Inliners International Forums General Discussion Engines Need to know exhaust pipe diameter for GMC 302.

51 GMC 1-ton

I replaced my 228 six with a 302 six. The 302 has a 2bbl. The 228 had a single bbl. The throat on the 302 carb measures 2 1/8" ID. The exhaust pipe that came with the 228 is 2" OD. I assume this is restricting my exhaust. What would be the proper exhaust pipe size?

Steve, I'll measure a 302 exh. pipe I have that came stock. I thing most 270/302's had the large 3 bolt exh. pipe fitting.

Ed Pruss
Longmont, CO

21/2" Joe

Sidehill,
The stock exhaust size is 2-1/2". If you kept the 2" pipe I think you would be restricting the engine. The 302 is an engine that needs to BREATH. When I installed the 302 in my 1/2 ton GMC, I sent the stock manifold to Kansas Kustoms and had it split for dual exhaust. Nice thing was that I already had one pipe out the back and all I had to have the muffler shop do was add another(cheaper). With the short dual Smitty's, it sure sounds sweeeeeeet.

Stovebolt651

Well, thanks again. Going from 2" to 2.5 is a 56% increase in volume. That must be why my motor has been running a little warm on hills. I also kinda expected more power, although it's not fully broken in.

Steve

What vehicle is your 302 in?

A stock 302 in good tune will make about 160 HP and signs off at about 3400 RPM. Adding a set of Nicson headers and dual carbs will make a big difference. The factory cam is all torque and economy, but the valve and port sizes are very generous so a performance cam will really transform a Jimmy.

Jack, What kind of cam do you recommend? I,m going to rebuild my 302 in the not too distant future. I,m running it stock right now except for the Fenton dual manifolds and the TBI injection. It is really worn out. I already have a set of .125 pistons I bought from Tom Langdon a couple years ago. With the lighter pistons, will it be necessary to have it balanced? If not, how much does balancing help? I'm not looking for high rpm, just a good dependable driver. It very seldom sees more than 2500 rpm. Thanks, Joe

Jack

I get yelled at alot because of this, but the 302 is in a 51 GMC 1-ton. I'll give the dual carbs and heads and cam some thought but I'll probably run it a while as is to see how I like it. The motors only got about 150 miles on a rebuild so far.

Joe, I wasn't happy with the Howard F298 cam in my last 302, it hurt bottom end torque and the powerband came in about 2700 RPM. With an overdrive trans I was having to backshift a lot to get in the torque band. I like a Harley-Davidson type power curve, so I talked with Tom Langdon at length about cam profiles. We came up with a dual-pattern grind with about the same exhaust duration, but shorter intake duration and less overlap. This after taking into consideration vehicle weight, drivetrain including flywheel and clutch weight, transmission ratios, gearing and carburetion.
There are so many variables, including your own personal preferences. Some of us like a lopey idle and lots of top-end power. So it's hard to make a recommendation without considering all these factors.

Tom then ordered a cam from Delta with his recommended profile. It came with a "Cam Doctor" printout so I was sure it was ground correctly (believe me this is not to be taken for granted!)I degreed it in carefully to be sure that it was working as designed. Good thing I did, it was 2 degrees retarded, easily corrected with an offset crank key.

I can't really say how it all turned out because the truck isn't on the road yet, but I feel like this is really the best way to avoid an expensive mistake. Talk to some cam grinders and if they don't ask you similar questions, they aren't doing their job.

BTW, reboring for those 1/8" oversize pistons is not something I would undertake without considering several things - it could take several passes on the boring machine, which can be costly. Also,it's taking a lot out of the block and could cause overheating. This kind of overbore was done routinely on race motors in the days when GMC's were cheap and expendable, but the extra 20 cubes or so won't make any noticeable difference on the street. I know GMC pistons are expensive, but it may be cheaper in the long run to stay with a mild overbore and new pistons. I also feel that balancing a street GMC, beyond matching the piston weights, is not cost-justified but I'm probably out-numbered in this opinion. Just my unsolicited 2.5 cents worth...

Thanks Jack I really don't like a lopey idle and definitely don't need high rpm power. My pickup weighs 4100 lbs. It has a 5 speed and a 3.08 rear axle. Approximately 2000 rpm at 70 mph. I pull a trailer some. I didn't know if a cam different from stock would be any better or not. As far as the pistons go, I bought them really cheap from Tom. I thought this would be a good way to go, but maybe not. Joe

Jack, i am interested in your reflections and may come to wish that i had seen them earlier. If one is building a daily driver, i think that what you have learned from experience is entirely correct. When we started out to build the 302 for my '52 Chevy, i wanted to replicate a 1950s street racer or whiskey runner--but at the same time, to make it as "bulletproof" as possible.

We have done all of the things that you suggest one should not do--4.125 bore, Howard F-298 cam (228 @ .050), fully balanced--but it has been assembled verrrrry carefully, with great attention to detail, and i am hoping that with proper care it will live a long time. i don't plan to drive it every day--$2.00 gasoline makes that a doubtful alternative in any case--but i am hoping to enjoy it fully whenever i have opportunity. It will run in front of a Saginaw overdrive, but the differential is Leo Santucci's old 4.88, so i should be able to keep the revs up.

In a few weeks--i hope--Vic Holley will run the engine in on the dyno, and that should tell us something about the cam and the power curve. i'll be interested especially in the torque numbers. With a 5,000 rpm absolute redline and 4,500 practical redline, the top end will not be anything to boast about, but she should get to it in some haste. The 4.88 should make it quick, but should also serve as a built-in governor (3.42 in overdrive). That's probably a good thing, given the age of the engine, the chassis, and especially the driver!

None of this is what we should do "now," but a hot 302 in a '52 Chevy is not about "now." It's about October 1955, runnin' against that rich kid drivin' a 210 Delray with the little orange funny-lookin' "v-thing" and "powerpack," fresh from the factory, and leavin' him behind . . . way behind. Just thinking of it makes me smile.

God's Peace to you.

d

Thanks Don, let us know what kind of numbers you see on the dyno. Is it a chassis dyno or engine dyno? I wanted to run my motor on the latter but would have to sacrifice a flywheel to bolt the dyno brake up to.

There's more than one way to build a Jimmy and while your plan differs from mine, the main thing is to get 'em on the road and drive 'em! The F298 cam didn't work for me but in a lighter vehicle with lower gearing, it might be just the ticket.

Also, my decision not to overbore was a little different as I started with a NOS shortblock, and just didn't want all that "meat" ending up on the machine shop floor.