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Discussion Starter #1
I have just got a Holley Mega Blower and want to know what it compares to? I believe it's a 420. Does that mean it's a 4-71, or is it closer to a 6-71 due to the air it flows? I just swapped the 177 that was on my 525sc with the new Mega Blower and would also like to know what kind of horse power gains I can expect to receive. It's got dual 750s and I have not decided on how much boost to run.

Thanks for all the help.
 

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No Sandbar To Far
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It's bigger than a 6-71 but smaller than a 8-71.
 

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Boost is Boost doesn't matter if it comes from a 177, 420, 6-71 or 14-71. you will make the same HP at the same boost. It is however a more efficient blower i.e. you dont have to run it as hard to accomplish the same boost pressure. When you get into high HP and large Cubic Inches you have to go to a bigger blower to flow enough air to make the boost. There is a balance, you would not put a 14-71 on a 350 to make 5psi. Too much parasitic loss to drive the big blower as you would not run a 177 on a 572 to try and get 15psi it does not flow enough air. You will be able to make more boost with your 420 than you made with your 177. Have fun. just my .02.

How much boost were you running through your 177 single carb set-up?
 

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Good comparison Sanger and BamBam. They are both correct. My very first blower was a 420 megablower but that was back when they were old and made by B&M. Last time I looked at one of the Holly units, they were identical down to the rotors. The 420 has a case comparable in size to a 6-71 depending on who makes it maybe just a hair larger. The 420 refers to the volume of air it moves per revolution. Once again, more air volume than a 6-71 and just slightly less than an 8-71. If you want specifics as too exactly how much air volume it provides versus a 6 or 8-71, let me know and I can tell you. Overall, it's a fairly efficiant blower. Anything over 10 lbs of boost, it would be much wise running a superchiller. Obviously if your bottom end isn't built with a forged rotating assembly and blower pistons, I would be safe and keep it around 5-7 lbs. This will most likely be debated my many but I have always believed that if you can afford to build a purpose built motor, build as much "insurance as you can afford" and be smart with the boost. It's all fun UNTIL that little ping you hear (detonation) destroys a piston. Once it detonates a bit, it usually does a pretty good job eating itself from within. Good luck and hope this helps.

"The General"
 

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Discussion Starter #5
This is all great info. To answer Bam Bam questions about previous boost...I don't know. The boat did not have a boost gauge but it was a stock Mercury 525sc. I will be putting a boost gauge in today and heading to Naci tomorrow for the weekend. I have 3 different pulleys to play with to reach a safe but optimal boost level. A Supperchiller will be next on the list of improvements but may be a month or two before I have the coin for it.
Thanks again for all the info. I would be lost without all your help and PB Forms....Thanks.
 

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I had a holley megablower 420. Just an FYI, parts may be hard to find as holley does not make them anymore. You may have to call a specialty outfit like The Blower Shop if you ever need parts.
 

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Boost is Boost doesn't matter if it comes from a 177, 420, 6-71 or 14-71. you will make the same HP at the same boost. It is however a more efficient blower i.e. you dont have to run it as hard to accomplish the same boost pressure. When you get into high HP and large Cubic Inches you have to go to a bigger blower to flow enough air to make the boost. There is a balance, you would not put a 14-71 on a 350 to make 5psi. Too much parasitic loss to drive the big blower as you would not run a 177 on a 572 to try and get 15psi it does not flow enough air. You will be able to make more boost with your 420 than you made with your 177. Have fun. just my .02.

How much boost were you running through your 177 single carb set-up?
Although I would have previously agreed with you, I have just proven it wrong. I just did the same swap. I went from a non intercooled 177 Merc 525SC blower and an 1150 dominator to an intercooled 420 with dual 825 Demons. I know, I went more CFM, and intercooled, but I dropped 1/2 psi of boost. I did not dyno before and after, but I could pull a 28" bravo 4 blade to just about 4100 RPM before, and now I can pull it to 5100 rpms. That takes more HP than an intercooler and 1/2 psi less bosst will make. Noticably more bottom end, and more top end. Very impressive. Wish I had dynoed it now!
 

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Although I would have previously agreed with you, I have just proven it wrong. I just did the same swap. I went from a non intercooled 177 Merc 525SC blower and an 1150 dominator to an intercooled 420 with dual 825 Demons. I know, I went more CFM, and intercooled, but I dropped 1/2 psi of boost. I did not dyno before and after, but I could pull a 28" bravo 4 blade to just about 4100 RPM before, and now I can pull it to 5100 rpms. That takes more HP than an intercooler and 1/2 psi less bosst will make. Noticably more bottom end, and more top end. Very impressive. Wish I had dynoed it now!
He probably meant "boost is boost"...at the same temp. The cooler (denser) charge from a slower blower and intercooler will produce more HP at the same boost level...so it's not apples to apples.
 

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Discussion Starter #9
I ran it last weekend with great success. 5 lbs of boost, 5300 rpm, 28 pitch prop, 86 mph with 5 people in the boat. With a little more tuning and less people should reach 90 mph easy. What do you think?
 

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He probably meant "boost is boost"...at the same temp. The cooler (denser) charge from a slower blower and intercooler will produce more HP at the same boost level...so it's not apples to apples.
i agree, sometimes big boost from a small blower is just the intake charge expanding from the heat, and that ain't gooooood:)sphss
 

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I ran it last weekend with great success. 5 lbs of boost, 5300 rpm, 28 pitch prop, 86 mph with 5 people in the boat. With a little more tuning and less people should reach 90 mph easy. What do you think?


Sounds great.I also run the 420's on my motors. I run 10psi with intercoolers and no problems with 91 pump, even with the heat of the summer. The Blower shop said they can rebuild them if needed, and there shouldn't be a problem with parts.
BTW, 90 should be no problem for ya with a lite load.


Darrell.
 
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