SBC 4.25 STROKE

[BEDNAR1320]

I AM GOING TO BUILD A NEW SHORT BLOCK FOR MY DRAGSTER,THE OLD COMBO IS A 432 (4.145X4.0) WITH CHEV 18DEG HEADS. IT RAN VERY WELL 7.65 AT 174 IN THE DRAGSTER AT 1650 LBS. I BOUGHT A DART TALL DECK BLOCK (9.325) SO NOW I CAN GO WITH A LONGER STROKE AND/OR A LONGER ROD,I AM USING THE SAME HEADS THEY FLOW 370 AT .700. I SHIFTED THE OLD COMBO AT 7300 AND WENT THROUGH THE LIGHTS AT 7600. I WOULD LIKE TO BUILD A 468 (4.185X4.25) WITH ABOUT 15:1 COMPRESSION (ALCOHOL) IT WILL HAVE ALUMINUM RODS AND DON'T MIND SPINNING IT A LITTLE HIGHER THAN THE OLD COMBO. I CAN USE THE 4.25 STROKE WITH 6.200 RODS AND A 1.0 CH PISTON WITH A DECENT ROD RATIO. I WOULD LIKE TO KNOW WHAT YOU GUYS THINK ABOUT A COMBO LIKE THIS,OR SHOULD I STICK WITH THE 4.0 STROKE AND JUST BUILD A LITTLE MORE RADICAL MOTOR. I WOULD LIKE TO PICK UP 2 OR 3 TENTHS.

[Jason G]

I've got Dart 9.325" deck Iron Eagle with a 4.125" stroke and Lunati Pro Billet 6.125" steel rods and only minor clearancing at the bottom of the bores was required. A 4.25" stroke would easily fit with these rods. It will require a lot clearancing with aluminum rods though. I don't know if there would be room.

Jason G.

[GOSFAST]

Built 2 similar setups about a year back.

Believe the first build made 732 HP and second one came off at 751.

Would be curious what the 751 HP build would do in that ride?

Just a tip on that relatively short C.H. and the aluminum rod's, you need to be very careful where the top of the rod comes in contact with the underside of the pistons.

It gets somewhat "tight" in there. The thickness above the rod-eye will most likely end up at about .200"/.220", if memory serves me correct! Don't have the "exact" specs here with me.

It's also a snug fit between the pin bosses with aluminum rods! With some GRP's and J&E's it took some fitting on BOTH the rods and the pistons!

Those numbers above are on 93 octane by the way! One is running around here on the N.Y. streets in the nice weather.

Thanks, Gary in N.Y.

[jmarkaudio]

I have a 447 SB2 (4.155x4.125) in the same block, Oliver billet rods needed the bottom of the bores cut a little. Aluminum will need much more unless you go to a 2" or Honda journal. I don't think I would go that long on the rod, mine has 6.125. I wouldn't be worried about rod ratio on a drag motor. Maybe even 6" for the 4.25 stroke to keep the compression height better. For this summer I have a 9.5 deck aluminum block to build...

[Abbottracingheads]

If you need that big of a motor build a big block. That much stroke in a small block might not run as fast as what you had.

[Hendrens Racing Engines]

Try changing to a used set of cup SB2.2 heads with your existing stroke.on our house 420 circle track engine 3.875 stroke 9.0 block at 15.0-1 on gas it makes 844 H.P. the same combination with a drag profile cam would make much more.
most of the late heads will flow over 400 cfm. Bill

[BEDNAR1320]

THANKS FOR YOUR INPUT BILL,BUT SB2 HEADS ARE NOT IN MY BUDGET,THE 18DEG HEADS I HAVE HAVE BEEN DONE BY MBE SO I HAVE SOME MONEY INVESTED IN THEM. I THINK I WILL GO WITH A 4 OR 4.125 STROKE,MORE COMPRESSION AND A MORE AGGRESIVE CAM PROFILE,I THINK IT WILL RESPOND TO THAT. THE CURRENT COMBO IS 13:1 THE CAM IS 276INT/284EXH AT .050 BUT HAS FAIRLY MILD RAMPS,LIFT IS ONLY .685/.690,I THINK I'LL GO WITH SOMETHING THAT HAS ONLY SLIGHTLY MORE DURATION AND LIFT AROUND .740 BUT QUICKER RAMPS TO GET THE VALVE OPEN MORE SOONER,I DON'T RUN THIS ENGINE ALL THAT MUCH SO I'M NOT TOO CONCERNED ABOUT VALVE SPRING LIFE,I HAVE A MILDER MOTOR THAT I RUN MOST OF THE TIME.

[John W]

THANKS FOR YOUR INPUT BILL,BUT SB2 HEADS ARE NOT IN MY BUDGET,THE 18DEG HEADS I HAVE HAVE BEEN DONE BY MBE SO I HAVE SOME MONEY INVESTED IN THEM. I THINK I WILL GO WITH A 4 OR 4.125 STROKE,MORE COMPRESSION AND A MORE AGGRESIVE CAM PROFILE,I THINK IT WILL RESPOND TO THAT. THE CURRENT COMBO IS 13:1 THE CAM IS 276INT/284EXH AT .050 BUT HAS FAIRLY MILD RAMPS,LIFT IS ONLY .685/.690,I THINK I'LL GO WITH SOMETHING THAT HAS ONLY SLIGHTLY MORE DURATION AND LIFT AROUND .740 BUT QUICKER RAMPS TO GET THE VALVE OPEN MORE SOONER,I DON'T RUN THIS ENGINE ALL THAT MUCH SO I'M NOT TOO CONCERNED ABOUT VALVE SPRING LIFE,I HAVE A MILDER MOTOR THAT I RUN MOST OF THE TIME.

I agree that you need to get more aggressive with the cam. Probably a good bit left in the current combo with just a cam change. The intake lobe in my 427 is 267@50 & .806" gross lift. Shift=7800 cross @8000. Valves are 2.18/1.625 SS. So far I have over 100 passes on these springs and quite a few dyno pulls (engine and chassis) and the springs still measure to listed specs. Spring is Crower #68860 1.635".