The GM/Merc Rectangle port head. Discuss.
01-17-2015, 12:13 PM
#1
The GM/Merc Rectangle port head. Discuss.
Lots of cylinder head talk lately. Lots of guys wanting to upgrade their cams, intakes, and other things. Lets face it, this cylinder head was used on everything from a 365HP 454 mag, to an 800SC merc, HP500's, 500EFI's, etc.
Of course this was what GM had to provide mercury, at a reasonable cost, and reasonable performance. A good head in its day, but lets take a look at what we have.
Most had 2.19 intake valves. Standard exhaust port location. 118cc chambers, with 325ish cc intake runners. The 325cc intake runners, were quite large, especially on a 365HP 454 engine, but they worked ok. Even still on the large side, for a 502ci marine engine.
We have had the theory of rectangle ports and lack of velocity shoved down our throats here on oso, for years. I get how that works, and I am sure you do too by now. But, blaming the intake port size, to this heads lack of performance, is unfair. This head has more going against it, than just the intake port size.
In 2015, this cylinder head, really does not hold a candle to just about anything aftermarket off the shelf. We have the cam profiles, but without the cylinder head flow capability, we are limited.
Here are some flow numbers for a 188 GM iron Head, with 2.19 intakes.
INTAKE/EXHAUST
.200 149/117
.300 197/150
.400 233/174
.500 258/184
.600 280/187
.700 287/187
A big block chevy head, that in stock form, cannot even hit 200CFM on the exhaust port, or 300CFM on the intake port. Think about something for a minute. We all sit here and stroke our penis's in regards to roller camshaft upgrades, more lift, duration, how a roller has faster opening and closing ramps, etc. We get erections talking about exhaust manifold upgrades, and break out boxing gloves when the single plane vs dual plane intake comes up. But, when talk of replacing these antique heads comes up, usually we hear crickets.
The average guy on oso is spending good money to rebuild these vintage heads. New guides, valve jobs, sometimes seats, resurfacing, and so forth. Then he will drop over 1k dollars, to upgrade from his flat tappet camshaft, to a hydraulic roller. In my opinion, and some may disagree, but I personally, would rather ditch these heads in favor of a good performing aftermarket head, and retain the flat tappet cam style, if doing both upgrades is not an option. Besides the obvious power gains from better airflow of the cylinder head, now you can go aluminum, possibly smaller chambers to bump your compression a bit, weight savings, etc, esp for the freshwater guys.
While I know many guys do not care for flat tappets, I do not think they are as horrible as some make them out to be. Hell, cummins and many other forms of motorsport engines are still using them today. As far as the whole lobe going flat thing, I think there are several things that need to be addressed, if going with flat tappets.
For starters, you'd want to use a quality, stellite faced lifter. No cheap chinese crap. Many machine shops have lifter cutting groove tools, to help get a little more oil to the lobes, as well as companies like crower selling grooved lifters. Also, as we know, break is crucial. Way to many guys forget to mention, that while they performed a 20 minute break in procedure, they also used valve springs that were entirely too stiff, didn't want to go thru the trouble of removing the inner springs, or installing break in springs, etc. Then they blame the oil, they blame the cam core, they blame the EPA, they blame their wife, etc. Bottom line, too much spring pressure at break in, is a good way to flatten the lobe. Of course its no secret, you want to use a good oil with proper amount of zinc. I know of many boaters still running flat tappets running 15w40 diesel oil, FOR YEARS, without any issues whatsover. I know of many flat tappet equipped vehicles with 200K+ miles on their origninal cam and lifters.
Comp cams, Crane, probably to this day, sell way more flat tappet sticks than they do rollers, as I am sure their market is the vintage car guys building up a budget small block/big block. Am I in anyway, suggesting that a flat tappet is better than a hydraulic roller, absolutely not! But, I also do not feel, that you must stick with low performance and deal with it, because you cant afford a roller setup. There are many respectable power producing flat tappet engines out there running around.
Of course this was what GM had to provide mercury, at a reasonable cost, and reasonable performance. A good head in its day, but lets take a look at what we have.
Most had 2.19 intake valves. Standard exhaust port location. 118cc chambers, with 325ish cc intake runners. The 325cc intake runners, were quite large, especially on a 365HP 454 engine, but they worked ok. Even still on the large side, for a 502ci marine engine.
We have had the theory of rectangle ports and lack of velocity shoved down our throats here on oso, for years. I get how that works, and I am sure you do too by now. But, blaming the intake port size, to this heads lack of performance, is unfair. This head has more going against it, than just the intake port size.
In 2015, this cylinder head, really does not hold a candle to just about anything aftermarket off the shelf. We have the cam profiles, but without the cylinder head flow capability, we are limited.
Here are some flow numbers for a 188 GM iron Head, with 2.19 intakes.
INTAKE/EXHAUST
.200 149/117
.300 197/150
.400 233/174
.500 258/184
.600 280/187
.700 287/187
A big block chevy head, that in stock form, cannot even hit 200CFM on the exhaust port, or 300CFM on the intake port. Think about something for a minute. We all sit here and stroke our penis's in regards to roller camshaft upgrades, more lift, duration, how a roller has faster opening and closing ramps, etc. We get erections talking about exhaust manifold upgrades, and break out boxing gloves when the single plane vs dual plane intake comes up. But, when talk of replacing these antique heads comes up, usually we hear crickets.
The average guy on oso is spending good money to rebuild these vintage heads. New guides, valve jobs, sometimes seats, resurfacing, and so forth. Then he will drop over 1k dollars, to upgrade from his flat tappet camshaft, to a hydraulic roller. In my opinion, and some may disagree, but I personally, would rather ditch these heads in favor of a good performing aftermarket head, and retain the flat tappet cam style, if doing both upgrades is not an option. Besides the obvious power gains from better airflow of the cylinder head, now you can go aluminum, possibly smaller chambers to bump your compression a bit, weight savings, etc, esp for the freshwater guys.
While I know many guys do not care for flat tappets, I do not think they are as horrible as some make them out to be. Hell, cummins and many other forms of motorsport engines are still using them today. As far as the whole lobe going flat thing, I think there are several things that need to be addressed, if going with flat tappets.
For starters, you'd want to use a quality, stellite faced lifter. No cheap chinese crap. Many machine shops have lifter cutting groove tools, to help get a little more oil to the lobes, as well as companies like crower selling grooved lifters. Also, as we know, break is crucial. Way to many guys forget to mention, that while they performed a 20 minute break in procedure, they also used valve springs that were entirely too stiff, didn't want to go thru the trouble of removing the inner springs, or installing break in springs, etc. Then they blame the oil, they blame the cam core, they blame the EPA, they blame their wife, etc. Bottom line, too much spring pressure at break in, is a good way to flatten the lobe. Of course its no secret, you want to use a good oil with proper amount of zinc. I know of many boaters still running flat tappets running 15w40 diesel oil, FOR YEARS, without any issues whatsover. I know of many flat tappet equipped vehicles with 200K+ miles on their origninal cam and lifters.
Comp cams, Crane, probably to this day, sell way more flat tappet sticks than they do rollers, as I am sure their market is the vintage car guys building up a budget small block/big block. Am I in anyway, suggesting that a flat tappet is better than a hydraulic roller, absolutely not! But, I also do not feel, that you must stick with low performance and deal with it, because you cant afford a roller setup. There are many respectable power producing flat tappet engines out there running around.
The following users liked this post:
JaniH (10-19-2020)
01-17-2015, 12:44 PM
#2
Registered
yeah,stan wiess has some good info.heres a print out
01-17-2015, 12:57 PM
#3
Registered
and to correct,,the heads were from hp500's so they thought the merc clean up was light porting and intake valve was actually 2.25"
01-17-2015, 01:06 PM
#4
Registered
iTrader: (3)
This is exactly where I'm at. The heads on my 500s are fresh so I don't really want to dump them. Swinging 555 NA underneath I know im choking them with that head. Thinking of a victor JR intake and 1000 4150 carb to better utilize the cubic inches I have.
Thoughts?
Thoughts?
01-17-2015, 01:24 PM
#5
Registered
I'd fluff and buff them,clean up/profile the casting bumps for the rocker studs and push rod tubes.also can cheat a little w/ back cutting the valves,but leave a .072" margin on the exhaust.and put a healthy cam in,,over 244,and have a split of 8 degrees
01-17-2015, 02:27 PM
#7
Registered
Thats why I put Dart Iron eagles on my tired old Merc 400's. With the HP 500 carbs and intakes on them and new roller cams I dyno'd at 475 and have gobs of torque at 3500 rpm.
01-17-2015, 04:44 PM
#10
Registered
iTrader: (1)
For whatever it is worth we used the stock heads for a reason. We targeted 580-610 hp for the race motors. Which we have 600. Everything we use is off the shelf and easy to get . Its easy to keep a couple sets of spare heads around ready to go. I do realize that we are at the max those heads will put out w/o a blower and that's fine. They idle perfect and run great, I would not change a thing unless I thought I needed more hp.
