Tips on selecting Pistons??

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Princess Valiant

Princess Valiant

A.K.A. Rainy Day Auto
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I have rebuilt engines before but typically I would find a suitable kit and buy the kit for x- amount of dollars and then had everything to put it together.

The kits usually came with pistons and who knows what kind of compression they were running.

This engine build I would like to know what kind of compression it has and more specs than would be with a generic kit.

So any tips on calculating compression and what could or should go into the engine.

The engine in question:

360 with a build date of 7 /24/74 ....presumably for a 75 vehicle. This engine came out of an RV. The intake was missing so I have no idea if it is a 2 or 4 barrel version.

I live at almost 6k feet in altitude so the compression here is not what it would be at sea level.

The engine is intended for a street car....a pretty mean street car. Not really into the race thing. Will be running street tires with a 4 speed and undetermined gears.

ok, let the discussion begin :)
 
Stock pistons for that LA model 360 will be around .110" in the hole. Typical SCR with a later Standard Felpro head gasket (thicker than the original) and a .030" overbore will be around 7.5:1..... nothing to get excited about and worse at altitude.

To get this number, I used the Pat Kelly calculator (on tab #3) with the following inputs:
Bore 4.030"
Stroke 3.58"
Chamber volume 72 cc
Gasket thickness .051"
Gasket bore 4.17"
Deck clearance .110"
Flat top piston with 9 cc dish volume

Use this calculator or get comfortable with another and learn the common parameters and you can check SCR and DCR over an over to your heart's desire with all sorts of parts choices. This calculator will also figure your DCR with the proper inputs.

For street use, I like to look for SCR over 9:1. This is likely to push you to very different pistons like Speed Pro H116CP (has similar piston and pin weight to the stock so probably no rebalancing of the crank), or KB232's for about 9.5 SCR (same thing on the weight..).

But all this will vary as you change heads and a few other things. Just keep in mind that a good safe region for pump gas is 9.5 SCR max (and low 8's DCR) for iron heads, and a bit more of AL heads.

So what do want in a 'pretty mean' street engine? That can have all sorts of meanings! Glad you are looking at this the right way: picking the application and building to that.
 
The very first thing I look at is compression height. After calculating block height minus rod and 1/2 stroke 1.687" is left. Many aftermarket pistons are in the 1.675" range, this gives .012" left for block clean up, zero deck or just the right head gasket for quench.

Once this is known head volume and such can be inputed to any compression calculator for an accurate number you can work with.
 
First thing I'd try to figure what cam range you'd be using and work backwards from there. And to figure that out gearing and powerband comes into play. 360 is a very capable engine usually don't have to go over 6000rpms to make huge power. Most probably peaking in the 5000-5500rpms (350-400 hp) with 260 to 280 cams. Once you figure out the cam not to hard to figure out CR needed.
 
You're moving out of the mild enthusiast spot into more of the specialized area. Best thing I can do really is give you what I'd be asking if I were working with you on it...
You said altitude, and "strong street car". Do you need power brakes? What can you do for regular maintenance? What can you live with for noise? What fuel type? Will you ever go out of the mountains with it? What's the actual budget?

In order to decide on internal parts - you have to know what you have to spend and what you want from it. RRR's idea is probably close although I prefer the Keith Black versions. But a forged unit might be better suited to your use depending on what you consider "fast" or what you need.
 
Bang for the buck I think RRR is spot on. If you ever plan to spray on it spend the extra $$ for forged.
 
Rani, what's the target/goal? E.T. Wise, do you have a goal?
Or are we just doing a simple mean machine build?

Answer me these;

Octane available
Car weight
Rear gear & tire size
Transmission to be used (I seen it said 4 speed, auto or manual)
Aluminum of iron headed
Ported heads or not
Valve sizes intended
Is there a limitation to observe?

Considering I am at sea level.....

General and typical rumblefish street build.

360 w/flat top Pistons at zero deck.
I have used the KB-107's a few times and now I'm building a 360 for the wife's Cuda using the above link RRR posted. If you can afford new rods, a little overkill here doesn't hurt. New scat rods are reasonable in price.
Head gaskets vary for edging the ratio up or down.
Have the block deck mills to achieve a zero deck height and for use with MLS gaskets.
750/800 carb/Dual plane
An Edelbrock RPM or a Weiand Action Plus are the best bets IMO. A 750 covers most bases. The 800 is also pretty good on top.
Well prep'd cylinder head,1-3/4 header - 2-1/2 exhaust
If the cam stays small, (read below) the head just needs good basic work. If your stepping up in a cam size, more port work bcomes a thing to really work towards obtaining.
Aluminum or iron headed, flow is flow and here is where the power makes or brakes an aggressive street combo. The rest of the exhaust should be free flowing and at least 2-1/2 inches. 3 inches gets a little loud but opens up some good power if your getting over a 475+ hp level.

The cam, converter and rear gear/tire size now need to be worked out for your car & the rpm range you want to be in. You could go as mello as a cam @ 230@050, a mild 2500 converter and 3.55's.
You could also step up in cam to a 244@050, 3500 converter and use 4.10 - 4.56. Your tire was increases a good bit.

And yet again, moving up to a larger cam and converter is possible. Though just a tad light on compression, you may also like to get some well ported heads at this point. But the point is, the base short block package is there and good enough to use in a pretty flexible mannor with a verity of changes. Simple, effective for the street, easy on the pocket.
 
Another shot of the above linked slug
 

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