hi guy i need your advice im building a sb 400 flat top pistons bored 40 over and looking to get a new set of heads [alu] i want to run on pump gas what i need to know is what kind of compression can i safely run.
bore 4.165
stroke 3.75
head gasket dia 4.2
head gasket thickness .071
combustion 64cc
piston dome cc -6. this gives me 10.33-1 do you think that would be ok or should get a thinner gasket and bump the compression up a bit or larger cumbustion chamber say 70 cc and lower it down
thanks anthony

I would use a head gasket no thicker than .040" with a zero deck to get the quench distance in the ballpark. Definitely go with a 70cc or larger chamber if you want to run street pump gas. A 76cc head will have you right at 10.2:1, 70cc's is closer to 10.8:1

what is the max compression ratio i can safely run on pump gas 93 oct

That really depends on what cam you choose.

Have you picked one yet?
Have you bought the heads yet?
What rpm range will this engine run?
and with what gearing and trans?

10.6:1 here with steel heads. Just be sure your timing is right and be on the watch/listen for detonation.

You're running aluminum heads so you should be ok.

Im running 10.5:1 now with steel heads and a 268H from comp. I have run 11:1 with alum heads and 292H from comp....As stated, listen for loading and detonation...

Iron aluminum brass fiberglass etc won't matter on head material.
Get your QUENCH to near 0.040". If its much more, you'll knock on 93 octane and 8:1 compression.
Also match your cam to the Compression and compression to the cam.
A huge cam needs high compression.
High compression and a real small cam will drive cylinder pressure to the moon and knock on anything less than 112 octane.
234º (@0.050") dur cam will handle 11:1 with zero problems

Who sells 93 octane pump gas? I haven't seen that since the early 90"s. Here in Cedar Rapids, Ia your choice is 87, 89, 91 and one place here sells 110 for the Stock car drivers. I can't afford to run that on the street.

Here in Illinois, lots of stations have 93. We're starting to see more E85, too.

There in central Iowa, you gotta have E85 all over, now there's some octane!

I see 93 at the Wawa here or was it BJ's ? Anywho... not everywhere here either.

I think Waterloo, Ia is the closest E85 to me. Thats about an hour drive.

Back to safe CR. You will be fine with 10.33:1, the Alum heads and 93 OCT...you MIGHT-MIGHT get away with 91...(shrug)

11.26/1 with iron heads.Pump swill Sunoco 94.Solid flat tappet 248 @.050.
Cranking compression is 170 psi.36* total all in at 2700 rpm with a 3500 flash stall.

Back to safe CR...

shrug)

They pissin ya off Harley?

hehe:)

No stinkin E85 here in Okla. None that I've seen. Course I'm like a Lab, sometimes it takes a 2 by 4 to get my attention...:D

I'm kinda wanting to try E85. You can run a little more advance with it can't you?

Last year we had 93 everywhere. This year 91.

Damn I wish I had a dog.:sad:

:D:beers::cheers:

Flat top pistons won't give you 10.33:1 in a 400. They're 10.1:1 in a 350! Check out the compression calculator at Ross Pistons. With 6cc's of valve relief on a zero decked block with .040 gasket and 64cc heads, your compression is 11.5:1.

I didn't see the flat tops :rolleyes: ut oh.

.071 gasket ?,64cc heads, -6cc pistons, + .040 400, and pump gas. :noway:

thanks for your help i think i have the wronge set up. i'v got a set of edelbrock etec 200 64 cc chamber and edelbrock air gap intake. that will give me too high a compression ratio... damage control.... so know i need to know which is better higher compression small quench or thicker head gasket lowwer compression larger quench. 91 octane is everyware but i can still get 93 at sunoco. thanks for your help anthony

AND don't go by duration at .050" numbers b/c it's the advertised duration number when the intake valve close's that counts!! This number create's DCR!!

B/c there are, FOR EXAMPLE ONLY!!, 295/235 cams out there as well as 265/235 cams!!

Try the 265/235 cam in a cast-iron headed, .060" quenched, 11 to 1 engine running on 92 octane and I bet it won't run around the block before blowing up!!

pdq67

are you asking what cam do i have if so this is the cam i have.
voodoo .489/.504 lift
268/276 duration

You really need a 70 or 72 cc head...

or a -14cc D Dish piston