Thread: 8-71 blown big block,questions

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I have a 402 bbc bored .040 over,in the last rebuild I put in trw forged pistons,427 steel truck crank with arp 2 bolt main studs,line honed,3/8truck rods that I polished beams,resized,best arp rod bolts,magged,shotpeened. heads are 781 2.19/1.88 valves,pocket ported by me,short turn work,polished exhaust ports,new arp head bolts,block and heads decked, I was running a B&M 174 blower on this motor for a season and half,i'm a regular weekly bracket racer,car is 1979 malibu 3450# glide,3.50 gear,ran high 11's with that setup,maxed that blower out at 8# boost,decided to step up,sold that blower for $1000,bought 8-71 for$2350 from dyers comeplete kit,my 60 foots went from 1.70 to 1.80's with 5# of boost(20 percent underdriven)average run is now11.0 123 mph,best time 10.68@127mph 1.58 60' on a real good air day,I now have 2 seasons on this big blower,I put 3.70 gear in,now 60' is 1.70 average,I think i'm makin 600 hp on good air days,i'm think carbs are rich,square 84 jets in pair of holly 750 dp's,NOW FOR THE QUESTIONS How much boost is safe to crank into this engine? I would like to run low 10's@130-135,I have the stock crank key with a very tight fitting fluidampr,I have pulleys for 6# boost or 8# boost,I also have a set of 3.89 gears I can try,I have 3 and 1/2 seasons on this motor now(about 300 1/4 mile passes)4800 stall convertor,transbrake,leave on 2-step at 4200rpm,no tire spin,shift at 6200 to preserve parts,but will pull to 7000,mild hydraulic flat tappet cam

The rods bother me a little, but I would think you could go to 10-12 Lbs without major problems as long as you don't severely "miss" the tune up

i have a blown 548 one key way in the crank fits gooodin tight with a fluidampr been that way for over 5 years it is 10to1 with 10 of boost I BEAT THE HELL OUT OF IT. so the one key way that would not worry me to much but the rods i would go up to a H beam and every thing looks good but if you pull it down why not go with some H beams and add a key way?

if I took it apart I would change out the rods and crank key,BUT it runs so perfectly now I hate to fix it ,IF IT AINT BROKE,I could just leave the boost low while I build a replacement,I have a nice standard bore 4 bolt 454,a454 steel crank,a bunch of 7/16 chevy rods,also have a 427 tall deck motor,steel crank,standard bore,I am trying to decide what size the replacement motor should be,I also have a nice set of 990 heads that will go on this next motor,I'm thinking 496 or 525 with 8# boost ,pump gas

well i have used the thumb rods/ truck rods in many big block builds and they have never gave me much trouble .but you would be getting to the point were a better rod would not cost much like 300 to350 alot cheaper then if the rod wants to come out for a visit . if you have the room i would build the tall deck long rod 496 with the 990 heads .well the 525 i have built that would work to

Originally Posted by pat mccarthy

i have a blown 548 one key way in the crank fits gooodin tight with a fluidampr been that way for over 5 years it is 10to1 with 10 of boost I BEAT THE HELL OUT OF IT. so the one key way that would not worry me to much but the rods i would go up to a H beam and every thing looks good but if you pull it down why not go with some H beams and add a key way?

what do you think of stock chevy 7/16 rods compared to say eagle rods?I have 19 of the 7/16 chevy rods,could sell em and buy better ones,sell my 454 steel crank and buy a 4.25 stroker,some 5.385 rods for my 4 bolt 454 block,I have everything to build to build a 454,stock heavy duty chevy parts,would aftermarket eagle 496 be better ?

i would sell the 7/16 there good if the old ones the newer ones have a thinner beam the bolt never fail much on the stock big block rods and if ARP or SPS i never seen a need for the 7/16 have used them on some builds just at the time they were handy .i am not a big fan of eagle stuff but it would be better then the stock 7/16 rods and a new 4.250 crank would get you the 496 and you could use the 6.375 rods but you can build the tall deck with a 4.500 crank and use the 6.535 rods with a srp shelf piston to .i would say that you could use the 7/16 rods but they heavy and the bolts get in the way with a4.250 crank you have to grind more on the oil pan rails and the rod is short you can get a longer rod if you go H beam and the cap screw bolt rod has more room so less grinding on the block

I have some very expensive hooker headers that might not fit tall deck motor,I would also like to re-use my internal balance dampr and flexplate,so the easier way to go is low deck internal balance 496 if that is possible,I'm lookin at 700+ hp,should I be lookin at usa made crank and rods?$$$$$$

any thing past that yes. i have built many engine with the 4340 steel cranks and H beam rods .if you have the money it would be nice to have the USA parts .but i have used many many sets of the H beam rods and 4340 cranks they work.you can go internal if so it better to use a 6.385 for more counter weight and you still may need one plug of heavy metal on each end . hard to say this would have to do with your bob weight

Originally Posted by pat mccarthy

any thing past that yes. i have built many engine with the 4340 steel cranks and H beam rods .if you have the money it would be nice to have the USA parts .but i have used many many sets of the H beam rods and 4340 cranks they work.you can go internal if so it better to use a 6.385 for more counter weight and you still may need one plug of heavy metal on each end . hard to say this would have to do with your bob weight

what import brand of rods and crank do you prefer and,what usa brand

Originally Posted by rabid rat

what import brand of rods and crank do you prefer and,what usa brand

scat rods and crank or the 8 plate ohio crank or the cat cranks .on usa stuff crower rods and cranks. howards rods and cranks .brc cranks callies cranks oliver rods

QUESTIONS How much boost is safe to crank into this engine?

It's kinda hard to tell how much more boost can be safely added without knowing the static compression ratio (or better yet, the dynamic compression ratio) and cam specs.
That being said, I would use the pulleys that give you 8 lbs of boost and slip in the 3.89 gears. Work on reducing your short times. They should be alot better than 1.58. They should be in the 1.4 area.
IMHO, I would not overdrive the blower without having a second keyway added to the crank. I have personally sheared stock woodruf keys and split dampers (including fluidampers). I had a guy who didn't even make it out of his driveway with a stock damper!

Originally Posted by Fastnuf

QUESTIONS How much boost is safe to crank into this engine?

It's kinda hard to tell how much more boost can be safely added without knowing the static compression ratio (or better yet, the dynamic compression ratio) and cam specs.
That being said, I would use the pulleys that give you 8 lbs of boost and slip in the 3.89 gears. Work on reducing your short times. They should be alot better than 1.58. They should be in the 1.4 area.
IMHO, I would not overdrive the blower without having a second keyway added to the crank. I have personally sheared stock woodruf keys and split dampers (including fluidampers). I had a guy who didn't even make it out of his driveway with a stock damper!

My static compression is 8.3 to 1,the cam is a crane powermax #134561 hyd. flat tappet,specs are 236/246@.050, .553/.571 lift, 112 LS, I dont know how to calculate dynamic compression, I noticed the 60' slowed at least a tenth when I switched from small 174 B&M blower to the large 8-71,however top end charge much better,I beleive the small blower was very responsive at lower rpm's but became less efficient at top end, I am a bracket racer so a slow consistant 60' is a good thing,and I like the big top end mph,I broke the sprag in my $800 4800 stall, BTE 9" race convertor mid season and borrowed a TCI 10" that stalled lower(4000),made no difference in 60', my current 3.70 gear/1.76 glide is not a lot of gear for this 3450# car,leaves like a snail,cross finish line at 5800,I need at least a 3.89 gear,or maybe 4.11,I have been making small changes,went from 3.50 gear to 3.70 ayear ago,60' improved a tenth,so I figure change to 3.89 should get me another tenth,I dont want to make alot of changes at once because I won 2 out of the17 races in no box modified at my track last year,combo is working well for racing,but not optimized. The car varied from 10.68 ET@near sea level density altitude on my perform-air weather station to 11.35 in mid summer @5000 density altitude,leads me to believe I should maybe put some smaller jets in there for mid summer hot weather racing,plugs always look perfect

one theory of mine is it 60' is slow because I am underdriving the blower 20 percent @5# of boost,I think it will really wake it up if I bump up the boost,crane says cam is good to 10-12 # of boost,so I am running it at 5#,I think this is like putting to big a cam in a low compression motor,which would also be slugish off the line. Does this make sense to you guys?

Originally Posted by Fastnuf

QUESTIONS How much boost is safe to crank into this engine?

It's kinda hard to tell how much more boost can be safely added without knowing the static compression ratio (or better yet, the dynamic compression ratio) and cam specs.
That being said, I would use the pulleys that give you 8 lbs of boost and slip in the 3.89 gears. Work on reducing your short times. They should be alot better than 1.58. They should be in the 1.4 area.
IMHO, I would not overdrive the blower without having a second keyway added to the crank. I have personally sheared stock woodruf keys and split dampers (including fluidampers). I had a guy who didn't even make it out of his driveway with a stock damper!

the fluidamper are known to crack in hard use with a blower. the ATI are better . a stock damper will not have the right amount of press if used that and it just iron .yes another key should be added but you are splitting the press? so then the press means nothing and you are working on two key ways ?? i think i am going to use a bigger key way on a sbc build i working on now. i will be going to 1/4 key rebroach the crank timing set and damper going to run one key way full length of the crank with a hard key see what happens customer did not want to buy a bbc nose crank for is sbc and i will not cut it for a nother key when it is to small from the go