Inliners International Forums General Discussion Engines GMC Straight Six Head Questions

Hi!

I'm all twisted up right now trying to decide what head I should put on my 270 GMC straight six. It's only going to be a mild street rod engine; 1940 Chevy Coupe, dual carbs, 3/4 race cam, no salt flat/ top speed car by any means.

Option 1 is either one of GMC's big port heads, being the 270 "H" head (2193417) or 302 "D" head (2193983). Option 2 is just leaving the origional small port head on it now (2194819).

I'm willing to spend the money for a new head if it's worth it. I've got a line on a 302 "D" head nearby. I've gotten mixed answers from everyone I've talked to so far ranging from "It's a must for any power!" to "it's a gas guzzeling waste of money!" so I thought I'd run it past the experts before I start throwing money away!

Is it worth it power wise and gas consumption wise?

Thanks a bunch! Great website!

Pete

Welcome! We invite you to join Inliners International, where some of us still know and value an inline GMC.

A 302 or 270H head can make significant power, and with head work it can make some more -- but for a "mild street" engine in a full-size car your stock 270 head would likely serve you just as well or, perhaps, even better.

In the day, "California Bill" advised his readers to stick with the smaller ports for the 270 in a street-driven vehicle. He wrote that his customers were often disappointed in the low- and mid-range performance of a head with larger ports.

If you find a good 302 head at a good price, you might buy it and hold on. As we progress with projects of this kind, we often come to feel that "if some's good, more's better" and "mild" becomes more and more "wild." Those of us who start out with "wild" in mind are often disabused by experience. It is sometimes better to move forward in stages until we find satisfaction. Some of us only learn the hard way.

God's Peace to you.

d
Inliner #1450

I agree with what Don said. I have a small port head on my '57 270. According to most it is the worst choice for performance. It has served me well since 1978 in my every day '53 Chevy pickup. I've never hooked to anything it wouldn't pull or loaded it with something it wouldn't haul. It will do 75 all day and get 17+ mpg. What vintage is your 270? Head choice will dictate piston choice. A little work on the small port head will serve you well on the street. If you can get the 302 head for a good price do it. If you don't use it you can sell or trade it.

The only thing about the 302 head is you will need suitable pistons. Like Don and Tom say, I would think that with dual carbs the small port head will give better driveability.

Tim

Okay, I'm sold! Small port head it is! Thank you so much for the detailed responses! It's exactally what I was looking for!

Don: I just printed out the form, I may turn out to be your youngest member at 17 years old! I can see what your saying about keeping my options open. I am one of the "learn by mistake" guys too. I'll keep trying to get ahold of the guy with the 302 head. How much would a "good price" be? My guy said his was boiled, magnafluxed and pressure tested with hardened exhaust seats. $600?

Beater of the Pack: Wow! Cool Truck! 17mpg and 75mph is perfect for me! My motor is out of a 1956 GMC 1/2 ton.

Titen: Perfect! Boy, that's good news!

Some other questions: Any recomendations on what I should do to the head? So far, it has been boiled, magnafluxed and had hardened seats put in it. (Done before I found out about the big port option.) I plan on putting the bigger Powerglide valves in it, milling a 3 angle grind on the intake and exhaust valves for better flow, and ROSS high-dome light weight cast aluminum pisons. I also plan on using the big port, single barrel, dual carb intake and the port matching trick that you, Beater of the Pack, posted on the "GMC 302 intake on 270 engine + carb choice?" thread a couple of months ago. (Video) I also plan on spending some time with a grinder and smoothing out the ports as much as I can. Am I doing this right? Any reccomendations???

Thanks again guys! I really appreciate the advice!

Pete

P.S. Should I post a thread on the engine/car build? If anyone's interested, here's the link to the build thread I have on the HAMB website: 1940 Chevrolet Coupe Build Thread

Sounds like you are on the right track. Thanks for the build link but I got there and wandered. The HAMB is a big place. When using 2 or 3 carb manifolds the small port one are easier to find. I only used the big port manifold because it was for a 4bbl. Next time around it will be 2X1.Welcome the Inliners we need young blood!

By all means, share your engine build and the ongoing improvements to your fine '40 coupe with us. You'll find a wide range of experience -- and opinion -- among us, and you might learn some things you'll want to remember.

In choosing to power your '40 with a warmed-up GMC 270, you have cast your lot with some Great Eccentrics. Some day you will appreciate that expensive experience and give thanks for it.

Your man has some time and labor invested in his bare 302 head. See what he wants for it. i should not want to pay more than $300 for that, but "it's worth what one will give and the other will take." For now, it's not essential to your build, but it might be useful in the future, to use or to trade for something you want.

The work you have outlined for your 270 head looks good. Larger valves -- 1.94-inch intakes and 1.6-inch exhausts -- and the three-angle valve job should be helpful. "Smoothing out" the ports is a good idea, but it is something you want to to undertake with care and patience. Haste makes waste in a hurry with a grinding tool! In the same way, take care to assemble everything correctly, with the proper clearances and torque specifications. A GMC engine service manual will serve you well.

God's Peace to you.

d
Inliner #1450

Beater of the Pack: Yeah! The HAMB is huge! If your building a car, there is at least 3 threads of builds just like them! It's pretty amazing. As far as manifolds go, does having the bigger port manifold make that much of a difference? I know a guy that sais he has ready access to either one of them, but I'm just curious.

Don 1450: Ok, I will! Should I put it under the "Truck Talk" forum? Okay, I'll see what he wants for it. I have been talking to him over the phone and he wants to show me his truck. I think he said it was a '57 GMC with it's stock 270. He has a 3 speed saginaw overdrive behind it, 3.38 rear end and sais he loves it! It's still up in the air, but it looks like we can get together within the next two weeks. I'll make sure to take my time on the rebuild. Haha, I've got a friend who built a 302 GMC for his Bonneville car- he put the rear crank shaft bearing in backwards and blocked the rear main. The motor blew up after one run! I'll be double checking those bearings for sure!

Oh, and one more thing! What do you guys think about plaining the head? My ideology is that the lower the head, the lower the pistons, and the lighter the pistons. How much is a good amount to plain? What do you think?

Thanks guys, I hope I'm not asking too many questions. If I am, let me know. I'll back off.

Thanks so much!

Pete

I'm following a few builds over there. Different name same avatar. Yours is a good start. Nice '40. I had a '41 for a short time. In '69 I had the loan of a 'bone stock '40 sedan in Bozeman, Montana. We wrecked my bride's Porsche on our honeymoon. She spent 4 months in the hospital. A friend loaned me the '40 in return for some work on it.

If you are going to run a duel carb setup on a small port head the small port manifold would be best. Sealing it would be a lot easier and it should flow better. The head on my 270 was milled .030" I think. It was a long time ago.

It sounds like the '57 GMC pickup would be a fun one. Get him to give you a ride. You will love it and you will know what to expect. The Saginaw 3 speed ODs are great street choices but getting rare and expensive. My pickup has an S10 T5. First gear is too low and OD isn't high enough. The rear end is 3.55 to 1. I have a 3.08 that I may throw in to try or I may get the right trans.

If I was putting a build thread here I'd use the Bench Racing forum sense the is no GM forum. If that is wrong Titan will let you know and help you find the right spot.

i should think that the "engine" forum, where we are now, would be the right place for your continuing saga of the engine build.

i have a Saginaw four-speed with an overdrive from a three-speed adapted to it behind my 302 GMC in a '52 Chevrolet two-door. With a 4.88 differential i use second gear (2.2) for low. (With a Saginaw overdrive we need all the pieces, but especially the solenoid!)

When i was your age . . . in the 1950s . . . my father built a '40 Chevrolet four-door for his wife. The engine was stock, and ran well for what she wanted, but after the paint and upholstery work was done, my father cut out the radio speaker panel and mounted a Frigiking air conditioning unit in the dash (painting it woodgrain to match), so they could feel that ice-cold air. The old 216 pulled the Tecumseh air-conditioning compressor quite well; it had plenty of low-end torque. i should not want to do that, but he did. Your coupe with a 270 might replicate much of what many of us wanted at your age. i'd like to have my father's '40 now, with a 302 in it and that air conditioner out of it!

God's Peace to you.

d
Inliner #1450

Several years ago a friend in Virginia City sold a '41 four door that I had asked him about many times. When he told me he had sold it to someone else he apologized saying"...I'm sorry but he offered $800 and I knew you wouldn't pay that." The car was perfect. It had never spent a night outside in it's life. My fault for not making a hard offer. I was trying to not pay $8,000.
Borg Warner over drives are great for street driven cars. But like Don says you need ALL of the parts. The Saginaw 3 speed has a one off solenoid that is hard to find and pricey when you do find one. Never throw away a broken Saginaw 3 speed OD solenoid or pass a cheap one up. They can be fixed. Also if you do a conversion like his you'll have to have the main shaft from the donor trans because the OD shaft is not the same as the standard shaft.

Go with the smaller port head and intake manifold. Sure there are many instances where people have used the "wrong" setups that contradict modern thinking and concepts, and they have seemed to have been fine with the results they got in return, but for low RPM driving like you will be experiencing 99% of the time, the smaller runner sizes and plenum volume will give you better and snappier throttle response and make for a more pleasant cruising machine.

Yeah, that's what the guy with the truck was saying too. I think I'll put the 4 speed I've got in it and keep my eyes peeled for a 3 speed. If I find one, I'll do what Don did. On paper, the car should do fine without it. With a pair of 7X10's on the back (~101in circumference) 2650 rpm would put me right about at 75 mph. The overdrive (.85:1 ratio), would only bring the rpm down to 2250 to go 75 mph. It helps, but I can live without it.

Okay, I'll stick with the small port manifold then. I can see how leaks could become an issue with the big port one...

My grandpa's "How to Hop Up Chevrolet & GMC 6 cylinder engines" book from 1951 shows that the head can be milled up to 1/8 (.125) of an inch. They also say that with the stock pistons milling off .080" will bring compression up to 7.5:1. I'm only looking for about a 9:1 compression, so I suppose we could do the .080" mill and the rest with high dome pistons. This seems like an awful lot though, I was thinking it would be around .030" too. The amount of weight we are saving is so minuscule that it may not even be worth the money to plain it at all. It could all be done just by the pistons... I'll ask around some more...

Beater of the Pack: Wow, that must have been a nasty wreck! Glad she's okay! What a honeymoon! What a steal for that '41! $800 bucks? That's incredible!

Don 1450: That old '40 would be pretty sweet with a 302!

One more thing, Beater of the Pack, some guys from the HAMB were disagreeing with you about the small port 270 head possibly performing better than the big port. My thought was maybe that the big port would flood easier, but I'm not sure. If you'd like, you can straighten them out. Here's the link: GMC Straight Six Questions

Thank you guys so much! I really appreciate it!

Pete

There is always someone on the HAMB willing to argue. Here too for that matter. Let them believe what they want. Here is the deal. If you modify an engine you must have a purpose in mind a goal. If the goal is 200 mph+ at Bonneville your engine build will not be the same as someone trying to turn a 9 second quarter mile. Neither of those engines would serve you very well on the street. They would be useless in town, burn tons of fuel, load up at low rpm and over heat in traffic. California Bill covers this in his book. You must be honest with yourself about what you want your engine to do and build it for that expectation. The components must work together so that each is a complement to the next. This takes thought and a plan. The whole intake side is a balance game. The factory strove for power and economy. They used single throat carbs and fairly small ports and valves working to maintain good velocity into the cylinder. The 302 had big ports.1 3/4",a 2 bbl carb and a governor. The big ports are only needed at wide open throttle. At all other times they tend to slow velocity and allow fuel to fall out of the mixture. Putting a big runner manifold between multiple carbs and small ports and valves makes little sense. Remember our engines are air pumps not fuel pumps. You can get all the fuel you need with one single bbl. You can increase valve size like Don suggested with mild porting achieve 9.0 to 9.5 compression with pistons,decking and head shaving, the right cam ground for your use, a good exhaust system. two carbs of the right size and strong ignition you will be happy and once it is together and running strong you can tell the pundits it's a "full race" 302. Most won't know the difference. A lot of folks have done otherwise and like what the have. There is no one way. Thats the beauty. I wonder how many of those experts I passed on my way to the Roundup a few years ago?

I went over and read that and I don't think I disagree with what was said. The big port heads will make tremendous power at at low rpms and through the full rpm range especially at full throttle. The problem is smooth transition from part throttle to full and back or smooth running at any position without flat spots. I have run big port heads on the street but I don't think I gained drivability. That's why I say if you can get the big head for the future it may be a good idea. If I passed someone it wasn't that Fox guy, he knows his stuff!

This root issue here is air flow velocity, especially if your running carbs which need the flow velocity to generate fuel signal. The long intake manifold makes for variable signal response especially at the end cylinders. Big cam big carb - stomp on it at low RPM and it'll fall on its face. Since this is intended to be a street engine you don't want to have to crappy low end response.

If you put the fuel delivery right at the port though > 90% of this goes away.

Big head, small to medium cam beats a small head big cam any day of the week.

In 1996 Chevy bolted the first set of factory high flow heads on the SBC - power and economy went up. Compared to the previous old style SBC head the L31 vortec heads outflowed them by >30%. power jumped close to 80HP - with nearly the same cam shaft timing.

Thanks guys, that makes a lot of sense now. Yeah, that RichFox guy said he was Inliner #9! One of the founding fathers of the Inliners Club! Pretty incredible.

I'm meeting with the guy that has the GMC pickup tonight. Should be pretty interesting. I'm sure I'll be chalked full of questions by the end of it, so get ready!

I'll post a thread of the car and engine build on the Bench Racing forum. Worst thing that happens is that it gets removed.

Thanks!

Pete

i have milt.302 head boilled-pressered tested surfaced -apart i use sb chevy ,manley valves 2.02 -1.600 x .100 long big port not a prob with right cam rember each port is feeding 100 cu.in. $125+shipping

Ah! Andy, your killing me! Thank you, I really appreciate your offer but right now, I can't accept it. I'm sure someone else on here could do well with the head, so feel free to chime in!

Again, I really appreciate the offer. But to be honest, I've already gone ahead and finished the head I've got and ordered pistons for it. I just have too much money tied up right now.

Thank you, and I'm sorry we couldn't make a deal.

Pete

From personal experience;I build a 302 about 8 years ago.302 head with Chevy V8 valves,Venolia 9.25 pistons,cam is 215 degrees duration at .050 lift,single 4 barrel carb,home made short tube header. My 37 Chevy truck with this engine was driven by many inline guys over the years and will out pull a stock 270- 302 from 1200 rpm on up. It has exception throttle response and fuel mileage.Any engine's performance is the tuning of the various parts.I sold the truck and 60,000 miles later still runs well.
A 270 H or 302 head by todays standards is pretty tame.Think about it,GM used these engines for the heavy duty truck service,they were not performance engines originally.
A small port head will do fine,no need to buy a overpriced 270H head for a mild street 270 engine.

Thanks Tony!

I bet that was a pretty hot '37! I've got the same cam, similar pistons, similar valves.

Thanks for the advice!

Pete

Pete welcome to the forum,Lowboygmc is about your age aswell. It's good to see you young guys gettin into the sixes!!

If you go with the small port, which by all and me best for the street, consider a large port intake. I helped a guy with a 270 in a 52 Chevrolet with the small port head and we put a large port Offy 4 barrel manifold by tapering the head with a mill to accept the large port size. We only milled the first 3/8-1/2" and then blended slightly after that into the port. There was plenty of meat to do this. It had the venturi effect we were looking for and worked great.

We used a 465 CFM Holley normally used for offroad trucks and crawlers. We used a 700R trans and the whole package was killer. Good Luck

I too am running a small port head on a 270 with dual Zeniths on a big port intake with adapter rings. 9:1 compression J&E pistons and dual cast headers and 264H grind cam from the place in Tacoma (can't remember the name, but not impressed with the cam) . . . I am also running SBC valves as big as would fit with blending of the bowls and light porting.

The engine runs great on the street with an A833 tranny in front of a 3.73 12 bolt posi in a 50 GMC pickup.

Great, Thanks guys! Keep the advice rollin' in! I'm all ears!

nln6pinto: Thank you for welcoming me! Cool! Lowboygmc and I will be talking. It's a pleasure working with sixes! V8's are too common for me.

What do you guys think of hand making an intake? Money is tight, and the material is cheap so I figured, why not!? A 3 carb would be easiest... Just 3 bent tubes with some equilizers inbetween. What do you guys think?

Thanks!

Pete

Sorry guys, I got another question for you. Stop me if I'm asking too much.

I stuck with the small port 270 head, bigger valves, hardened exhaust seats, 3-angle grind. Here's the question: at what angle should the sealing surface on the exhaust valve seats be? The original angle was 30 degrees, however the standard practice on most motors today is 45 degrees. What should I do? Are there any advantages/ disadvantages to the 45 opposed to the 30?

Thanks a bunch guys! I appreciate all the advice, it really helps. I wish I could contribute more to the sight, all I really ever seem to do is take, take, take. I don't have much to give...

Pete

I know this is quite late, but I just saw this post. On the 30 degree seats, Jim Hand (Joe H's dad around here and at Stovebolts site) is a Pontiac guru and has a super how to hop up Pontiac V8 book.
In it he explains why he uses their 30 degree intake seats rather than switching to the common 45 seats (more a mfg econo move than to make anything "better"). This is a good tip for a street motor--wish Tom Lowe could do a flow test at the least on this matter.
Jim Hand is super good and thorough on his testing and research. It does well to remember other makes and engines got good stuff going for them too. I'm not above listening and copying when I can.
In a nut shell 30 degree intake seats have more flow at the early opening and late closing and can be more beneficial than having better flow in the .500" lift areas (more "total flow", its free and it happens twice per intake cycle)

The Pontiac's have a fully machined combustion chamber also,just like a Chrysler Hemi is, so you get a more precise and even flow in and out of the ports and chambers from cylinder to cylinder, so its more even. Plus, as you mentioned, they tailored the valve angle to maximize their agenda, even though it was different than what other GM engines were doing at the time. Most full race cylinder heads don't use angle cutters for seats any more, so the days of the angled valve seats are almost a thing of the past.

Under .500" lift: 30° intake seat.

You'll want the seats to be clean and fresh so might as well spring for a 3* to 5* valve job.

"Most full race cylinder heads don't use angle cutters for seats any more, so the days of the angled valve seats are almost a thing of the past".

what does that mean? is there a word missing? are you meaning they dont use 30 degree angled seats any more? I know, but other than having to stock/use different degree cutters, or stones, or whatever they use to grind, cut, or place valve seats into heads these days, why not get the most bang for your valve job buck?

Now a talking point for a 45 degree seat is the "self-cleaning" ability to better cut thru any foreign surface that may try to adhere to the seat surface and not let the valve completely seat-thus "burning" the seat and valve sealing surfaces.

If you can improve the total flow past the valve, with a seating surface thats a little different than current cost cutting thinking, why not entertain that idea--it aint like a 37.5 degree angle or a 63.7...30s were used in all 216,235 and 261 Chevys and virtually all Pontiac V8 engines for many, many years prior to the "bean counters" rise to power in the industry.

The more total flow idea means if you have a better flowing seat angle thats "gooder flowing" up to .500" lift, and you have a cam with .512" lift. Then you will have more total flow into that cylinder with the "under .500" lift seat angle because you are only in the "bad" flowing lift area (.500 to .512 )for a very brief part of the cycle. It would behoove you to go with the 30 degree seat--Not so if you have a .750" lift cam. "just sayin'"

To expand on the above mentioned vortec heads-- those '96 through '99 vortec 350 pick up engines had sequentially fired individual "spider-legged" port fuel injection mounted within a long branched intake manifold, and the vortec heads giving them 255 hp. The '95 350 pickups had the more conventional heads along with a throttle body fuel injection system mounted atop a pretty conventional two bbl looking manifold, they had 190 hosses.
Arguably the bigger feature of the vortec head was the fast burning small heart shaped combustion chambers. Run them babies on your hot rod small block and you had best dramatically reduce your total timing or you'll have to start buying wheelbarrel loads of pistons. Ask the roundy-rounders who didn't.
My '99 3/4 ton Checy pickup pulling the fifth wheel camper will "rattle" up the longer hills in the Texas summertimes even with it's computer and knock sensors on regular gas. Put in some 89 octane and it will get silent and drop a little engine temp doing it, even while running 4.10 gearing with the o/d locked out to protect the 4LE80 tranny.
The higher flow helped the hp, while the fast burn combustion chambers helped the economy (if you can call 9mpg good--but is better than 7, or 8).
hmmm, a stalker mayhaps in yon alley?

Those concepts may hold true for the OEM heads you mention because they were designed for one specific function, and that was the computer compatible world of everyday driving. So to say that a particular seat angle is optimum for the vast majority of engines isn't possible, because too many other variables come into play that can affect it. But in the race world, what I meant earlier about them not using multi-angle seats any longer was simply this. A standard base angle of some degree is still used to cut the seat, but instead of additional angles(hence 3 or 5 angle valve jobs)cut into the seat, they have been replaced with a multi-radius cuts instead, which greatly enhances the flow.

I've seen one of those things at the Rehr-Morrison race shop --really neat and can insure much faster and precisely uniform seat/neck shaping. But aren't those things custom made to a specific application?

heart shaped combustion chambers: Harley-Davidson, 1926

The exhaust side is the one that really benefits from the single angle valve seat as you want the gases to leave quickly and with as little restriction(ie.turbulence) If you look at any of the Top Fuel, Pro Stock engines the only multi angle seat is on the intake side to swirl the charge across the pistons surface. For a street engine the multi angle exhaust seat does seal better for low rpm use.

Wait, wait, wait. I'm talking about the angle of the sealing surface of the actual valve seat and valve itself--not any of the other angles that form part of a "three" angle or a "five" angle valve job. The seat angle of a bunch of stovebolts and the Pontiac v8s were thirty degrees, and not 45 degrees like most of the rest of world's motors got now. All this is stemming from research, (maybe I should say "rumor") by Jim Hand, who's credentials outstrip even ol' Hank's!

Hand's statement reads "...the 30-degree seat has more real opening between the valve and the seat than does a 45 degree setup. Above approximately .400 inch of lift, the 45-degree seat will show more peak flow. But unless very high valve lifts are used, the total flow from seat to peak lift will be superior with the 30-degree seat".
This is the whole basis for my efforts to share with you guys--take it or leave it. I have never heard of anyone else ever researching this aspect of flowing a head. And lets reinterate that this is referring ONLY to the valve and it's seat, not the finish of the combustion chamber, the size of ports, or any other etc. factors. Just this isolated part. If you guys have heard of this portion of flowing treated before, lets hear it.

Makes you wonder why GM in the later years only made the Pontiac's have that different valve angle, while the rest of the GM stuff adapted the 45° angle. Was that a case of the right hand not knowing what the left was doing. If it was that beneficial of a discovery, why wouldn't they have used in on their better platforms like the Chevy stuff, for instance?

Maybe one bean counter found out how much money they could save by not having to do two different seat angles? For that he got the "junior woodchuck award" of the month cap. That was the same fellow that was later promoted to the v-6 projects and said "we can save 50 cents a block by using a 90 degree vee blocks and run them down the v8 machining lines, and use the same kind of common crankpins and only lose a little bit of smoothness, over the smooth correct 60 degree configuration" He went on to retire leaving GM in ruin until Obama helped out. Just my guess, of course. I wasn't there, they didn't ask me or Ed Cole.