*

Postby lexcrob » Sun Feb 12, 2012 1:18 pm

That may have come out a little ignorant or fuzzy as im not much of a writer but take it with a opinion based grain of salt. Im also by no means a professional or experienced just like to try to get to the nuts and bolts of stuff.

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Re: Static compression and boost*

Postby superDorifto » Sun Feb 12, 2012 9:36 pm

They are not a gimmick. It all comes down to metallurgy and piston design.

Stock KA pistons fail because they are more brittle than forged performance pistons. They also fail because the compression ring land, the upper most land is very close to the crown of the piston. This is good for MPG, and emissions, but leaves little room for error if detonation or pre-ignition occur.

Same goes for the rods, they work great under normal operating conditions but fail when subjected to extreme events. You are not buying yourself anything Just cause you lower the static CR, you still need to add boost back into the equation to make power. All you are doing is making the car slower and less efficient out of boost.

2jz rods and pistons are a better design....thats why those bottom ends hold up better in stock form...

why try to reinvent the wheel, you can get forged rods and pistons for almost nothing?

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Re: Static compression and boost*

Postby lexcrob » Mon Feb 13, 2012 11:39 am

Agreed about the ring land and the fact that their obviously not as "strong".

What i took from reading that site was that if you want to make big power on pump gas you have to lower the compression allowing you to run more boost. The pay off of running lower compression is that you'll have a denser charge or higher boost level. On the other hand if you have high compression then you cant run high boost or make gobs of power without higher octane fuel.

^ i guess that was really my assumption

In other words say you found a compatible lower compression decent design cast piston putting you at say 8.1 then you could make more power than say a forged fully built engine with stock compression 9.5:1. Am i wrong on that theory? PUMP gas 93 octane or course being the limiting factor for the stock compression built motor.

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Re: Static compression and boost*

Postby superDorifto » Mon Feb 13, 2012 5:19 pm

The forged vs cast is an irrelevant part of the conversation.

You are running a higher boost level....it will make more power.

The trade off you're talking about would give you LESS power out of boost, and offer no additional protection when in boost. Forged bottom ends are about insurance, they are more ductile and forgiving than their cast counter parts. The forging process aligns the metal grain into a structure that is better able to deal with boost....thats all it is - insurance.

Heres another way of thinking about it...
You could make the pistons out of balsa wood, I'll make mine out of forged aluminium.

You run 8:1 at 15psi

I run 9:1 at 10psi

At see level, the boost ratio for your engine =2.02 (while at max boost)
On my engine I only make 1.68 (again at max boost)

under ideal conditions your engine is moving a little over 2X the air it would be NA, I am moving less. If you run 15 lbs of boost vs my 10 lbs, you would make more power. My set up would however be much more robust, and anywhere not close to full boost I would make more power.

Cast pistons buy you nothing.

This of course ignores the fact that better designed cast pistons are not readily available...but forged options are.

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Re: Static compression and boost*

Postby lexcrob » Mon Feb 13, 2012 7:23 pm

You make some good points but lets not forget forged pistons as far as a street car arent very "friendly". You have to run higher piston to wall clearances in most instances. They are lighter bringing on the need to have your rotating assembly balanced. Alot of them i've seen for our motor run larger wrist pins calling for new rods of course. Not that any of the later is necessary, but it would be recommended to really take advantage of these "robust pistons".

Then of course you have to get all the arp goodies and MLS headgasket and by that point you're in pretty deep. For assurance of your investment you better have it bored and honed with a torque plate along with a machine shop that even has the ......expertice to handle such a animal.


That was mostly a rant that will help nobody just laying out the reality that its not just slap in some pistons but maybe im wrong and its fine to do it my buddies car has some slap em ins....


So say you assembly one of these super motors how much more power on 93 octane would 8.5:1 pistons make compared to 9.5:1? **with compressor map matched turbos**
What are your thoughts on studs or oem bolts and headgasket for a 250hp goal?
How bout this ..... (toward bottom of page is some good ones STOCK bottom end dyno queens)
http://www.phearable.net/videos/

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Re: Static compression and boost*

Postby WDRacing » Mon Feb 27, 2012 4:35 pm

I read awhile back something that Kenne Bell wrote, you will always make more power by reducing static compression (within reason) and adding boost.

Stopping knock IS the key to making power. With a good tune the KA has been taken to over 500whp. You just can't have any knock. The rods in the KA bend before the pistons break btw. All assuming the tune is good and there is no knock.

WD

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Re: Static compression and boost*

Postby lexcrob » Wed Feb 29, 2012 11:44 am

IM ALMOST ABOUT TO BREAK OUT THE CAMERA AND BREAK OUT THE PHOTO BUCKET FOR MY BUILD THREAD. I found out piston deck height is .000 via FSM for the ka24de. I measured it best i could with a dial caliper (dont have the correct tool) and i got .005 with the OEM pistons .015 with my new ebay pistons.

I did some reading on quench race street engine recommendation is around .035. The factory headgasket is roughly 1-1.1mm or ~.043 may combined mill is .008! WHAT DOES THAT COME OUT TOO.= .035

^That is assuming the FSM recommends .008 mill from the minimum head height and block height. I would assume that it does but reguardless im getting another crank and set of rods for my other block to measure for reference. This will slow me to measure static compression ratio for the factory set up and compare to my engine thats bored out. I have to do this since the FSM lists dynamic compression ratio and i have no idea if you could even calculate that. To add i cant use "fourm" data since all blocks and head heights will be different (very slight). Lastly, ill be able to clay both engines and find out what my piston to valve clearance can be.

I wanted to have it together by the end of the month but i still have to buy a frigen buret.....They run like 100$

Im also going to cc my combustion chambers/intake and exhaust ports since i did some very very minor gasket matching and deburring. Again I also have a oem cylinder head that i will use as a reference
.


Trying to get some legit documentation for the ka24de. Its really annoying that "s13 has 42cc s14 has 46cc blahh blahh blahh nobody knows so I PLAN TO FIND OUT!

Last step will be degree wheel and adjustable cam gears to compensate for my milled block to get .035 quench and then s14 intake camshaft because its supposed to close earlier (creating a higher staic compression) hence the rated power increase in 95.


AAAAAAHHHHHHHH I HATE BUILDING MOTORS SOSOSOSOSO MUCH TIME AND PIDLY WORK

BUT NEXT IM GOING FORED ALL .40 OVER BIG BAD BOY SO TYRING TO LAY DOWN THE FOOTWORK!

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Re: Static compression and boost*

Postby C-Kwik » Fri Mar 09, 2012 11:48 am

superDorifto wrote:Forged bottom ends are about insurance, they are more ductile and forgiving than their cast counter parts. The forging process aligns the metal grain into a structure that is better able to deal with boost....thats all it is - insurance.
While ductility is important, it only helps avoid a total failure of the material (fracture). However, a design failure is going to occur once you exceed the yield strength, at which point, plastic deformation is occurring. Ductility generally just allows one to address an impending failure before it becomes catastrophic (perhaps before you throw a piston through your block); which is of course, a good thing.

It should be noted that forging generally causes a material to become more brittle. I can't speak to the different properties of pistons actually made, but if available forged pistons are more ductile, there are probably some differences in the chemical composition of the material that help maximize ductility. Which is likely why there is a change in the thermal expansion coefficient (necessitating the need for the bigger piston wall clearance that someone already mentioned).

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Re: Static compression and boost*

Postby lexcrob » Sat Mar 10, 2012 1:15 pm

In other words if you DO NOT ALLOW DETONATION what so ever and have extreme provisions against it to protect your "balsa wood pistons" your better off than the guy that just slaps in some forged pistons!

Im aware forged pistons intent is to be stronger giving you more room for error or problems. Im just trying to say their often used in applications where cast units would be fine. Now I have no idea how to explain the next weakest link as for as the ka engine. I believe i heard rod bearing failure is next on the pecking order.

*so say someone wanted to break 500hp arp rod studs "stretch torqued" and lower compression pistons would help get you closer than just forged pistons. AND KNOCK PREVENTATIVE AND DETECTION OUTMOST IMPORTANCE! OBVIOUSLY

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Re: Static compression and boost*

Postby superDorifto » Sat Mar 10, 2012 5:41 pm

C-Kwik wrote: It should be noted that forging generally causes a material to become more brittle. I can't speak to the different properties of pistons actually made, but if available forged pistons are more ductile, there are probably some differences in the chemical composition of the material that help maximize ductility. Which is likely why there is a change in the thermal expansion coefficient (necessitating the need for the bigger piston wall clearance that someone already mentioned).
The generally available forged pistons have differing amounts of Silicon in the Alloy. I forget which way it tips the scale, but more or less makes the piston more or less resistant to detonation, but also drives up the thermal expansion coefficient overall.

Its all about material selection for your application/use/driving style.

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Re: Static compression and boost*

Postby superDorifto » Sat Mar 10, 2012 5:47 pm

lexcrob wrote:In other words if you DO NOT ALLOW DETONATION what so ever and have extreme provisions against it to protect your "balsa wood pistons" your better off than the guy that just slaps in some forged pistons!
No.

You never design a system without accounting for all the possible failure conditions - or ways in which your carefully laid plans can blow up in your face.

That is my point.

The lower static CR just shifts the powerband to the left....it does nothing else to hold the motor together.

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Re: Static compression and boost*

Postby C-Kwik » Mon Mar 12, 2012 12:04 am

lexcrob wrote:In other words if you DO NOT ALLOW DETONATION what so ever and have extreme provisions against it to protect your "balsa wood pistons" your better off than the guy that just slaps in some forged pistons!

Im aware forged pistons intent is to be stronger giving you more room for error or problems. Im just trying to say their often used in applications where cast units would be fine. Now I have no idea how to explain the next weakest link as for as the ka engine. I believe i heard rod bearing failure is next on the pecking order.

*so say someone wanted to break 500hp arp rod studs "stretch torqued" and lower compression pistons would help get you closer than just forged pistons. AND KNOCK PREVENTATIVE AND DETECTION OUTMOST IMPORTANCE! OBVIOUSLY
When engineers choose a material for an application, there is a factor of safety they use to account for imperfections that they can't control or are too costly to control (cheaper to build less perfect parts with more material than to try and get more perfect materials and reduce the material needed). As a result, there is usually some room for adding power without failure, but it depends on how big of a factor of safety they use against how imperfect (in terms of crystal and grain structure) your particular set of pistons might be. In short, there is a bit of a gamble where your odds start to stack against you the more power you try to make.

Certainly, lowering the CR helps control detonation, but I don't think we have much statistical data that could be used to determine if lowering the compression (using same material as stock pistons) can be enough to allow the engine to stay in one piece @500whp. I tend to think rod failure will occur before then (from what I recall of a few examples of failures where detonation did not occur), and I'm not aware of any specific examples of anyone using stockish pistons of lower CR, let alone enough of them to come up with any statistical average of when such a piston might fail. One could try and make an analytical determination, but unless we have a lot of the engineering data, its unlikely we can make such a determination.

As sD stated, analyzing all potential points of failure is necessary. There is some factor of safety for every part, but if you start to add more power, your level of risk increases all around. In terms of the parts you mentioned, I'm not sure rod studs would be first on the list. Rod bolts/studs are under the heaviest loads under tension When the rod sees compressive loads, the combustion energy is transferred to the crankshaft). And tensile loads do not increase with more power (unless you increase engine RPMs) since the highest tensile load is going to occur as you reach TDC on the exhaust stroke. Lower CR, while may be a valid argument is a little bit impractical. Why pay money for lower CR pistons when you might invest a bit more and get forged pistons instead? Not even sure what kind of availability there is for lower CR pistons for the KA. And I would not feel comfortable having material removed to lower the CR either. I understand your argument, and I don't necessarily disagree with you, but from a practical standpoint, there is little reason to invest too much time in them.

And cheers, there is a reason I always loved the KA-T forum. These kinds of discussions are always a lot of fun even though, I haven't tinkered with a KA since 2004. :)

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Re: Static compression and boost*

Postby WDRacing » Mon Mar 12, 2012 3:41 pm

The KA will make gobs of power with the factory internals, that's just fact. You don't "plan" for knock. You do everything you can to remove it. There aren't ANY parts designed to withstand detonation. A forged slug with only hold together a touch longer then a factory piece when the motor suffers anything over mild knock. I'd venture a guess that only 10% of all engine failures have anything to do with parts failing at peak torque, which is where load is the greatest IIRC, but rather from detonation or some other uncontrolled event. I'm not sure how to figure piston speed and such into the equation when it comes to load, Chano will no doubt have something to say on that.

My point is this, forged or not, the only thing that is important is removing knock. You're not materially deficient when it comes to hp and the KA, you're octane limited. Which brings the discussion back to where it started, compression vs boost etc.

Choose your poison, meth/water/E85/C3H8 or any combo. I wouldn't lower the compression any if I were using forged internals, I would modify my effective octane ratio by using water/meth in a 60/40 mix. If we're removing knock by increasing our effective octane rather then decreasing our static compression the motor will be tons more efficient both on and off boost.

My .02
WD

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Re: Static compression and boost*

Postby lexcrob » Wed Mar 14, 2012 7:47 pm

WOW that was alot of good information

I wanted to add a few things ive found to this.

This is an opinion from a local machine shop when I asked for an opinion about resurfacing my block to get some quench.

" It would be a waste of money with such a low compression engine. On a higher compression engine small increases in compression are more beneficial. On that engine your too far from where you would want to be and the gains would be very negligable. "

His opinion blew my mind! I related seeing substantial gains found from throwing on a high compression headgasket on a honda. To me the idea that compression gains are linear in a sense was a legit opinion.

Next i asked HOW DO YOU FIND DYNAMIC COMPRESSION RATIO

" Its easy you just find the "degree" that the intake valve closes and measure it from there. "


Anyways some more good info I hope it at least adds some more juicy debate topics.


I really want to supercharge it but im having trouble finding information for affordable fuel control seems FMU is a bad idea although paired with a BTM i think it be a win. Then there is what size to run....... shooting for 230-300 and i dont think the little m90 can do it. Ive been over at the miata forums burnin through their stickys. LET ME TELL YOU THOSE GUYS ARE ON ANOTHER LEVEL AS FAR AS maf hacks/fmu's/and trickery!-- Honeslty if there was somewhere i could bring it and get it tuned and taught how to make minor alterations for myself I would spend the money to go prom tuned. Why cant they make a friken cobb for our cars. Maybe ill wire up a friken power cammander!!!! Or maybe ill just use the fmu combined with lots of meth to create a "richer mixture bbahhh blahhh .................DONT EVEN SAY IT I KNOW ILL BE DOING IT RIGHT THE SECOND TIME.

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Re: Static compression and boost*

Postby C-Kwik » Fri Mar 16, 2012 2:41 pm

WDRacing wrote:I'm not sure how to figure piston speed and such into the equation when it comes to load, Chano will no doubt have something to say on that.
To my knowledge, the load you need to be concerned about with regards to engine speed is the tensile load on the piston. This is only a function of engine speed and not how much power you are making, so there isn't much concern for it unless you are increasing redline. And if you go with stronger rods/pistons, chances are they will have a higher threshold so there is no concern there. Its possible a heavier piston might change things somewhat, but I recall at least JE aimed to match the OE piston weights. Can't speak to other companies, but I suspect they would try to maintain similar weights to OE for drop in applications as changing the mass would change the a major variable in both the load and natural frequency of the motor.
lexcrob wrote: This is an opinion from a local machine shop when I asked for an opinion about resurfacing my block to get some quench.

" It would be a waste of money with such a low compression engine. On a higher compression engine small increases in compression are more beneficial. On that engine your too far from where you would want to be and the gains would be very negligable. "

His opinion blew my mind! I related seeing substantial gains found from throwing on a high compression headgasket on a honda. To me the idea that compression gains are linear in a sense was a legit opinion.
I'm thinking you misunderstood what he said. Milling the head or block to raise compression would have a smaller effect on a lower compression motor because the change in volume divided by the original TDC volume would be small. In a high compression motor, the change in TDC volume divided by the original volume would be greater. Its not necessarily that raising compression in a lower compression has less effect than in a higher compression motor. Ultimately, and increase in compression ratio increases the expansion ratio by the same amount. This allows a greater amount of energy to be converted into mechanical energy. That said, if the goal is to raise or lower compression, using higher or lower compression pistons is the right way to do it. Resurfacing can be used to make a small compression ratio change, but since the actual length of the rod changes slightly due to heat expansion and due to strain, you need to make sure you don't cut off too much material.


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