Inliners International Forums General Discussion Blown and Injected Chevy 250 4.1 Integral Head Combustion Chambers

Does anyone have experience working with/testing the Chevy 250/4.1 integral cylinder head? I've been doing some research for a turbocharged build (mild boost, >10 psi) and it looks like the combustion chambers on the integral head are better suited to performance modifications (i.e. more airflow). They are more of a wedge shape than a bathtub shape, like the older style non-integral heads. This seems like it would be better suited to a higher airflow application like an engine that will see boost. The more swept-back wedge shape does not shroud the intake valve as much as the bathtub shape does. Documentation here https://www.gmheritagecenter.com/docs/gm...vrolet-Nova.pdf shows that these heads have a 71 cc chamber, which is similar to the bathtub style chambers, so no loss of compression ratio would be seen. The head pictured here is casting # 370696 from a 1975/76 Chevy 250 4.1L.

I know, I know, the integral head is "worthless" because you can't modify intake/exhaust...but I already have plans to address that. I'm specifically asking for input on the combustion chamber shape only.

Shrouding in the open chamber non-integral 230-292 head is not an issue even with larger valves like it is in the closed chamber 194 cylinder head. So shrouding shouldn't be much of a concern for you.

I seem to think the integral Cyl heads are lighter casting and have a tendency to crack much more than any other cyl heads for the 194- 292 engines. Also the Brazilian 12 port heads are thin castings and tend to crack. Turbo6 here on the BB blew out the complete combustion chamber on the 12 port Brazilian head, IIRC.
One better thing I seem to remember about the integral cyl head, they have a better short turn radius. The combustion chamber is not really any better than the standard larger chamber heads.
If you where to add metal to the chamber and make it look like the newer Chevy Vortec V-6 & V-8 heads 1996 and newer ( heart shape chambers) now your talking about an improvement.

MBHD

Blue flame 292 I don't know about the chamber being better or not. Are you going to saw off the intergrated part? If you have any pics of the exhaust manifold could you please post them? I haven't took mine apart yet would be curious what it looks like. Thanks Jay

I usually throw those heads in the iron pile. There are 2 different versions of it. Maybe 3.
1 BBL - 1BBL carb and single exhaust
2 BBL - 2 BBLcarb and dual exhaust

Not sure which one you have pictured.

Did notice the chambers are slightly different than a standard head. See how the chambers are tilted to the left and right? Regular heads have the chambers oriented straight.

Can you provide a better pic of just the chamber?

The casting is the same thickness as standard heads. The reason they fail so often is because of the extra casting near the partial exh manifold cast into the head. Thermal stretching causes the cracks.

When I worked for Headrick, we had a local telephone company that had inline engines in their fleet vehicles that all had the integral heads on them. Everyone they pulled off when they serviced the engines had the head cracked. They didn't even have to be magnafluxed to see the huge cracks in them. In the 8-10 years we did the machine work for them before they upgraded to the newer Astro Van, I never saw one single integral head of theirs that was reuseable. Most of the times the cracks were both external around the exhaust and intake area as well as the seats and chambers. As Tom said, the thermal expansion of such a large unique casting contributes to its demise. We had a large scrap pile of them also.

Tom please give me access to the "iron pile"! That is a 1bbl head. If it is a 76 the wide valve cover is required. I would love to be able to take one of those 1bbl heads and see if I can machine it to take a 2bbl carb like has been done with the small Ford six. The only 1bbl head I currently have is on the car but just looking from the top it would be a job. Jay

I wonder if the similarly designed small Ford six integral head has similar cracking issues?

Not that I have heard, but I am far far far from being knowledgeable about the small Ford 6. One thing that they have over the Chevy intergrated head is less of the exhaust manifold is part of the head casting. Ford had that engine out from about 1960 and a lot were used in mustangs. One other thing that hurt the Chevy was when the 2bbl head came out in 79 the carburator was set VERY lean with the first year that steel plugs on the idle mixture screws so they could not be adjusted. Maybe the extra heat caused by the lean mixture contrubitued to the cracking and burning of valves. Jay

We had a hard time even finding used replacement heads for the fleet vehicles because of the same cracking issues found in non-commercial vehicles. Fortunately at the time, the integral head could still be purchased new from GM.

Seems to be a lot of effort for small gain. There are modern inline alternatives that can be had for $1200 to $1800. 2X the power and mileage.

CNC-Dude I know what you mean. When my 79 GMC had the head fail due to burnt valves and cracking, I couldn't find any used heads that weren't cracked. The Junkyards at the time told me that the 250 2bbl head and a head for an Escort were impossible to find. This was in 1988. The local Chevy still STOCKED a new head which ment that they were selling a lot of them.
efi-diy I know what you mean. We have a Trailblazer that might get parted out someday to power something else! Can't answer for anyone else but for me I just like these old engines. Jay

CNC-Dude - I've seen some test results using 1.94/1.60 valves on a 230/250/292 head, and yes there are definitely CFM benefits in doing that. But I think that there could be more CFM potential with this chamber and 1.94/1.60 valves. Unfortunately the GM PDF I linked to in the first post shows that the integral head has less material at the valve throat area than the non-integral head (something like 1.3" vs 1.5" diameter). So I'm uncertain whether the integral head could even be machined for larger valves. I'll have to find a cracked one and cut it apart to find out (unless someone on this forum has already done that).

Hank - From looking at photos, my hope is that the integral head has a better short turn radius. I need to get my hands on one and make a mold of the chamber & port so that we can find out.

integrated j 78 - Yes, the plan is to cut/grind off the intake log and machine flats on the side of the head where I can mount flanges for a custom intake.

tlowe - Here's a link to a larger image of the chamber side of the head: http://i.ebayimg.com/images/g/BM4AAOSwpoJXEBYD/s-l1600.jpg I also attached one, but it's not as big as the original. Yes, I did notice that the chambers were tilted - maybe that's for better airflow around the intake valve? Under higher velocity, the mixture tends to flow towards the far side of the valve, so rotating the chamber about the intake valve could reduce shrouding IF the intake port is set up correctly (which obviously it is not with this engine family). Or maybe it's to point this high velocity airflow directly at the spark plug.

efi-diy - You're 100% right - I'm attempting to make horsepower the hard way. I could probably find a used Atlas 4.2 and throw a pair of cams in it to make the same horsepower for less money. But sometimes that's not as rewarding as designing your own head/intake combo. Also, that engine wouldn't fit with the theme of my build (nostalgia). I definitely have a soft spot for these old inlines.

no cams needed upto 450hp - just add turbo + bigger injectors/tuneup...

Blue Flame, I went through every page in the PDF file that you liked to in the first post. The only drawings that I could find we're of V8 heads in the power train section. Please if you get the chance tell me what page the drawing of the 6 cylinder head is on. How much cfm was gained by putting in larger valves? I have heard quite a lot on the ntake side but haven't heard anything about what happens when 1.60 exhaust valves are installed. Thanks for the close up picture. Jay

There wasn't one specific for the 6 cylinder head, it was just a generic drawing with the values for dimensions placed in it at "B", or "G" for instance, for the V8 and 6 cylinder head.

Here's a post on Facebook that does a better job of summarizing why I believe this is a good combustion chamber from an airflow standpoint: https://www.facebook.com/BenAlamedaRacin...e=3&theater

It is to my under standing One other thing GM did with that head was to add a small lump to the ports That is why the short turn is different then that of the non-integral head.