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Boost with iron heads and no intercooler
« on: February 17, 2021, 03:46:46 PM »
How much boost would you run??   8:1 CR, 6-71 no stripped blower, Iron heads, no intercooler, 242, 248 .646, .653 110 lsa solid roller cam, AV gas.

I had it dyno'd and it made:

741 hp and 693 torque at 31 degrees timing on 93 octane
727 hp and 682 torque at 28 degrees timing on 93 octane
^ both peaks were right at 5900 (hp) and 5300 (tq) and 8 LBS BOOST

I want to increase boost and make more power if I can. This is a lake boat ripper if you will. Not a drag boat, but I run it really hard all the time. I probably run it 20-40 minutes at a clip. All between 4000-6000 rpm. Never goes below 60 psi oil pressure with 20w-50 gp-1 synthetic blend driven racing oil. I DO NOT run it on 93 octane. I ran it all last summer, and will for the future, run it on 100% 100LL AV Gas (no mixing) Factoring in a compression ratio of 8:1, IRON heads, NO intercooler, and Av gas, how aggressive would you guys think I could go? ( am not going to intercool or switch to aluminum heads or go to a larger slower spinning blower anytime soon because I already blew a bunch of money on the build last year, the pump build this year and the new exhaust. Maybe later but definitely not now)

-  I made 8 psi with the 2.5% overdriven setup which put my effective compression ratio at 12.4 (the altitude I run the boat at is usually around 1100 above sea level which should take .15 off the final compression ratio which is nothing, but I wanted to mention it)

-  If I overdrive the blower 5%, I should see about 8.6 psi putting me at 12.7 effective CR
-  If I overdrive the blower 8%, I should see about 9.2 psi putting me at 13.0 effective CR
-  If I overdrive the blower 10%, I should see about 9.8 psi putting me at 13.4 effective CR

The new exhaust I had custom made, is a set of swept back shorty 2.5" primaries w/ super short collectors (I know those are probably too big, but I just did it thinking I can always use them later if I go bigger at some point). I figure these will lower the boost number a bit since the previous set of pipes were 2 1/4" bassetts.

To finalize my question:

-  It ran on the dyno on 93 octane, 8 psi, 31 degrees timing. no detonation, no issues.
-  I put 20 hard hours on it over last summer running 100% Av Gas, 8 psi, 31 degrees timing. no detonation. no issues.

I appreciate all advice, information, real world results, opinions. Obviously you can't just say "oh yea, it'll be perfectly fine!" or "It will blow up with the first turn of the key!" but I cannot make up my mind for the life of me. Thanks a lot fellas!!!

Engine specs:

472 gen IV iron block 4 bolt main studded
Machine work done by Andy Jensen at Jensen's Engine Tech
Cast case non stripped rotor 6-71 blower
SRP pistons (calculated 8:1 compression now)
Total Seal file fit rings (gapless 2nd ring) .033-.034 ring gap
Forged shotpeened rods
Scat forged balanced crank
Clevite bearings
Melling m77 oil pump
10 Qt dooley oil pan
World Products Bill Jenkins 1-043C 119cc iron heads
Cometic Stainless MLS 5 layer .070 headgaskets
dual valve springs with dampers
Comp 1.7 ratio roller rockers
Comp .135 wall 3/8 pushrods
Crower solid roller lifters
Comp 11-771-8 solid roller cam (242, 248 .646, .653 110 lsa)
Blp 3 piece timing cover
Rollmaster torrington timing set
MSD 6M-2L, MSD hvc 2 coil, and pro billet hei distributor (locked out, 31 degrees BTDC advanced timing)
Moroso Ultra 40 8.65 wires
NGK heat range 8 plugs
Jomar Professional stud girdles
Two side mounted Holley 850 four barrel carbs with 78 jets square (currently.  will need to change based on boost and exhaust size)
Holley 190 GPH billet mechanical fuel pump (7 psi) 1/2" lines

Main bearing clearance is .033
Rod bearing clearance is .028
crank endplay is .0045
cam endplay is .004
Ring end gap is .033-.034
472 cu. in.
7.97:1 comp ratio
VE: 74.4%

Thanks Guys!!!!

  • Boat #1: 1975 Hallett Bubbledeck
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Re: Boost with iron heads and no intercooler
« Reply #1 on: February 17, 2021, 08:40:01 PM »
I would say you leave it where itís at. 6-71 is a small blower that you are spinning at 4-6k rpm for long periods will generate a lot of heat plus you have iron heads which donít help dissipate heat either. Hot air being crammed in the engine doesnít make power. Adding an intercooler will give you cooler dense air giving you more power. Just my .02

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Re: Boost with iron heads and no intercooler
« Reply #2 on: February 18, 2021, 05:23:06 AM »
I'm sure staying at the level I'm at, or even going to 6 lbs will be the safest in the long run.  I guess I just feel like the carbs will help keep the charge air cooler (not as cool as an intercooler, but that is one of the principles carbs aid in.)  I was told but TBS, that the 6-71's  don't start to become INefficient until 7500 rpm.  Makes me think that spinning it at 6000-6600 rpm (2.5% O/D - 11% O/D) isn't really that bad. I hear people running 8-71's running at 13% O/D, 8:1 cr on straight av gas, no intercooler, (yes with aluminum heads, but that allows for just a little more leeway), and seem to be fine. I plan on bumping up the jetting one for the bigger exhaust, maybe a few more to richen it back up on avgas (I never jetted up from 93 before, but I saw 9-10:1 afr when on the 93 so I was probably still  in the safe zone when I put av gas in then). Perhaps lower the timing to 28*    (BTW, engine made 11 less hp at 28* timing compared to 31* on 8 lbs boost)  Thanks for the help, fellas
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