Best way to lower compression on 12.5:1 440

[76maverick]

I believe I am getting a good deal on a 451 with 12.5 compression. The down side is that is has 12.5:1 compression. What would be the best/cost effective way to lower the compression to 10.5:1-ish so I can run it on pump gas ? Here is the specs I have: 12.5:1 compression steel 440 Crank LY rods, Ross 14.00 CC dome Pistons, balanced rotating assembly, 452 heads, extreme energy cam 510 lift.

Thanks !

 

[Cornpatch MO]

I believe I am getting a good deal on a 451 with 12.5 compression. The down side is that is has 12.5:1 compression. What would be the best/cost effective way to lower the compression to 10.5:1-ish so I can run it on pump gas ? Here is the specs I have: 12.5:1 compression steel 440 Crank LY rods, Ross 14.00 CC dome Pistons, balanced rotating assembly, 452 heads, extreme energy cam 510 lift.

Thanks !

I think about the only way you can do that is to change pistons or use a thicker head gasket......................MO

 

[rumblefish360]

You'll need to know exactly what pistons are in there, where the sit in the hole and how many cc's the cylinder head is. The current gasket size. Which would be bore and thickness.

Once you KNOW it is a 12.5-1, then you can work out gasket thicknesses and possibly purchase a set from Cometic.
I ordered a .075 thick set for my engine. It was a off the shelf size. A fella over at FABO order a set at .100 thick. It was a custom order that took a little time to get made up.

 

[GTXperience]

I think about the only way you can do that is to change pistons or use a thicker head gasket......................MO

In order to drop a full two points, a thicket head gasket may not do it on it's own. Yes, a piston change will get you there. I woud aso look into an open chambered aluminum head (along with a thicker head gasket). It may not get you all of the way but may get you closer to where you want to be. I'm trying to recall what the cc size is for the 452s. I'm thinking 90-92. They are popular OEM iron heads for a street build because the induction hardened seats but that 12.5 compression is not a common CR for the street. Also, after a valve job or two, the hardness is gone and the lack of lead will do what it does to other OEM BB iron heads (you sunk my battleship or in this case some valves). So, maybe a set of alums might be the answer (again, if it along with a thicker gasket makes sense). GL.

 

[62 Dart Convertible]

Question to those more in the know - can one have the piston domes milled down and then of course re-balance?

 

[Jeremiah]

I doubt you could have the domes milled enough to get it to a reasonable pump-gas friendly compression ratio. If it's a 451 (400 block, 440 crank, 6.76" rod) it should use a 1.32" compression height piston, which is pretty common. Either a flat top or a reverse dome will get you where you need to do. ICON, Diamond, and JE all stock 1.32" CH pistons in various Chrysler over-bore sizes.

 

[polywideblock]

don't they normally recommend a higher compression for aluminium heads ?seem to remember something about 11:1 being the norm due to thermal conductivity or something like that . could this be made to work with your 12.5 pistons and a thicker head gasket ?

 

[Jcharger]

This may be a stupid question, but can you bleed off some compression (dynamic) with a longer duration cam?

 

[JRL]

You can "lower" compression with a later ivc and a wider lsa, but this seems too much.

 

[IQ52]

If you want a pump gas engine, buying an engine that is already 12.5:1 compression is not a good deal! It is a pain in the butt and always will be unless you tear it down and rebuild it with lower compression pistons and probably a different cam. Look for an engine already built correctly. Oops no, a different cam might not be necessary.

No again, .120" gasket 10.5:1, iron heads and that cam, it is still going to be detonation city. You'd need to get a gasket that was .140" thick or more. Then stack .176" worth of intake manifold gaskets so the intake will line up. Bad idea, bad idea.

 

[dieseldazzle]

I'd figure out the actual compression then have the Pistons milled. I thought I was at over 11 to one with me -12cc pop ups but once I measured it out I was barely 10 to one

 

[Mike67]

They do fly cut pistons but in your case the motor would have to be disassembled. So like others have said either buy new pistons or buy a motor that meets your needs...
If the motor is new it shouldn't be too expensive to install new piston if your getting a hell of a deal on the motor vs a complete build.

 

[dvw]

E85

 

[Cranky]

Machining the pistons isn't exactly cheap either plus you'd most likely would have to rebalance the rotating assembly. You would probably come out just as good to sell the pistons that's in it and buy ones that would give you what you want. And if the new pistons are a bit heavier than the old ones, all you'd have to do is bring them down in weight to match the old ones and stick em in....making sure piston to wall clearance is right of course.

 

[lewtot184]

Question to those more in the know - can one have the piston domes milled down and then of course re-balance?

as long as the domes aren't hollow this is very doable and probably the most economical way to go.

 

[Cranky]

as long as the domes aren't hollow this is very doable and probably the most economical way to go.

If the domes are machinable, what would you figure the weight difference would be? What would the cost be to machine them in your area? Been awhile since I checked here but the quote I got last, the shop wanted 2 C notes to machine the pistons then another 250 to balance them and the crank again. What chaps my butt around here is that I balance my own pistons and rods so all the shop has to do is set up and spin the crank but he never gives me a break. At least I know my pistons and rods are on the money so I kept doing it....

 

[76maverick]

Guys I really appreciate the advice and suggestions ! The aluminum heads are not in the budget unless I come across an abnormally good deal on a set. The engine I got I paid 1500 for it and it has less than 2000 miles on the rebuild. My though was that it was still a decent deal even with doing some reconfiguring. I`m going to tear it down and do some math as per MR GTX`s suggestion and figure out if its more cost effective to machine (if possible) the existing pistons or just buy a new set. Seem like the correct plan of action ?

 

[Cornpatch MO]

Guys I really appreciate the advice and suggestions ! The aluminum heads are not in the budget unless I come across an abnormally good deal on a set. The engine I got I paid 1500 for it and it has less than 2000 miles on the rebuild. My though was that it was still a decent deal even with doing some reconfiguring. I`m going to tear it down and do some math as per MR GTX`s suggestion and figure out if its more cost effective to machine (if possible) the existing pistons or just buy a new set. Seem like the correct plan of action ?

You don't have to tear it down more than taking off the heads to get your measurements to do the math for what the compression ratio actually is , and to determin what you can do to get it to what you want. Could be as simple as changing head gaskets..........................MO

 

[64 Post]

Probably quite a few piston companies out there stocking a hypereutectic shelf piston for about $500/set.

 

[lewtot184]

If the domes are machinable, what would you figure the weight difference would be? What would the cost be to machine them in your area? Been awhile since I checked here but the quote I got last, the shop wanted 2 C notes to machine the pistons then another 250 to balance them and the crank again. What chaps my butt around here is that I balance my own pistons and rods so all the shop has to do is set up and spin the crank but he never gives me a break. At least I know my pistons and rods are on the money so I kept doing it....

i'd have to look at my last receipts on balance but i'm thinking it was a little cheaper here. even if one had to pay $400-$500 for everything that's cheaper than pistons. another advantage in getting rid of a dome is better flame propagation. completely removing a dome would probably be over 30-40 grams. i guess one has to pick their poison.