how much boost can i run on a big block chevy with 10.4 to 1 compression

Steal crank
Forged rods
Forged Pistons the block has not been O ringed the motor has ZERO miles on it

 

I take it with that compression you are planning on running Alchy...

Can run as much boost as you can till it detonates, Not very viable if you are planning it to run on gas.

Steven

 

94 oct... how much boost would be safe

 

Very little if any. larger combustion chambers are more prone to detonation. If it was a small block I'd say ~6-8psi. For yours, maybe 4-6psi. If it has a big cam, it will tolerate more boost. Aluminum heads will also help.

Rich Krause

 

IT HAS A 600 LIFT CAM but it has steal heads so i guess 4psi of boost will make no power

 

A true 10.4:1 big block with iron heads will barely run without detonation on pump fuel when normally aspirated. I would recommend no boost unless you plan to run race fuel.

 

Probably better advice than mine, especially with iron heads.

Rich Krause

 

on 94 octane your throwing money in the air imo by turbocharging that motor. I wouldn't waste my time on 4 psi on a bbc with iron heads.

Steven

 

why are bigger chambers more prone to detonation??? so it means its better to have a dish piston and a small chamber?

 

Without writing a book on the topic, I'll try to answer your question.

As the mixture ignites, the cylinder pressure starts to rise. During this time, the remaining unburned fuel can spontaneously combust (preignition) or suddenly ignite (detonation) due to the high cylinder pressure. The smaller the chamber, the less time it takes for the flame front to travel across the piston top & the less likely that this will happen. A larger chamber volume at time of ignition will require more octane for a given compression ratio. Yes, small chambers & reverse dome pistons may allow more compression for a given octane.

For example, my Yamaha R1 has 11.8:1 compression & runs well on 89 octane, partly due to combustion chamber design & the fact that the pistons are the size of soup cans (74mm). It actually makes less power on 94 octane.

 

Yeah, I'm planning on a 350 crate with only 9:1 compression and all forged stuff, and there have been people telling me that I might have to run only around 4 psi, and thats with boost retarding control and 93 octane gas. My guess is that if you run boost, you'll have to run race gas and timing control.

 

I agree with the others in that your compression is too high to take much boost on pump gas. I would take a hard look at your actual cc's when figuring your compression ratio. Deck clearence, valve pocket depth and radius, chamber volume, dome height etc. can all have small impacts on total volumes which can make a big change in your end result. As for 4 psi not making much difference? We installed a Weiand 8-71 on a 460" BBC with iron oval ports, with default 5-7 psi pulley's, on my buddie's 69 El Camino. It ran 12.40 with an 800 carb on it.....with the blower and no other changes it ran 11.04. This is at close to 4000lbs. and on pump gas. We have since installed a pair of iron rectangle port "990" heads and it has gone 10.80's against a 40 mph head wind. Keep in mind my buddy just drives it on the street and does not tune it at the track. He puts 26-28 degrees of total timing in it and just drives it.....the Holley 750's are just as they came out of the box. Not sure how much boost it is actually getting because I can't get him to install the gauge!! I was skeptical also when he told me the 5-7psi default pulley's were going on it, but damn that thing is fun to drive!! LOLOL
Jay

 

Just an observation, it doesn't take much boost with 454 cubic inches to have some fun.
As for the BBC boost question, get some BIG chamber alum. heads, or change pistons. With iron heads 8.0cr would be good and 8.5-9.0 with Alum would work. The 9.0 is assuming a newer head design which would have a more efficent chamber. With what ever heads you chose ensure that the chambers are CNC'd to reduce detonation. You maybe able to drop your current CR to 10.0 with some good CNC work. Add a thicker gasket and things may not be too bad afterall. Cam selection is important too. Most big blocks with aftermarket cams dial in a 108-112 LSA and thats not very efficent with a blower. But a 108 with a less than 10.0cr may work, but risky, I would very timid about getting on it in warm weather esp. with no intercooler.