MILD THUNDER

iTrader: (3)

Flat tops in a 4 inch stroke, 4.280 bore, will not yield 9:1 unless you have around 100cc chamber heads. More like 8:1.

endeavour32

iTrader: (4)

I could have missed it here, but did you measure how far in the hole your pistons are? If you have not measured that, please do! I'm in the process of rebuilding my engines, and my pistons in one block were in the hole .025. Typical for an uncut deck is .020, however you cannot assume that is where you are at. Also unless you cc'd your heads you really don't know that spec either. Very few stock heads actually cc at 118, you are more likely around 120-122 cc's or you could be less, unless you cc you will not know. All of these little variances will make a difference in your compression ratio. Here is an example:

13.8 cc domes
118 cc chambers
.020 in the hole
.039 gasket
This will give you 8.96 cr

Now lets keep everything the same but-
120 cc chambers
,025 in the hole
Now your cr is 8.75

Which as it sounds is exactly where you want to be. Before you do anything else- check all of these measurements.

One of my engines, as it came from merc, only had a cr of 8.66 after I did all the measurements. The advertised ratio of 8.75 is not guaranteed.

MILD THUNDER

iTrader: (3)

Measuring your deck height now with the current pistons, wont be the final number. Once the machine shop determines how much needs to come off the deck of the block to make it true, is unknown. Then, once the new shortblock is assembled, you will want to measure your deck height with a deck bridge, and take into consideration piston rock. . Once you know where you're piston sits in the bore, write that down. You will also want to cc the chamber on each head. I've seen where shops have surfaced only one head out of a set. Heck, my two sets of dart heads, out of 4 heads, they each had a different cc chamber. They were all milled to match the smallest of the chamber size. As stated, there can be variances, esp if the heads have been surfaced. Once you have those numbers in your hand, you can now choose a head gasket, that should get you close to your ideal compression, but be within the bounds of an acceptable quench.

If you come in at 8.75:1, or 8.9:1, you'll likely not notice a difference from either a peformance standpoint, or reliability standpoint. The rest of the setup is far more crucial, such as fuel mixture, timing setup, intake air temp, spark plug design and heat range, engine water temp, etc. Also, you may want to have your cylinder head guy take a look at the chambers. A good cylinder head specialist can reshape the chambers slightly, unshroud the valves a bit, which helps airflow as well as combustion swirl. While it will take away a few cc, the small drop in compression is worth the gains. Also, a GOOD valve job and bowl blend is always a good idea. Just depends on how much you're willing to spend on those heads.

I think the bottom line here, is the 2399 .030 sized, will put you in the ballpark, which you can zero in on with head gasket choice.

MILD THUNDER

iTrader: (3)

In my merc manuals, it lists the 1989 454 Mag engine, to have a static compression ratio of 8.6:1.

mike tkach

iTrader: (3)

glad to see some facts instead of incorrect information like some guys post up.

mike tkach

iTrader: (3)

GOOD COP-BAD COP,lol.

sutphen 30

so your the bad cop.

mike tkach

iTrader: (3)

im not sure.

dereknkathy

but it also says the 330 is 8.5. all the same numbers with 0 dome volume comes out 7.73:1.

endeavour32

iTrader: (4)

Exactly- put in all the spec depths and cc's on a 454 mag and you come up with a ratio of 8.86 CR. We all know the chamber sizes on stocks heads vary, and that was the point of my post! You don't know what you have until you measure.