The Stovebolt Site Forums The Shop Area The HiPo Shop Hot Rodding a GMC 270

What can I do to make my stock 270 perform better. I bought a Clifford intake; Fenton headers; 2barrel carb ; HEI ignition.
Thinking a 3/4cam?.? Should I have any head work?
Thinking I should wake it ...

Thanks in advance

Hot Rod Lincoln is one of our resident performance gurus. I'm sure he will be along soon. If not then send him a PM. Sounds like a fun build.

Find an H head.
Aluminium pistons.
You could bore it out to 302...
270's are not inexpensive to hop up but when they get there thier smokin.

okay...

The really good hotrod stuff for GMC's was produced back in the 1950's when there were thousands of WW II surplus 270's available dirt cheap from the 6X6 military trucks. Diehard stovebolt fans loved them because they were the only engines that could be easily adapted to Chevrolet cars that could almost keep up with the flathead Ford V8's. The 216's and 235's just couldn't make the cut on the dirt tracks. The 270's had lots of torque, so much so that they were still getting wheelspin off the turns when the Fords were halfway down the straightaway. The successful GMC racers eventually learned to gear their cars high and lug them off the turns to minimize the traction loss.

The trick setup on a GMC engine is a 12-port cylinder head- - - -if you can manage to find one. They're scarce and EXPENSIVE! There are lots of go-fast parts out there, but be prepared to mortgage the homestead to buy stuff like pistons- - - -mostly they're special-order custom items with a fairly long wait for delivery, and a high price. Have fun!
Jerry

There was a guy a while back on Inliners that built one of those HAMB racers with a 302 in it. It dynoed at close to 350 HP. Bits and pieces of his build are on both sites if you can find them. I've got his cam specs and head flow data written down somewhere. It used a stock cylinder head, if that tells you what kind of potential is available from these types of parts.

FYI (for your inspiration)
Nakid H head
And then it all goes wrong
But I kinda like this one...

Hot rodding today is much easier, yet much more difficult than in the 1950's and 1960's; both because of the internet.

It is now much easier to ask questions rather than measure; and yet sometimes the answers we get are not understood because we either asked the wrong question, or didn't supply sufficient information when the question(s) was/were asked.

Jerry's comments about steering/stopping are VERY important! But with the internet, information about engine/transmission weights, dimensions, mountings are readily available. Find this information about the engine in question, do your measurements on your vehicle; and then spend some time with the math.

A few things to think about:

Weight - what will the new weight do to spring rates, weight distribution, brake load, etc.

Power - will the additional load on u-joints, rear gears, etc. require upgrades. How about traction with stock wheels/tires.

As far as asking questions about fit, I will relate a personal story, before the internet. Some 35 years ago, I decided to build a 1974 Pontiac GTO 350. I spent a couple of years in research, and pretty well had my build laid out, but I wanted to upgrade the exhaust. I personally detest headers on the street! The in-cabin noise is bad enough, but changing header gaskets with every moon change gets old in a hurry! Pontiac had offered some high performance exhaust manifolds on one of the factory ram air engines in a larger chassis. I knew it was going to be tight; but I also knew a couple of guys that were running the same chassis with performance builds, and I called (before the internet) each of them. Both had the performance manifolds on their cars. So I started looking for a set.

After building the engine, put the engine in the car. Simply could NOT line up the drivers side exhaust manifold! Tried everything I could think of. Pulled the engine, and installed the manifolds then reinstalled the engine. No way could I line up the mounts.

Finally, in desperation, called one of my friends, and we talked a couple of hours going over everthing. Finally it came to light that he had manual steering, mine was power. Got out the interchange manuals, and found a manual steering box. Sure enough, the power box was a quarter inch wider than the manual box. Ah success. Not so fast grasshopper!!! The power box used a different size pitman arm; and while the manual box was relatively easy from a different vehicle (Hollanders Interchange Manuals are a great aid), the pitman arm was unique, and more difficult to find than a needle in a Kansas hayfield!!! After several disappointments (wrong parts shipped, and returned) and a year and a half, I found one, for $200. plus freight (this was about 15 years ago). What can you do, I bought it. Finally, the problem was solved.

So the initial problem was mine, in NOT supplying sufficient information when I asked the question (will it fit), and not understanding the answer, when the answer (yes, it will fit) was what I wanted to hear.

So if someone tells you it cannot be done, listen, but do your own measurements; and if someone tells you it CAN be done, listen, but take your own measurements. The internet, mathematics, and physics are all your friends - use them.

My final build more than doubled the original engine's horsepower (same engine with some severe mods) and took over 800 pounds off of the cars original weight. So I KNOW about spring rates, shock loads, etc. from experience. I also improved the factory weight distribution from roughly 60/40 to 53/47. Really a nice driving "street sleeper".

Jon.

Good story, Jon, and thanks for reinforcing my viewpoint, but what does it have to do with hotrodding a GMC? Did you reply to the wrong thread, maybe?
Jerry

Thanks once again.
Now not sure if I should locate a 292 or 302(if they still exist)
somewhere. I thought I would just beef up what I have. But it seems parts for the 270 might be just as scarce.

Gonna keep my 6cylinder because v8 are a dime a dozen these days.

From a purely practical (translation- - -"less expensive") viewpoint, the 292 would make more sense to hotrod than the 270. It fits the same bellhousing as the Chevy V8, and the 292 engine will be 30-something years newer, therefore more available and less expensive to modify. There are lots of go-fast parts for the 292, and at least some of them are shared by the V8's. Back in the late 1960's, a fiberglass-bodied T Model roadster campaigned on the drag strips around middle Tennessee. It was powered by a beaned-up Chevy 292, and it showed its rear end to a LOT of V8 powered drag cars. It ran 3 Weber side draft carburetors and had a lot of internal modifications, but it was a real surprise to its competition.
Jerry

Jerry - the story has to do with hotrodding anything, and not doing one's homework properly.

The more planning one does, the better (and easier) the result; but there can always be "gotchas".

Jon.

Just like the old carpenter's adage- - - -"Measure twice, cut once!" Most of the times I've royally messed up a project were when I decided to take a shortcut, or just ignored an obvious flaw in my reasoning because I didn't want to admit I had made a mistake. That kind of thinking can get expensive and embarrassing!
Jerry

I would give Jim Carter a call. http://www.oldchevytrucks.com/blog/index.php/category/high-performance/

If you already have the stuff you list in your first, you are pretty deep into it anyway. A set of 9.5 Venolia Pistons and a cam will make you a pretty nice engine that is streetable.

A 302 is really just a bored out 270 as far as 'existence' goes.
And there is the real possibility you have plenty of beef already if your past was filled with 216's and 235's.

Go to oldGMCtrucks.com....the search function is your friend.
Also check out Patrick's Antique motor parts, as they seem to be the most complete supplier of Jimmy parts.

Not familiar with a Clifford 2 barrel manifold. What carb is that?
The Fenton headers will require a little grinding work to properly fit a GMC.
The stock head on your '58 should be #2194819. Smaller ports
and chamber than the 302, but same valve size. Some street guys seem to prefer it for high velocity and compression boost.
A cam change should be at the heart(brain) of your plan.
Delta Cam in Washington just reground one for me. $125 including lifter resurface and return postage.
If your short block is solid, money can be saved as performance pistons are a major expense.
But ain't none of it cheap !! Even the machine shop wants a premium for jacking around these huge chunks of iron.

Thanks Cren,
I didn't see the pistons on the website, but maybe I overlooked them.Used Patricks in the past, is Jims better to deal with?

Cliffords 4barrel intake with adapter , weber carb

Switching to a 292 would require a complete drivetrain changeover(trans,driveshaft,rear).
Looks like you have most of the performance pieces already. Add a beefier cam and a quality valve job and put it together.
For your application I doubt that porting/polishing would add much. Stainless Chevy V8 valves are a drop in btw.

Yeah , thinking I should do a compression test also- but I guess it really doesn't matter if am putting a beefier cam in it , right?

Just a suggestion:

Since you have the Clifford 4-barrel manifold, believe you would be MUCH happier with a small 4-barrel than any Weber 2-barrel.

Jon.

Jon ,
I was just cutting in half what I have on my 235- meaning getting only one. What readily available small 4barrel would you recommend?
Can always try& sell the Other.

Thanks , Danny

Compression check is quick, simple and easy. If the numbers are low a new cam will not correct that. A leak down test can zero in on weaknesses. These are not high comp. engines in stock form, less than 8to1 I believe. Increasing that factory ratio helps to wake them up, but requires, expensive pistons or head shaving or a combination of both.
I recently paid over $500 to have a 302 head cleaned, magnafluxed,surfaced
guides replaced, seats ground, and stainless valves supplied.
P.S. I see you are already posting on the GMC site

Thanx 68,
Yes I have posted on several forums.

The Inliners bunch is another good group of enthusiast's to source parts and information from as well.

John

John ,
You are correct. I haven't used that site as much as some other sites; but I am a member...

Danny - guess my question would be does your "readily available" mean readily available or inexpensive?

If you read my posts, you would probably infer that I like Carters, and you would be correct. The Carter 9400s which was superceded by the Carter 9410s are both rated 400 CFM (about the same as the Carter WCFB that was used on the Chevy 265 V-8 in 1955).

Not really a Holley guy, but some are; and Holley offered a 390 CFM 4-barrel.

There are other O.E. carbs out there if you look hard enough, that would require more tuning than either the Carter or the Holley.

Either the Carter or the Holley would have primaries which are SMALLER than the two-barrel for better low RPM performance (higher venturi velocity), yet have the secondary for higher RPM. Not sure what activates the Holley secondary, but the Carter secondary is an "on demand", meaning the secondary is activated when the engine asks for the extra air.

Holley tuning parts are available at every auto parts store in the world (does this mean that every Holley in the world needs tuning?? ); while the Carter uses the 10-201 strip kit (generally available by main order only).

I personally ran a Carter 9400 with excellent results for over 400,000 miles on my last shop truck (non-Chevrolet) with 300 CID 6 before the salt/cinders of Missouri winters destroyed the body for the third time. Once on the truck, it was never removed (until I junked the truck).

Jon.

Danny,

Have you checked out the oldGMCtrucks.com discussion board? You have to be a member, but the initiation ceremony is not painful.

Bill (Miles) recently had a 302 "built" and professionally dynoed. All of the work/testing/adaptations are chronicled in a thread on oldGMCtrucks.com. The information posted there is not about old "foreign war/engine" stories - it is about a GMC 302.

I think that one problem they had was the #6 cylinder piston got tight because of non-uniform engine/block heating/cooling.

You might be surprised/impressed by the compression and horsepower discussions/considerations.

If you join that site, and read that thread, think about where/how to buy the parts/pistons (and, why).

Good luck - keep us informed on your progress.

Regarding an interesting/dependable carburetor, Bill uses a GMC Holley AA-1 2-barrel carburetor on his 302. Adjustable jets are readily available.

I subsequently put the the same set-up AA-1 on my stock 261 Chevrolet engine (bored to 270). I can easily adjust it to perform well at low and high rpms.

Tim - something to always consider with the Holley type AA-1 carb:

The economizer valve (a.k.a. power valve) which is made from neopreme, is located in the bottom of the carburetor bowl, and has a direct passage into the intake manifold. If the economizer valve fails, the bowl is drained directly into the intake manifold. If you are forced to use ethanol-based fuel........

Jon.

Ok , what 4barrel would you suggest? Am also looking at that Clifford dual setup again instead of a single carb- but I don't know. Am a member at OLDGMC and have a post there also....

Thanks

A 600 CFM carb will be plenty for your 270. You may even try to find a 390 CFM, but they tend to be a little pricey. There are a lot of formulas you can use to calculate the max CFM required based on the RPM the engine sees. So that would be a good starting point to find a baseline.

Thanks 12

As stated in my earlier post, I would suggest either the Carter 9400s or 9410s (both are 400 CFM); but if you like Holleys, the Holley 390 would be less initially. Final cost would depend on how much you drive; the Carter will probably get 25~40 percent better fuel economy than the Holley (I personally got more than 20 MPG AVERAGE over 400,000 miles on my 300 CID 6 in my shop truck.

Jon.

Thanks 4 schooling me