fred

Anyone looking for one pair of GM large oval port #049 casting heads that have had 2.19 in and 1.88 ex valves put in them with just some pocket porting and gasket matching complete with comp-cams valve springs retainers and teflon seals ready to bolt on done by Valako call me, these haven't been ran they are fresh, Fred
Days 517-267-1171
[email protected]

blue thunder

Ok Bo, know I am confused. If I was trying to fill a barrel with water I would want max flow. Velocity would not help. I would remove the spray nozzle to achieve this. It is a fact that an engine performs best when max flows are achieved.

In a boat you are primarily concerned with the torque curve of your setup, not necesarily where peak hp is made, although they are certainly related.

BT

Dave F

Assuming your engine can use the flow that the heads can achieve.

The issue is the size motor and or rpms it runs. If the heads can supply more than the motor needs its not optimal.

Velocity always helps, always.

When the intake runner size is too large for size/rpm of the motor you sacrifice low end torque some because you sacrifice vaccum which sacrifices velocity.

DAVE

Gary Anderson

BT,
The water/barrel analogy doesn't exactly work because your filling it through a valve that is only open part of the time. Someone referrence an article online that explained the relationship of torque curves, valve timing, and porting. In the lower and midrange, the higher velocity with smaller ports will "stuff" more fuel/air into the cylinder. It's only at higher rpm the port size becomes restricting. I've heard that 454 breath fine with ovals up to about 5000 rpm, after that the oval ports get restrictive.
I'll see if I can find that article, it explained it a lot better than I can.
My $0.02
Gary

tomcat

The short answer to this question can be found in the Edelbrock catalog. They show two dyno curves on a 454, one with oval port heads and intake, and one with rect port heads and intake. Everything else is the same, carb, cam, compression, headers etc. The oval port makes a peak of 540 HP @ 6000 RPM, the rect port makes 533 HP @ 6500 RPM and is still rising. At 6500 RPM the oval port is already down to about 515 HP. So for most boats the oval port is better, BUT you have to know that the oval port heads being tested are Edelbrock aluminum. They flow just as well as rectangular ports and have a better exhaust port than the GM cast iron heads.

blue thunder

I in fact have eldebrock 6055 aluminum high perf. rect port heads on my twin engine 454 magnums. I guess you all are saying it would be better for me to remove these and install the gm cast iron ovals ports that I have? Either that or wind my engines up higher? I run them to 5k rpm now.

If this is true about rect vs oval, than why did mercruiser use rect port heads on the magnum and call it high performance. I believe the cam is the same as the 330 hp 454. Only other performance difference is in carb., valve size and maybe combustion chamber. They clearly knew the WOT that these engines would see. Were they misguided?

In my example with the water barrel I was envisioning the rectangle ports as the open hose filling the barrel and the ovals as the nozzle. Ovals will cause a restriction in flow within the runner, possibly causing more co-mingling of the air/fuel fixture. I think I can see that. I think I can see where excess flow of rect. ports may not be efficient from a utilization standpoint, but other that the atomization process improvement caused by ovals, I don't see a negative impact to rect. ports.

BT

excaleagle42

hi bt. hows the bravo coming? hope you'll be ready for canal day.
my little boat has rectangular ports, my big one has ovals. in my opinion, the rectangulars don't decrease the velocity enough on their own. the decrease or slow down if you will, of the fuel/air charge would be more noticable if your exhaust was too restictive or other similar factors that impede the flow or if you had huge runners that were ported too much for low rpm torque.
the ovals flow well too, and the truth is it's not the oval runner that is most restrictive, it's directly under the valves that need the most attention when porting. they will make close to the same power as previously stated spinning 5K. the ovals may have a little more down low but not much unless your combination of parts is way off.
yes, maybe you should take off those crappy edelbrock aluminum heads and sell them to me for cheap Fran

tomcat

blue thunder - You have good heads, and wouldn't change them, but if they were 6045s instead of 6055s it wouldn't make much if any difference. In Edelbrock's comparison they actually used their 6055 rect head and the 6045 oval head, both in aluminum. At 5000 RPM it looks like the oval port heads made ~20HP more. I've got the air flow figures for both heads. At 0.400 intake lift, a point often used to compare heads, the oval head flowed 240 CFM, the rect head 248 CFM, not that different. Both heads used large valves. So the little bit of extra power of the oval port head is due to something else (velocity).

I have often wondered why Mercury used the rect port head. The best answer I have heard was that GM wasn't selling the oval port head, just the peanut port or rect port, so Mercury had no choice for their high performance engines.

mcollinstn

BT, the 330/454 and 365/454mag do not use the same cams. The magnums (454 and 502) both use a bigger cam and the rect port heads and aluminum 2plane intake. I have the cam specs around here somewhere in the Mercruiser service manual, and they are definitely different.

Just went to the kitchen and dragged out the manual (I keep it under the pancake syrup).

7.4L/330hp/454MagAlphaOne:
Intake .470" lift
Exhaust .488"lift
ValveSprings 86#closed 312#open


454MagBravoOne/502Mag:
Intake/Exhaust .513"lift
ValveSprings 110#closed 316#open


M

JimV

iTrader: (1)

Guys
There's a little more to it than filling a barrel with water. Remember air is heavy, add fuel and it gets heaver, once put into motion the weight of the charge carries into the cylinder even after the piston stops at bottom and starts to the top. This is known as inertia supercharging. This is how the engine can achieve over 100% volumetric efficiency. It is not uncommon to see engines produce 104% and above V/E. In a relitively low rpm marine engine with large ports the air is lazy and the V/E numbers are low. To cover this a cam must be used to keep the airspeed at a higher rate, add compression, raise the rpm level or a smaller intake runner. The 454mags have higher compression and operating rpm.
The GM ovals are a good start for the low budget guy and they work great after you spend the money for larger valves, chamber porting and some heavy port work. I would not recommend bolting a stock set of ovals on a motor you may be dissapointed in the results. The last 454 motors we did made well over 500ft lbs. The airflow numbers exceeded the rectangular port heads but the cost is relitively high. The down side is the intake airflow peaks at around .525-.550 lift.
I am guessing but I think merc used what they could get their hands on from GM. I dont think GM had an engine assembly with the combination of oval ports with the LS6 bottom end(steel crank dimple rods forged pistons) so merc took what they could get and made the best of it. They did a great job selecting a camshaft. I am sure money played an important role in this as well. as always this is my opinion JimV