Blown Hemi questions?
#2256610
02/20/17 03:31 PM
02/20/17 03:31 PM
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Joined: Dec 2007
Posts: 12,592 Great Neck,LI,new york
hemi-itis
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What is the ideal CR for a street/strip roots blown Hemi.I thought I had 8.5 to 1.Now that the engine is out and we are measuring what was there it is coming up at 7.4 to 1.The piston is 100 thou in the hole.This is NOT was I expected to find.The camshaft was ADVANCED 6* also.And the rings were Moly. Not too happy with my findings BUT,,,,,I got 5 years and well over 10k miles before the ring failure. The iron heads I had on there are 170 cc,,the aluminum ones I want to use are 183 cc and un shrouded. So the new pistons will make up for the shortcomings of the last build.Now,,,make it a true8.5:1 or higher??? Wonder what I was lacking with that cam 6* advanced,,,,,,,,,
HEMI-ITIS has no cure. My condition is fully BLOWN!!
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Re: Blown Hemi questions?
[Re: hemi-itis]
#2256701
02/20/17 05:26 PM
02/20/17 05:26 PM
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Joined: Dec 2007
Posts: 12,592 Great Neck,LI,new york
hemi-itis
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Thanks Dave.This vision has been in my head since attending the '71 Summer Nats at National Speedway here on Long Island.I was 13 YO What better place to find pointers and info than HERE!! Thanks to Tom
Last edited by hemi-itis; 02/21/17 01:40 AM.
HEMI-ITIS has no cure. My condition is fully BLOWN!!
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Re: Blown Hemi questions?
[Re: hemi-itis]
#2256770
02/20/17 07:54 PM
02/20/17 07:54 PM
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Joined: Aug 2003
Posts: 43,292 Bend,OR USA
Cab_Burge
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What type of fuel and how much boost are you planning on running for your street driving? The last blown pump gas Street hemi motor I built and dyno tested(4.250 bore with 4.375 stroke) had 9.2 to 1 mechanical compression with a Littlefield 10.71 12 % under driven for CA pump gas with a set of the Carb shop 1050 CFM, I think custom built blower Dominators carbs. That motor made 927 HP at 7300 RPM with 33 degrees total timing on 1999 91 Octane CA pump swill with 7 lbs of boost We had made a bunch of pulls testing the timing and jetting first to get those results We switch the upper and lower pulleys to make the blower have 13% over drive which made 12.5 lbs at 6500 RPM , we did first switch the fuel to Union 114 octane race gas and took 8 degrees timing out of it so it had 25 degrees revved up, the motor started to go into detonation at 6600 RPM so we stop pulling it. That pull made 1024 HP and was still gaining HP at 6500 and started loosing it at 6600 RPM We made one more pull and moved the timing up to 27 degrees and the motor hated that IHTHs
Last edited by Cab_Burge; 02/20/17 07:56 PM.
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: Blown Hemi questions?
[Re: hemi-itis]
#2256965
02/21/17 01:48 AM
02/21/17 01:48 AM
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Joined: Dec 2007
Posts: 12,592 Great Neck,LI,new york
hemi-itis
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Started at 30* and was down to 24* when the rings failed. Mike,I never did a compression test on it.BUT,,,,leak test before my last track day was already 18%.2 passes and it was done
HEMI-ITIS has no cure. My condition is fully BLOWN!!
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Re: Blown Hemi questions?
[Re: hemi-itis]
#2256971
02/21/17 01:52 AM
02/21/17 01:52 AM
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Joined: Dec 2007
Posts: 12,592 Great Neck,LI,new york
hemi-itis
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I'm also on the fence about coated bearings & pistons
Last edited by hemi-itis; 02/21/17 01:55 AM.
HEMI-ITIS has no cure. My condition is fully BLOWN!!
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Re: Blown Hemi questions?
[Re: DusterDave]
#2257181
02/21/17 02:58 PM
02/21/17 02:58 PM
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Joined: May 2011
Posts: 4,489 northern,Ohio,USA
Clanton
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FWIW, the static compression ratio on my old 528 blown Hemi was 7.8:1. Personally, I wouldn't go any higher than 8.5:1. Here is a pic of one of the Ross blower pistons I used, with tapered tool steel pins. Notice how far down the ring grooves are. Nice piston,not to thin by the valve pocket where itis had trouble.
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Re: Blown Hemi questions?
[Re: hemi-itis]
#2258163
02/23/17 12:51 PM
02/23/17 12:51 PM
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Joined: May 2011
Posts: 4,489 northern,Ohio,USA
Clanton
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Mike,that's where it needs to be.Found my pistons to be 100 thou in the hole. With the piston .100 in the hole you are far away from the edge of the piston[valve relief] and could have a much thicker edge there.Will you do a clay check on the valve to piston?
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Re: Blown Hemi questions?
[Re: hemi-itis]
#2258188
02/23/17 01:54 PM
02/23/17 01:54 PM
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Joined: Dec 2007
Posts: 12,592 Great Neck,LI,new york
hemi-itis
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I want to raise the top of the piston,not sure if I should make it zero or leave it down a little,,,,,,, 9.5 to one isn't too high for a mainly street driven engine.Guess I could underdrive to 3 or 4 lbs for pump gas steet driving.Would the higher compression give me more power with less boost?How does that change the intake manifold air temp???
HEMI-ITIS has no cure. My condition is fully BLOWN!!
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Re: Blown Hemi questions?
[Re: hemi-itis]
#2258256
02/23/17 05:39 PM
02/23/17 05:39 PM
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Joined: Dec 2007
Posts: 12,592 Great Neck,LI,new york
hemi-itis
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So if I start with 9.5:,it will respond better to higher levels of boost with more octane?I would also think that at that static CR,The pump gas limit would be,,,,like 8 lbs I'm also replace the STOCK '66 iron head with can old set of Direct Connection Dart raised port heads that were ported by Total Flow.
HEMI-ITIS has no cure. My condition is fully BLOWN!!
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Re: Blown Hemi questions?
[Re: hemi-itis]
#2258315
02/23/17 08:47 PM
02/23/17 08:47 PM
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Joined: Aug 2003
Posts: 43,292 Bend,OR USA
Cab_Burge
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I Win
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Bend,OR USA
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If your going to aluminum heads you can raise the mechanical compression ratio a little bit and still be safe on pump gas, I wouldn't though Keep it safe at 9.5 to 1 and use the O.D. ratio on the blower to make more power on better fuel when you need it As far as what is safe at 8 lbs. of boost will be determine buy which spark plug heat range, how much timing, the jetting and the local weather conditions, if it starts to not feel right, lift up your right foot!
Last edited by Cab_Burge; 02/23/17 08:54 PM.
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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