10.5:1 273 pistons with 360 heads compression ??

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ProStocker273

ProStocker273

MoPar N Jeep
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Well I was looking at throwing on a set of 302 heads on my 273. They turned out to be 308 heads. My short block is running factory bore 2bbl pistons zero decked with a felpro gasket (4.06 x .039) I believe. I'm not looking forward to the compression drop as I'm looking to go with a better/bigger cam. I got a few leads on some commando pistons (10.5:1). I can't seem to find a way to calculate my compression with the 10.5 pistons and 308 heads. I was just trying to guesstimate with published chamber cc numbers but still unsure. Also I plan to use a gasket with a (3.7 x .028) with the 308 heads and 10.5s

Where will I end up roughly compression wise with this combo?
 
I'm not entirely sure what the height differences are between the two but I believe that's where it's made up because the hipo 273s have reliefs, the lopo don't. If I had to guess I'd say you'll gain 1/2 point with the pistons, but you're losing a full point with the heads. Instead of pistons, you might want to have a set of 273 bore thin head gaskets made up. They won't affect the balance, might be around the same price, and will get some of that compression back without too many side effects like additional milling.
 
The factory rating for compression for 273-2s was 8.8 to one where as the 273-4 was 10.5 to one. The heads and gaskets were the same.
 
My question is using 10.5 pistons with open chamber heads and a thin head gasket( .028 x 3.70 ). Where would my compression be at roughly?
 
My goal is to keep my compression up with the 360 heads.

Now I have (zero decked block with 2bbl pistons and 4.06x.039 head gaskets

Option one (2bbl pistons with 3.7x .028 gaskets and 360 heads and cut heads)

Option two. (4bbl pistons 10.5s and 3.7x .028 gaskets and 360 heads)

I would prefer to not cut the heads or tear into a fresh motor for a piston swap but I gotta keep compression up..

I believe the difference in cc between the 920 and 308 heads 8-12cc's
 
I think your CR will be in the toilet. I'm using domed pistons from Kanter with closed chamber heads and mine will probably come in a little above 9:1.

I would also be concerned about possible interference between the valves and the block, given the small bore of the 273.

To calculate your CR you will need to know the compression height and dome volume of the pistons as well as the chamber volume of your heads. Deck height would be good to know as well.

I have piston measurements in my engine build thread for Egge, Kanter, and old TRW forged pistons: http://www.forabodiesonly.com/mopar/showthread.php?t=199120&page=2

As for chamber volume, I can tell you that every closed chamber 273 head I've cc'ed was significantly larger than spec. I believe spec is 57cc's but most measure around 63-64cc's. So you really need to cc your heads as well.

Not sure how you got to zero deck on your existing build. I'm not aware of any zero deck pistons for 273's so I'm guessing you've milled the block. Most blocks left the factory taller than spec. Mine was .010" taller than spec. Do you know what your deck height is now?

This is the CR calculator I use:
https://www.uempistons.com/index.php?main_page=calculators&type=comp
 
That's my problem... the 8.8:1 piston are shorter and would not have been flush with the deck. The block was decked to make them "0". So the block's shorter than it would have normally been. When you stick the taller 10.5:1 piston in there, it's going to pop out the top of the block by the amount of difference in compression heights between the lopo & the hipo pistons.

Anyone know that difference in compression heights?
 
ProStocker273 said:
My question is using 10.5 pistons with open chamber heads and a thin head gasket( .028 x 3.70 ). Where would my compression be at roughly?
Click to expand...

I used 360 heads on a 318 that had too high of compression with fel pro .040" gaskets. It brought the compression down to 9.2

I would think that you would be somewhere in the 8.8 - 9.0 range without doing the calculation myself. You have a couple of different variables that would be difficult for me to calculate.

What is the current cc of your original heads?

What is the stock head gasket thickness and diameter of the fire ring of the head gasket for the 10.5 pistons and head cc's.


Also, you will have to notch the bores to get the larger valves in the 273. Are you going to do that?


I can do the compression calculation if you provide the information that I need and are going to notch the bores. I don't want to go through all of that if you are not going to go through with it. I am willing to do it if you are serious about it and can give me the missing information.


What is your target compression? I can do the calculation backwards and tell you how much clearance volume you will need to go from 10.5 to whatever your target is.
 
Well this is where I was saying estimate and using published numbers. 360 valves clear. I don't see how 10.5s and open chamber would end up with low compression just not sure where it would end up. I am using a small bore thin gasket as stated. I don't know deck height as I got the motor and just replaced bearings gaskets heads cam (normal rebuild stuff) but the pistons measured .003 down in the bore and yes they have 4 valve reliefs. I'm assuming the deck was cut as most 273s I've measured are -.015+ down in the bore.
 
Motor as it sits
3.63 bore
Deck height unknown
2bbl Piston -.003 down
Gasket size 4.06 x .039
920 273 heads 62cc chamber (guess)

Option one
3.63 bore
Deck height unknown
2bbl pistons -.003 down
Gasket size 3.70 x .028
308 360 heads 70cc chamber (guess)
(Maybe cut head -.020. -.030)

Option two
3.63 bore
Deck height unknown
4bbl pistons +.139 height difference
Gasket size 3.70 x .028
308 360 heads 70cc chamber (guess)
 
ProStocker273 said:
I don't see how 10.5s and open chamber would end up with low compression just not sure where it would end up.
Click to expand...

The 10.5 compression pistons and the closed chamber heads are a matched set that produces the 10.5/1 compression ratio in other words the piston and the combustion chamber produce the compression ratio. With a 360 head you have a couple things working against you. They are open chambered and not closed chambered. The 273 heads are listed as 57cc whereas the 340/360 heads are listed as a 70cc head. There's a lot of compression loss no matter what piston you use. Also because of the larger bore of the 360, the chambers of the 360 heads are huge compared to the small bore/ small chambered heads of the 273.
 
Thanks tmm. I'm aware of that info just trying to keep it above 9:1. I figure if I'm gonna run the 360 heads the commando pistons where my best bet to keep compression up.
 
To get 9.5 compression, you will need a clearance volume of 66 cc.

This means that you should have 66 cc "above the piston" at TDC. This includes the volume of the head combustion chamber, the volume of the head gasket and the volume above the piston.

Standard 360 heads that I've measured are 72 cc's.

the head gasket at 3.70" bore and .028" thickness is 4.9 cc's.


So you will need:

72 + 4.9 - 66 = 76.9 - 66 = (-10.9 cc's)


This means that your piston must displace 10.9 or call it 11 cc's above the head deck.

Using 3.63" for the bore, you will then need your pistons to stick up .062 inches ABOVE the head deck. This assumes a flat top piston. If you use a piston with valve reliefs, then you will need to tell the piston manufacturer that you need your piston dome to displace 10.9 cc above the deck including valve reliefs.
 
X2^ Veloicty is king with our small bore short stroke 273's. You will give up alot of TQ down low (& we dont have alot to begin with!) with the huge open chamber 360 heads! Just do good port work to your current heads o some 302 casting 318 heads, youll be alot happer with the throddle responce man! (spelling sucks sorry)
 
Thanks krazy kuda though I'm not paying for custom pistons and I'm not sure where the commando pistons are as far as cc's
 
I'm running 4.56 gears with a 4speed and a tunnel ram. I'm working on getting motor to wind up. I can't find any closed chambered heads and ended up with a set of 308 360 heads so I'm trying to make em work. My current heads are great heads but one has a small crack near the front of the motor. I don't think I'll need the bottom end power as much. With the gears a t-ram and light car this combo will like more rpm I'm sure.
 
Yeah the thing is I got the heads and the motor and I'm a tight ***. I'm not gonna spend alot of cash on a 273 when I got a 360 under the bench in the shop. But hey it's in the car it runs strong now and I think it's got more on the table if I can make the 360 heads work. My cam I'm looking at is a higher rpm cam and needs some compression.
 
To get 9.1 compression, you will need a clearance volume of 69.3 cc.

This means that you should have 69.3 cc "above the piston" at TDC. This includes the volume of the head combustion chamber, the volume of the head gasket and the volume above the piston.

Standard 360 heads that I've measured are 72 cc's.

the head gasket at 3.70" bore and .028" thickness is 4.9 cc's.


So you will need:

72 + 4.9 - 69.3 = 76.9 - 69.3 = (-7.6 cc's)


This means that your piston must displace 7.6 cc's above the head deck.

Using 3.63" for the bore, you will then need your pistons to stick up .043 inches ABOVE the head deck. This assumes a flat top piston.

For a flat top piston to be 9.1 - 9.5 compression, it will need to stick up .043" to .062" above the deck. If you want to use pistons with a dome between .043" and .062" and you will have to take a little off for valve reliefs.

The piston will need to stick up a little higher to compensate for the valve reliefs. So an educated guess, that pistons with valve reliefs should stick out somewhere near .050" - .067" range.

Put your piston in the block and see how far it sticks above the deck.

If you use 360 heads, you will need a piston that sticks up above the deck about .055" average (give or take a little).

If you don't have pistons that stick above the deck, then you will want to use the 273 heads to keep your compression up.
 
Need to know chamber volume; get a Burette, measure the chambers. Pick a cylinder, put a piston down .500 measured off the flat of the piston, calculate the volume, then with the Burette you just used on the chambers, measure the volume, subtract that from the calculated, gives you dome volume. If the piston sticks up out of the hole, measure from the flat down to the deck, calculate that volume. Calculate the sweep volume of the piston.

Add sweep, chamber, gasket volume, subtract dome, add/subtract deck (piston in, or out of the hole), divide that by chamber, plus gasket, subtract dome, add/subtract deck. That will give you your compression ratio. Pretty much anything less is like playing pin the tail on the Donkey.

According to "How to HotRod Small Block Mopars", the pin height is the same between 2 and 4 bbl engines at 1.79 inches.

The conversion factor from cubic inches to CC's is multiply CI by 16.387; do that before you do all the addition/subtraction above.


I would not recommend adding 2.02 intakes; you can if you really want, you'll just have to notch the block for clearance, another compression drop, 1.88's may need notching also.
 
I put 70cc W-2 heads on my 273 several years ago. I had an old school set of 13.2:1 domed pistons. I had to fly cut the tops of the cylinders so the 2.02 valves would clear. I also used an old school .027 thick 273 head gasket. (Those things were actually available from JC Whitney back then....) I never figured the actual compression ratio but it wound up having 180 to 182 lbs of cranking cylinder pressure. Of course a lot of the cylinder pressure is dependent on the cam but thought this info might give you a little better idea of what you're up against. You can definitely rule out any low end but once it hits 4000 the fun begins!!
 
The Rebuilder's Choice Domed Pistons from Kanter are taller than the Egges so we'll assume those. I cc'ed those pistons and the dome volume is 11.5cc's. The compression height I measured was 1.769" Assuming your deck height is right on spec at 9.6" and assuming all the numbers you gave above, the static CR calculates to 8.75, which is not as bad as I expected. The smaller thinner head gaskets are helping you quite a bit.
 

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I think there is a good chance that block has been decked below spec. What do you think about pulling out a piston and measuring the compression height. Then you could back into the deck height, assuming stock stroke and rod length.
 
You guys need to remember also, just cause you buy "10.5 to 1" pistons, DOES NOT mean the engine will be true 10.5 to 1. Most of those engines were lower in compression than that. Its a blueprint spec...
 
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