BlueBlock350

I am trying to purchase a 403 cid crate engine and i was wondering if someone could explain to me what a compression ratio is. I was asked what I wanted the compression ratio to be and im just curious. Thanks.

Rallye469

It's the ratio of volume in a cylinder when the piston is at it's lowest point compared to it's highest point.
It's basically how much an engines' air and fuel get 'compressed'.

If you have 60 cubic inches of volume(in the cylinder) when the piston is down and 6 cubic inches of volume when the piston is up you have...
a 10:1 compression ratio.

The guy basically boiled down your needs to an arbitrary fact. Kind of...
He probably just wants to know what you'd like the engine to do.
If it's 8.5:1 it's basically just a stock build, not too much in the way
of performance.
The higher the # (like say 13:1) the more power you're probably making.
A nice running street/strip engine will probably have anywhere from
9.7:1 up to 10.7:1.

11:1 is the ragged edge of streetability. It starts causing the mixture in the
cylinder to ignite from the heat of compression rather then the spark of a plug. That's called detonation.
Aluminum heads(they say), and things like quench & head chamber design can help eliminate detonation as well...but 11:1 still seems to be the accepted divide.

Did I explain anything here?
...feel like I'm rambling.

BlueBlock350

yeah you did. thanks. there are a few compression ratios that you can pick from and they are as follows: 8:5:1, 9:0:1, 9:5:1, 10:5:1, and 11:0:1. I was thinking maybe 9:5:1 or 10:5:1? What do you think? Again, it's a 403 cid motor (if that matters or not). So, I take it that you don't really recommend 11:1 or 11:0:1?

Rallye469

That's tough...no 10:1?
I don't know the characteristics of a 403 with iron heads.
10.5:1 sounds iffy...it's not crazy but it is high. You can always go for a cam with some overlap to bleed off a bit of the pressure.
This sounds like a really specific 403 question...contact some 403 guys like FCR.

ricebasher302

The general rule is that aluminum heads cool better and thus will allow 1 point higher compression that an iron headed engine. I would go NO higher than 10.0:1 with iron heads and 11.0:1 with aluminum. So, based on your choices, I'd choose 9.5:1 for iron heads. It would be possible to get away with 10.5 and iron heads, but you'd need a good cooling system, premium fuel, and precise tuning to prevent detonation. Rallye469 is right about the cam specs, as they will determine how much of that compression is actually taking place. These are just general rules. The 403 probably has a few tendencies of its own that you may need to look into.

BlueBlock350

9:5:1 is probably what I will go through. I want something that I wont have to worry about constantly tuning all the time due to the fact that I have so much limited to time to actually work on it. It will basically be a weekender but I want it to have some "kick" when I need it or when I see a little honda civic or when one of those little rice canisters try to race me. Or I guess I could keep what I already have which is a 350 and work on it. Im not sure yet.

marcar1993

9.5 is good for what your looking for. Make sure you get a cam to match. The more compression the more radical it can get, if you go to radical with not enough compression, you'll make it slower. That would be the next discussion, also what heads do you plan to run? All these factors need to be thought out before hand. Building a motor is like building a house, you complete ALL the blueprints before you ever start to build.

BlueBlock350

Im not sure about what heads to get i was looking at hot rod magazine when I noticed that edelbrock has a power package that you can get that has the heads and everything with it. do you have any suggestions as to what heads I should get based on the 9:5:1 ratio (403 cid motor)?