Rec vs Oval port?
03-19-2002, 10:52 AM
#11
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Oval ports are in fact better performers than rectangle ports because of the low rpm the boat motor see's.
The reason they create more torque and hp is because they make the charge into the cylinder a higher velocity which swirls the mix, creating disturbance in the cylinder, creating a more efficeint/complete burn.
Griff,
GM does have a large oval for the Gen 5 and 6. I'm going by memory here so forgive me if the part number is a little off.
It's 10101137. Large oval port head, around 265 cc's with around 110cc combustion chamber. That will bump up the compression to around 9.5 or so with stock pistons.
They also have a large oval port that measures 290cc for the 502/502, but they're aluminum.
DAVE
The reason they create more torque and hp is because they make the charge into the cylinder a higher velocity which swirls the mix, creating disturbance in the cylinder, creating a more efficeint/complete burn.
Griff,
GM does have a large oval for the Gen 5 and 6. I'm going by memory here so forgive me if the part number is a little off.
It's 10101137. Large oval port head, around 265 cc's with around 110cc combustion chamber. That will bump up the compression to around 9.5 or so with stock pistons.
They also have a large oval port that measures 290cc for the 502/502, but they're aluminum.
DAVE
03-19-2002, 09:00 PM
#12
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Thanx for the input (exept pudder) What was the cost of Jim V wedges and mild port? What is the compression and cam spec you have, aftermarket exhaust?
03-19-2002, 09:25 PM
#13
heads
The GM ovals work great but need port work to get them that way. I dont think any of them come with a hardend exhaust seat with the exception of the later vortech head, which would be my first choice if you have a flat top piston. Even the vortech head needs exhaust porting and chamber work to optimize.
Thanks for the good words Guys
Thanks for the good words Guys
03-20-2002, 07:40 PM
#16
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Andy,
Gen IV are blocks produced between 1965 - 1990
Gen V are blocks produced between 1991 - 1995
Gen VI are blocks produced between 1996 - present
The heads are not interchangeable so if you had a Gen V you could not use the large oval port GM heads because they were not made for Gen V motors. You would have to go with after market heads. Sorry Griff but I couldn't resist taking this one.
Bo
Gen IV are blocks produced between 1965 - 1990
Gen V are blocks produced between 1991 - 1995
Gen VI are blocks produced between 1996 - present
The heads are not interchangeable so if you had a Gen V you could not use the large oval port GM heads because they were not made for Gen V motors. You would have to go with after market heads. Sorry Griff but I couldn't resist taking this one.
Bo
03-20-2002, 08:12 PM
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You guys apparently know this stuff better than I. I guess what you are saying is that twin 330 hp 454s would perform better than my twin 454 magnums with rectangle ports. That defys logic to me. Are you considering gear ratios and prop pitch in your thought process? Rect port will offer a higher hp. A higher hp will allow more optimal gearing to reach max torque at optimal rpm. A lower hp curve restricts this optimisation... no? Maybe I am missing something, but more is better in this case as long as you are geared for it. You cannot beat hp more hp no matter what.
BT
BT
03-20-2002, 08:34 PM
#18
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Gen Mixing....
I'm running Gen V rect. port heads on my Mark (Gen) IV motor. Had the heads massaged a little, and valve relieved the block. Should wake my motor up a bit. I'll let you know when it's back in the boat!!!
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03-20-2002, 10:22 PM
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Blue Thunder.
Your 454 mag has more H.P. & will perform better than the 330 H.P 454 because it has better heads, more compression, bigger cam, 4 bolt mains, different carb & intake, forged pistons & a lot more. Now heres the head situation. The 330 horse engine has small oval port heads with an intake port volume of 200cc. It's good for low rpm power but wont flow enough to make H.P. above 4500 rpm. The rectangle port heads have 320cc runners & bigger valves which flow a lot more air & will produce peak H.P. above 6000 rpm. At the lower rpm's that most boats run at, 4600 - 5200 rpms, what happens is there is so much volume inside these big runners you loose the velocity of the air flowing to the engine. Think of water spraying thru a garden hose nozzle, then take the nozzle off, more water flows thru it but it has no velocity! Same principal. The big rectangle port heads are better than the 330 horse small oval port heads but there not the ideal heads for the job. Another prime example of where bigger is not always better! A large oval port head with the big valves & intake runners around 260cc would be a good choice for an engine running around 5000 rpm's. I hope this helps explain things a little.
Waterfoul,
I believe the stock GM heads will not interchange between GEN V & GEN IV , but I believe some of the after market heads will.
Bo
Your 454 mag has more H.P. & will perform better than the 330 H.P 454 because it has better heads, more compression, bigger cam, 4 bolt mains, different carb & intake, forged pistons & a lot more. Now heres the head situation. The 330 horse engine has small oval port heads with an intake port volume of 200cc. It's good for low rpm power but wont flow enough to make H.P. above 4500 rpm. The rectangle port heads have 320cc runners & bigger valves which flow a lot more air & will produce peak H.P. above 6000 rpm. At the lower rpm's that most boats run at, 4600 - 5200 rpms, what happens is there is so much volume inside these big runners you loose the velocity of the air flowing to the engine. Think of water spraying thru a garden hose nozzle, then take the nozzle off, more water flows thru it but it has no velocity! Same principal. The big rectangle port heads are better than the 330 horse small oval port heads but there not the ideal heads for the job. Another prime example of where bigger is not always better! A large oval port head with the big valves & intake runners around 260cc would be a good choice for an engine running around 5000 rpm's. I hope this helps explain things a little.
Waterfoul,
I believe the stock GM heads will not interchange between GEN V & GEN IV , but I believe some of the after market heads will.
Bo
03-20-2002, 10:35 PM
#20
The big problem with the oval port heads is that the port is at the bottom of the head not toward the top to promote a better radius going into the valve opening. If you take rec port heads and put wedges in the floor of the intake ports you get the small port volume and high velocity but you get much more flow than the oval port head due to the larger radius entering the valve opening.
