what is the lowest compression a SBC can go with?

reallycoolcorvette · 04-25-2003, 04:25 AM

say a 350, 383 and a 406?

I am talking about how low of a compression ratio is available out there for these combos?

I ususally see 8.5:1 but if the answer is get larger CC heads for lower than I guess thats it.

Monty · 04-25-2003, 03:30 PM

Why do you wnat to lower the compression so much? Forced induction? Detonation resistance?

The lower the compression, the less efficient the engine. The same is tru for overly large combustion chambers. If you look at most modern engines and cylinder ehads, they trend is towards a small combustion chamber in an effort to improve combustion efficiency.

I would not go much lower than 8.5 or so, anything lower than 8.0 or so, and the engine gets really lazy and performance/efficiency really suffers.

reallycoolcorvette · 04-26-2003, 04:07 AM

i definetly agree, just curious to thoughts on this matter.

Forced induction is the topic, for a full out race motor and 8.5:1 overall is the best way to go.

CFI-EFI · 04-26-2003, 10:03 PM

Let's see, now. A set of 6" rod deep dished blower pistons, swinging on a set of 5.565" 400 rods, 76cc chambered heads, and a thick head gasket, I bet we can get you down to 5.00:1- 6.00:1 pretty easily. I haven't done the math on this combo, but I'll bet we can get it even lower if necessary. Is the price of kerosene going up?

RACE ON!!!

OatBoy · 04-27-2003, 03:28 AM

that is a question i don't think i've ever heard before, even with a lot of forced induction 8.5 is really the lowest you would go, as stated above.
i want to know ....why?>>?

Vetterodder · 04-27-2003, 03:58 PM

Quote:

that is a question i don't think i've ever heard before, even with a lot of forced induction 8.5 is really the lowest you would go, as stated above.
i want to know ....why?>>?

Going much lower than that will hurt torque when the engine doesn't have boost. Not so much of an issue for a dedicated race motor though as they only need to make power at high rpm where full boost is already there. On a street car though, where you normal use lower rpm more than high rpm, the engine could feel like a slug.

reallycoolcorvette · 04-28-2003, 04:10 AM

kyah, i've been playing around with different cams and looking at their effects on cylinder pressure.

Final c/r will compliment the cam and vice versa.

MikeC · 04-29-2003, 03:42 PM

My 350 had a CR at around 7.75:1..... ran really good.......14.6 sec in the 1/4 mile( not blistering but smog heads and a CR like that I figured it was pretty good.)

reallycoolcorvette · 04-29-2003, 09:18 PM

7.75:1 compression?

that is pretty low for n.a.

WashingtonRacer · 04-30-2003, 04:52 AM

Monty hit the nail on the head, I have been looking into this whole senario lately too, as I had to finally determine exactly what I wanted for a CR. I found it was not only the CR thats the issue, but the even greater issue is Quench height, which is just as important as compression ratio it's self. they both go hand in hand, and basically you will have to find a happy median between both to get the best setup.

CFI-EFI · 04-30-2003, 09:51 PM

Quote:

Quench height, which is just as important as compression ratio it's self. they both go hand in hand, and basically you will have to find a happy median between both to get the best setup.

Yes and no. Quench distance is MORE important than compression ratio. A shorter quench distance is likely to allow a greater compression ratio "work". Lowering the CR by increasing the QD, is more likely to cause problems (knock) than the origional (higher) CR. Little compromise is necessary if you plan ahead. Final tweaking, if necessary, can be accomplished by adjusting the combustion chamber size. This can be done by matching the chambers to the cylindar walls, unshrouding the valves, or just grinding back the plug side of the chambers. Piston dome/dish can also be massaged. Good luck, and...

RACE ON!!!