Thread: 8-71 blown big block,questions

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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 rabid rat

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?

well yes more boost will make every thing work better .i have a big cam idle it has 2inch,s of vac at intake at the carbs it has about 15 and the carbs work out of the box just fine carbs think they are on a mild engine the blower helps out

I have been told by several people over the last couple years that my 60' time is slow relative to my finish line mph and calculated hp, one thing all these people had in common was they dont own or even know anyone that has driven a carbed blown car down a dragstrip,all their info comes from a computer program, I have recentlytalked to a guy with a simular combo,all steel 55 chev,427 9# boost best time 10.07@137,1.53 60' this looks right in line with my10.68@127,1.58 60' at 5# boost,in fact the guy with the 55 is running glide with a 5500 stall and a 4.56 rear gear,One could conclude giving my car more gear wont help my 60' much,but I think I need to talk to a few more people with a simular combo before I can consider this a fact,I dont think these computer programs are very accurate for blown cars,do any of you guys out there actually have a blown car that you run down a track,and have time slips,I would like to compare notes with you

Originally Posted by rabid rat

I have been told by several people over the last couple years that my 60' time is slow relative to my finish line mph and calculated hp, one thing all these people had in common was they dont own or even know anyone that has driven a carbed blown car down a dragstrip,all their info comes from a computer program, I have recentlytalked to a guy with a simular combo,all steel 55 chev,427 9# boost best time 10.07@137,1.53 60' this looks right in line with my10.68@127,1.58 60' at 5# boost,in fact the guy with the 55 is running glide with a 5500 stall and a 4.56 rear gear,One could conclude giving my car more gear wont help my 60' much,but I think I need to talk to a few more people with a simular combo before I can consider this a fact,I dont think these computer programs are very accurate for blown cars,do any of you guys out there actually have a blown car that you run down a track,and have time slips,I would like to compare notes with you

Here's my computer program!
http://www.calvertracing.com/gallery/gadavis.mpg
BTW, that was a 1.35 60' ON THE BACK TIRES!

Originally Posted by rabid rat

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

I agree with your not wanting to make to many changes at once. One at a time.
5800 RPM is a tad low for crossing the finish line. Maybe the 4.11's would be better than the 3.89's.