Blown 440 motors on the street, how much boost and compress?
#2866668
12/30/20 07:12 PM
12/30/20 07:12 PM
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Joined: Aug 2003
Posts: 43,157 Bend,OR USA
Cab_Burge
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I've got a customer who has a BDS 8.71 blower with two four barrels on one of his street pump gas 440 motor cars, it had a valve train failure and I'm getting to fix it for him I didn't build this motor originally I've built blown pump gas hemi, BB Chevy and SB Chevy but no Mopar wedges. My question to you that have built and driven them is what static compression ratio do you have and how much boost on pump gas? This motor did not have a boost gauge so the customer has no idea what boost it was making, I'll fix that for him He has a 30 tooth and a 32 tooth pulley on the motor which according to my calculations is right at 6.2 % ratio under drive or over drive, am I right on this math The old combination was stock stroke with 906 heads with right at 8.8 to 1 compression ratio, he wasn't happy with it and wants it to have a little more power and better tire spinning power for burn outs after the local cars shows are over I'm planning on better new aluminum heads with 440 intake port size to be able to reuse is his current blower intake manifold and increase the cubic inch to 505 C.I.(4.350x4.250)and probably increase the compression ratio to between 9.2 to 9.5 to 1 ratio, what say you about these thoughts Thanks in advance for your help and comments
Last edited by Cab_Burge; 12/30/20 07:15 PM.
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: Blown 440 motors on the street, how much boost and compress?
[Re: Cab_Burge]
#2866750
12/30/20 10:06 PM
12/30/20 10:06 PM
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Joined: Oct 2007
Posts: 4,206 New York
polyspheric
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New York
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8-71 means no intercooler. I'm guessing he wants to keep the same drive if possible. What does he have for ignition control? Is boost retard possible? Is the radiator big enough? How do you feel about water/methanol injection?
Unless you know different, I suspect the manifold's discharge area is an open box. You could reduce intake charge temps and clean up mixture distribution some by masking the manifold down to a triangle located under the front of the rotors.
The triangle actually looks suspiciously like a NACA (submerged) duct, with the sides parallel to the rotor angle, and the base located directly under the rotor noses. Yes, the area is much smaller than OEM but flows more air.
Last edited by polyspheric; 12/31/20 11:10 AM.
Boffin Emeritus
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Re: Blown 440 motors on the street, how much boost and compress?
[Re: Cab_Burge]
#2866882
12/31/20 10:49 AM
12/31/20 10:49 AM
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Joined: May 2011
Posts: 4,489 northern,Ohio,USA
Clanton
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Soften the chamber on the iron heads and polish it,Port the heads[mine had 280 cfm] try to keep the swirl in the cylinder.How does the piston look with carbon to indecate fuel flow,quench?These are some of the things I did with my iron heads but I was on race gas.My Isky sft cam .570ish lift 110 had a lot of tq but also a lot of overlap[70 deg] ps:as you can guess this cam suffered from the overlap and was a tight lash.
Last edited by Clanton; 12/31/20 11:16 AM.
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Re: Blown 440 motors on the street, how much boost and compress?
[Re: Twostick]
#2867010
12/31/20 03:00 PM
12/31/20 03:00 PM
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Joined: Aug 2003
Posts: 43,157 Bend,OR USA
Cab_Burge
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I'm going to use a set of aluminum heads with either 75 CC or 84 CC chambers which means having a reverse dome piston with either a 32.0 or smaller reverse dome piston to get the compression down instead of going to a lower deck height and loose quench. Speaking of softening the chambers Edlebrock does make a head with the softening done to allow a quench dome piston Thanks for bringing that up I'll do the math on that head next
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: Blown 440 motors on the street, how much boost and compress?
[Re: B1duster]
#2868568
01/03/21 05:52 PM
01/03/21 05:52 PM
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Joined: Dec 2007
Posts: 12,587 Great Neck,LI,new york
hemi-itis
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Final compression ratios in excess of 12.4:1 are not recommended for use with "pump gas." The higher the final compression ratio, the higher the octane rating of the fuel must be in order to help prevent detonation and serious engine damage. The formula for calculating your exact final compression ratio is as follows: Final Compression Ratio (FCR) = [ (Boost÷14.7) + 1 ] x CR
Boost = Maximum blower boost 14.7 = psi at sea level CR = engine compression ratio
Altitude plays an important role in determining compression ratios. If the altitude in the area where you normally drive is significantly higher than sea level, then your compression ratios will also vary. To determine the effects of the altitude on a calculated compression ratio, use the following formula: Corrected Compression Ratio = FCR - [ (altitude÷1000) x 0.2 ]
= Pump Gas = Good Gas
HEMI-ITIS has no cure. My condition is fully BLOWN!!
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Re: Blown 440 motors on the street, how much boost and compress?
[Re: hemi-itis]
#2868721
01/04/21 12:25 AM
01/04/21 12:25 AM
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Joined: Aug 2003
Posts: 43,157 Bend,OR USA
Cab_Burge
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We, him and I, live in central Oregon which is above 2500 Ft to the furthest north he will take this car and around 4500 Ft above sea level to the south down near Klamath Falls, OR. My house is right at 4300 ft. and his is a little higher, I think. He may have moved since I was over to his house to the south and east of me 10 yrs. ago The car and engine where built by someone else a long time ago, stock 906 heads, forged +.030 Seal Power flat top pistons down on the hole around - .090 with stock LY rods with ARP bolts I'm still working on making up the parts to order list but it will get a new set of aluminum 440 heads with stock size 440 intake ports so we can reuse his BDS blower manifold, a 4.250 forged 4340 steel stroker crank along with a bigger and better solid roller cam, better 7.100 long rods and a better ignition and fuel system to make sure he doesn't hurt it due to lack of fuel supply at WOT More to come later
Last edited by Cab_Burge; 01/04/21 12:28 AM.
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: Blown 440 motors on the street, how much boost and compress?
[Re: Clanton]
#2868898
01/04/21 03:17 PM
01/04/21 03:17 PM
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Joined: Aug 2003
Posts: 43,157 Bend,OR USA
Cab_Burge
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Please do post those pictures I ended up ordering some Eddy E street heads so I can reuse his BDS blower intake and Comp Cams SS rocker arms, I'm having Bullett make me a new cam that is quite a bit bigger in lift and duration, old lift was right at .512 and the new cam should have around .620 net lift. Their grinding it on a 112 LSA and I'll probably install it between straight up and no more than 3 degrees advance on the intake lobes to start with. I'm still debating on what compression ratio to start, probably around 9. to 1 depending on the current block deck height
Last edited by Cab_Burge; 01/04/21 03:19 PM.
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: Blown 440 motors on the street, how much boost and compress?
[Re: Cab_Burge]
#2868918
01/04/21 04:14 PM
01/04/21 04:14 PM
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Joined: Jan 2003
Posts: 28,067 Irving, TX
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Irving, TX
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Expect your boost numbers to drop with those heads and cam.
You likely need to remind the guy that boost is resistance to flow. The new setup will use more air so less of it will stack up in the intake. You're always on the right track when you make more power with less boost.
He has to get out of the mindset of talking boost numbers. Someone else will be packing more boost and make him think he's missing out.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind. - Stu Harmon
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Re: Blown 440 motors on the street, how much boost and compress?
[Re: hemi-itis]
#2868952
01/04/21 05:24 PM
01/04/21 05:24 PM
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Joined: Oct 2007
Posts: 4,206 New York
polyspheric
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Do I have to explain this all over again?
Since the cylinder volume and the combustion chamber volume are both pressurized to the same extent, the original compression ratio remains unchanged by boost pressure, and the compression ratio is not increased. However, cylinder pressure and knock sensitivity are increased due to greater charge density.
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