Rec vs Oval port?
 

I have a 1978 25ft Magnum heavy lay up.It has a 365hp Mag BB chev with rec port heads. TRS drive, would I see any improvment in acceleration or wot mph gain if I swiched to oval port heads? I get 61 mph at 5050rpm now with a 23p re- piched to 24p, old quicksilver cleaver.

hmmmm thats odd. As far as i know teh rectangle port heads are bigger than the performance (large) ovals so i woudl tend to hink smaller ports might just slow you down.

You coudl pobably have your curent heads worked a little to make them flow more air andtheat might help you pick up some HP and speed.

You might call and talk with JimV. He can put 'wedges' in your heads and mild port work and really wake them up.

Just read an article in Family and Performance boating that said they tested 2 identicle motors 1 with rec ports and 1 with oval,,, the oval porst made more HP and torque, and in a more usable power band.

Jim Valekeo did my heads. I am running the stock GM rec. port and Jim put the wedges in them and 2.3 intake valves and a bunch of grinding later and my engines really came alive. Jim can set you up with a complete package deal....

I've been doing a LOT of research on this because I'm thinking of putting some different heads on my motors. What I've come up with is the oval port high performance heads have smaller intake ports but they make more torque & horsepower at lower rpm's. Unless you plan on running above 6,000 rpm's you will make more horsepower with them & go faster. If you do buy new heads somethings I would look for are heads with intake port volume around 260cc, combstion chamber volume around 118cc, valve size 2.19 intake & 1.88 exhaust, and as long as you have them off I would get them mildly ported, unshroud the valves a little, clean out the bowl & runner area, & make sure the intake & exhaust line up. As far as aluminum vs cast I think I'm going with the cast because of the price difference & because I've heard a few stories of head gasket & corrosion problems with the aluminum. Did you have a particular head in mind ? Please keep us posted! good luck !

obnoxus,

what type of oval port heads did they use for the test??

What generation block are you using?? If its a Gen V, then going to GM large oval ports won't work. If it's a Mark IV then they'll be fine. On a Gen V, you would have to go with an aftermarket heads.
Most of your performance increase would be in the lower and midrange area, with maybe a slight increase on the top.

CR max,
they were Edelbrocks,,, the bare casting prt # was 7161 which did include valves,, I did not get a price,,,, for complete heads with Inconel exhaust valves ready to bolt on were just under $700 a piece,,, like $683 I think,,,, did not get p`ricing on cam and injection work yet.

I saw an article by Teague in PB a few years ago. They pretty much agreed with Bo. Better midrange with the ovals. And unless your pushing over 500hp, there wasn't much of an increase in hp with rec. port. They also mentioned that even in higher hp applications they liked to stay with oval ports on single engine boats for the added low/mid range torque.
Gary

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

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?:confused:

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

Hey Andy,

How come no thanks for puder ??

Bo

Griff: how can I tell what generation block I have and why would it make a difference on head selection? P.S. Thanx Pudder

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

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

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!!!

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

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.

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]

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

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

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

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.

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

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:D Fran

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.

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

heads
 

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

JIM V

I am running one of the 502/502 partial kits from G.M. in a 25ft switzer. It has the large oval aluminum heads (225 intake / 188 exaust.) 9.6:1 comp. If i understand what you are saying, the heads I have are good but some work could really wake the motor up. (we ran 79.1 on gps @5250+- rpm )with an 850 double pumper and a victor jr intake.

what is the weight of your boat? Mine is a 24ft Magnum approx 4500lbs with min 390hp and I can only get a true 61 mph max after 15 hrs at wot or the full length of lake Huron(just kidding) is your hull that much better or can 110 hp realy give you 18mph

Yes, it is true, so I've read, that Merc basically had to use what it got it's hands on when it came to using the heads discussed. Oval's weren't prevalent in those years.

With extensive work done to ovals, they can out perform rectangles to 6000 r's on the 454's and 502's. When you get into the 540's and above, I would use the rectangles.

Jim basically reinforced what I said, with a much better explaination. That's why he's the expert though, huh?

Velocity is actually the key. Like Jim said, the air gets lazy without it.

As I've said 100 times, you have to match your parts with your application. Ya just cant through a bunch of parts together and expect them to be optimal.

DAVE

Heads
 

hpbigblock,

Yes, by modifying your heads you can increase power. The heads are capable of flowing as much or more than the rectangular ports. Your heads in particular, are superior in design and would be a great starting point. Your 502 would benefit changing to a 2.300 intake valve. If the airflow numbers coinside with the valve sizes you will make more power. Depending on where the numbers are now you can make another 40 hp.






Dave has been making this point since I have been around and I agree with him 100%

JimV im wondering if you have done any wedges and cam changes to a other wise stock 500hp carb motor and what results have you gotten. i see on your web page that you are doing a 502 with 650hp but ther hasnt been much updates. were do us guys with 500's go beside supercharging. im interested in what kind of numbers your getting with the wedge modified heads. im assuming the reason for the wedges is for velocity because the rectangle port head the port are to large.
thank you in advance

Andy

I dont know much about magnums. when i bought the switzer I knew nothing about them and still know verry little because the company went out of buisness in th early 90's and they seem to be somewhat rare (at least there is no info on the web about them that i can find.) but are a light boat. I did find an article in an issue of Power Boat from the 1991 year that rated them stern drive performer of the year.(I didn't know this untill after i bought the boat for a song and dance. Idont think the owner knew what he had in tearms of performance) The 79.1 was done on flat water and this boat lives for 2ft chop. So on good water who knows what it will do.

Great thread we've got going here !

Blue Thunder, There is a point when air is flowing thru a head that if you have too much volume ( rec heads ) the air stalls and you loose some of the velocity needed to move the air faster thru the port. The water hose reference was for velocity, volume demonstration purposes only. For best performance the intake port should be no bigger than needed to supply the cylinder with all the air & fuel it can use at the highest RPM it is to be run at, any more & performance will decrease.

I've never heard of the wedges put in the rectangle port head but it makes a lot of sense. How is this done & how much does it cost ?

The reason the rectangle port heads are used instead of heads that would be more marine friendly is because GM never really paid much attention to the marine industry, when building marine high performance engines they just grabbed what was on the shelf that they always used for automotive high performance engines. Unfortunately its not the most ideal choice. The GEN VI engines got cylinder heads that, although the're not perfect, are a lot better for marine use than the earlier rectangle port heads.

Bo

Yes, like Dave F said, matching the correct components together is crucial to max performance. It is also the single biggest challenge I have with my current project boat. It is so mismatched its not even funny. Next winter I'll have to do some sprucing up in that area. Thanks for all the great info.

Excaleagle42... the bravo is progressing, but very slow. The SB engine is re-assembled and back on board ready for fire. A little warm weather would help... Just installed my newly fabricated dash plate with all new gaffrigs and switches... pretty happy with that. Then I need to reinstall all the upholstrey and clean everything... then theres the trailer brakes. Wow, good thing I love doing this stuff ;) . It should be in respectable shape by Canal Days. We will have to hook up and have a cold one. Your in a black eclipse 42'? See ya later.

BT :cool:

Ahhh, the C&D canal.

God, I can't wait to see you guys down there this summer. If you see me first wave me down to say hello.
The upper Bay and canal area is where I spend my time.

DAVE
242 ls Formula
"Blue Bayou"

Ah yes.... the c & d indeed. Abosultely my favorite performance boat adventure. Blue bayou... cool name, I can remember that. See ya therre :)

BT :cool: