[BDS 502]

I have just recently acquired a World Motown block and a few other parts and decided I am bound and determined to build a sbc that will compete with my blown 502. I will either go with a 4.155 or a 4.185 bore. The rods I have are 6.00" Carillo and a Lunati 4340 3.875 crank. Since I have to buy new pistons either way my questions are do I need to go with a shorter rod and a piston with more compression height? a crank with less stroke (3.75, 3.48 )? and keep the long rods or a mixture of both. I am fairly new to large displacement small blocks and need a little advice. Or should I just build a 377 or something with a short stroke big bore and let the blower do the work? I will be using a Vortec V7 YSI blower, Demon 950 carb, and a set of Dart 227 CNC ported heads. Not sure of a camshaft yet. Please give advice on what you think I should build. My goal is in the neighborhood of 1000 hp. 800 ft lbs.

[pdq67]

Why don't you buy a copy of the book that is out now called "How to build big small blocks", or something like that.

It should help you figure out what you need to consider before starting a project like this.

I bought the "How to build Big BB's" book when I started thinking about my 496..

pdq67

[Eric68]

I would have to suggest ditching the 3.875" crank. (actually I might be interested if you decide to sell). The reason to get rid of it is because piston selection is nil for that stroke with a 6" rod. I have not to date seen a dished piston for 6" rods (and I've been looking) with that stroke, and a flat top leaves you at 10.5:1 with 76cc chambers. I personally don't care for the r/s ratio that results from a 5.85 or 5.7" rod with the big stroke.

I would just build a 406 or 412 (3.800" stroke x 4.125" or 4.155" bore). Use a 3.800" stroke (makes for a zero deck without any milling on a 9.025" tall block) and keep the bore as close to 4.125" as you can. The extra displacement from boring just thins the walls (I guess not really a big issue with an aftermarket block) and reduces the number of rebuilds you get from the casting. You are making your power from boost anyways, so getting exotic with the displacement serves no purpose IMO.

With a more "normal" 3.800" stroke you can just use an ordinary 400 CH piston and there are zillions to pick from to help you get the compression ratio you want. If the 3.800" seems odd to you there are a few mfrs that are making this stroke -- a 3.75" would be fine too IMO, but then you'll have to deck the block.

[greg moreira]

The only issue I see is your current 502 is said to be street drive....and its obviously real healthy running in the 8's. So, I agree, its tough to build this thing with a big crank, rods and such cause of piston options.....and even by giving up displacement, boost done right can run just as fast as your current 502. The big prob though......it will be as fast but it will be tough making 100 less cubes as streetable as the 502. A comprehensive fuel injection system should help and I suggest you get with some real good guys on that.....along with a real good cam grinder like UDHarold(hes on the chevelles board). If you want to compete with the 502 on all levels(Fast and streetable), it will take a little bit of trickery to have your cake and eat it too with the smaller motor.

[BDS 502]

Originally Posted by greg moreira

The only issue I see is your current 502 is said to be street drive....and its obviously real healthy running in the 8's. So, I agree, its tough to build this thing with a big crank, rods and such cause of piston options.....and even by giving up displacement, boost done right can run just as fast as your current 502. The big prob though......it will be as fast but it will be tough making 100 less cubes as streetable as the 502. A comprehensive fuel injection system should help and I suggest you get with some real good guys on that.....along with a real good cam grinder like UDHarold(hes on the chevelles board). If you want to compete with the 502 on all levels(Fast and streetable), it will take a little bit of trickery to have your cake and eat it too with the smaller motor.

I agree 100%. My 502 is streetable with 9psi of boost not the 15 psi I throw at it on the track. I know it would be tough to match torque and horsepower, but I would be happy if I could get close. Streetable is not really an issue, heck I am lucky to put 50 miles on this car in a year. Thanks for the reply, also thanks PDQ67 AND E.68 I will look for the book. I already have parts to build a 426 with this block, I may just put it together and run it normally aspirated until I can get the rest of my blower parts in order. Thanks guys!

[Eric68]

What kind of pistons do you have now for an NA 426?

[BDS 502]

Originally Posted by Eric68

What kind of pistons do you have now for an NA 426?

They are JE. 14.7:01 with my 62cc chamber heads.

[Eric68]

Guess then you don't plan on using them with the blower

[BDS 502]

No, that would probably not be very wise.

[camaroman7d]

Have you considered running alcohol? You could pull it off with that compression (or close to it). You canmake more power and still drive it on the street. The reason I mention this is because of your goals and you will not find a piston/head combo to get your compression down to where you will need it for pump gas on boost. With so little street use, you could pull it off (meet your goals).

I don't see how you can get there on gas with the rod/stroke combo you have in hand. You are going to have to make big boost to get close to your goal and as far as I know there is not a piston made (or that can be made) to get the compression down to the 7.5:1 area, which is where you are going to want to be to run the boost you will need to reach your goal.

I had a VERY hard time finding a piston for a 6" rod 3.75" stroke (383), that would get my compression to a decent level for boost. Once I found a piston that would be "close" I had to open up my chambers to get the compression to where I wanted it.

I have not played with any large small blocks "yet" so I am not sure what all is out there for them, I just don't see how a dish piston can be made for such combos. At some point the wrist pin is getting to close to the crown of the piston. Isn't there a better selection of pistons for a shorter rod? Something has to give, rod/stroke ratio and/or boost. I would give up a little on the rod/stroke ratio in favor of boost.

[BDS 502]

Originally Posted by camaroman7d

Have you considered running alcohol? You could pull it off with that compression (or close to it). You canmake more power and still drive it on the street. The reason I mention this is because of your goals and you will not find a piston/head combo to get your compression down to where you will need it for pump gas on boost. With so little street use, you could pull it off (meet your goals).

I don't see how you can get there on gas with the rod/stroke combo you have in hand. You are going to have to make big boost to get close to your goal and as far as I know there is not a piston made (or that can be made) to get the compression down to the 7.5:1 area, which is where you are going to want to be to run the boost you will need to reach your goal.

I had a VERY hard time finding a piston for a 6" rod 3.75" stroke (383), that would get my compression to a decent level for boost. Once I found a piston that would be "close" I had to open up my chambers to get the compression to where I wanted it.

I have not played with any large small blocks "yet" so I am not sure what all is out there for them, I just don't see how a dish piston can be made for such combos. At some point the wrist pin is getting to close to the crown of the piston. Isn't there a better selection of pistons for a shorter rod? Something has to give, rod/stroke ratio and/or boost. I would give up a little on the rod/stroke ratio in favor of boost.

I am seriously considering running alcohol. I will have to buy another carb anyways so why not. When I build it for the blower set-up I will probably use a 3.75 or 3.80 stroke and a piston with a little more compression height, this should help the rod ratio a bit.