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Old 03-03-2004, 06:19 PM
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Question Increasing HP on my Blown SBC with Cylinder Head Replacement

Currently running a B&M 144 Blower on my 355 4 bolt main SBC. Dyno'd 340hp at 4700rpm with 9 lbs of boost using a 3" blower drive pulley. Engine has a mild cam, stock 882 heads with low flow numbers, 700cfm Holley Double Pumper Carb, Block Hugger Headers. Question is this........I'd like to increase my HP by changing to an Aluminum Head. I've been told to use a larger chamber head 76cc thus lowering the compression will gain HP. Also told I don't need to change anything except heads to see a siqnificate increase in HP. Any suggestions out there as to what I should do to increase my HP.

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Old 03-03-2004, 06:38 PM
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Don't lower your compression unless you are running into detonation problems with your current heads. Try and find heads with the same chamber ccs.

Get a cam designed for use with supercharging. Ask the cam companies techs for help in selecting the right cam.

Get some AFR or Trick Flow heads. Depending on how fast you want your car to be, get 200cc or higher intake runners. I would look for 220cc heads with 2.08" intake valves.

If you do all that you will need more carb. Go with an 800 to 850 CFM carb. Holley makes supercharger carbs now. Look into those.

The most important thing is to do your homework and ask around. Try and find out the general consensus on what will make your engine work the way you want. You can't just throw a bunch of parts at an engine and expect it to make serious power on its own. See what the techs say at the various parts companies.
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Old 03-04-2004, 01:11 AM
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Hold on a second. Blown engines still work similar to N/A engines. You don't want to put a huge head on a 350 (meaning runner size) with a small blower, you will have to spin it to the moon to make boost (torque). his will create heat and cause more problems than it's worth.

The 882 heads should already be 76cc if I'm not mistaken. Most atermarket heads are 64-72cc. What I did was take my 72cc heads and "worked" the chambers so now they are 75cc (which put my compression right where I wanted it).

With aluminum heads I would expect ou to be able to run a little more boost (all other things equal).

A set of better flowing heads would be a great idea and you will make more power with the same boost level. So switching from the "smog" heads to a set of performance heads will really wake up your engine. Since it is a 350 I would keep the runner size 190 or below (probably 180 for this application). While you have the heads off swap out the cam to take advantage of the new heads.

You need flow and velocity, this is why too large of a head will hurt performance. Just about any head you choose (aftermarket) will out perform those 882's hands down.
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Old 03-04-2004, 05:33 AM
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And as mentioned, be sure it's a blower cam. Too much exhaust duration and you'll dump your boost out the rear.
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Old 03-04-2004, 06:41 AM
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okay... here's where people say: "bigger chambers are you nuts? That will lower compression"

Yeah... that is the goal in blown applications. What people dont seem to understand is that with more volume in blown applications you will yield more gain than loss. It is pretty simple, the bigger space you have for air/fuel, the more the blower can push in there. You will not see any improvement by increaseing the static compression ratio. In blown applications no matter the chamber size, the dynamic pressure will always be similar so long as the blower can maintain the same boost level with more volume to fill.

Get bigger chamber heads and port/port match them, this will yield you much better performance.

Also static compression ratios should be around 7.5-8.5:1 in blown engines. Remeber that something like 8.5:1 will yield dynamic compression of around 12, 12.5:1.

Last edited by 87442lover; 03-04-2004 at 06:57 AM.
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Old 03-04-2004, 11:00 AM
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Thnx Ya'll

THANK YOU ALL for your input.
Everyone seems to be of a different opinion on what I should do. One feed back says big intake numbers, another feed back says small.

The engine already has a blower cam, 700cfm Holley double pumper blower carb, blower pistons and as far as the 882 heads having 76cc chambers I couldn't say. What I've read about the 882 heads is that they are notorious for producing low flow numbers, this is why I'm looking for better flow numbers in the heads.

As far as the blower producing more boost, I can switch to a smaller drive pulley to make the blower spin faster.

Im building a full custom pro street car which I'd like to take to the strip and pull some low et's.

If anyone is interested I have a '69 Corvette Aluminum Headed 427 L88 NOS Roller motor dyno'd at 580hp on pump gas / 930hp w/ 350hp nitrious shot.

Check out my project Stude at
www.angelfire.com/super/1950bulletnose
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