Hot Rods big block chevy head swap

They used Oval port intakes [with rectangle port heads ??]
Not rectangle port heads with oval port intake ??

The Mercruiser GM intake #333841 is oval port



If you're going to swap heads to open chamber oval port, change the intake to oval port [you don't want reversion upstream]

Personally I would change the cam and leave everything else as-is
Consider a retro hydraulic roller so you can have high lift and little valve overlay and get a cam with a tight LSA [105°] and an adjustable timing set ,to advance the cam.



@55 chev straight axle If you decide to go down the oval port open chamber route, put some 2.300" intake valves into the heads [but leave the throats as-is]
And cut a nice 3 angle seat job. and back cut the valves slightly. The whole circumference of the valve becomes a high flowing venturi at about 0.100" lift
Doing this creates an insane amount of flow "off the seat" even though maximum lift flow remains unchanged.
The effect is more overall cylinder fill for the same cam duration.

My 454 with L88 heads and GM alloy intake was 130 hp just off idle [@1200 rpm]
Horsepower wasn't much though! At only 612hp but it was starved back with a 730 Holley

454's are one of the easiest engines to make power from

 

I seriously doubt Mercury Marine would do something like that. I would not wonder or contemplate for any length of time building an engine with such an obvious miss match of parts. To do so would ignore decades of research and development by the factory engineers and tens of thousands of racers around the world.

Edit; I see Kerrynzl has posted the obvious technical note.

 

This Rectangle port intake on Oval heads thing is not new. I know it doesn't make any sense, but there has been several engines built that make great power with this mismatch. I guess it started with a racer who had a built rectangle port motor that had a serious failure of a valve. Had access to a nice set of oval port heads but wanted to keep his tunnel ram and dialed in carbs. The thing ran great, no huge power loss. Long ago on Speed talk there was a discussion among some savvy engine builders that have tested and raced them, and the results are better than you would think. There's some high-speed port flow dynamics that go on in those ports that are not fully understood, but they do work! Not saying that the correct matching manifold is not better, it is better. Some guys just work with what's available, and they get lucky at times.

 

As always , there's guys who Talk about engine building & those who Do engine building , you Always hear from the Talkers ,not so much from the Doers .

 

Thank you

 

I love BBC folklore and wives tales

 

Not sure what you mean?

 

He's probably referring to the "Top Secret Grumpy Jenkins Trick" of putting a rectangle port intake onto oval port heads.
It was so secretive you probably never heard of it until now


@55 chev straight axle . If you're going to swap heads, change the intake [it is a cheap bolt-on component]

 

It was so secret not even he knew about. Probably only works on slow boats

 

I am using a Weiand stealth oval port intake. The heads in question have been nicely done with installed height spring, multi angle, C-Ced, new guides, port matched, new seats, new tulip valves and seals. I chose them because of the semi open design. The only negative is the smaller valve size.

 

For a street brawler , just bolt them on. You already have matching components.

 

I run this intake with these heads, it made pretty good power

 

Myths are always fun to read. And guys will tell you it doesn't matter, and it probably doesn't for the average street driven BBC head and intake matchup. But there have been plenty of test engines run on dynos with mismatched ports going either direction that show that matching intake to head ports for oval or square port BBC heads/intakes makes a difference in HP on a dyno.
Do whatever you like, but I'll always match mine up.

 

I won't lie, in hindsight I wish I had used a better head, it would be more efficient, but the stubbornness in me wanted to use heads I already had.

 

I slapped a sq port 4bbl OEM on some oval ports a long time ago. It was what I had and I was replacing an iron Q-jet so I could drop a Holley 780 on it. Yes, it did make a difference you could feel. Was it right? I didn't care, I had em both, I only spent money on gaskets. Would it be better with an oval port? Of course. What made it work? More overall intake volume is my guess. Why did it improve what I had? It was what I used to call a "suburban motor" since it was a 454 from 2500 suburban. It only spun about 5500 RPM, no race motor, headers and a Holley.
On manifold match, I prefer a manifold that's just a whisp smaller than the head. I believe it makes for a nice airflow boundary layer during cam overlap and reverse pumping when you're building a fire breather. Whatever the static physical blockage is (I had a 16th overall) it's increased by the square of the speed. At high RPM that reversion is reduced and the intake charge is cleaner going back the other way. Do I have thousands of dollars in dyno time to prove it? No, and I also didn't do A-B-A testing as in with and without. What I did have was a 1050 Dominator that had nearly ZERO carbon in the bottom vs my peers carbs which always had a notable amount. Of course at the exhaust side I also hogged out the header inlets larger than the ports in the head for the same effect in reverse. No legends or wives tales, it just seemed logical and best of all only cost me a little grinding time on the head and some welding/grinding on the header. Anyone else ever do this?

 

Thats what it's all about. Love the story. In some strange way I think we all tried it.

 

Boat engines and car engines while the same, aren't. The environment they live and operate in are night and day different. This being the case, what works in boats isn't always the best practice in a car. Mismatched head ports to intake ports may make the acceleration curve erratic. But you will never notice it in a boat. Boats go from idle to wide open rather than gradually accelerating like a car. Props slip very easily and you wouldn't notice if a sudden power spike made it slip unlike what might happen in a car. Boats are also usually ran at a high throttle position or WOT most of the time they are running. At WOT hot spots and intake charge flow become a concern in cars as detonation issues are not good. Boat engines have extra clearances built in as the blocks and heads are usually lake cooled and never get hot while the pistons still do. This helps with detonation problems. And since the throttle is usually on or off, who cares if there is a low power point in the curve.

Jet boats are another issue alltogether. As a jet pump is like running the engine on a dyno all day, as long as the engine has enough power to spin the pump you can go faster. Most jet boats will never turn more than 6500 as pumps become less efficient above that and the impeller in the pump will be set to limit you to that point. If you have more power than that, the impeller is usually changed to still stop you around the same RPM. This being the case, there are a lot of big block oval port headed jet boat engines out there. And if you are looking for a little extra, you bolt on whatever intake you find and try it. Boats are usually toys and people rarely car about efficiency and every last once of power. Boats measure performance by smiles and the seat of the pants dyno. If you're happy, congratulations it works and you continue to run it. And someone else will follow suite as everyone wants to be happy and have fun.

 

" Interesting" suppositions ..