[baron_]

Currently running a stockish 389 with ported 6x-8 heads
I happened to have the heads, so was easy

I'm guessing I'm at like 7.5-1 compression ratio, which I realize is a bit silly low. block isn't zero decked or anything...and probably 98cc chambers
But, it was an easy combo to make...to learn on, tune...and be safe with
I've learned a lot


Next is a 461, 400 stroked. 4 bolt splayed, filled, all forged/ross pistons etc
What would you guys shoot for?
I'll push 15 lbs of boost or so
Methanol, intercooler....twin turbo HX35's behind my 5spd

I'm looking at head choices.
6h or 6S might be around 122cc
4x around 114cc
7m5 at around 110-114cc
I like the 7m5's with 2.11/1.77 valves. I realize i'd have to convert to screw in studs and add the extra header bolts....but might be worth it, rather than starting with 6S or 4x heads

Should I shoot for 8.5-1 or closer to 8.0?
I'd rather not run on the ragged edge...rather have a bit of wiggle room



Wasn't planning on zero-decking this block either, to give a little safety room


Thoughts or suggestions? It's not a racecar, but a fun street car that can hopefully be impressive at the track

[BruceWilkie]

Clay

Unless aluminum heads I'd try to keep it right in the 8:1 range. You may want to keep the 6x-8's and dish the pistons a tid to put compression where you want it. The more compression you run the more compromised your ignition becomes. Just like NA retarding a high compression motor for lower octane fuel costs more power than the compression increase gives. FYI (iirc) turbo69bird runs 6x-8's on his 455. I'm not ruling out using mine but more inclined to use Aluminum. Even with aluminum I plan to stay at 8.5 or less. I'd rather turn up the boost more than tweak up compression. Detonation kills a motor pretty fast if it occurs near max boost and rpm conditions. My old turbo vair ran out of fluid for the water injection under those conditions. The motor was toast faster than I could react to the noise. Actually the car felt like a pavement related vibration just before I heard anything. A crappy day for me.

[baron_]

i think turbo69bird had an error about that, he's actually using the 6x-11 heads (big chambers)??


Looking like I need to go for 122cc or so heads, then?
I don't know if I like the idea of dishing those ross pistons. They're already pretty lightweight and thin, it seems. I'm thinking that would be too much of a dish

[BruceWilkie]

Are the Ross pistons speced for blown use? Do you already have them? The std forged TRW SpeedPro SealedPower can take a pretty good dish and lose weight at the same time yet still be strong. Avoid longer than stock rods too.

[baron_]

yea, already have the ross pistons from a project long ago
They're the ones made for a 6.8 rod and chebby 2.2 sized

All came in a stroker 400 kit from Ohio Crank, when I wasn't a turbo person


So, I'm thinking that I should go with appropriately sized heads, rather than the dishing and whatnot



jeff and his supposed 6x-8 heads had me messed up for a while too
I'm actually the one that sent him mail about that, asking how he was running that kind of boost with that kind of compression. He had it listed in his profile improperly

[Tom Vaught]

If you had an aluminum head you could run 10-1 at any boost without issues with methanol. Marty ran over 36 psi with a 10-1 engine.

To be safe stay around 8.5 to 1 and then if you go back to gas you will still be good to go for 15-20 psi if using a cast iron head.

Tom Vaught

[David Holmberg]

To me, for a streetcar, I'd run closer to 9.1

The car will have more power down low off boost, where normal driving happens, and it will spool quicker.

For a true street car I'll trade a small amount of boost for a car that's more fun to drive everyday. And if you're running an intercooler and methanol, you won't give up any boost on the top either.

[RUDOLFSSON]

Jeff runs 8.2:1 compression ratio.
My combo has 8.1:1 cr and 15lbs of boost with pump gas on the street, iron heads, no cooler or meth injection yet. No issues so far. No signs of detonation after a close inspection and fresh up on the motor.

[baron_]

Interesting variance of opinions.

I could probably hit 9-1 by just using the 6x-8 heads and not zero-decking, maybe slightly less (i'm assuming not zero-decking would give me a few cc's too?)

Or slightly dish the pistons (only 5cc or so) and use those 6x heads for closer to 8.5-1?


Lots to think about, so I appreciate opinions.
I'm going to do some searching on zero-decking.....thoughts on how much I'd lose/gain there?

Other opinions or suggestions?


I'd like to err on the side of safety and ease of tune


I'm going to be using 'the crutch' holley (solenoid operated boost referenced power valves). So, I can easily run it pretty rich and not worry about lean issues.

[BruceWilkie]

Originally Posted by David Holmberg

To me, for a streetcar, I'd run closer to 9.1

The car will have more power down low off boost, where normal driving happens, and it will spool quicker.

For a true street car I'll trade a small amount of boost for a car that's more fun to drive everyday. And if you're running an intercooler and methanol, you won't give up any boost on the top either.

With the right cam you would be hard pressed to notice the difference from 8:1 to 9:1 in normal driving with a well matched turbine housing/wastegate you wont have any problem spooling. No different than NA running 10.5 compression pump gas and compromised timing curve than 9.5:1 and optimum timing. If your running pump gas there is no gaurantee that the pump labled 93 doesnt have a batch closer to 91 or diluted accidentally with 87. Again set up under 8.5:1 and be comfortable with a safety margin.
I'm also talking iron heads here. Aluminum can usually handle .5 to one full point higher.

All out race put methanol to use and go for 10:1+

[Marty Palbykin]

10.5 to 12.5 on alcohol should be no problem under 36 psi of boost with out an intercooler with a good tuneup.