Compression too high?

[Roppa440]

We have 95 and 98 in the gasstations,

I live in the UK with the 98 same octane gas. I run over 10:1 static compression ratio with iron heads (pressure equivalent to over 11:1 on aluminium heads). Using a solid lifter cam with 237/242 duration @50 thou. Stock stroke and rod length. Timing is 35 degrees total and 18 initial. The pistons are those KB ones with the raised "D" shaped dome to fill the quench area. Set to 40 thou clearance using a steel shim gasket.

No compression issues. However if I use the 95 or 94 octane gas I need to back the timing off just a touch.

[Belgium Cuda]

The composition gasket doesn`t live well with the drastically different expasnsion rates of Alum & Iron , this is why every 2.2 / 2.5 engine needed the head gasket changed just about as often as tune up / spark plug changes , even the awesome 1009 FelPro gasket doesn`t live for me , the solution is the Cometic MLS gasket with 3 steel layers allowing expansion by allowing the steel shims to slide instead of tearing at the fibers of the composition gasket ultimatly failing the gasket

Can you recomend a vendor who can supply the right dimensions?

[Chryco Psycho]

Cometic has a .029 thick gasket

[Belgium Cuda]

Cometic has a .029 thick gasket

Can you buy directly from them?

[moper]

I disagree with CP onn this. There are literally thousands and thousands of composite gaskets living very well with different head and block materials. There are reasosn to run an MLS gasket.. but there's no reason to in this case. In terms of compression, I run 11.8:1 for use with 91 octane... but you have to have a cam that bleeds off pressure to run that. 10.9 is not a big deal even with the smallish K60.

[Belgium Cuda]

I just want a maintenance free engine with a lot of torque and minimal tuning because that's not my thing. Concerning the compression I can
run with a higher octane as long as I don't have to set the timing everytime it's to hot or to cold outside.

[440mike]

this is a little off the topic and my boss wont tell me why he does this ? but on bb dodge motors over 10 1/2 to1 with aluminum heads for the street he welds "lumps" beside the valve guide on both side walls in the bowl down to the floor, then i have to shape and polish blend them down a bit, all he says is to prevent detonation and pinging ? but does not tell me how this works ?
he does not do it below that 10 1/2 to 1 compression ?

the really dumb part is it must work because they make killer power on the dyno but he does not tell me how this works ?
my 451 motor made 569 hp with a hydraulic cam, it has these bumps to,
can anybody tell me how this works ?

boss likes those layered steel gaskets for the aluminum head engines to.

[Chryco Psycho]

Well I live in extremely cold climate , -35 in the last week , & compostion gaskets between iron blocks & alum heads Will not live Period , if you live in Florida maybe ......anyway I recommend MLS gaskets & hate having to change them in the car
Your Call , I figure $200 for gaskets is far cheaper then the teardown time to replace 1 bad gasket
 do what you want
 as for the welds beside the guides it would pick up air speed by running through a venturi & probably increase both swirl & turbulence which both help reduce detonation

[440mike]

yeah was real cold here to the shop door froze closed but warmer now, time to start work i hate mondays

[aarlucas]

There is a lot of good info in this thread.  But one thing that was was not brought up about the compression ratio that is more important than the "calculated" compression ratio is the cylinder pressures.  Having a good tight quench will buy you a little more pressure safety as well.  As someone else stated, a good quench is around .040 (you can go as low as .035 with good steel rods, but that is pushing it.) and should be NO MORE than .055.  Anything above .055 and you loose the quench effect and benefits.

You can have a 12-1 "calculated" compression ratio if the cam has enough duration ( at the right timing events) to release the cylinder pressure down to a managable point.  You can also take away a little timing to control detenation as well. There are many things that need to be taking into consideration for this type of engine to squeeze the maxumum performace from it and still be safe with pump gas.  The rule of thumb with aluminum heads is approx 185psi.

For example, my engine is a 440 with edlebrock heads.  I have a zero deck flat top piston.  A solid roller cam and I have a quench of .041 and my cylinder pressures are 195 psi.  I run 93 pump gas and have never had any detenation issues with it.  I found the best timing is 36*.  I know I am puching the limits.  But I race my car as well as driving it on the street.  So I squeeze every ounce of performace out of it.