I'm putting a 302 gmc in my 31 chevy pickup project and want to put a turbo on it. can anyone help with what size i should look for? its just going to be a street car at about 4-6 psi boost thanks for any suggestions, Jay D.
First, I’m a big fan of turbo’s, I’ve owned 7 of them. All factory fuel injected high performance stuff. But those are engines made to take high levels of boost with extensive electronic fuel and timing controls. I’ve also run a couple McCulloch/Paxton units, and an Eaton unit on basic stock or street performance engines. At low boost levels the McCulloch (or similar centrifugal unit) is hard to beat. They look right, it’s not that big a deal to set the carb up for blow thru, pressure is easy to manage (even some mechanical pumps), and ignition retard is easy with even a simple MSD unit and Hobbs switch. Low boost levels like you’re talking about are much easier that way. Those big GMC’s make good low end torque and the low boost makes pressure at 2500-3000 rpm (depending on the pulley drive ratio). You might sit back and think about that.
A very rough-guestimate would start with Size = 302" (straight 6, yes?) Rpm(max) = ? Boost = ~ 4-6 psi Where you want the boost( @ rpm) = ? Blowthru or Drawthru = ? Both work, both have advantages & dis-advantages; from packaging standpoints, using, components required, & drivability. What's important to you? & how much can you build &/or scrounge, or prefer all-new? Math-out the cfm required, then look for the same thing in OEM cars. CID, rpm & psi. It's all about airflow, but at what amounts & what rpm? For more efficiency, then I'd check compressor maps. 4-6psi might not sound like much, esp for bragging rights, but it'll be *very* noticeable & put a smile on your face for sure. Most stock mills can take that level w/o issue, other than mild tuning - including jetting, ignition curve & fuel pressure(you do *not* want to get into lean mixtures nor esp detonation). 6psi shouldn't require an intercooler. You won't need or want the hi-rpm(race) components for a street mill. Stock or heavy-duty will work just fine, actually. & the stock cam is actually good for this. They just require some light modifications to work. A short exhaust pipe from exhaust manifold to turbo works, although not as pretty nor technically good as headers, but a lot cheaper. & believe it or not, plastic "potty" plumbing works for intake tubing. Esp good for "try-outs/experimentation", you can always do the SS tubing later. Keep the factory rpm limits, add boost if you have to later(it *is* addicting... . You have been warned. . You won't find much for drawthru(turbos need a proper seal in the shaft) in oem, although there were some in the 80 n earlier. Most JY turbos are for blowthru, can be adapted fairly easily & cheaply - *if* you do your own work. & adapt other oem parts as needed - but this'll require you to learn & understand the function of those parts. Sadly, Dick Datsons' websites(hands-on DIY JY adaptations) about turbos/etc, went down when YahooGroups went away. To see how-it-could-be-done, I'd suggest reading thru the old TurboStude site. It's a good read & still applicable, esp in terms of thought-process. Everything was done w/o electronic controls. There was a site called JYTurbos, maybe still around? Also, see if you can find some of the old Dick Datson booklets. They are out-of-print, but also still useful for the techniques. The GatorSupercharger wouldn't work well for your size engine, but the Blowthrough turbocharging book would. Ebay, maybe? There are also a handful of turbo-ing books out there, but tend to the hi-cost low-boost side of things. Marcus...
thanks Marcus its a 302 GMC INLINE SIX i'm thinking 4-6psi boost starting around 2000rpms max rpms is going to be below 5000 its just going to be a street cruiser daily driver i would like a draw through for the simplicity of it, but open to ether way. i can also build to suit, compression and so on. i'm not new to boosting just turbos this is my 53 its a 270 gmc six View attachment 6098891 View attachment 6098891
^^^ Well, I see you're familiar w/boost-itis... . & nice cars. BS, do you purposely want to use some older oem-type stuff(image?), or do you want to make your own manifolds n headers, & use a better more efficient turbo? Not sure what you're after, either in looks, quality, or components. FWIW; before the GrandNational Buicks, Buick had a "sporty" T-type, that had a v6 (231 cid?) w/a drawthru turbo n Q-jet carb. Neat idea, but the space limitation(s) gave it a weird tight 90* elbow coming out of the compressor. & iirc, the carb-to-turbo-manifold/adaptor had the carb sitting too low, so it'd get some fuel puddling, although I think that was heated for better driveability. They worked ok, but it'd be better, I think, to make a better-functioning copy. IIRC, boost wasn't much, didn't come in 'till after hiway speed. I don't think there was 5psi at wot, if that. But considering the turbo turbine was fairly large for that little mill, it might work ok on your 302, coming in early enough to be fun w/o being a danger. You'd still use over-boost-protection. There was also a system sold be Accel, that used a flapper in a box, under the carb, to direct either n/a, or boost when it came on. But that only worked at upper rpm, & was low-boost anyways. Both were examples of hi-cost/hi-rpm/low-boost. For the older-look, there are also, of course, the Corvair turbo & Buick 215 turbo. A pair of 140cid 'vair turbos might work, but don't know how low the boost'll come in at. Most of the60's turbos seemed to be AiResearch w/a round 4bolt exhaust flange, if you want that look. If you want a good matched new T3/T4Hybrid turbo, the 1st guy I'd talk with, goes by the handle Shag55 on another OT board. Knows his stuff. I'll look up his info if you wish. Lots of ways to adapt, if you're going to build headers & maybe intake, I'd suggest "weld-ells" for the exhaust. Very hd weldable castiron(maybe SS, too) elbows, made just for turbo header building. The only thing I'd caution on drawthru, is from the last I've heard/read - you *really* want a carbon-seal on the shaft to prevent oil being sucked into the intake under vacuum, leads to easy detonation. Oh yes, *no* intercoolers for drawthru, at least after the carb, & it's usually not needed for drawthru anyways, due to the fuel/air absorbing so much heat(Latent Heat of Vaporization effect). The suggestion of a McCullough/Paxton isn't bad, but unless it's one of the new-types, won't move enough air for a 302. Stude guys found that out in the late 60's, even dbl-ing them up only went so far. & that was on a 289. A single one gave ~ 5psi just after the blower outlet, & ~ 3psi in the manifold under the carb. Still a whole lotta fun, mind you, but since they are rpm-dependant, again, well above hiway speed. It really is too bad that DDs' site went down, as all of it would've been up your alley. One possibility I haven't done, is use the WayBackMachine.com to find archived copies of it. Once you figure out which way you want to boost, then you can start looking at the modifications. Shouldn't be all that many, not real hard anyways, for 6psi. & stock(or abit better) compression works well, so does a stock cam(extra lift usually doesn't hurt). Also returns good driveability. Porting the head works, but stock w/a good valve job is fine. You do want good quality bearings, rings, valves n seats, at least. May as well through in a good balance job. At 6psi, even stock pistons work - just be careful on the tune n quality of gas. Either way, it should be a fun project. Marcus...
A mid sized T3 or T4 turbo would be fine on a basically stock 302ci 6cyl. But there are lots of factors... carb or efi? intercooled? fuel type? If it were me, I'd build a sheet aluminum intake with a stock LS throttle body, 3 or 4 stock uncapped LS fuel injectors, a Holley Hyperspark Distributor and Terminator X EFI universal system. For the exhaust, a simple flipped header into the turbo with a good external wastegate and downpipe. Run the cold side through an intercooler and have a submerged 255-340lph fuel pump in the tank and it'll be a riot. You could also run e85 fuel and ditch the intercooler at those low boost levels. Any higher boost you should look at upgrading valve springs.
Marcus thanks i'll try ad answer some questions hers what i would like to do not necessarily have to do. i would like to go draw through as it would be simple for my application, i have unlimited room in a 31 pickup i'm going to run just the top of the hood or i can run no hood at all. as for all the manifolds and bracketry i can fab myself BUT i'm not opposed to buying items. the turbo i would like to be new. i have in my mind that i would like to draw through with a 600 Edelbrock 4 barrel, i have one on my blower and all the parts and knowhow to work on them. limiting boost is something i don't understand with a draw through. i here bad words about a draw through but its about the same as my blower motor the fuel passes through the rotors and into a manifold just the same. my blower motor starts and runs perfect. it will set there and idle for an hour and never mis a beat it runs as good or better than an old stocker. in the end with a turbo i would like to get somewhere around 250 300 hp with boost coming on around 2000-2500 and rpms and keeping the rpms limited to 5000 keep in mind this car weighs almost nothing. if you could hook me up with this Shag55 i would like to talk with him. my main objective is to get a turbo matched to my needs. just throwing anything on there is a recipe for disaster. i can handle most of the other stuff ( fab work ) then theirs always you for questions, thanks! Jay D.
This is the turbo I am running on my Ford 300. I hope to get a bit more than 4-6 psi though.... Intercooled and blow through throttle body injection. https://www.summitracing.com/parts/TNT-12028 The motor is done, just waiting for my slow ass self to finish up the chassis....
I’ve never ran a blown GMC of any kind but I’m very familiar with stock head and 12 port GMC’s naturally aspirated on gasoline and nitro methane. If you are going to use a stock 7 port cylinder head remember that 2 of the shared port pairs, 1 and 2, 5 and 6 do not even fire only 3 and 4 do. This can definitely make a determination on your induction system whether sequential port or some type of blow thru or suck thru like a conventional roots blower. I can guarantee one thing I would do is o-ring the block/cylinder head. I would also use headstuds. They are 1/2” but only 4 per cylinder. ARP sold a kit at one time but it’s no longer listed so I buy single studs by length along with washers and nuts. With todays manufactured lifters if you are going use a flat tappet cam are usually made outside of our country and supercharged engine need more valve spring pressure which can become a problem with the available replacement parts. It’s pretty easy to get a cast iron cam reground for your application and possibly a steel one if you have a military core but the chilled iron lifters for them are pretty hard to find today. . good luck and have fun with your project.
this has been suggested what do you think??? T-4 GT3582 Specification: Compressor wheel: 65.7/82.6/85.3mm Compressor Housing: 0.72 A/R Anti-Surge, 4" Inlet, 2.5" Outlet Turbine wheel: 68mm/62.3mm Turbine housing: T4 Inlet, V-Band 3" Outlet, A/R 0.82 Oil inlet:7/16-24UNF Jay D.
Hey, bs; I'll pm you Shag55s' info, just gotta find it. The reason I was asking the Q's, is because everyone has a different idea on what they want to work with, how they want it to look, & perform. For new, Shag55 should have the answer(s). I don't think I'm good enough to make a very specific reco on any given turbo. I'd have to go chasing the charts(which I haven't done in a long while) to kinda narrow it down, & then I'd still ask - if I was going new. JY is cheap enough to make mistakes & change sizes +/- . Sorry, no ego, nor anything to sell you. Your preferred rpm-band should be real fun on the street, & you won't have to do the dick-swinging-bragging deal on how much boost you get. The smile on your face should get 'em wondering... . Your carb choice sounds like a good start, & a good aftermarket manifold might also be a good start for proper fuel/air distribution. Can always go custom-made. Since boost(it's *not* airflow) is nothing more than a measure of resistance to flow, it's almost meaningless, but at least a reference place to start from. From what I remember, ~ 1 atmo(~15lbs) results in a dblg of power, it's not exact...), so @ 6psi you might count on ~ 50% increase(very rough guesstimate). I don't know where you're at w/the 302 now. Limiting boost is done(preferably) by an exhaust waste-gate(The other way is literally choking the engine intake, done in the 70's/80's = rather bad idea). Dumps exhaust as needed, to keep turbo from out-putting more than desired, & can easily be made adjustable if you want. W/a blowthru, you don't want a BOV(BlowOffValve), as that dumps intake-side air, & since a DT intake will contain air/fuel that's well mixed, not the best idea. This is gonna be a very nice street-mill. I think you'll be happy w/drawthru, just don't have any manifold-flow-patterns(from carb-to-ports) be low spots. It has(was) been found(sometimes) having right angles &/or T's in some of the manifolding promotes turbulence, slowing fuel-drop-out. Turbo beats it to a fine death, but still... Don't forget to ask about carbon(or other) seals on the compressor-side. There are a *lot* of good reasons to go new, taking advantage of the tech now available. Marcus...
Late model have a little advantage in the block over the GMC’s. A 12 port heads also doesn’t hurt performance..
I have a large port head that should help, although its just a hot street cruiser not a dragster. Jay D.
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