360 engine, which pistons with Magnum Heads?

[360duster]

Hi Guys,

i´m thinking about rebuilding a 360 SB that i already have for my Valiant. I want to use magnum heads, which pistons should i use to achieve between 9:1 to 10:1 compression ratio? I´m not sure if KB`s 362 are the way to go, with this piston i have to machine the small end of the rods and this will change the balance, am i right? Can anybody suggest a Piston that doesn´t need this modification to the rods and gives me the compression ratio i want to have? thank you for any input!

Michael

 

[MopaR&D]

Just get zero-deck flat tops. Even though it'll give you around 10.5:1 compression the large squish pad of the closed chambers in the Magnum heads will keep detonation to a minimum on pump gas. A couple people here can attest to that, one being dgc333.

In fact going with any below-deck piston would probably increase the potential for detonation because you don't have the squish or quench action when the piston reaches TDC.

 

[360duster]

Hi,

so you would suggest the KB 107, even with the smaller chamber? Or are there any other good choices with the same potential?

thanks Michael

 

[dgc333]

I have the KB107's with the block machined to put the pistons at zero to deck. With the 64cc magnum chamber and an 0.039 thick head gasket I have 10.6:1 compression.

I am running a Comp XE268H cam which gives cranking pressure of between 180-190 psi. The ideal quench of this set up lets me run 89 octane and I have used 87 in a pinch (but I do need to take it easy with 87).

The KB362 will still give you the ideal quench with lower compression from the dish under the chamber.

If you want to go this way and would like a set of magnum rods so you don't have to machine your LA rods you can have them for the cost of shipping. They have 5.2 pistons on them now so the cost of shipping will include the weight of the pistons.

NOTE: The magnum rods and KB362 pistons will be a lot lighter than stock so it will be imperitive that you balance the rotating assembly.

 

[360duster]

Hi Dave,

thank you for your offer, your a nice guy! But i think i go your way with the KB 107 Pistons. I always thought that the magnums have 60 cc chambers, so i was scared to reach over 11:1 compression.

Another Question: is it necessary to balance the rotating assembly with the new (KB 107) pistons?

greetings Michael

 

[Coyote Jack]

Dave and I have very close to the same setup. I can attest to his findings and recommendations. I run a little different cam but all in all we have the same engine internals and it works great.

Jack

 

[dgc333]

Hi Dave,
Another Question: is it necessary to balance the rotating assembly with the new (KB 107) pistons?

I would, the KB107's are quite a bit lighter than stock. When I built my engine I also used Eagle SIR rods which are lighter than stock too. To balance the rotating assembly almost a pound of metal we removed from the crank. Even if only 1/2 of that needed to be removed for the pistons that's an awful lot of imbalance.

 

[sharpie]

I'm running Speed Pro STL-H405CP30 pistons. Haven't fired the engine up yet, but I'll update when I have to see whether they're good. They sit about .06 below the deck.

 

[dgc333]

.06 below the deck is about the same as stock and will make for a low compression motor, down in the 8.5:1 range with open chamber heads and low 9's with a closed chamber head like a magnum head.

 

[sharpie]

.06 below the deck is about the same as stock and will make for a low compression motor, down in the 8.5:1 range with open chamber heads and low 9's with a closed chamber head like a magnum head.

That's why I'm running closed-chambered heads and a thin gasket, so my quench doesn't exceed the tolerance.

 

[dgc333]

Once you get past 0.050" you loose much of the quench benefit. Even with closed chamber heads and a thin gasket 0.060" in the hole isn't going to provide much if any quench.

 

[fishy68]

That's why I'm running closed-chambered heads and a thin gasket, so my quench doesn't exceed the tolerance.

I'm not an expert but have read several articles on the effects of "Quench" and designing it properly and from what I've read you don't want much farther than .040~.045" or it won't be very effective.

 

[LXguy]

Interesting stuff. Lots of good advice in this thread.

This motor did alright with the stock .050 below deck piston arrangement:

http://www.hotrod.com/techarticles/engine/hrdp_0804_small_block_mopar_engine/index.html

Steve

 

[fishy68]

Interesting stuff. Lots of good advice in this thread.

This motor did alright with the stock .050 below deck piston arrangement:

http://www.hotrod.com/techarticles/engine/hrdp_0804_small_block_mopar_engine/index.html

Steve

There's no doubt they do good even in stock form. That's what got me loving 360's back when I bought my first C body with one in 1985. But when set up for a good quench they'll quite a bit better. My 74 Fury was a low comp., pistons way down in the hole and I still had to run 89 octane and keep the timing at no more than 35 degrees or it'd spark knock. I believe that was due to the open chamber, no quench effect. DGC333's engine is a great example of how you can run high compression on 89 octane when taking advantage of properly designed "quench".