Hello,

I'm considering putting Edelbrock aluminum heads on my 305, but I'm worried about the compression ratio since my block isn't zero decked and I have dished pistons. I have to measure the dish and the gap between the piston and the deck, but until then how much of a part does compression ratio play in power? I know that the stock ratio of the LB9 was 9.5:1 so how much of a drop in power would I experience if the compression ratio was lowered to 8.0:1?

Thanks,

Matthew C. Hall

 

It would destroy power. Think of it this way, blow a ziplock bag up with a bunch of air, and seal it. Then compress the bag so that there is less space in the bag, but the same amount of air. The more you compress the bag the more resistance you will have, which when combustion occurs, that resistance = more power produced due to one of two schools of though. First, school is more power being produced due to less space. The second school is more power being produced by more air in the same space. It's a toss up on which you choose to beleive, but the end results is the same.

Now, high compression ratio's will lead to more pumping loses, but you will more than make up for that in power gained. Let's say you lose 10 HP by going from 8:1 to 9:5:1 due to pumping loses, you might actually gain 20-30HP which means, by the time you factor in the pumping loses, you've actually gained 30-40 HP. That's just hypothetical, I'm not saying that what will happen.

So, GENERALLY SPEAKING, in MOST cases, high compression will yeild more power. (Supoerchargers work based on this principle by forcing a lot of air into the cylander, which gets compressed as if it had a normal N/A amount of air in it (which is generally 75-85% efficient), cause more pressure, causing more force on the downstroke of the piston. In fact, this is why superchargered and turbochargered engines are often built with reduced compression ratios, so they can handle the extra air without cracking or cause an event of two flame fronts coliding in the chamber, which in a supercharged situation will almost always lead to a rod, piston, or head giving, if not a cylander wall.

 

The main problem you are gonna run into with swapping directly to aluminum heads is the slower burn rate of the cumbustion chamber. Aluminum dissipates heat faster than iron which results in a slower burn rate of the air/fuel charge compared to a iron headed engine with the same CR. I cannot tell you how many people I see bitchin about how they put on aluminum heads and did not see a performance increase. You will want atleast around 10:1 with an aluminum head (or more if you want) to get comparable burn rates to an iron headed engine with 9.5:1.

As an example, I think the LT1 engines come from the factory with 10.3:1 (not sure - someone will correct me if I'm wrong but I know I'm close). The increase in compression will give you more power on its own, but the main reason for increasing it is to maintain the efficiency of the engine which is severly affected by the burn rate of the air/fuel charge which is affected by the type of material the heads are made out of.

Slow Iroc has already stated all the benefits of raised compression so I will not go into that.

Hope that helps........

Laterzzzzzzzzzzzzzzzzzzz

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Matt

1987 GTA L98 MD8 GW6

"Stop Lights timed for 35Mph are also timed for 70Mph"

 

Typically raising compression by 1 point 9:1 to 10:1 gives 5 - 10 % increase in power, closer to 10. When switching from iron to aluminum heads it is generally recommended to go up one full point to compensate for the faster heat dissipation as explained by Matt.