The Stovebolt Site Forums The Shop Area Engine Shop 270 GMC engine exhaust system

I plan to install a 270 from a 1950 GMC fire engine into a 1952 GMC half-ton pickup. I also have a 1951 GMC parts truck with the stock 228. The 270 exhaust manifold uses a 2 3/8" I.D. pipe and the 228 uses approx 1 3/4" I.D. pipe. Both the 270 and 228 appear to use the same intake manifold and carb. Questions:1. Would the 228 exhaust manifold fit on the 270? 2. If so, are there any engine performance issues associated with the smaller exhaust pipe diameter? 3. Did GM use the larger diameter pipe on a larger truck to accomodate the additional length of the overall exhaust system or to gain extra horsepower? It just seems to me that identical carb and intake manifold air flow in 228 and 270 engines ought to equal identical exhaust systems. My thought process on this question assumes that a 1 3/4" exhaust system ought to be cheaper and fit better than a 2 3/8" system. I would appreciate anybody's thoughts on this matter. Thanks!

The manifolds are interchangeable. Conventional wisdom says a bigger exhaust is less restrictive. With a 1bbl carb I doubt there will be any tangible difference in performance.

If you have an exhaust in your truck, use the 228 manifold on the 270 motor. If you need an exhaust, use the larger 270 manifold. The price difference of the larger pipe again will be negligable.

If you ever want to add a carb or two or maybe put a 500cfm 4V on it (the 270) you'll want the larger pipes not the smaller ones.

Thanks for the feedback. Dave

the carbs for a 228 and 270 on the outside look the same but internally thier not. so where did the carb come from the 228 or the 270? if it came from the 228 you wont get the best from the 270 because you cant get all the air and fuel thru the carb needed to run the 270 at optimal performance. what are the numbers on the carb next to the big filter nut on top? there's a tag there which then will cross over to what engine it belongs on. as for the pipe id put the bigger one on like john said

carb king (john) explained it a while back.

This particular 270 was partially stripped of it's external goodies when I bought it at the salvage yard and was missing it's original carb. Same for the 228 on the parts truck. I'm using a Rochester RP on this engine right now but I also have some Rochester C's to choose from. All are missing the numbered brass tag if those are the numbers that you're referring to. I live at 7600' elevation so leaning out the main metering jet to accomodate the thinner air is the only thing that I have done to modify a Rochester 1 barrel. I get it that a 270 needs a larger jet than a 228. What else should I be looking for?

Zenith carbs are great and were used on most if not all of the GMC 228-270 engines originally. I'm not familiar with models and numbers, but hopefully someone else can chime in. I think the 4barrels were Rochestors? and were used in plenty of 302's but I can't say whether or not they were used on 270's.

Jeff

The intake manifold on the 270 is the same as the 228 parts truck motor. Narrow spacing on the carburetor mounting studs the same as a Chevy 216. Not the wider spacing like a 1956-62 Chevy 235. I attributed this to it being a 1950 GMC 270 rather than a later model. Perhaps I don't have the correct intake manifold on this engine? Dave

I saw another 270 in a junkyard truck over the weekend and it had the same small intake/large exhaust combo as my current engine. It must be OK.

My 1950 gmc firetruck came with 270 with the "H" head, with bigger intake valves/ ports. This might be the case why it has bigger exhaust. It could pump more air.