Can someone explain the effects of lowering Comp/Ratio with more boost?

[468LC]

I am looking for a good technical explanation of why I should use blower pistons in my next shortblock and what will I gain in power going from 9.5:1 down to around 8:1? Boost will be in the neighborhood 11 or 12 PSI.

I figure if I put my new crank pulley on now with my current compression I will have to use race gas and or no timing.

Any thoughts would be appreciated.

[MikeyMustang]

well.


I am not shure about "blower" pistons but fordge is a must!!! NOT hyperutetic....maybe they are calling fordge "Blower" pistons.....as for compression ratio...i think they want you to stay away mostly from detination and run pump gas......rember i have trickflow heads that have been milled 30 thous...and raised my compresion about half a point or so...and i run 94 and all is well....

btw check out my new videos of tues night.....ALL with NO traction one even hitting the rev limiter (6250)...and still running 11.91 or less!!

http://mikeymustang.no-ip.org/610.wmv

Now i want to pull the wheels like Ukfiveo does!!!

hey Steve forgot to ask you about the box intake guess it didnt work out well?


Mike
http://mikeymustang.no-ip.org/

[468LC]

Here is the type of situation I am asking about mike.:)

I looking for more of a pros and cons type of answer to this one.

I am also looking at it from a standpoint of making the most power between the two scenarios.

9.5:1 and 11PSI with 4* timing = X amount of power safely

8:1 and 11PSI with 14* of timing = X amount of power safely.

Who comes out on top and why?

Are there any advantages to cylinder filling with a dished blower piston?

Yes they will be forged.





As for the box intake I picked up 2+ MPH with it but that was on my stock long block and stock lower intake. I ended up selling it when I bought my new combo because I figured that the stock lower would be a huge cork in the system even with the box upper intake. Man I miss the box.

[mygt8a4re]

the only advantage i can figure is the capability of running more boost safely. i would think that the lower compression would hurt performance with the same amount of timing & boost on both applications. i would say to plan a build up using dished pistons for lower compression would only be good if you have plans to run alot more boost & possibly nitrous in addition......i dont know all the pros & cons sorry, i just wanted to add that the hope for some serious boost should be added into your plans for use with those pistons.........

edit: sorry steve, i went back & read your posts, i misunderstood your point. i saw you were wanting to add a larger crank pulley w/o the use of racegas. im not sure of the loss from the compression drop in proportion to the gain from "x" amount of boost from the crank pulley. but i can add that the dished pistons could help aid in all the future boost addition plans you may have w/o the worry your confronted with now!

[regattacoupe]

Well with 8:1, naturally emissions will through the roof. But you " should" be able to run more aggresive timing. torque will also be missing till you get into boost.

[frank woodall]

First of all a 'blower' piston is not just a forged piston. The top of the piston is much thicker and the skirts are much stronger than a regular forged piston that is not for blower/turbo applications. The reason a blower piston is approx 8-1 is because, say your ultimate goal is a cylinder compression of approx 22-1. If you start of with 10-1 pistons, it would only take about 6lbs of boost to reach your goal and all parimeters being the same if you start of with 8-1 pistons it would take about 13lbs of boost to reach your goal of 22-1. Most chargers are efficient enough to go from 6lbs to 13lbs and only create about 10-15 degrees more heat. So raising the boost level does not effect heat enough to worry about. Dropping piston compression and raising boost levels raises your engines efficiency. For each lb of boost fed to your engine equates to approx 13-16 hp. (w/fuel). With 10-1 pistons you can't compress as much air/fuel into a cylinder as you can with an 8-1 piston (the dished style piston allows more compressed fuel and air to enter the cylinder) which gives you more power. So after all, isn't this why we supercharged our cars.....to get more HP, so yes if you can afford it deffinetly buy dished low comp blower pistons, you will make a ton more power with lower comp and more boost!!!!!!!

[5150 LX]

Steve, drop Brian with ADPerformance an e-mail. I picked up my stroker kit and he may be able to shed some insight on this for you.

his e-mail is: scatstroker@hotmail.com

[468LC]

Do you mean Brian that goes by Eagle on Corral.net?

He is a pretty straight shooter and he is somewhat local to me.:)

[mustangmark]

Here's the sum of it:

high compreassion and low boost = Improved throttle response, better off-boost gas mileage.

low compression and high boost = More total power, less off- boost gas mileage. Less off boost power.

This is assuming you are comparing the two on pump gas. On race gas, high compression and high boost levels are possible, in which case you'd be able to make more power than with low compression and high boost (for obvious reasons).

high CR and low boost is beter for autocrossing, rallying, and road racing because you get better fuel economy off boost and good power from low mid and high rpms. Whereas low CR and high boost gets most of it's power at the mid - top range. ;)

I could be wrong, this is what I was told by all of the guys I race with. Most of them are road racing/autocross nuts though ;)

[5150 LX]

That's him Steve, and yes very knowledgable and very reputable. If your are considering a purchase, give him a shot.

[fanglemeister]

Static compression ratio means very little, you have to look at dynamic compression, that is the actual amount of air being captured and how much it is compressed, along with the temperature requirements of your available fuel. Dynamic compression is determined by adding or subtracting for cam overlap and intake runner harmonics, combustion chamber quench area, then factor in boosted atmosphere adding heat, aluminum vs iron pulling heat back out, and dozens of other factors. Looky here:

Combustion actually begins as soon as the intake valve closes. As the compressing charge absorbs heat, oxidation of the charge increases. Oxidation is always an exothermic ( heat releasing ) operation. But - early combustion is always an endothermic ( heat absorbing ) operation. The charge will absorb heat from combustion as fast as it is released to create new chemical events. This leads to cool or "blue flame" combustion at around 500c. After a period of time, the amount of energy released by oxidation will be sufficient and the charge will begin to rise in temperature, terminating in a full flame.

Spark ignition always occurs during the cool flame/early combustion/induction period. The spark does not start the reaction - that is already underway. What it does is avalanche the reaction by releasing free radicals and creating a hot spot.

All of the heat needed for combustion prior to the spark comes from charge heating - particularly compression. The warmer the coolant, the warmer the combustion chamber, the less time it takes. Ditto for adding boost, the more heat you start with, the less time it takes. Compression is the major heat source for combustion. The higher the compression, the greater the heat, the faster the charge starts to burn. High enough compression will result in the charge going to full flame without a spark, usually much earlier than is ideal wrt angle of the crank in rotation.

So more heat = shorter induction period. The more the external heat in the charge, the less lead angle ( spark advance ) needed for ignition. ( Always assuming that you wish the peak pressure to occur at the optimum point ).

Now we get to iron vs alloy. Remember the truism that iron heads make better milage but aluminum heads make more power. Iron conducts much less heat away from combustion than aluminum. If you are detonation limited, the aluminum would carry more heat away during this stage, delaying HCCI until past spark release, thus allowing a higher compression or more intense charge. The Iron, conducting less heat - would require less fuel in throttled conditions.

50% of all compression - regardless of the geometric ratio or the intake supercharge - occurs in the final clearance volume - from 2 clearance volumes to one clearance volume at TDC. ( Because of previous charge heating - this can actually be significantly more than 50% ). Virtually all of the work is done very close to tdc.

Now lets look at the chemistry taking place during the induction period, as the charge heats up. Hydrogen is being oxidized. Very little carbon. Hydrogen oxidation is significantly faster than carbon oxidation. Carbon is buried in the hydrocarbon molecule. All the hydrogen must be stripped from the particle before the carbon can be oxidized. Free hydrogen is important. Methods to break hydrogen free are heat on weak molecules and spark ionization. We use both in otto combustion. First we build enough heat to cause some molecules to break down and release hydrogen. Then, sometime later, we avalanche the breakdown by an ionizing spark. Adding octane slows down the burn by requiring more heat to break it down.

Keep absolutely in mind that combustion effectively starts with the intake valve closes and the piston is moving upwards. Heat is being added to the process and the action has begun. A spark is not necessary for combustion to occur. Anyone who has seen an engine "diesel" and keep running after the ignition is turned off knows that it can and does happen.

Combustion is temperature controlled - more heat - faster combustion. Typically, the engine runs out of time before combustion occurs. The spark ionizes a portion of the charge - increasing nicely the number of free radicals and provide a localized hot spot that carries a small portion of the charge to enflamement. Then the rest of the charge does its thing. The
safest way to think of the spark is not ignition, but as avalanche. It does't do anything about putting the snow on the mountains, it just triggers the release.

So back to your question - low static compression plus high boost allows more charge to be burned than low boost plus high static compression, only due to the properties of the available fuel. The reasons for this are buried in the rant above.

Anyway, I have rambled enough, apologies for boring everyone with the hard science of hotrodding. Thanks to the internet it's all out there to read, just have to find it!

[468LC]

Sweet post chris.:)

[sinner]

Here's a link with some more info
http://www.motorsportsdigest.com/forced2.htm

[speedytang]

I also have not understood why you want a car that has less power when not in boost, which is whats going to happen if you lower the compression. To me less heat with a 9.5 compression at 6psi than a 7.5-8.0 compression at 12psi. The other thing I was thinking about is it takes 35-40hp to turn a supercharger so why have a engine that has 40+less horsepower to turn that blower your now 80hp in the hole.