pistons for forced induction

[Arctc Wolf]

It's this simple, compression and boost pressure are related exponentially. as you reduce compression ratio, boost level achieveable goes up exponentially. this means that you can fit a lot more air and fuel into each cylinder, causing more power to be made. Of course there are many other variables to take into account, such as combustion chamber design, camshaft selection, header flow, intake flow, air temp, combustion chamber temp, cylinder bore, piston design, spark plug design, humidity, ect. that all affect the detonation threshold of an engine, but in general, what I said above is correct.

[camaroman1181]

if you are looking to make an all out drag car, you want as high a compression ratio as you can support before knocking the engine to pieces. Higher compression ratios will always give more hp simply by increasing the efficiency of the engine.
If you want to get extreme, domed pistons tend to detonate easier because they deliever a less than ideal quench area. dished pistons are much more idea because they keep the majority of the gases in the center of the combustion chanber near the valves and plug.
If you felt like getting custom pistons, get your heads fully cnc'ed, then have pistons with the tops made to match the combustion chambers with just enough room for the valves to move. this would be idea as far as just pistons are concerned.
another area to look at is thermal coatings. one of the main causes of detonation is "hot spots" in the combustion chamber. basically, any sharp edge in the chamber will lead to a heat concentration that can cause an errant flame front to untimely propagate thru the chamber, thus detonation. coatings help reduce these effects by not letting the heat soak into the metal as much and allowing the parts to hot hold as much heat. they work especially well on exhaust valves which regularly see temps over 1200 F.
for a street car, i wouldtn run much more than 9.5-10.0:1 for moderate boost levels and that would only be with a fully coated and polished combustion chamber. maybe a little higher with custom pistons. but thats my opinion.. good luck if you plan to pursue this. id be very interested with the results. personally, im just going with the ross dished for my turbo buildup...

[phoenix64]

Porsche also X-rays every single one of their cylinder heads to make sure the coolant passages and the combustion chambers are perfect, all to controll detonation.

Lower C/R motors are more durable and more powerful. They are less efficient at part throttle, thats the only disadvantage.

[camaroman1181]

lower c/r motors are not more powerful.. more durable yes.. but by no means are more powerful there ahve been repeated tests on this. add the fact that the only thing that effects teh otto cycle efficiency is compression ratio, i dont see how a lower compression ratio is more powerful. if that was the case, then top fuels and funny cars would run 6:1, not 17:1+...

[nocutt]

Lower C/R motors are more durable and more powerful. They are less efficient at part throttle, thats the only disadvantage.

I think you have either your fact backwards or your opinion on a tangent way yonder...

From your statement...I could just use the L67 as an example, take the pulley for the SC off, drive and then try that on an L36...you feel the difference...now drop the Compression to about 7:1 and you will see a snail...
Now if we add forced induction to the mix...assuming everything is equal...you still CANNOT surpass the throttle response, the MPG, the fun factor...the power...etc, I can keep going on but I am sure you get the picture :D
LC engines + FI is ONLY IMO for those who race...(I am not going to get into that)...if you are building a street car...and you want pistons...keep it no lower than 9:1...in a HC engine, be conservative with timing...if you get aggressive then compensate with a much cooler charge AND octane...

[phoenix64]

Fun factor, MPG, Response, are all different ways of saying part throttle efficency aren't they?

[nocutt]

Originally posted by phoenix64:
Fun factor, MPG, Response, are all different ways of saying part throttle efficency aren't they?

If you road race, drag race, street race...then NO! if you turn your car on...then drive to work and home...then yes ;)

[camaroman1181]

i still dont see what you are trying to prove phoenix... its been dyno tested that all things equal, higher c/r's put out more power across the board than lower c/r's, na or boosted. personally, i dont care bout the power my car has at quarter throttle cuase when would you ever race at anything but full throttle?

[98Camaro3.8]

Originally posted by camaroman1181:
i still dont see what you are trying to prove phoenix... its been dyno tested that all things equal, higher c/r's put out more power across the board than lower c/r's, na or boosted. personally, i dont care bout the power my car has at quarter throttle cuase when would you ever race at anything but full throttle?

At the same PSI, high c/r will make more power, that's a given. I think what he's saying is that when you lower compression but raise the boost to the point you start getting detonation, you will have more power than if you ran high compression and lower boost (but still to the point of detonation).

[Turbo V6 Camaro]

high c/r = boost = top fuel
low c/r + more boost = drag/street cars

[phoenix64]

Originally posted by nocutt:
</font><blockquote>quote:</font><hr />Originally posted by phoenix64:
Fun factor, MPG, Response, are all different ways of saying part throttle efficency aren't they?

If you road race, drag race, street race...then NO! if you turn your car on...then drive to work and home...then yes ;) </font>[/QUOTE]What?

[RicE-ter]

i agree, WHAT did u say?

ne way back to the topic. im guessing that l/c pistons last longer and make better power because as u compress air it heats up which causes detonation in the combustion chamber instead of an even burn spreading away from the spark. if u didnt know a diesel works by compressing the fuel/air mixture enough that it ignites on its own w/ no spark plugs. Theres a saying that u would rather have 7lbs of quality boost than 12lbs of hot boost.

[nocutt]

The point is HAS THE POTENTIAL of creating detonation is NOT the same AS CAUSING
To even the field...if everything remained equal except pistons (CR), octane and timing...then for every pound of boost...the H/C motor would have more power throughout the power band than its counterpart L/C motor...PERIOD!
Now that isn't the real world, because in the real world too many variables come into play...however the rule of thumb here is, the lower the compression the more tuning window is at ones disposal...on the other hand...the higher the compression on the motor, then the smaller the tunning window becomes...the idea is looking for the compromise for the street or the like.
If you are under the precept that L/C motors live longer and make more power then you are only looking thru a small window...

[ June 16, 2005, 02:12 PM: Message edited by: nocutt ]

[phoenix64]

Its great to talk about smaller tuning windows and such, but as far as I know I'm the only person who has done any extensive tuning with a wideband.

No one is arguing with you nocutt. I'm talking about max power for a given octane(pump gas). Your talking max power for a given boost level.

[camaroman1181]

it doesnt matter about a given octane or not. assuming no detonation, higher compression ratios always make more power. this is not max power for a given boost level, this is max power for any boost level. boost level is irrelevant. higher compression makes more power. the draw back is higher compression leads to detonation without precautions (more octane, tuning, material coatings.) detonation eats motors alive. so for street apps, you err on the safe side, sacrifice some overall power and lower the compression ratio.