Inliners International Forums General Discussion Engines GMC 302 intake on 270 engine + carb choice?

Hi,

I want to switch intake on my 270 to one from a 302 to be able to install a brand new 2bbl holley 350cfm carb without the hassle of 2bbl-1bbl adaptor.

My question:
will the holley 2300 (350 cfm carb) bolt to the 302 intake (the intake has 4 mounting holes for the carb)

I understand that there might be a port size difference between my 270 head and the 302 intake, i believe Larrowe and Sons have "Port Ring Adaptors" readily available to solve this issue.

Any input would be appreciated, thanks

The port size matching is different, but Larrowe only makes the adapters for the Chevy Stovebolt 216/235 engines, not the GMC engines AFAIK.

If your head is a "270H" then the port size is the same as a stock 302 -- 1.75 inches, as i recall. Other 270 heads offer the same port size as the 228-248-256 series.

Close on 60 years ago, in his GMC Speed Manual, "California Bill" argued that the small ports were better for low- and mid-range driveability, especially on a basically stock engine.

Seems to me that changing the manifold with the "port ring adapters", were they available, would be more of a "hassle" than the "2bbl-1bbl adapter" on the 270 manifold.

God's Peace to you.

d
Inliner #1450

I run a small port 270 head with a 4 bbl manifold for a 302. I made adapter rings. The problem comes in sealing it to the head as the gasket surface on the head is smaller than the manifold. 270 gaskets are too small and 302s are too big. The head could be milled but that is just more trouble. Even with a 2bbl adapter you have the same size hole on the manifold. That said, I ran a 2bbl through an adapter for a while and the 207 liked it.

A friend of mine used a larger port intake (Offy) on a small port and set the head vertical on a mill and machined a 45 degree angle until the port opening was the same size as the intake. He had no gasket problems and it worked great. The head with this angle made the flow in-rush thru a type of vortex and really made the engines low end torque awsome. It was a 270 in a 51 Chev coupe using a T-700R and 3.55 gears. He never needed to run it over 4000 and it cruzed right at 1800. I believe he used a 465 CFM offroad Holley. Not a well known carb model but perfect for a lot of inlines. I'm thinking of using one on my 292 Y-Block...........Good Luck

LIKE THIS?

Thanks for the answers guys, always amazed over the knowledge people have.

The Port-ring adapters i mean are these:
http://prostores1.carrierzone.com/servle...ort-size/Detail

Regarding the intake gasket i realize there might be trouble, but since i dont know what head i have i will need to get this looked at.

Scored a nos 302 2bbl intake on ebay yesterday for $75, thought in a weak moment that i could use that with the stock exhaust fitting and a brand new carb and get away with it for under $500 in total..
We'll see =)

There are 4 heads. Two have old style chambers two have flatter chambers. Two have big ports 1 3/4". Two have small ports not 1 3/4". Old round chamber and big ports =270H head. Flat/D chamber and big Ports =302 head. Flat/D chamber and small ports = Later 270 and maybe 248 head. The other one is everything else. Patricks catalog has the casting #s and photos.

Those rings look like the ones I made. They work just fine. Sounds like a good eBay deal.

Update: turns out i have the small port head (819).
All parts ordered, will check out how it will fit.
Contemplating opening up the head if the seal is poor.

Beater,
I know this thread is old but just came across it.
Can you elaborate - chamber shape (is the early one fully open, the later one a flat D quench like say the 283 Chev), valve sizes, pictures (or a link, I couldnt find it on Patricks), casting numbers for each?
Much thanks

Here is the 983 Head. It has a "D" shaped chamber and 1-3/4" OD intake port opening.

CNC,
That helps thanks.
I have read the 302 head is 118cc?
Trying to calculate (guess) the cc's of the 4 GMC heads from guessing everything - what I THINK I know:
Deck Height 11.156 228-236-248-256
Deck Height 11.250 256-270-302
Rod Length 7.000 all
Compression Height UNKNOWN so I'm guessing at deck clearance
Gasket ~.040" factory

Head cc's
Small Port open 130cc?
Big Port open (270H) 130cc?
Big Port closed (302) 118cc?
Small Port closed (late 248-270) 99cc?
Not sure of the valve sizes on all 4 heads either!

Pistons
0cc (flattop) for closed heads
31cc dome, 270 open head

Not sure why the early 302 shows 7.20CR, then 7.50cr

The 983 casting head has 1.94" intake and 1.500" exhaust valves. The 1-3/4" intake port opening that is always attached to these heads is kind of misleading, that is the OD of the counterbore for the port rings. The actual intake port runner size opening is more like 1-5/8".

Don't mean to but in, but this is an interesting conversation. i'm about ready to build an engine for my 31 and have some 270 and 302 blocks, also a 819 small port rebuilt head. I have the 270 ready with stock pistons, but I would really like to use a 302 I have that's .060 over I believe. this is going to be a torque motor probably never see 4000 rpms. I would like to get the compression up to 8.0- 1 - 8.5-1 with stock pistons. i'm just not sure if this combination will work, are the small port 819 heads 99 cc ? and what would this get you on the 302.
thanks Jay D

With so many variables contributing to the compression ratio formula, you will have to sit down and run some numbers through the calculator formula based on your actual combo to see. The basic numbers you need to calculate this accurately are below.

Bore size
Crank stroke
piston deck height
head gasket thickness and gasket bore
combustion chamber volume
piston dome/dish volume

Badsix,
CNC is right, the way to do it is measure all that stuff on your engine (and post back, the numbers would help me gathering info for the stock GMC history).
I got 118cc for the 302 head from the HAMB site.
I am GUESSING at 99cc for the late 270 (small closed) head, back-calculating from its factory CR of 7.50 to 7.75.

IF those are true, then going from 118cc to 99cc would raise your CR from about 7.50 for a stock 302 to 8.65 with the smaller late 270 head. IF everything else is compatible and I'm not sure about that.

The smaller ports might cost you 10hp or more though compared to the bigger 302 ports.

Well you can see why I am searching/guessing for all the stock GMC numbers, they are very hard to find but they matter in order to calculate anything!

great I will try and get some numbers this week end
thanks Jay D

Stock valve sizes, at least through 270 is Intakes 1 5/8 (1.625) Exhausts 1 3/8 (1.375) California Bill called for Intakes of 1 15/16 (1.9375) and Exhausts of 1 11/16 (1.6875). I'm not sure if the 302 stock valves were the same. The 302 was pretty new to the hot rod scene when California did his book. We sorely need a new GMC Speed Manual that covers all the specs and old race tech as well as new tech from MOPAR main seals to turbos and fuel injection. It would even be a great help to be able to easily retrieve the wealth of information buried on this site and molding away in stacks of the TPN.

Im getting focused back on some of these unfinished projects. I am trying to meet a publishing deadline right now, so once I can get the ball rolling I am going to finish the performance series I had planned to do.