Best compression ratio to run w/6851s(68HTA).

[Thecarfixerguy]

I already ordered Wiseco 8.1:1 pistons for the build, but I’m not sure if that’s the best ratio to pair with this turbo. The turbo is rated 51 Lbs/min. I was thinking maybe a little higher ratio might be appropriate for this turbo. Is was going to up the compression a bit with .036” Cometic head gasket, but not sure how much that changes things or if I’m even going the right direction. I’m looking to make a fun street build with quick spool times.

 

[Spleen8urLSX]

Don’t focus so much on compression. On a turbo in that class for a street car compression isn’t going to be a deal breaker, and unless you’re using e85 or race gas much more compression will just make it harder to tune. Also unless you’re having the head resurfaced that isn’t the head gasket you want, the felpro composite or oem composite will hold up to whatever that turbo can throw at it as long as you’re tunes proper, while acting as a “fusible link” (ex: if some kind of severe detonation does occur) to where the head gasket will be more likely to pop in an incident opposed to an mls gasket holding it in and resulting in a torched head. Plus the oem composite (can’t speak for thickness on the felpro) is .051” thick, that .015” difference in thickness isn’t going to make or break it.

 

[Thecarfixerguy]

Gotcha. I do general auto repair for a living and anything other than MLS gaskets feel cheap to me, but I see where you’re coming from. The head is at the machine shop getting a valve grind and resurface, and the block deck is clean so I’ll probably stick with the MLS I’m used to. The advice on compression ratio is noted since I plan on tuning myself which is also something new to me, and I hope to avoid any expensive mistakes.

 

[curt-s]

Block and head surfaces need to meet a specific RA value and be near perfectly flat for MLS, not just "clean".

 

[Justin DuBois]

I’d start with the octane requirements if it was my choice to make.

 

[Thecarfixerguy]

Block and head surfaces need to meet a specific RA value and be near perfectly flat for MLS, not just "clean".

I don’t remember what the RA spec for my machinists new mill was, but I think he said the machine was upwards of $60k. I think the head surface will be “clean” enough. The original milling pattern on the block is still clear and there’s no pitting or measured warpage. I’ve installed several hundreds MLS head gaskets without issues. I’m pretty comfortable moving forward with MLS with what I’m working with.

I’d start with the octane requirements if it was my choice to make.

That’s good input. I plan on using premium pump gas. This info is what I’m here for. I was looking at the common range of pistons people use and wondering if I should be at the low end. Am I right to assume the people using higher compression pistons(like ~9:1) are also running fuel that is more tolerant to high compression?

 

[Justin DuBois]

I run 2G pistons 8.5:1 CR on 1G rods, 6 Bolt, and run 94oct E25 mix. California has 91 oct E10 premium, and it, umm how should I put this, It sucks. Knock was easy to find. When I started blending in more E85, to up the Octain, the knock went away.

So IMO If you go full corn fed E85, or 105oct race gas, go for the 9:1

Else 8.5:1 is pretty snappy on 93oct.
And if you’re in California maybe 8:1 on 91oct would be intersting, but this is just guessing

 

[Spleen8urLSX]

Gotcha. I do general auto repair for a living and anything other than MLS gaskets feel cheap to me, but I see where you’re coming from. The head is at the machine shop getting a valve grind and resurface, and the block deck is clean so I’ll probably stick with the MLS I’m used to. The advice on compression ratio is noted since I plan on tuning myself which is also something new to me, and I hope to avoid any expensive mistakes.

To put it into perspective, guys are going 8s on the felpro composite. In short if you don’t know 100% your surface finish on the head will be proper composite is what you want.

 

[JusMX141]

As high as your build and fuel type will allow without paying much attention to the turbo itself. Consider a 68HTA to be a better-flowing and more-efficient 16G...it's not like you're bolting some 72+mm unit on there and hitting "send". The right compression ratio will maximize efficiency on any build...it's how the great Curt Brown was able to make 499whp with a 40 lb/min turbo 15 years ago- remove the turbo and that car probably still would've made 300.

You must be logged in to view this image or video.

 

[donniekak]

Lower compression actually makes more power with forced induction, and spools sooner. It will However make the car feel like a dog out of boost.


The reason is simple. Displacement is usually figured with swept volume. This is because naturally aspirated engines must suck in their own air. With forced induction air is blown in the cylinder filling up the total displacement including the cc, and piston dish. This effectively increases displacement.

Kiggly has posted data before about this with a highly data logged setup.

 

[slugsgomoo]

As Donnie mentions, for a racecar, there's a lot of data for going super low compression, but on the street, it's a lot less fun in my experience. I've had 7.8:1, 8.5:1, 8.8:1 and 9:1 motors, and by far the most fun street setup was the 9:1 with HKS 264/272 cams on an Evo3 16g. I would expect a 68HTA to magnify the good there, and flow a bit more on top.

While you have to manage timing properly with higher compression, it's still possible to run 22psi on a 16g with plain old 92 octane and 9:1 with no issues.

I'm not sure how my new setup will be with 92 octane because I haven't gotten to run it yet, but I'm going 10.5:1 with a 2g head on a 2.4L I promise you can do 9:1 with no problems. I certainly won't ever build a street car with less compression than a stock 2g (8.5:1) ever again because as mentioned... they're dogs off boost.