Wiseco 8.3:1 or 9.0:1???

[spidy3]

wiseco 9.0:1 or 8.3:1 comp ratio?


current set up is in profile....

additions will be...

Scat rods
comp 1012 cams
fully ported head
manley springs & retainers
1mm oversized exhaust valves
removed balance shafts
AFC
pocketlogger
(want to run 22 psi on EvoIII)

Which is the more resonable/reliable compression ratio to go with for a daily driver / weekend track warrior???

need to decide this weekend, ordering parts monday

Dom

 

[delta448]

Q: What makes a piston move?
A: Pressure.

Whether you get the pressure gain from the increased CR or from packing in more boosted air, it should take the same amount of cylinder pressure to create the same amount of torque (if temperature, etc. remains constant). The real issue at hand is about the heat inside the cylinder before ignition and at what point it causes pre-ignition (read: before higher octane is required). If you have a shitty intercooler, running more boost will cause as much problems as a higher CR will. Higher compression heats the charge as it's being compressed, the more compression you have the hotter the charge gets. Pushing more boost also makes more heat. With a lower CR the charge doesn't increase as much in temperature as it would with a higher CR, post intercooler. With a higher CR, the heat is added more by compression in the cylinder than by the lack-of-turbo/intercooler efficiency combo. If you have a good intercooler and a very efficient turbo, I'd definitely recommend the higher CR pistons. You'd have better low end spool up abilities too.

 

[GVR4592]

Higher compression doesn't spool the turbo quicker.

 

[delta448]

Higher compression doesn't spool the turbo quicker.

Please explain. I was always told that the higher off-boost cylinder pressures of a higher CR would decrease the rpm level needed to provide sufficient exhaust pressure to begin spool-up. If this isn't correct, I would like to understand why exactly. Not a challenge, I just want an explanation.

 

[blackGSX2g]

If you guys are having trouble with 8.5:1 and 20 psi, it's a tuning issue.

I agree. I'm currently running 30psi daily on my 50-trim with 9:1 compression on pump gas with about 18-19* of timing at 7500-8000 RPM; 0 knock with a lousy tune on my DSMLink.

 

[JayRolla]

30psi on pump with that much timing and no knock. Man I hate my evo turbo.

 

[GVR4592]

Please explain. I was always told that the higher off-boost cylinder pressures of a higher CR would decrease the rpm level needed to provide sufficient exhaust pressure to begin spool-up. If this isn't correct, I would like to understand why exactly. Not a challenge, I just want an explanation.

Higher compression doesn't increase exhaust pressure or heat. It actually lowers it. I'm not a scientist so I don't know the physics behind it, but along with compression ratio there is an expansion ratio. When the exhaust gas expands, it cools. Apparently with higher compression the expansion ratio is also higher, and the exhaust gases are cooler because of this.

 

[nightspeed87]

Higher compression doesn't increase exhaust pressure or heat. It actually lowers it. I'm not a scientist so I don't know the physics behind it, but along with compression ratio there is an expansion ratio. When the exhaust gas expands, it cools. Apparently with higher compression the expansion ratio is also higher, and the exhaust gases are cooler because of this.

i have heard of this also,^^ and i know it doesnt actually help turbo spool.

The thing im curious is, does more compression give you a difference of torque ( Id think it does ) To me feels like a car with more compression generally feels torquier. Is that true. ( and im not just talking about off boost torque.) And would more compression give you better gas milage, or less?

 

[GVR4592]

Higher compression will make more torque. More torque requires more fuel, more fuel being burned per power stroke means worse mpg. Even though it takes more fuel, I doubt you would notice it at the pump.

 

[nightspeed87]

makes sense, thanks.

 

[delta448]

Thank you Luke. I had never heard that before. It makes perfect sense now.

If you still had a chicklet count, I'd recommend you for rep points.

 

[blackGSX2g]

Higher compression will make more torque. More torque requires more fuel, more fuel being burned per power stroke means worse mpg. Even though it takes more fuel, I doubt you would notice it at the pump.

When I switched over from the stock 7-bolt with bolt-ons to a built 9:1 6-bolt with the same bolt-ons my MPG went up from 19-21 to 24-25 around town. There might be a few other factors that could change this number a bit, but I noticed the difference almost immediately.