Induction system for 350cid with headers

[rich weyand]

OK, so I put a new GM Performance 350 in my '78 when the original started blowing green water out the tailpipes. I kept the Edelbrock 1406 carb, but put an Edelbrock 2601 air-gap intake manifold under it. The air-gap manifold lifts the carb and fuel runners up off the lifter box cover to keep the engine oil from overheating the carb. The problem is it also keeps the engine from heating the carb.

Somewhere along the line the original induction system, with the thermostatic air cleaner and heat riser and all that, was removed and they put one of those "two chrome pie plates" deals on it. This solves the problem of what do you use for a heat stove and heat riser when you put headers on the engine. But, it gives you no carb heat.

"Carb heat" is what the flyboys call it. They put a wrap around a header, called a carb heat muff, and run a duct to the air intake. This has a pull cable to the interior so the pilot can select between ambient air and heated air from the carb heat muff. They also put a carb temperature gauge on the airplane.

The issue is that gasoline, when it vaporizes, takes on the heat of vaporization from its surroundings, in this case the carburetor. In the right weather conditions, say 50 degrees and high humidity, the carb can condense ice and freeze up. I didn't have those problems, or haven't yet, but I had poor performance on the cruise circuit and bad gas mileage -- only 7.5 mpg on a 350! -- until the carb heated up, which could take a long time, or not at all depending on ambient temperature.

OK, so I fixed it, and boy does it make a difference. No mpg numbers yet, but driveability is vastly improved.

I started out by getting an air cleaner body, lid, heat riser tube, and front bulkhead fitting from the local junkyard, which has several late 70s c/ks in stock. I cleaned them all up with EvapoRust and Aircraft Paint Stripper, starightened them out a bit and repaired some thigns with JBWeld, then repainted with Rustoleum Rusty Metal Primer and Gloss Black straight out of the can. Nothing fancy, it's a truck and a DD. I also got a new PCV breather line, cap, and filter, and a new air cleaner thermostatic vacuum switch. The existing air door and motor worked fine. I also needed a 3/4" air cleaner spacer to fit the stock air cleaner body over the Edelbrock, due to a conflict at the Edelbrock's fuel entry.

Installation of all the above was straightforward. For the cold air induction line from the air cleaner to the front bulkhead, I used an aluminum dryer vent tube. For the heat riser, I used a Gibson 500415 Polished Stainless Steel Exhaust Tip. This is a "turndown" style. I had a local machine shop mill off the turndown part to be flush with the bottom of the straight part and mill three 1/2" wide, 1/2" deep slots across the bottom side of it. This I band-clamped to one of the header pipes. The Hooker headers have one pipe that heads back before it turns down and it was perfect for that.

The heat riser was an issue. I tried a "flexible stainless steel exhaust pipe repair kit" in a 1-1/2" diameter, but it would not bend tight enough to get out from under the air cleaner snorkel without hitting the valve cover. What I ended up doing is cutting the original heat riser pipe up into its component elbows, and using them with sections of the flexible between them to build up what I needed. I slotted the upper flange so I could clamp it to the stub on the bottom of the air cleaner and take any strain off the header, and assembled the whole thing with JBWeld, primed, and painted.

So this is what I got:





As I say, driveability is much improved. It takes about 30 seconds at idle for the header to heat enough to start warming the inlet air drawn through the heat riser, which is much faster than using a stock manifold because the header tubes are so thin and they heat up rapidly. It takes about five minutes at idle for the air door to open. It does not just flip open, but is proportional. Very nice. And when you get on it hard, the vacuum motor loses vacuum and the air door opens the four-inch pipe from the bulkhead whether the air cleaner is warm or not.

Before I had the worst of both worlds: hot underhood air when the engine and weather are hot, and cold underhood air when the engine or weather are cold. Now the inlet air temp is regulated, which means all the carb tuning I did with a warm engine will actually be spot on whenever I drive, hot, cold or in between.

We'll see what the mileage numebrs come out to, but I can tell you right now that I am using much less pedal to get around. I drove over to Nashville, IN and back tonight and was maintaining 60 mph on the carb's cruise circuit with maybe 1/4" of pedal. Maybe not that much.

So, a couple things for you all. One, do not use the air-gap manifold unless you have a thermostatic air cleaner setup. The carb will run so cold under most conditions that you will get terrible carburetion, poor performance, and bad gas mileage.

Second, if you want to put headers on your engine, and want to keep the thermostatic air cleaner setup, it is not hard to mod the existing heat riser pipe and connect it up with a turn-down style exhaust tip with a little machine shop work.

Happy Motoring!
Rich

[LeoP]

THANK YOU, THANK YOU, THANK YOU. I have been trying to tell folks about this for years and the icing of the carb venturis, no one would listen. OK you folks that think an open element Air cleaner is better, you are incorrect.

[Fast305]

THANK YOU, THANK YOU, THANK YOU. I have been trying to tell folks about this for years and the icing of the carb venturis, no one would listen. OK you folks that think an open element Air cleaner is better, you are incorrect.

The 1980 C10s 305 with a Q-Jet and GMPP Vortec Q-Jet manifold has a very similar setup. Only the air cleaner is a dual snorkel from a L82 Corvette and has a second 4" duct to the driverside air inlet hole in the core support.

[rich weyand]

The 1980 C10s 305 with a Q-Jet and GMPP Vortec Q-Jet manifold has a very similar setup. Only the air cleaner is a dual snorkel from a L82 Corvette and has a second 4" duct to the driverside air inlet hole in the core support.

Interesting. There is a second induction port knockout on the driver's side of the core cupport on my '78, but it is smaller, like it was for the six. Also, both knockouts were still in place, and it looks like they were in place from the factory. The K-10 was over 6000# GVW, so exempt from things like catalytic converters. Perhaps it did not originally have the warm-air/cold-air thermostatic induction setup, which is covered in the service manual under "emission controls". What emissions did it control? "Unburned hydrocarbons in the exhaust". Unburned hydrocarbons? Oh, you mean FUEL. I don't have hard mileage numbers yet, but I was getting 7.5 mpg consistently (all my driving is around town), but I filled both tanks today. I had 100 miles on one tank, and it took ten gallons, and the induction system change was made about halfway through that 10 gallons. Could I actually be getting 12.5 mpg (to get a 10 mpg average)? Great if so! We'll see when I run a full tank on this setup and get hard numbers.

[WMBigs]

Rich,That sounds like the engine combo I would like for my 76!
What GM engine did you start with?
Thanks,
MikeB

[rich weyand]

The stock engine is available from GM for about $1500. Gm Performance Parts provides several flavors of more powerful engines, the least expensive of which is #12499529 for about $2000. They go up rapidly in price from there. I bought #12499529 from JEGS, which is one-day shipping to me. http://www.jegs.com/i/GM+Performance...99529/10002/-1 This engine has greater lift and duration than the stock engine, and a very fat and flat torque curve that drives very well.

I put the Edelbrock 2601 Air-Gap Manifold on it, the Edelbrock 1406 carb on that, and Hooker headers and duals all the way back under it. Note that the air-gap manifold keeps the carb and intake runners from getting overheated by engine oil from the bottom when hot, but it also keeps the carb from warming up when cold, so the thrmostatic air cleaner as discussed above is necessary if you use the air-gap manifold.

The correct tune for the Edelbrock 1406 with this setup appears to be -- primaries: jets .092 (#1425), rods .065 x .057 (#1461); secondaries: stock; step-up springs: 8# (comes with #1464). You will also need to replace the airhorn gasket (qty 5 is #1499) when you change the primary jets. Adjust the float bowls while you are in there, as they will be wrong. I got this carb setup using an NGK air fuel metering system I bought from Amazon for $250. There are cheaper ones, but this one is NGK and has the excellent NGK five-lead oxygen sensor. http://www.amazon.com/NGK-Powerdex-A.../dp/B0018MUNTM

This setup should give you about 300 peak hp, with 300+ ftlbs from 1500 rpm all the way to the redline, peaking at 340 ftlbs.

Rich

[vic_V8]

Rich, I honestly can't believe you haven't moved on to TPI for a nice effective way of gaining HP and TQ, not to mention miles ahead on drivability. That coupled with the 700r4/NP208 would probably have you in the 16-17 mpg state.

My blue truck with 35s was knocking down a corrected 13-16 mpg with a quadrajet I rebuilt while half drunk and pissed off at the float design, and a cheapo Elebrock Preformer intake.

Said TPI on my 450 HP 383 for your inspiration

[rich weyand]

Rich, I honestly can't believe you haven't moved on to TPI for a nice effective way of gaining HP and TQ, not to mention miles ahead on drivability. That coupled with the 700r4/NP208 would probably have you in the 16-17 mpg state.

Well, there's a lot I could do if I wanted to spend a lot more money and change the truck much more away from its basic nature. The 700R4 certainly occurred to me, and is up for consideration if the TH350 dynamites. Switching to a Quadrajet did as well, but that's pricey. I don't mind the NP203, but if it dynamites, then some rethinking will be appropriate there, too. You have to balance the cost of the mods with the cost of gasoline, and I only drive about 6000 miles a year. Bloomington's a small town, and we're pretty close in to town.

As for fuel injection, it has a big disadvantage in my eyes: it requires a computer. A solid-state thingy, in any case, whatever you want to call it. I deal with computers a lot, and don't need any more of that. There's just something so retro about what I have now, and I kinda like it that way. 1978: last year without catalytic converters, last year without an ECM.

But I do think about it once in a while. Like today at the gas station.

Rich

[rich weyand]

Follow-up note. The GMPP #12499529 engine I used is the same 8:1 compression ratio enging as a GM Goodwrench #10067353 except for the cam. The cam in the #12499529 is reputed on the net to be the cam from a 60s Corvette 350 cid 350 hp engine with 11:1 compression ratio.

My gas mileage with the #12499529 engine in primarily low-speed, low-RPM driving is about 8 MPG. The carb has been dialed in to a fair-thee-well with an NGK A/F ratio meter, and others on the net complain about fuel consumption with this motor. I suspect it is a case of some serious valve overlap combined with low RPM.

For all that, it has lots of horsepower and lots of torque and drives nice.

Some other options would be:
- the GM Goodwrench #10067353, same 8:1 CR engine without the Corvette cam, giving 260 hp with a clean induction system.
- the GM Goodwrench #10067353, and have them pop in a Comp Cams cam before they put it in. Voids the warranty, but what the hell.
- the GMPP 350 cid 330 hp #19210007. The same as #12499529 , including the Corvette cam, but with Vortec heads and 9:1 CR for 40 more hp.
- the GMPP 350 HO engine #19210009. This engine is the later block with one-piece rear seal and Vortec heads and 9:1 CR.

But just so you know, the Corvette cam will cost you at the gas pump with low-RPM putting around, due to the valve overlap. Basically, intake mixture makes it to and out the exhaust valve before the exhaust valve closes. At higher RPMs, there isn't time for that to happen, but at low RPM and when idling, fuel consumption will be about 20% higher than with the same engine without the Corvette cam.

Happy Motoring!
Rich

[SUBURBIAN]

Your HEI ignition setup is solid state also, in case you didn't know. If you fear solid state electronics failure, better switch to points/condenser ignition setup.

Looks like you've learned why GM put that ugly heat stove and tube on so many vehicles, and experienced all or some of the difference between an all out/hotrod build and one for fuel economy and cold dreivability. The hotter you can get the intake, the more mileage you'll gain. That's why the old iron manifolds get the mpg's they get...they hold the heat IN. Mileage and performance are two different birds. I'm not a fan of the "airgap" style manifolds.

You may have an mpg or two in tuning your distributor's spark advance curve.

[rich weyand]

I can keep a points/condenser distributor laying around. Alternator, too, as the voltage regulator is solid state.

No vacuum advance on this engine. With that high-lift, overlap cam in an 8:1 CR engine, it takes straight timing and a ton of advance. 32* total timing, so 12* BTDC or so at idle. Or were you thinking tuning the centirfugal advance?

As for the heat stove, not too ugly hooked to the headers like I have it, above. And it's not so much getting the intake hot, as keeping the intake within a temperature range. Once it's up to temp, I am getting pressurized cold air through a four-inch pipe from in front of the radiator, but when it's cold, it gets warm air. The air-gap manifold plays its part, too, by keeping it from getting too hot. Sort of like insulating your house, then putting in a thermostat for the furnace and A/C. The air-gap manifold insulates the carb and runners, then the thermostatic air cleaner uses whatever mix of hot header air (furnace) and stone-cold outside air (A/C) it needs to maintain itself within the operating range set by the thermostat. Since that's where the carb was tuned, that's where the mixture is right.

Rich

[SUBURBIAN]

I just read your post from earlier. 8 mpg? Something's not right with your tune. Sure, you're running lower CR, and different heads than a late 60's 350/350 vette (L46/L82), but even still...8 mpg? I've had 350's with more cam and overlap and carb than that pushing 15+ mpg.

[rich weyand]

I just read your post from earlier. 8 mpg? Something's not right with your tune. Sure, you're running lower CR, and different heads than a late 60's 350/350 vette (L46/L82), but even still...8 mpg? I've had 350's with more cam and overlap and carb than that pushing 15+ mpg.

I don't think so. The A/F ratio is dead on. Timing is dead-on spec at correct RPM. When I tuned the carb, I went from 6 to 7.5 mpg. Adding the thermostatic air cleaner got me to 8. That K-10 is almost 5000 pounds, and there are guys out there running this engine in their vintage Corvette's, which weigh half as much, and they're getting 10 mpg.

Bear in mind I do next to no highway driving. My normal drive is 1-1/2 miles at 25 mph on a narrow country lane, where the last half-mile is a 10* hill that goes up 130 feet or so. I normally drive that with the NP 203 transfer case locked to circulate oil to the output shaft bearings. Then I pull out on a state highway and accelerate to 50 quickly enough to keep from getting run over. There's only half a mile at 50 mph to the first stoplight going into town, then four more stoplights in the next 2 miles, all at 40 mph. Trip back is the same, except when I get home I have to go up my driveway, which is a 13* slope that goes up 60 feet. All in all it has to be the worst drive ever for gas mileage, and it's even worse with a high-overlap cam because so much of it is at 25 mph loafing along at low RPM in third gear on the TH350.

So I don't think overdirve would help much, it would just decrease the revs and increase overlap losses. All tuning parameters are dead-on spec, with quality instruments.

One thought is to change out the cam, but the engine is still under warranty, so I will consider options for a while. I should also take it out for some highway driving and see what I get for mileage, though my normal driving is not highway driving so it would purely be of academic interest.

Bums me out because I love the performance and the way it drives, and I spent $500 extra on this engine over the 350/260 hp engine that is identical except for the cam. Best bet would probably have been to get that engine and have the cam swapped out before it ever went into the truck.

Rich

[SUBURBIAN]

Timing is dead-on spec...where is the timing set? I'd also say that you're probably lugging the engine too much...which will kill mileage. What are your rpm's going 25mph in 3rd?

[rich weyand]

Timing is dead-on spec...where is the timing set? I'd also say that you're probably lugging the engine too much...which will kill mileage. What are your rpm's going 25mph in 3rd?

Timing is 32* BTDC at 3000 rpm, which is fully advanced. That should be about 12* BTDC at idle, and it checks, so the centrifugal advance is working properly.

RPMs at 25 mph? No tach, no clue. It's pretty much idling along, maybe 1000 rpm, maybe less. I am running (apparently, according to the RPO card) 3.43:1 axle ratios with 31" tires and a stock TH350 for shift points.

Lessee, 25 mph is 440 inches per second, with 31*pi rolling distance and 3.43:1 should be 930 rpm at 25 mph in 3rd.

[vic_V8]

holy crap that's bogging the motor down

[SUBURBIAN]

Yup, you're killing that thing at that engine speed/load. That engine/cam should have a crapload more initial/total advance than what you have in it, too. At least 14 to 16 initial, maybe even 18. Total should be 36 or more. I'd have all advance in well before 3000 rpm if it was mine. Time to tune the distributor, then go back to the carb. Until you can do that, get the revs up at low speed by running in a lower gear, and see what kind of difference that adjustment alone makes.

[rich weyand]

Well, I'm not the one who decides when to shift! I guess I could put it in second when I'm driving the lane, but I thought that's what an automatic transmission was supposed to do -- decide when to shift and what gear to be in.

As for timing, that's the GM spec -- 32* at 3000 (full advance) -- so that's where I set it. At 8:1 compression, it'll run on dog p*ss, so I suppose I could put more advance in it without knocking. Does that help mileage?

[vic_V8]

advancing my camaro's helped a little bit. too much will take away from cooling though.

[1998454]

8:1 compression with a larger cam is inefficient. Couple that with the low speed driving and your spending most of your time at a horrible rpm for that engine. The vacuum pulses at that speed are likely messing with your carb as well. A holley is better suited for an engine with any oversized cam. Just my .02