Mercruiser questionsoriginally

[MOWtown]

I am told that my 454 merc originally came with oval port heads and intake. Furthermore, that oval port heads will produce more torque (better for boats) than square ports. Also, is it true that if I am only going to be spinning no more than 5,200 rpm, oval ports are actually better than square ports...that square ports are actually "overkill" if I'm not spinning 6,000+. Any thoughts...?

[kc0000]

Mercruiser 454 Mag MPI 385hp is rev limit 4800-5000
Mercruiser 454 7.4 MPI 310hp is rev limit 4500-4600

Both come with the same "left over" 325cc intake square port.... They over compensate for the large square ports by restricting the intake manafold and making the runners as long as possible. The 310hp is a truck motor and can not be built up without jepordizing the pistons. The 454 Mag and 502 Mag use the same internal components and the same intake manifolds, just different bore, short block probably good for 600hp+.

As far as the original Mercruiser 454 coming with ovals, left over parts from the 427.....

KC

[NordicHeat]

As KC pointed out, the 454/502 Mag motors all came with the rectangular port heads left overs from the past era of high compression, 7000 rpm motors.
However, from appx 1985-1997 the standard Merc 7.4L (300 hp carb and 330 MPI) motors had the small peanut port heads that are extremely limited for power output.
The 1998 and up 310hp MPI came with the newer Vortech oval port which some people have made work well. These also have a small cumbustion chamber (105cc) raising compression almost a full point.
Better heads, more compression and 20 less hp. Mercury Marine could never clarify that one for me http://www.lasvegashotboats.com/iB_html/non-cgi/emoticons/rolleyes.gif

If you have to switch heads to achieve your goal there are many aftermarket heads with smaller rectangulars and some very good oval port heads that would work very well.
If you have the peanut ports, don't bother trying to get any performance out of them. They simply won't work.

Your hp desire and rpm range will dictate the best head for your application.
In my personal application, I run Edelbrock OVAL ports (extensively worked by Chapman Racing) on a 540 c.i. that spins 5500 rpm very happily. http://www.lasvegashotboats.com/iB_html/non-cgi/emoticons/biggrin.gif
Good Luck,
Dave

[MOWtown]

Thanks guys. FYI, I have the 7.4 MAG.
My block is sound...heads thrashed. I have a set of fresh oval port heads so it seems logical to use these heads and just buy a new intake. As usual, just trying to find a balance between increased performance and $$$. I've read that this motor is 330 HP stock. I'd like to get atleast 360-380 out of this rebuild (OKAY, I'd really like 500 but that's the next motor!http://www.lasvegashotboats.com/iB_html/non-cgi/emoticons/wink.gif but, considering that I'm not spinning more than 5000 rpm's, am I hurting my performance by going with the ovals OR am I helping because they tend to provide more torque at lower RPM's? Should I buy the new intake or cough up the money for new square port heads and use the original intake? Or does it f#%^ing matter at these sub 5,000 rpms? BTW, 21' Eliminator open bow

[kc0000]

i have aset of 310cc intake Dart Pro-1 Alum heads that would make a world of difference on you motor.... brand new.
KC

[kevnmcd]

</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (MOWtown @ Feb. 13 2003, 2:49 pm)</td></tr><tr><td id="QUOTE">.....BTW, 21' Eliminator open bow[/QUOTE]<span id='postcolor'>
Let's see pics? Does it look like mine?

[MOWtown]

Sorry, Kevin. Can't post pics right now since the outdrive is at the shop, the hull is in my mechanic's yard, the motor is at the machine shop, and half of the upholstery is in my garage!!! I should have it back together within 3 weeks though...

Hopefully, I'll have enough time left before boat season starts to do the upholstery but I'll nurse another year out of it if it means not missing any weekends!

It's very similar to yours...mine's an 88 pre-EAGLE open bow...burgundy, black,silver,white colors.

[MOWtown]

KC, thanks but I'm trying to use what I have and save some money for the upholstery and trailer.

Mike

Speaking of trailers, Kevin, how is yours coming along? What did you do for brakes...disc? Do you have the same older Competitive trailer that I do with loads of bondo on it to form smooth edges?

[kevnmcd]

</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (MOWtown @ Feb. 13 2003, 10:36 pm)</td></tr><tr><td id="QUOTE">Speaking of trailers, Kevin, how is yours coming along? Â*What did you do for brakes...disc? Do you have the same older Competitive trailer that I do with loads of bondo on it to form smooth edges?[/QUOTE]<span id='postcolor'>
Yes, I have the Competitive with tons of bondo on it. http://www.lasvegashotboats.com/iB_html/non-cgi/emoticons/sad.gif Speaking of brakes....yes, I put disc on it and will be picking it up from the trailer shop today. I was going to do the conversion myself since I had all the parts but just didn't have the time, so this place installed them for me for $275. Money well spent, I think. I will be doing some metal work on it this weekend (a few rust spots http://www.lasvegashotboats.com/iB_html/non-cgi/emoticons/crazy.gif ) and hopefully painting it soon. Then all that is left is to do the lights, wiring, and carpet. Should look much better when I am done.

[MOWtown]

$275 is worth it. Did you go with a new Attwood actuator or just modify your existing one? Did you dig out all of the bondo permanently or float it back in to original design? Do you now puke at the site of Bondo http://www.lasvegashotboats.com/iB_html/non-cgi/emoticons/biggrin.gif ?

[kevnmcd]

</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (MOWtown @ Feb. 14 2003, 2:27 pm)</td></tr><tr><td id="QUOTE">$275 is worth it. Â*Did you go with a new Attwood actuator or just modify your existing one? Â*Did you dig out all of the bondo permanently or float it back in to original design? Â*Do you now puke at the site of Bondo Â*http://www.lasvegashotboats.com/iB_html/non-cgi/emoticons/biggrin.gif ?[/QUOTE]<span id='postcolor'>
The only part I replaced on my actuator was the master cylinder. Everything else was good and didn't need to be changed. I will have to do some minor bondo repair on around the actuator as some spots have flaked off. It was well worth it to pay someone else to do that...it would have taken me probably two whole weekends to complete it when they did it in two days.