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, nothing over the top. 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 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 you guys before I start throwing money away. Is it worth it? Thanks! Pete
From my own personal point of view. Since its only gonna be a mild street engine i wouldnt bother trying to find a gmc h-head. I would just build the engine with tripple carbs, dual exhaust, and port the head a little. Maybe a slight bore but not even that. Unless you stumble accross a h head laying around that someone doesnt need i would take the money and spend it on something else.
Hey, this is the H.A.M.B.! If some is good, more is better and too much is just right! But then, you're going with a 270" instead of stepping up to a bored 302". For the street, your head will be fine. jack vines
That's what I'm starting to think too. Patrick's Antique Cars and Trucks are the only people I've talked to so far who say otherwise. Thanks guys! Pete
If you have a 270 head that isn't cracked in the exhaust ports I would say run it. I don't believe pistons made for a 270 open chamber head will work with a 302 head anyway. A 270H head is kind of rare and $$$$.
I believe any performance manifold will have ports for the 270H/302 head. You'll have to use the port adaptors to align the intakes to the small port head. You can also mill out the ports, I think there is a YouTube video showing someone doing that. For RichFox -- where exactly in the exhaust ports do these heads crack?
I'll probably have ROSS cast me some high dome, light weight aluminum pisons. I want a compression around 9:1. Can't have them made untill I decide on a head though... Yeah! I read about that on inliners.org! They mill out the ports with a 45 degree angle. They say that it creates a kind of vortex and makes it have a really mean lower end! Here's the video: http://www.youtube.com/watch?v=uAwz44_6mTk&feature=relmfu After reading this and the thread I posted on the inliners website, I think I'm going to stick with the small port head. It's got better gas mileage, smoother (possibly better) performance, that intake vortex and good lower end acceleration. Plus, I've already got one! The inliner's guys have got some great evidence and advice too, I can't post the exact URL, but if anyone's interested, go here: http://inliners.org/ then click on, "Bulletin Board, then "Engines" then look for the thread titled, "GMC Straight Six Head Questions." Thanks so much guys, I really appreciate the advice! It was very helpful! Pete
Try this link: http://www.inliners.org/ubbthreads/ubbthreads.php?ubb=showflat&Board=1&Number=73794&Searchpage=1&Main=9140&Words=%26quot%3Bgmc+straight+six+head%26quot%3B&topic=0&Search=true#Post73794
Nothing wrong with the small port head for a street engine. But the bigger port head making less low speed power? No.......I built a GMC 302 10 years ago with Venolia 9.25 pistons,a cam of 215 degrees duration at .050 lift, Chevy V8 valves, home made header and a small 4 barrel carburetor.I'm no expert engine builder but the built 302 would out pull a stock 302 from 1200 rpm on up in an actual real world comparison.It's now owned by a guy on this site.
I used to think that, and my mid '50s McGurk catalog only shows GMC intakes for large port heads, but then I bought a McGurk dual intake that is identical to my large port McGurk, except it's machined for the small port heads.
I have seen several 270 heads cracked right along the casting line in the exhaust port. Even killed a few myself. But the thing still has an El Mirage record.
If I were to go the expense of buying a large port head, I'd go for a 302 head rather than the 270H. The 270H was a big deal when it came along, but that was two years before the 302 existed; the combustion chamber design of the 302 head is much better than the H head.
Thanks! How'd ya do that? Plus they are way more abundant... and cheaper. I'd agree with you, but that's what I heard on the Inliner's website. Click the link, read the second post. I think maybe it has to do with the big port head flooding the motor easier, causing the low end acceleration to be dodgy, but I'm not sure. I'll ask. Would hardened valve seats help this problem at all? Thanks! Pete
Here's what the inliners guys said. Easier for you to read it there than for me to summarize (probably screw it up) and plagiarize. http://www.inliners.org/ubbthreads/ubbthreads.php?ubb=showflat&Board=1&Number=73794&Searchpage=1&Main=9140&Words=%26quot%3Bgmc+straight+six+head%26quot%3B&topic=0&Search=true#Post73794 RichFox: Thank you! Thanks! Pete
Big port floods the engine making it doggy at low speeds? That's nonsense,GMC installed the 302 as an option to the 270 or the standard engine in heavy trucks.You think GMC would use an engine that's doggy at low speeds for heavy trucks? A small port head will work fine on a typical modified street 270 making around 180-200 hp. The 302 head offers better power but it's not night and day.A street 302 makes about 220-240 HP if tuned carefully. The word is street engine,not a lumpy race only engine
Blackjack said -"I'll probably have ROSS cast me some high dome, light weight aluminum pistons." ===== small point. http://www.rosspistons.com/information/faqs.php#faq6 "Can ROSS make hypereutectic or cast pistons? No. A hypereutectic piston is a cast piston; ROSS only manufactures forged pistons."
Look in the exhaust port from the manifold face at the back wall. That is where I have seen many cracks. Next to the casting seam. Magnaflux is best.
Help! Another dilemma! I stuck with the small port 270 head, bigger valves, hardened exhaust seats, 3-angle grind on the valve seats. Here's the question: at what angle should the sealing surface on the exhaust valves be? The original angle was 30 degrees, however the standard practice today is 45 degrees. What should I do? Are there any advantages/ disadvantages? Thanks a bunch guys! I appreciate all the advice, it really helps. Pete
It all depends on how much cash you have.... Low budget rebuild? Special ross pistons with stock bottom end. Later small port head that you have. It has a good looking combustion chamber and staped rockers. Get a good cam hyd or mechanical. The Bigger port head flows better, but not worth the money people are asking. (h head) I would rather put $1000 into the lower end or save up for a 12 Port. NO NO NO hardened seats!!!!. Unless the seats are sunk so far down that they need it. The seat area has been hardened and 3 or 4 degrees grind to touch up and lap is suffencient. The hard seats are usually harder than the valves and then the valve beginds to deform instead of the seat. Don't listen to the machinist hype about lead in the old fuel. Yes it is true that the lead helps reduce friction but does nothing for the seats. Best to run Marvel Mystery through the carb before each oil change or service. 2 or three carb setup will work good. Non vintage use a cifford with 4bbl. I prefer steel tube headers over fentons or nicsons. Both available and worth the addition. And of course straight pipes or leagally run cherry bombs...or smitty's Open driveline with the 4spd Sag will be a blast and at a good price too. If you got the money go for a t-5. look at the inliners.org page about the hybrid. Think about it this way... You can always upgrade when you got the cash. Keep 'em inline...
Thanks so much for the input! About the hardened seats... I'm putting larger, newer SBC valves in it. Will this help the problem at all? Thank you for the support! I'm glad someone thinks I'm going the right direction! Amen to that. Pete
larger valves will require new seats. thinner the better. seen too many heads cut for thick seats an go into the water jacket.
The Jalopy Journal
