Compression vs Boost FYI

[pgtpwr]

ok check this out if you got 2 engines with the following set-ups:

9:2.1 CR + 12psi = 16.7 DynamicCR
8:1.1 CR + 15.7psi = 16.7 DynamicCR


are both engines producing the same POWER. even if one engine allmost 4psi more boost but lower compression.

It depends. Which specific engines/heads/turbos are you looking at?

If you go lower than 8.5:1 you're going to have a dog of an engine if you're not in boost. Any higher than 9.5:1 and you're going to have to run premium and back off the timing. Or thereabouts. :-)

Dan

[pgtpwr]

Staying on the theoretical side: how could you create two engines that were completely identical *except* for their static CR? Does that mean they have the same bore & stroke, but different piston shape? To keep everything else the same, you need to change the combustion chamber volume. How do you do this, and still call the two engines "otherwise identical" ?

Assuming you have an answer to that question, then clearly the higher boost engine will produce more power at a fixed dynamic CR, because it ingests more air mass than the other.

To lower the compression ratio you can either increase the combustion chamber volume, or use pistons with the wrist pin mounted closer to the combustion chamber (better for large drops in CR).

Doing either one of these increases the displacement of the engine by a small amount. And you got it, the one with a lower CR will injest more air as a result. Once you turbocharge it to reach the same dynamic CR, you've got a LOT more air in there, making more power than a higher compression engine with the same dynamic CR.

At the high static CR end, you're going to run into detonation so you will have diminishing (or DEAD!) returns. At the low static CR end, the displacement becomes so large that the combustion no longer occurs in the combustion chamber in the head, but rather throughout the cylinder... and you start scoring cylinder walls and you don't have a good flame front... so you again have diminishing returns. Showing again that all of the theoretical crap doesn't make a difference - it's what someone actually builds that matters. :-)

Dan

[JoKER93pgt]

HC=small turbo applacations Stock IHI...
LC=T3orBigger Recommended..
Dose that sound about right?

[cadvis3d]

HC=small turbo applacations Stock IHI...
LC=T3orBigger Recommended..
Dose that sound about right?

well i'm running a t3 with stock compression and its fine.....pulls like a train....

[The_Tiredguy]

just a question but have any of you thought of using high compression pistons and turbos and trying to achieve pressures where gasoline will combust like diesel... using only pressure and no spark... just a thought but that would DEFINITELY pull the most power out of things... if you could keep the pistons/rods/block/head/valves/rings/etc from falling apart of course

[veltpak6]

just a question but have any of you thought of using high compression pistons and turbos and trying to achieve pressures where gasoline will combust like diesel... using only pressure and no spark... just a thought but that would DEFINITELY pull the most power out of things... if you could keep the pistons/rods/block/head/valves/rings/etc from falling apart of course

Gasoline burns too fast when it detonates. Far too fast to be safe for an engine. It would be uncontrollable. Diesel contains more BTU's of energy litre/litre and it detonates much slower.

[Richard]

just a question but have any of you thought of using high compression pistons and turbos and trying to achieve pressures where gasoline will combust like diesel... using only pressure and no spark... just a thought but that would DEFINITELY pull the most power out of things... if you could keep the pistons/rods/block/head/valves/rings/etc from falling apart of course

Well normal gas will combust like diesel and when it dose not a forged pistons or Rods can hold the power becuase when this happens the piston is just before TDC and the explosion send your piston back down the wrong way and your rod breaks. This is called detnation and it is what kills anycar. The only thing that causes it to combust is the heat generated buy the high pressure. If you compress air it makes the molicual move faster which makes heat. The whole point of running boost is to get the most valum without going past a sertin pressure. Once you go past this pressure which I believe is some where around 400 psi your engine will detnate with pump gas. If your air temp is too high or you don't have enough fuel to cool your air charge you will reach the cumbustion point way before 400 psi......bottom line you want the greatest valume of air and fuel possible without too much pressure at TDC. This is the hole reason you go with lower cr. Anything elce is just stupid and ignorint. High cr being better for boosted cars is the biggest myth out there. There was a time when I also thought that a higher cr was better and the more I reserched it to prove those who didn't agree with me the more I was proved wrong and that lower cr is better for boosted motors. If high cr is better then why do Top Fuel dragsters run 6:1 cr and make 6000 hp. Why don't they run 12:1.......O that's right there motors would never get past 800hp if they did that. Like wise your 4 cylender will never get past 290 hp on pump gas even if you had forged pistons with stock cr of 9:1. Now if you only want to run + 14's then I guess stock cr is good enough for you. IMO high 12's is what I plan on some day and I am gonna use 8.2:1 cr to get there.

[dodatdamnthang]

It depends. Which specific engines/heads/turbos are you looking at?

If you go lower than 8.5:1 you're going to have a dog of an engine if you're not in boost. Any higher than 9.5:1 and you're going to have to run premium and back off the timing. Or thereabouts. :-)

Dan

Not really the stock 1st gen gt is 7.6 to 1 and its not a dog in the vac bout as fast as my 2.0 was stock

[FRCFD6]

Not really the stock 1st gen gt is 7.6 to 1 and its not a dog in the vac bout as fast as my 2.0 was stock

disconnect the turbo and try again.

[Rizzo90]

Well, I've just taken the time to read this entire thread.....and am DAMNED GLAD I purchased some stock pistons for my rebuild rather then gettin higher cr forged pistons....just .030 over....Looks like my plan now is to rebuild, PORT EVERYTHING and get me a t3. thanks !!! informative read!!

[man/plansPGT]

very useful information here

[Richard]

I guess you can defiantly see both sides of the issue.

[supra4gen]

idk after reading the first page i still wanna boost a ze motor for my escort...

[Probeson403]

I just bought a 93' Probe GT.. it has a V6 Mazda JDM in it. Would anyone know how I could go about finding out the size of the motor and its proper name? plz

[Lewis7789]

Man, this thread made my head hurt. haha.

So just to clarify, I am putting a KL in my Protege5 so I need to buy either a ZE or DE and I'll boost it with a T3/T04 around 8psi with MegaSquirt and a proper tune.

Should I go with a ZE or DE? I'm leaning towards the ZE since the motor will have a standalone and tuning. Seems like a lot of cats just go with the DE since it's easier to work with.

Any help would be appreciated. Thanks!