What the hell 8:3:1

[garputo2]

I got my other question answered so I have decided to go with the 851 compression and push more boost to gain my goal hp of around 400 that I got but most places sell 831 or 881 only one I can find is ross at 851 so what I wanna know is why would they make a 831 that will cause some loss in hp right? or 881 I just want good forged pistons and rods going with a new stock crank.

 

[Indian]

I went with Ross 8.3.1 to run more boost because I'm in cali and the highest pump gas available to me is 91 octane. It depends on your goals and how much boost you want to run. Ross also can make a custom compression ratio to suite your needs. You can also go with weisco.

 

[Rockbmx1]

K, first off use puntuation when talking about compression ratios. 8.8:1, 8.5:1, and 8.3:1 cr. Second off, if you need to ask what a compression ratio means then you should worry about it. The 8.5:1cr Ross pistons you are talking about are stroker pistons, the 8.3:1cr is the one for a 2.0L. The compression doesn't make much of a difference in power when you are talking about such low comp to start with. Just get some standard Ross pistons (the 8.3:1 if you are lost) and use those. Unless your motor is blown, then I wouldn't even worry about pistons right now.

 

[Indian]

K, first off use puntuation when talking about compression ratios. 8.8:1, 8.5:1, and 8.3:1 cr. Second off, if you need to ask what a compression ratio means then you should worry about it. The 8.5:1cr Ross pistons you are talking about are stroker pistons, the 8.3:1cr is the one for a 2.0L. The compression doesn't make much of a difference in power when you are talking about such low comp to start with. Just get some standard Ross pistons (the 8.3:1 if you are lost) and use those. Unless your motor is blown, then I wouldn't even worry about pistons right now.

8.5.1 is the stock compression ratio. Both the 8.3.1,and the 8.5.1 can be for the 2.0 block. The compression ratio DOES make a difference in power,and how you want to use it.

 

[garputo2]

I have the motor out block has been honed and cleaned by the machine shop I have a new crank and its because of spun rod bearing so what I want to do here is build one bad mother but I know that the new cars have alot of hate when you start changing anything from stock on the systems like fuel/air ratio. So I want to get a nice set of pistons and rods as I plan to run a 20g turbo and beef everything up. So I want a block that can handle the 400 hp+ goal. my turbo bearings were bad so I am going to order the turbo also. clean the head and throw it all back together. I am just concerned about the smaller ratio since the 2g spec is 8:5:1 rather that 8:3:1 I want to use alot of boost like 25-30 psi so what the heck is recommended with what I want

so to add to the last response if I go with the 8:3:1 it will handle alot more boost that 9:0:1 thus getting to 400 hp by using lots of boost

 

[DSMopar]

its more dependant on what fuel you intend to use. If you're going to run pump gas just throw the 8.3:1s in there. If you really 'must' have 8.5:1 get the higher comp pistons and adjust with the head gasket thickness.

I wonder how many of you guys actually check your chamber volumes/ deck heights. Biggest n00by mistake is to grab a set of advertised piston compression numbers and assume that's what you have...

 

[Rob10_99]

edit, called out with no actual experience or proof whatsover, not worth my explanation.

 

[juanpac0]

Higher comp =/= faster spool.

 

[LiquidX]

Higher compression = Need of better tune to prevent knock and also good choice to run meth for the higher boost numbers.

 

[flinguist]

Better for E85 also.

 

[dsmfa9nutter]

There is no possible way in hell that you would feel any difference in the seat of your pants between 8.3 and 8.5. Seriously. Go with the 8.3 and leave your tuning margin bigger.

 

[dSmgs-T]

this thread has way too much misleading/wrong info.

 

[GRAVEDIGGER]

this thread has way too much misleading/wrong info.

At least the title is RIGHT (sorry, had to say it)

 

[firebirdvert305]

Im on 8:3:1 ross/eagle rod setup on E85. Numbers are too the left.

 

[cody92talon]

its more dependant on what fuel you intend to use. If you're going to run pump gas just throw the 8.3:1s in there. If you really 'must' have 8.5:1 get the higher comp pistons and adjust with the head gasket thickness.

I wonder how many of you guys actually check your chamber volumes/ deck heights. Biggest n00by mistake is to grab a set of advertised piston compression numbers and assume that's what you have...

why do you have to check your chamber volume with different pistons an adjust things with your head gasket thickness

 

[jrohner]

Because those are the things that effect compression ratio -- bore, stroke, pistons -- dome/dish/reliefs, how far down the hole the piston is, head combustion chamber volume, head gasket thickness. You can't know your exact compression ratio just by what pistons you have. Plus, if you get the 8.3:1's with a size of .040 over, the pistons will probably be basically around 8.5:1's.

If you have your head resurfaced, the material you lose raises the compression ratio. If your head gasket is slightly thinner, it raises your compression ratio. Both of these also have a slight effect on cam timing, which also effects your actual cranking compression (dynamic compression) but not the static compression ratio.

 

[DSMopar]

Because those are the things that effect compression ratio -- bore, stroke, pistons -- dome/dish/reliefs, how far down the hole the piston is, head combustion chamber volume, head gasket thickness. You can't know your exact compression ratio just by what pistons you have. Plus, if you get the 8.3:1's with a size of .040 over, the pistons will probably be basically around 8.5:1's.

If you have your head resurfaced, the material you lose raises the compression ratio. If your head gasket is slightly thinner, it raises your compression ratio. Both of these also have a slight effect on cam timing, which also effects your actual cranking compression (dynamic compression) but not the static compression ratio.

Exactly, advertised piston compression ratios are based on assumed dimensions. Factory dimensions aren't always what they're said to be (deck height and head chamber volume) from the factory. On top of that say someone broke a T-belt and had the head rebuilt... if they surfaced the head the chamber volumes decrease. Same goes if someone rebuilt the engine machined the head surface. If someone ported the head (properly) the chamber volumes could increase. If you're using a thicker or thinner head gasket from the assumed dimensions this could also change your compression ratio.

Point is, don't go buy a set of pistons advertised at 8.5:1 and assume it's 8.5:1... for all you know it could be 7.5:1 or 9.5:1 or farther off... You need to check your head chamber volumes, check your deck height and choose your pistons/head gasket based on these.

 

[cody92talon]

so your saying i made a little mistake wen i did mine because i put weisco 8:3:1snd got the head an block decked and used a standard thickness MLS headgasket ? how much will this affect me ?

 

[LiquidX]

so your saying i made a little mistake wen i did mine because i put weisco 8:3:1snd got the head an block decked and used a standard thickness MLS headgasket ? how much will this affect me ?

It's not really that big of a deal. When they deck the head and block they are only taking thousandths off. You'll be fine alright, stock compression on the 2g is 8.5:1 where as the new Evos have it at 8.8:1. I'd say you just went closer to 8.5:1, that's all. It's "easier" to tune with lower compressions pistons for knock issues but you can still achieve the the same if not better with higher compressions. Just takes a little more.

I can't say I've ever really heard of anyone really checking the volume of the chambers to make sure of their compression number, but a few have. All in all, you're A-OK

The guy above was just stating that without checking the number, you don't know for sure what you are running. Just because the piston says x:1 doesn't mean it actually is x:1 on YOUR block.

 

[DSM_PWR]

Thread is full of FAIL

 

[dsm-onster]

We'll to put to rest whether or not Wiseco pistons are really 8.3:1 as advertized, they are If you don't deck your head, then rest assured the wiseco 8.3s are correct. Their shelf pistons are made to exact specs to fit. If they can't get the dish right, that would mean they have more problems then what the results out there have proven otherwise.

Resurfacing the head once won't change the specs .1 CR either.

Yes, 8.3:1 vs. 8.5:1 isn't enough to notice. maybe 1-2% in thermal efficiency on the CR efficiency curve. Actually 8.8:1 isn't that much different. Teh higher you go the less thermal efficiency is recovered. And it's only .3 over 8.5 and 8.3 is only .2 under. . . Now going from 7.8:1 to 8.8:1 is a healthy jump. No stockish 1g timing curves allowed on pumpgas.