Will increased compression ratio inrease my Boost #'s?

[Sweet '96 Cobra]

Will increased compression ratio increase my Boost #'s?

A short summary of my setup first... I asked for 8.5:1 ish compression ratio from the shop that built my shortblock, I get it back friday and he tells me it is now 9.15:1 compression ratio with the .20 over Manley 11cc pistons. Im installing some cnc ported heads that have 53cc combustion chambers with stock valves and cams still. Im also running a 2.8KB mammoth setup and installing new longtube headers this time.

Im a little worried about the compression ratio as we can only get 91 octane here and my car is 99% street car. Will this increase my Boost #'s and will it be an optimal compression ratio or should I get them to switch out the pistons and go back to 8.5:1? Any thoughts?

Thanks,
Jared

 

[Bdubbs]

Boost is boost so I don't see how raising your compression will increase boost. If anything a bigger s/c pulley will lower your boost levels. Someone correct me if I'm wrong.

 

[Quick Strike]

Your boost may actually decrease as boost is a measure of restriction and you have reduced restriction with the head porting and long tubes. You have not reduced the CID and have actually increased it a few cubes with the overbore. The true issue is that extra squeeze of higher compression could exceed the fuel's tolerance resulting in detonation. You could always change to cams with longer duration if it were to detonate. The extra .5 in compression ratio should be ok for 93 octane, but I don't know about 91.

 

[s351]

The only thing you have to worry about is your tune, The raised compression may not enable you the run the same amount of boost on 91. But you should be able to make the same power or more with a little less boost as your motor on its own should be making a little more power. More compression = more heat especially with a twin screw. Thats not a huge compression jump but you might have to either lower the boost and up the timing or drop some timing and leave the boost the same.

 

[Sweet '96 Cobra]

Thanks Guys, ordered up some Cometic head gaskets at .066 thick and 3.70 cylinder diameter to lower my compression down to 8.55:1 . The stock style mls gaskets are .036 and 3.63 in diameter.

 

[SlowSVT]

Thanks Guys, ordered up some Cometic head gaskets at .066 thick and 3.70 cylinder diameter to lower my compression down to 8.55:1 . The stock style mls gaskets are .036 and 3.63 in diameter.

Is this a .020 over 4.6? If "yes" I would not run a gasket designed for a 3.700 bore for a few reason.

I wouldn't sweat the .6 bump in compression ratio. You just won't be able to run quite as much boost on 91 which is generally limited to 16 psi at 8.5:1. 14-15 lbs should be safe with 9.1:1

 

[Sweet '96 Cobra]

Is this a .020 over 4.6? If "yes" I would not run a gasket designed for a 3.700 bore for a few reason.

I wouldn't sweat the .6 bump in compression ratio. You just won't be able to run quite as much boost on 91 which is generally limited to 16 psi at 8.5:1. 14-15 lbs should be safe with 9.1:1

Yes it is a .020 overbore. Can you elaborate on reasons not to use the 3.70 bore gasket? The shop that built my shortblock recommended the 3.70..

 

[SlowSVT]

Yes it is a .020 overbore. Can you elaborate on reasons not to use the 3.70 bore gasket? The shop that built my shortblock recommended the 3.70..

Using a 3.700" bore gasket in a 4.6 bore block won't have as much of a robust seal of the smaller bore gasket which will limit how much boost you can run and will be more prone to blowing head gaskets. This is the #1 reason for not doing this! The shop should know this better than anybody.

The larger bore gasket will leave a small grooved cavity where the head meets the block. The charge trapped in the gap may not combust completely. Overtime carbon build-up may accumulate there.

It reduces the clamping area between the head and the deck of the block.

.066 is a pretty thick gasket and will be more exposed to the combustion pressure which in a supercharged engine can be quite high.

The only treason I can think of why the shop is using this gasket is because they ordered the wrong one :nonono:

 

[R.D.P.]

You could run a quarter or half bottle of Torco per tank to bump up a few octane points to be safe. I wouldn't mess with the headgaskets, just either drop a little boost to balance it out or add some Torco and make even more power than before at the same boost level. If my motor ever get's rebuilt I'm going at least 9.5:1, but it will be on e85.

 

[Sweet '96 Cobra]

Using a 3.700" bore gasket in a 4.6 bore block won't have as much of a robust seal of the smaller bore gasket which will limit how much boost you can run and will be more prone to blowing head gaskets. This is the #1 reason for not doing this! The shop should know this better than anybody.

The larger bore gasket will leave a small grooved cavity where the head meets the block. The charge trapped in the gap may not combust completely. Overtime carbon build-up may accumulate there.

It reduces the clamping area between the head and the deck of the block.

.066 is a pretty thick gasket and will be more exposed to the combustion pressure which in a supercharged engine can be quite high.

The only treason I can think of why the shop is using this gasket is because they ordered the wrong one :nonono:

I appreciate your concern on the choice of head gaskets. I actually spoke with a Fellow from Cometic this afternoon about my situation and asked him about the gasket bore size my machine shop had recommended and ordered for me and the Tech Rep from Cometic said that they will be fine and that its common for this type of use so that made me feel a lot better. He went on to say that the extra thickness will be fine also and that they always recommend the head studs from ARP which Im also using.

You could run a quarter or half bottle of Torco per tank to bump up a few octane points to be safe. I wouldn't mess with the headgaskets, just either drop a little boost to balance it out or add some Torco and make even more power than before at the same boost level. If my motor ever get's rebuilt I'm going at least 9.5:1, but it will be on e85.

I thought about that also, but the more I thought about it the more I wanted to be able to fill up at the pump and not spend extra for the Torco every fill up, thats not to say that there wont be a 20+psi tune in my future.