Let me reiterate. The 4* advance is on the 9:1 compression as I stated “10-15whp of 10.5:1 with 9:1 compression” on the same boost level for 9:1 as the previous 10.5:1 compression.

 

We understood that the first time, but what wasn't disclosed is what happens when the same 4° ignition advance was applied to the 10.5:1 SCR engine.

 

That is a valid point and to clarify what was done in stepping down only in compression and then they only made fuel and timing (added 4*) adjustments to the tune and were able to be 10-15hp of the previous 10.5:1 compression and it’s most efficient tune up which had 4* less peak timing advance.

 

The obvious question concerns the big disparity in timing, but no need to get too deep into the weeds with any single anecdotal experience. This is where the expression "YMMV" applies.

 

Thanks everyone for your help. I am leaning toward the 10.5:1 compression. After my deeper research I found out that wiseco pistons 10.5:1 compression are 10.5:1 with .034 head gasket which is evo 8 head gasket, I will use evo 9 head gasket which is .045 and that will lower compression slightly from 10.5:1 to 10.3:1 and if I port cylinder head chamber very slightly from 43mm to 44mm that will gibe me 10.1:1 compression ( correct me if I am wrong) . In other hand if I do the same think with 9.0:1 compression it will lower my compression to around 8.6:1 which is a little too low for e85. So I am thinking to order 10.5:1 pistons.

 

Do not port the combustion chamber - it was designed to create a specific flame front and quench, which could be ruined if you have no idea what you’re doing. You risk actually ruining the head beyond just reducing its efficiency, too. There is nothing wrong with 8.6:1 on E85, it’s still going to make a lot more jam than 91/93 octane is capable of. There are lots of head gasket options out there, don’t feel restricted to the OEM gasket for your specific application, especially since you want to put a lot of boost through it.

 

If spool up is nearly the same with high/low compression, why not buy yourself the extra insurance and go with low compression? Its easier on parts and has a larger room for error. You can run more timing etc.

8-9:1 for 800whp+ will live a long time.

Its better to add some stroke. A 94mm crank will out weigh any compression change within reason compared to a 88mm crank.

 

I think you are right here. I was doing some calculations and lets say engine number 1 is 10.5:1 compression running 40psi boost making 800whp and engine number 2 is 9.0:1 compression and on same boost and timing making 4% less power which is 32whp less so this engine is making 768whp, its seems a lot less but on lower compression engine I can easily add 1 more degree timing and 2 more psi boost and I am pretty sure I will get those 32whp back and this lower compression engine will be more happy than the higher compression. correct me if I am wrong.

 

I have been reading this whole thread and its very interesting. When I was building my motor, I went with 10.5:1 pistons at the suggestion of my tuner to improve spool; which has been stated that it doesn't do in this thread, but in my experience it does seem to have some effect. I moved from a MHI green on the stock bottom end to an MHI red on a built 10.5 2.0 bottom end and see VERY similar spool characteristics, like to the point of being able to overlay data logs and have the boost lines on top of each other.
I guess this is probably more due to the fact that with the built bottom end, timing and stuff could be more aggressive in the spool window because there is less worry about making too much torque?

Either way, I am very happy with the outcome for my particular use case. Its a street car and it feels a whole lot different on the built high comp motor than it did on the stock bottom end. I just find it interesting that it was suggested by my tuner for exactly the reason that it doesn't appear to matter for lol.

 

Those different turbos spool similarly simply because they actually do spool similarly.

More timing doesn't make a turbo spool faster, just like more compression doesn't.

 

Sorry for late response. I am wondering why not to port combustion chamber. I did port couple of them already similar to this video with great results. My current motor also has ported combustion chamber by me many years ago and car is making great power. I think those sharp edges must be ported as shown on the video.

 

He built my 2.4 and I will never do business with him again. Bad experience with them. I found bolts left loose on my intake manifold, swiss cheese for wiring under my dash etc. He did my 12.5:1 and and yeah he definitely used no timing up top just to keep it alive.

 

David L. of Headgames is a bad *** cylinder head porter and shaping the chamber is a must.

 

Dang that sucks to hear. 12.5:1 is insanely high for force induction outside of INDY and F1.

 

There really isn't much different between the 2 really but lower compression will keep the car happy and you would be able to run more ttiming and boost.