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.

yeah,stan wiess has some good info.heres a print out

http://i59.tinypic.com/33a776c.jpg

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"

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?

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

I've got a couple 1000cfm 4150's for sale....

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.

the iron eagle head is a good upgrade over the gm head but if it is in the budget i would go with a set of afr 305, with cnc chaimbers from bob madera,they are not that much more and the power gain is worth the xtra cash imo.

what is your compression ratio?

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.

What was the build recipe if you don't mind sharing?

[QUOTE=sutphen 30;4251341]yeah,stan wiess has some good info.heres a print out

http://users.erols.com/srweiss/table...hevy_Big_Block

[ATTACH=CONFIG]535800[/ATTACH]

[QUOTE=Rookie;4251520]Thanks a great website. I've been pouring over the data on there for the past few weeks trying to figure stuff out on my new builds.

Just screwing around this morning having coffee, playing with my dynosim program. How accurate is it, who knows.

But, I built a typical 502 engine. 9:1 static compression, single plane intake manifold, the ever so common crane 741 camshaft, and a single 950 carb.


Stock GM 188 Heads. = 532HP.

Dart 308 Iron Eagles = 591HP.

AFR 305 CNC chamber =615HP.

Edelbrock Performer RPM Rect =551HP

No other changes made besides the head flow and appropriate valve size. Whether or not the program is accurate, or the data Im entering is, the numbers seem to be in line in what I've seen. Any thoughts?

joe,does the program factor in things like timing or air/fuel ratio.also water temp.

It does factor in timing, air fuel ratio, air temp, no water temp though. AFR was left at 12.5:1, as thats where they say max power is made on gasoline.

if that program is correct,and i would say it is in line, the iron eagle head is the best bang for the buck.i still like the afr cnc,d chaimber 305 and i think in reality it would do a little better espically in a marine application because we can run them a lot cooler than in a car.

Just did a 540ci, 9.5:1, AFR 315 cnc heads, 680 lift crane hyd roller, single plane with 1100 CFM carb. 715HP. Changed to 800 Carb, 685HP.

Tunnel ram with twin 750's, 762hp :frog:

I agree with ya on both parts. I added the Edelbrocks performer RPM rectangles to the prior post.

That dynosim is interesting for sure. Thanks for sharing that.

Have played with these as well, and for most apps 600hp or less, the large oval ports work better, like 781's, with little work and a decent matched intake #'s are impressive...

awesome post,,I'd say it looks really close.

What program is that?I think I know but rather not guess.

yup,got ~640hp out of 10.5-1 compr heavily massaged oval ports,ports and combustion chamber.cam was pretty aggressive.

Its "dyno 5 sim" . Got it from summit or jegs I think for around 100 bucks.

joe,you just cost me money,i just ordered one from summit,102 bucks.

should have search Amazon,saw it for $53.

make sure you get the right version they offer two grades with different options! $50.00 or $102.00. also offer 2 drag racing applications as well. I love mine it works very well!

This is the one I have. Sold under comp cams label.

http://www.summitracing.com/oh/parts/cca-181501

http://proracingsim.com/dynosimmainpage.htm

I see "desktop dyno 5", for 50 bucks. But, not same one I have.

http://www.summitracing.com/oh/parts...6011/overview/

Thanks Joe, that's what I was trying to get across.

MT, you wanna take a desktop stab at mine with the current 800cfm and soon to be 1000cfm carbs? Curious what it will say.

555
9.3.1 CR
88 heads
Dart 6200 single plane
Stock hp500 cam

I'll plug it in for ya this evening

As I went back to look at what you have there, I happen to notice you said "HP500" cam??? OMG.

Dude, what in gods name is that cam doing in a 555 cubic inch engine? lol.

Anyhow, the program with your combo is coming back around 490HP. Going from an 800 Carb to a dominator, only picked up about 7hp, which im not surprised. Your just not making enough power to take advantage of a bigger carb.

Going with the crane 651 specs, bumps it up slightly over 1HP per ci, to 570HP with a 1050.

Switching to a set of AFR 305's with CNC chamber, still with the 651 crane cam, made the numbers jump to 671HP with a 1050.

Bought the boat set up this way with M3s on it. Went back NA with what I had to enjoy it for the rest of the season. Now in the process of cam, carb, intake hunting. Don't really want to lay out the $$$ this year on new heads.

Thanks for spec'ing it out for me. Any other suggestions from the group are welcome on which direction I should go.

and this exactly where this thread started. with MT pointing out-with flow numbers-that it is time we stop relying on 60's technology for cylinder heads. 570 to 670 hp...

If new heads aren't in the budget, I would strongly suggest doing a cam upgrade at minimum. Just my opinion

Point taken

Agreed

Should be noted, that this is just bench racing with flow numbers. As we know, port shape, runner size, and a whole bunch of other things factor in.

I don't mind at all. Its a bit old school but it works. 509 cu , 10.3 comp, 630ish on the cam (roller of course) single plane, 850 race prep carb , msd, gm crank, good rods 32 degrees timing. There is a bit more little stuff but that kinda covers it. No porting, just built tough. They run like a clock. Knock on wood.