D-port head question

[MANIC MECHANIC]

After reading several discussions here about oval port heads being the better (stock)454 head to use on a N/A 6000rpm or less motor,( I have read the same thing in ***boat's tech section repeatedly) I am left wondering why Merc put them on most if not all 454's?
I have six 454's of varying HP and year in storage and they all have the D-port heads.
http://www.***boat.com/image_center/data/500/1191454_s_and_350_s-med.jpg
http://www.***boat.com/image_center/data/500/1191OB_s_and_454_s_in_shelves-med.jpg
Why did they use the D-port heads?
:confused:
Tim

[Moomawnster]

Hey Tim ! get in touch with Wet Dream ..... he needs a 454 !

[gnarley]

Originally posted by MANIC MECHANIC
After reading several discussions here about oval port heads being the better (stock)454 head to use on a N/A 6000rpm or less motor,( I have read the same thing in ***boat's tech section repeatedly) I am left wondering why Merc put them on most if not all 454's?
I have six 454's of varying HP and year in storage and they all have the D-port heads.
The D port shape is only what the exhaust port looks like. If you were to pull the intake manifolds you might find oval & squares depending on the OEM HP rating

[MANIC MECHANIC]

I have pulled the intakes, the heads have the big rectangular ports.
My question remains;
Why did Merc use big port heads on these unblown, low rpm motors?
Tim

[gnarley]

Originally posted by MANIC MECHANIC
I have pulled the intakes, the heads have the big rectangular ports.
My question remains;
Why did Merc use big port heads on these unblown, low rpm motors?
Tim
Tim from what I know GM only offers the small peanut port oval head & that comes on the low HP motors. The next largest flowing port is the square port so Merc uses that, as there is no large oval port in production for OEM use on a Gen V or VI, there is only a choice of two heads, thats it! So I guess Merc feels that the square port will work better at 5000 than the oval's & they don't loose a whole bunch of low end drivability.

[Dennis Moore]

Mercruiser doesn't build engines for the regular production sterndrives, only for the HP series of engines. They simply buy an engine assembly from GM. They have no control over the design of the GM engines.
With the mild camshaft timing used on the rectangle port head engines (the engines are undercammed to prevent water ingestion) the performance is increased slightly with larger intake ports and valves.
It is not until you start killing intake port velocity with a long duration camshaft for high horsepower at the (relatively low) engine speeds of a marine engine (5500 rpm max for example) that the huge intake ports start to really hurt low speed throttle response.
Stated another way;
If a short duration camshaft is necessary to prevent water ingestion, the only other way (and not a good way) to build horsepower is to increase port and valve size (rectangle port heads).
Dennis Moore

[MANIC MECHANIC]

Interesting.
Thanks for the replies.
Tim

[gnarley]

Originally posted by Dennis Moore
Mercruiser doesn't build engines for the regular production sterndrives, only for the HP series of engines. They simply buy an engine assembly from GM. They have no control over the design of the GM engines.
Dennis, doesn't GM build the engines to Merc's spec's? And isn't the reason that Merc uses the square port head & the mild cam due to the fact they cannot get the engine built with an oval port head that is better than the peanut port head? GM only offers a couple of different head configurations on factory built OEM engine units, the oval peanut port head & a large square port, there is no large oval port available. This is the real reason I think that Mercury uses the square port head, there is no in between like there used to be with the Gen IV engines so to make the square port head work at lower RPM's they put in a smaller cam as you said to keep the signal up.

[Liquid Invader]

Manic Mechanic,
You could have saved a lot of typing buy asking this and the olds questions today when you were by. Or don't you trust me anymore?
I think you just like to write.
Later 383
:D

[MANIC MECHANIC]

Just checking up on you.:)
Tim