Compression Ratio ?

[mustangmendez]

I am trying to figure out what compression ratio my 460 would have ?
Info :
Block DOVE- A .030
4.390 Bore
3.85 Stroke
10.300 deck height
Heads DOVE -C
75.8cc
Pistons KB Hypertech
22cc Dish
Head Gasket
.040
Any info ?????
Thanks Tony

[jschiller]

Virtual Engine 2000 gives 9.66 to 1, assuming a head gasket bore equal the cylinder bore, i.e., 4.390

John

[nanard289]

Hi Tony,
To know your exact compression ration, you have to specify:
- the exact rod length,
- the exact piston compression height.
(we don't take care about the small clearance volume between the fire ring and the piston top).
These values will permit to calculate the exact clearance volume between the block desk and the piston top. However, if you don't know it, you can refer at the John's calculation that is a very good approach.
Kind regards.
Bernard

[blykins]

Give me the diameter of the head gasket hole and how far the pistons are down in the cylinders at TDC...and I'll calculate it by hand.

[mustangmendez]

I dont have the engine together yet ,I was hoping to fique somthing out before I bought the pistons .
With a 10.310 deck higth the CR :is 9.9 to 1
That is a 71 and later 460
Mine is pre 70 10.300
Does that help any ?
Tony

[blykins]

Well it would help if you knew right off hand how far the pistons sit in the hole (even on a stock engine) and I will definitely need the head gasket bore.

I'll see if I can lookup some stuff and find the info I need....if you happen to run across it before I do, just post.....

[blykins]

Alright....I found some stuff....Head gaskets have a 4.500" bore, and the pistons usually sit .010" down in the hole for your year. You will probably need to know that when you order your pistons anyway....especially if you have to have the block decked....

Give me a few minutes and I'll work this up for ya.

[blykins]

I'm showing 9.59:1 with the above specs, a head gasket with a 4.500" bore and .041" thickness and the piston sitting .010" in the hole. That's with a 22cc dish. Want me to see what the dish needs to be to get you the 10:1?

[blykins]

I went ahead and did it anyway.....a 17cc dish right on the button will give you 10:1 with the above specs.

Hope all this helps...

[mustangmendez]

Blykins

Were your figures with the 10.300 or the 10.310
The machine shop did mention somthing like 0 deck height.
With the 10.300 Block
Thanks good work
Tony

[steelcomp]

Mendez...
To really "know" your compression rate, you need to be able to measure your dome volume. This is the part of the piston that is above the top ring, including dishes, valve reliefs, pop up domes, etc. and you should consider the small space between the piston and cyl, above the ring to know your true ratio. Do you want to KNOW the CR, or do you wnat to have someone else tell you? Or just guess? Without knowing, you're guessing. It's easy. You should find out the exact (as close as possible) volume of your combustion chamber(s) as well. The head gasket mfgr can tell you the volume of the head gasket, since few are round. With this info, the formula is easy. If you're interested in learning how to "know" your engin and it's specs, let me know. I'd be glad to run you through it.

[blykins]

Hey MM,

I used the 10.300" deck height. If you have the piston made for a 0 deck, then that would add to the ratio a little bit. That .010" deck that I used, was .159 cubic inches...so if you factor that out, then the compression ratio goes up to 9.8:1. I have a BBF reference book, and even with the 10.300" it shows a .010" piston to deck clearance. I don't know if you were buying off the shelf pistons or having them made, but if you buy custom ones, you can just tell them what you want.

steelcomp: I agree with you, but he's not at that stage yet. I don't think he has all the parts for the engine buildup....so we're just trying to get a general feel for what dish and specs he needs before he orders them. When he gets the pistons he wants, then if he desires, we can calculate the compression ratio again based on his piston/cylinder clearances, volumes above the top compression ring, etc. But I think he's just wanting to know about what it would take to hit the 10:1 mark.

[trularin]

I used the piston at the top and got 9.701 to 1. but I figured .050 chamfer on a diameter .030 over the stock. I use the relative data in this thread.

If you want more, visit the machine shop.

[steelcomp]

In my experience, it's not a really reliable source to refer to the mfgr's numbers. Case in point. (Bear with the treasonous example, here) BBChev...TRW pistons "advertised" with a 119 cc chamber, and a given dome volume, as a 12.5:1 CR piston. I was looking for max compression without angle milling, or any other exotic techniques. I went to zero deck, and surfaced the heads (after polishing and cc'ing the chambers) back to 118.8-119 cc's. When I did a cyl pour to actually measure the piston dome, and calc'd the CR, it was 11.2:1. So now, when I see the TRW's advertised at 12.5:1, or someone tells me that they have "12 1/2 to one cr" in their BB Chev, I know better.
You won't know untill you measure, or you talk to someone with the exact set up who has. To me, when buildiing a motor, this is preliminary. Gioin strictly by mfg'r specs, even as a base line, has been proven unreliable.

Just my $.02

[mustangmendez]

Thanks guys

I agree with you, but he's not at that stage yet. I don't think he has all the parts for the engine buildup....so we're just trying to get a general feel for what dish and specs he needs before he orders them. When he gets the pistons he wants, then if he desires, we can calculate the compression ratio again based on his piston/cylinder clearances, volumes above the top compression ring, etc. But I think he's just wanting to know about what it would take to hit the 10:1 mark.

Yes I just want somthing between 9:5 to 10:0
Most calculations are done for the later model head 93cc .
I will order the 22 cc pistons and get the short block together and ask for some more help ...
Thanks again .
MM