Inliners International Forums General Discussion Hi Performance gmc 270 build...upgrade parts?

I have an '59 GMC 270 motor that is in stock form at present. I am building it to go in a hotrod 53 chevy truck. I haven't built one of these before so was wondering if anyone wanted to give some initial thoughts or experiences as well as any suppliers for parts. I know Patrics probably carries some stuff. I will run a blower, but haven't totally decided on injection or the old carb. Other than that it will be pretty basic. I have nice big tube headers on my 235 ('57 model) now, can these be tweaked to fit the gmc as well? Boost will be kept to a minimum, 8-10 lbs or so. Unfortunately I don't have the H head, so i will be porting and adding as big valves as feasable, maybe some homemade roller rockers. Seems like there should be some piston that is close in size that could be bought reasonably??? With teh 270 and 302 having the same stroke....how far can you take the bore on a 270...just assuming it can't be bored all the way to 4 inch?? This will be run in front of a T5 and 57 chevy truck open drive rear. thanks in advance, looking forward to this build...hans

You could get pistons and most of what you need from Patrick's. Stovebolt and Clifford also sell GMC performance parts. I'm sure others here will add to the list. Search the old posts here for this topic there is a lot of info here. Try to find California Bill Fisher's Books. The little GMC Speed Manual is a good one. There have been a lot of things figured out since these were written in the '50s but most of it is still good. He says that 3 15/16" is max bore for a 270. 4" is possible but it may weaken the cylinder walls. Yours being a '59 it has the D shaped combustion chambers with small ports. The big ports are really only desirable, in my opinion, on a race motor. The small ports work better on the street. With a blower it wont matter any way. If you run that much boost you will have no problem filling your cylinders! On the street I'd think half that would be plenty. You would want a mild cam, stock would work. Your engine should have the stamped steel rockers which are pretty strong. These are by nature not high revving engines,6,000 is a lot! I have a '53 Chevy Pickup with a '57 270. I built it in '78 so I don't remember all I did. It's Bored .030,I think I'll see tomorrow, balanced. It runs a 500 cfm Edlebrock,Fenton headers, and electronic ignition. There was some mild porting done on the head. The cam is the hottest one Clifford sold at the time that was usable on the street. I haven't seen the cam card in years. Chevy manifolds can be fitted to GMCs but it takes some work. My Fentons were actually cast for a Chevy. I would think that your tube headers could be made to fit. That's what we do! I too run a T5 with a '68 GMC Dana 44 limited slip with 355 gears. It'll run 70-80 all day. This one loves to run at 3,000 and I have to think to keep it around 2,500. I hope this was of use to you ,good luck!

A GMC 270 will bore safely to 3.9375 for 292 cid. You will not want to exceed that number on the street, especially with a supercharger! The GMC block is 1.5 inches longer than your 235. Patrick Dykes sells "Venolia" pistons and other parts for the 270, and cams ground by Donnie Johansen. Tom Langdon sells reproduced "Nicson" cast iron headers and used intake manifolds. Memphis Equipment sells NOS parts for military 302s, many of which will fit your 270.

With a four-inch stroke, you are well advised to keep the rpm to a maximum of 5,000 on the street. Torque, not horsepower, is the great strength of these engines, and torsional vibration is their great enemy. A Fluidamper is an excellent investment. Take it easy on the blower boost. If the engine is breathing well, five pounds will likely be more than enough. Nothing shortens engine life more quickly than the notion that "if some's good, more's better."

God's Peace to you.

d
Inliner #1450

Great info guys, thanks. I have been pouring through the old topics finding as much info as possible as well. I have seen Donnie Johansen mentioned a few times in regards to cams. WHo does he work for or how do you send him inquiries? Can they regrind on your original cam or do you have to buy a cam blank as i read one of the other guys has done? I don't need much of a cam if any with the blower and as said I won't be twisting it high rpm, so just a bit more lift would be nice which I might can do with some custom rockers. It's a shame it's so much work to get the piston pin a bit higher in the piston, I would love to have a more efficient piston....oh well. Hans

Donnie Johansen grinds the cams for Patrick Dykes to sell, and Patrick deals with the customers. They grind the cam you provide using Howard Johansen's recipes. i provided a NOS military 302 cam that i bought from Memphis Equipment Company.

God's Peace to you.

d
Inliner #1450

Ah, Ok. Is there any reason to get a NOS cam and regrind vs using an old one,provided the old one has all it's lobes...;). maybe the 302 has more lift or duration form the factory so it has more material to work with???

If your 270 cam is in good condition, then it should, so everyone says, take regrinding without difficulty. If the lobe and bearing surfaces are worn or scarred, then you should find another cam to regrind. i provided a new cam for regrinding because i wanted to do that only once, eliminating any potential problems and getting the best possible results.

A military 302 cam is not likely to provide any improvement in "high-speed" performance over the stock cam in a 270 for a civilian light truck. "Speed" is not what a six-by-six offers, although many a soldier may wished for it when under fire.

i don't have the manual in front of me, but as i recall the stock GMC 302 produces its maximum torque at 1600 rpm, and peak horsepower at 3600 rpm. On the dyno, with two Holley-Weber carburetors, my engine produced maximum torque at 3050 rpm and horsepower at 4390. The cam (228 @ .050) and head work are most of that difference. More carburetion might raise the horsepower peak, and i may get to that if i live long enough and can afford the gasoline.

God's Peace to you.

d
Inliner #1450

Not positive on the '59 but your chamber may not be a "D" but one that followed the valves. Not exactly a heart shape but close. I think the compression was about 8-1 which would be great for a blower. Most GMC heads I have used could have .100" milled off without a problem. This may help you get the comp you want. The small port head is great on the street as indicated above. A 1.87 intake and 1.5 exhaust would work well with a blower. I would also o-ring the block/head if you going to run any boost over 10#. Studs for sure. ARP sells them.

I have used a set of Clifford 235 hearders on my GMC's before by splitting the tie bar between to 2 front and then back exhaust ports and they worked perfectly.

Since you will probably be buying forged pistons I would advise you to purchase them in a diameter of an easily purchasable ring size. I would contact Ross, Arias, etc for them as they both work with us 6 cylinder guys. I have a 270 out to 292 3-15/16 and it needs another sleeve maybe 5. The only 270 that could be bored to 4" was a 1940, the first one. If you go to 3-15/16 have it sonic tested in case it has a core shift which I'm sure was my case.

Donnie J is available for consultation by calling Chet Herberts I believe. He ground a blower cam many years ago for me from one of my steel stockers. I'll look up the cam card and see what it was. It is a dual pattern for a 270 flat top piston blown engine I wanted to put in a 50 sedan but it was never completed. I was also going to use the same head you may have. I have all the Comp cam part #'s for using a 1.5" diameter valve springs and Chev V8 length valves if you want or need them. I never did complete the engine but still have all the parts. I believe it has JE pistons/rings and would be about a 277".

I was planning B & M Blower on my own manifold so it would have the stock length snout

Good Luck with the project.

huffin, Something we haven't said yet is to make it full flow to the oil filter. I think there is an article in the tech tips. I pulled my 270's head today and looked into the cylinders for the first time in 30 years! It looks like it was leaking between cylinders. I don't know if running on 3 or 12. I was right it's .030 over. The cylinders and valves look good. As long as It's down I'll freshen it with rings, bearings an lap the valves. If it lasts another 30 years I'll be 93 and won't know if it's running or not! I'll modify the timing cover so I can get it off without removing the pan. I have plans to put a McCulloch and 2 Zeniths on it this fall but I'm not ready to do that yet. Earson Cams In Carson City, Nevada will grind a cam for you. An Erson rep was at our last club meeting I'm going to talk to them and have one done for the supercharger. I be glad to talk to them for you when you decide what you want. It's close by. And as Jimmy six said the chamber shape not a true D shape. Its sort of a D with a notch in the straight side.

[quote=jimmy six #35.
Donnie J is available for consultation by calling Chet Herberts I believe. He ground a blower cam many years ago for me from one of my steel stockers. I'll look up the cam card and see what it was. It is a dual pattern for a 270 flat top piston blown engine I wanted to put in a 50 sedan but it was never completed. I was also going to use the same head you may have. I have all the Comp cam part #'s for using a 1.5" diameter valve springs and Chev V8 length valves if you want or need them. I never did complete the engine but still have all the parts. I believe it has JE pistons/rings and would be about a 277". [/quote]

Jimmy Six, Do you still have this info? The cam card specs, Comp # for springs and valves. Do you have the engine? Do you have a JE piston #? Thanks Tom