All,

I will assume responsibility for the head gasket selection.

I advised ealshire of the RA/RZ surface requirements for proper durability of the MLS gasket and they were complied with.

The reasoning for the choice was twofold.

I checked my data. I have entered 142 pairs (284 gaskets) of Cometic gaskets into service in the marine environment. To date there have been zero reported, part and/or material, failures to date. That is a fairly large data sampling that would indicate to me that, with proper handling and installation, this product would insure the highest reasonable expectation of success. I stand by those numbers and would repeat that component selection.

They also offer the largest selection of thickness and bore diameter to optimize the mechanical compression ratio that is critically important to these enhanced naturally aspirated, small blower applications.

Bob

in a build i reciently completed i noticed the port alignment was not real good.this engine was a 509 with gm rect port heads and a 2.3 whipple with intercooler.it made 765 hp and 840 lbs torque so in my mind perfect alighnment is not always necessary.alex,i also am going to mention that your comment[internet engine experts]also includes yourself as you are here responding to this thread.if that comment was made to belittle some of the people who are posting on this thread maybe you should keep in mind that this is the internet and people are intitled to there opinion,even you.

i was amazed when i took one of these apart and found this.just because merc did it does not make it right!

correct me if I'm wrong but doesn't Alex build motors for a living and is his profession? I can tell you that i have heard phenomenal things about him from reliable resources. Grouping him in with Internet expeers isn't only insulting it's flat out disrespectful. That comment right there is why guys like Alex that have a wealth of data and real life experience don't post on forums anymore. I didn't even see that being directed towards you for what it's worth.

Darin Morgan touched based when asked this question on another forum.

Question

Darin...or anybody else;
Why because of results with their testing MERCRUISER uses a Rectangular port B.B.Chevy intake manifold on an OVAL port head with a giant missmatch because in their words, "mid-range torque and top-end horsepower was increased without changing the low-end output".
Ford Marine also did the same thing with using a Cobra-Jet intake manifold on a regular D0VE port size head on their big-block to create the same giant missmatch for somewhat the same reasoning -

WHY did this occur...???

I was told that this mismatch was large enough to create TWO seperate sound wave pulses; can this happen...? Is the runner being ever so slightly smaller than the port better than a perfect size matched transition? Assuming you could get the manifold on in the perfect location.

Reply

The simple answer is No. Having the intake runner slightly smaller is not the correct approach, its the safe approach. Its the lesser of two evils but given both options I would prefer the port and manifold be sized properly and matched perfectly. If the port is to large and not sized perfectly for the engine combination, having a manifold that is smaller than the port can have a band-aid effect on the air speed and help the engine produce power. This was most often seen with Pro Stock cylinder heads in the late 1980s and early 90s due to using larger than necessary square port heads. Many engine builders (as is still the case today) had no concept of proper sizing and tended to over valve, over manifold and over port an engine. They found that leaving a step (most often on the floor) would increase both torque and power. They reasoned that this power increase was due to a decrease in reversion. There theory was only partially correct. Yes it did lower the amount of reversion but it did so by increasing the average air speed in the system not by the blocking effects of the step. I also surmise that there where some atomization benefits to the step as well but that is unproven.
Its all ancient history but I do see some people still using this technique today. With proper port sizing and induction system design there is absolutely no need what so ever for steps or miss matches of any kind.

Another question to Darin

I have a slight lip going into the head port (approximately 1/16th of an
inch) as the intake runner is slightly ported. This was purchased used,
and I was basically too lazy to match the mating port.

Reply from Darin

You can not have the port present itself to the air flow. Period. Don't let this happen. Take all measures to remedy this. There are epoxy resins capable of fixing this with very little time and expense. Never , ever, ever,ever bolt an intake up to a cylinder head that is larger than the port. Its horsepower suicide!

I dropped off all the parts to my builder this morning, we matched up the gasket to it and it's no where near as bad as the one pictured, the bottom doesn't overlap at all, but the top does and requires attention, so it will get the attention it needs and be blended.

at this point in time i wish i never responded to this thread.i am really thinking about keeping my opinions to myself and just say nothing.maybe oso will be better off without the input from this [internet engine expert] as alex put it.

Mike, I've found your thoughts and opinions to be quite valuable. I'm glad that you chimed in, and I've learned a lot (from everyone).

I think that a lot of the bickering and poo-slinging is just folks' dealing not being around our boats and the freezing temperatures. Have to keep the blood warm somehow, right?!

Anyway, that's just my take, but as the starter of this thread, I feel it's important to speak up.

Thanks,
Tom Zell

In an attempt to mitigate anecdotal, subjective perceptions regarding the Roval line of cylinder heads I thought it best to reach out to the person that designed both heads. Tony Mamo.

These are all his words in reply to a number of questions posed regarding this very debate. He has seen these conversation on other forums and addressed them as follows. He just replied to post this on his behalf;

Bob

First Question:
So I'm going to order some heads next week and I'm still debating between the AFR 290 oval ports and the 305 rectangles. They both have their pros and cons. I know the 290's will make more power and torque....



First Response to posted question:
Go with the 305cc with the CNC option you can install them in a stroker motor later.







Tony Mamo’s Response:
I would argue exactly the same but insert "290" where you have 305....

The only "con" to a 290 in a comparison of these two heads is price after that nothing but upside.

The 290 has more intake flow....more exhaust flow....and a smaller runner for high airspeed. In short, it will make more power (and especially torque) on any displacement you would naturally consider either to be a good fit on.

Don’t forget guys the 305's are tested on a 4.600 bore and the 290's on a 4.310 bore.....the disparity in airflow is even greater than it looks on paper.

The 290 is an extremely optimized and vary capable small runner.....don't confuse the fact it’s considered an "oval port" into thinking it’s just a good street head. Better to view it as our smallest race head that compliments a small to moderate displacement street engine extremely well. This head flows 370 CFM guys.....I have flowed some 345 -360 cc aftermarket rect. ports that wished they could muster that on a 4.310 bore or perhaps only slightly eclipsed it (with 60+ cc's of volume!)

The rect port design simply has more CSA at the entrance than most of the heads/applications we are discussing need. Till you start moving 400+ CFM's on aggressive engines with very large appetites for air the rect port opening of a BBC head is larger than it needs to be. That’s not where the air is restricted anyway.....it’s all about the bowl and negotiating the turn into the valve and chamber area.....that’s where the port is very sensitive to shape, CSA, and valve job/chamber design.

Ported Vic Jr with a one inch open or possibly tapered spacer assuming you have the room for a spacer.

Perfect intake for that cylinder head
Hope this helps....

-Tony