compression height

[larrym]

Buddy of mine is putting together a motor I'm trying to figure out where the piston is going to finish relative to the deck. Experts please point out glaring errors.

JE pistons eagle rods.

So assume you have a block with a 9.510-inch deck height, 5.967-inch connecting rods, and a stroke of 3.400 inches. First, divide the stroke by two and add that to the rod length: 3.400 / 2 = 1.700, and 1.850 + 5.967 = 9.517. Next, subtract that answer from the deck height: 9.510 – 9.517 = +.007. So the answer is that the piston compression height should be .007 inches out the hole?
Based on this what sort of compression with stock combustion chambers

 

[pacecarta]

d

 

[larrym]

He plans to run a stock Felpro so I think they are at .060 compressed according to the Machinist there might be a couple thou of the heads.
The valves are new but still have a slight dish.
My numbers look right on the calculation for the compression height?
I used a calculator online for compression and came up with 8.5 to 1 but that seems low to me I thought it would have been closer to 9 to 1

 

[earlbrown]

So the answer is that the piston compression height should be .007 inches out the hole?
Based on this what sort of compression with stock combustion chambers

No, the compression height should be whatever you're shooting for.

In your example, those pistons with your crank and rods just happen to put you .007" out of the hole.

8.5 is probably pretty high. It seems the JEs have a pretty good sized dish. If you have valves with a dish on the bottom that's going to lower it too. 9:1 is actually a pain to achieve to get with common off the shelf pistons. That's why I designed my forged slugs with smaller dish CCs and infinite compression heights.

Until you CC your heads and pick out the exact gasket, there's no way to know (or even guess closely)

 

[pacecarta]

s

 

[earlbrown]

I was thinking the stock heads were like 49?? It seems like with my OS Ferra's sunk into the combustion chamber as far as I could get them netted me 46cc. Granted I only took .004" off the deck just to true them and I polished the combustion chamber.

 

[PSID6]

Well if the piston is out of the hole you have to subtract those cc volumes, because it is a positive number, in order to get your true CR. The -29.5 dish cc's are the actual dish of the piston itself, not the whole piston sticking out of the hole. Just as a reminder compression height is the measurement from the piston pin centerline to the top of the piston,not how much the piston is sticking out of the hole. It's not going to be much but I thought it might be worth mentioning.

 

[larrym]

So my calculations are not out to lunch?
Piston relative to deck height ?

 

[PSID6]

No there not out to lunch but not accurate either.

 

[earlbrown]

So my calculations are not out to lunch?
Piston relative to deck height ?

No, if the numbers you used are accurate, 007" out is correct. You'll notice if you build it, they all won't be exactly 007" due to the block not being perfectly square and variances in stroke and rod length.

 

[larrym]

Thanks guys good info we shall see what the machinist finds once things are assembled.
So even if it is .007 or less, as long as he runs a stock head gasket he should be good.

 

[PSID6]

I was referring to CR.

 

[earlbrown]

right. The general rule is you want the head at least .040" away from the piston at TDC. With only 7thou protrusion, it's easy to stay off the head with a normal gasket.

 

[turbodave231]

Calculating the piston deck height is Great to see where you might end up.......but you might be wrong if you don't do a trial assembly and measure with a depth mike (or some other method).

Set the crank in the block with bearings, Assemble the rods to the pistons without the spirolox and don't put any rings on the pistons. Drop all 6 in place and snug the rod bolts.

I just finished measuring my latest build. Same JE pistons, 3.400 stroke, 5.960 Molnar rods, and a finished deck height of 9.510". The pistons range from .006 to .008 in the hole. Measure the parts (including CCing the heads) before you order your headgaskets.

 

[larrym]

Calculating the piston deck height is Great to see where you might end up.......but you might be wrong if you don't do a trial assembly and measure with a depth mike (or some other method).

Set the crank in the block with bearings, Assemble the rods to the pistons without the spirolox and don't put any rings on the pistons. Drop all 6 in place and snug the rod bolts.

I just finished measuring my latest build. Same JE pistons, 3.400 stroke, 5.960 Molnar rods, and a finished deck height of 9.510". The pistons range from .006 to .008 in the hole. Measure the parts (including CCing the heads) before you order your headgaskets.

Thanks Dave his rod length is 5.967 so if all things work out similar to yours he would end up close to 0 deck that would make my buddy happier

 

[chriss4655]

I'am Happy just to have a cam with lobes !!! LOL Great info here !!!

 

[larrym]

I'am Happy just to have a cam with lobes !!! LOL Great info here !!!

Lol you are going to be amazed at the difference Chris.
Here is a pic of his wiped cam!

 

[chriss4655]

The car had a funny idle every since I have owned it,the motor never had a valve train tick, the thing ran pretty good. Good power down low , just never had the top end charge i thought it should have!! There are a few wiped lobes the one in the pic is the worst and it is #3 exhaust !!!!! Some pic's of a few of the parts for the new motor

 

[chriss4655]

After square/parallel decking .006 out of the hole

 

[larrym]

My math was good!
That means my build should be close to zero deck.