Headers vs Cast Manifolds for Low RPM

[Chris in Idaho]

I have a 1986 short bed 4x4. The previous owner pulled the original 4.3 v6 out and installed a Goodwrench 350 with a Quadrajet on top of a square to spread bore adapter on an Edelbrock Performer EPS intake manifold, and it has full length headers. It idles smoothly, so I assume the cam hasn't been changed. It has a SM465 transmission and NP208 transfercase which give it a nice low gear ratio for crawling/trails.

My question is this:
For what I like to do with this truck I need smooth controllable throttle response and good bottom end torque (I need to be able to lug the engine way down... like 300 RPM without dying). Right now the engine runs more like a hotrod than a truck. It has a lot of power, but the throttle response is touchy... it kinda explodes into action, and below 1300-1500 rpm it doesn't really have the torque to lug the truck around. I think the intake manifold (especially with the adapter) has too much volume to keep the intake charge velocity up at low RPM. I just got back from the junkyard where I paid $13 for a non-EGR Quadrajet intake manifold, and I'm wondering if I should have grabbed a pair of cast exhaust manifolds too. The headers are kinda cool, but I'm not really concerned with top end horsepower. My uncle always tells me that headers destroy bottom end torque... is he right? Would it work better with stock manifolds?

[Chris in Idaho]

If cast manifolds are the way to go, are there some better than others? The ones that looked like good candidates to me were on a TBI pickup. The driver's side manifold had an O2 sensor, but there was no air injection or any other crap. Someday I might want to run TBI so picking these manifolds now might be helpful.

[Pyrotechnic]

You CANT lug these motors, especially a 350. A hydraulic flat tappet will start the power at 1900-2000 RPM. A hydraulic roller will start at around 1700-1800. Not only that, lugging under high load is what destroys bearings. You will be doing a rebuild before you know it.

Lugging down to 300 RPM ? What are you doing with this truck and why aren't you willing to run at a higher RPM in a lower gear ?

[Albatrossian]

Headers are far more efficient than cast iron manifolds. There are different kinds, short and long tube, long tube headers will give more torque. I honestly can't see any gas engine lugging down to 300 rpm without stalling. For crawling, a QJet is actually the better choice other than FI for that.

[Chris in Idaho]

Originally Posted by Pyrotechnic

Lugging down to 300 RPM ? What are you doing with this truck and why aren't you willing to run at a higher RPM in a lower gear ?

I'm using it for rock crawling, and I'm already in the lowest possible gear which comes out to be around 70 to 1 final. I'm not lugging it down that far all day long, and I'm not pulling any power at that RPM either so there's no great load on the bearings. There are just a few times when I need to go that slow to avoid breaking something or rolling, and there are many cases when I just need a tiny bit of umph to get over something but I don't want to be launched into the next obstacle.

So far it sounds like headers are ok.

[Chris in Idaho]

I had a 283 with a 2GC 2bbl and the intake manifold had very small runners, and that engine did way better at low RPM than this 350 does. I think my problem is mostly with the big intake manifold. When the engine slows down the intake charge looses velocity and the fuel separates out, but if the runners were smaller the intake charge would still move faster.

[heavychevy79]

Your carb adapter may be causing the touchy throttle problem.
Those open adapters/spacers kill low end power and cause jerky throttle,
four hole adapters/spacers give a more stable throttle responce and better power.

Btw those 4 hole phenolithic (sp?) spacers work great, but they are not
for adapting to spread bore.

[Pyrotechnic]

You might wanna check out what kind of adapter it is as said above. The open ones screw up the vacuum signal to the primaries, so you need a 4 hole version.

Also, it could be a carb tuning issue. Q-Jets came closely tuned to their original application. I once swapped a mild RV cam into a 350, which was a very mild cam but the primaries were very lean and did not run right.

Ignition timing is important. Make sure it is set correctly.

Another thing is vacuum advance. I've found that full manifold vacuum at idle gives smooth throttle response and more torque off idle, whereas ported or timed vacuum gives jerky response and makes it feel gutless till the motor gets some speed. This is whole different topic with lots of debate but there are some things you need to do before you switch to manifold vacuum. Do you know what it's currently connected to ?

[Chris in Idaho]

Thanks, those are some good insights. I wouldn't have thought of trying full manifold vacuum for the distributor advance, that's really interesting. In the morning I'll get out there and see where everything's actually going. What do you have to change before you can switch to manifold vacuum? I think it may be the original quadrajet from the 4.3 v6 so it's probably not tuned for the v8. Also, weren't the 80s Qjets computer controlled? If there was originally an o2 sensor or anything like that it's gone now so the carb may have issues. I think Qjets are cool carbs, but I don't know much about them... has anybody looked at the Cliff Ruggles book "How to Build and Modify Rochester Quadrajet Carburetors"?

[Pyrotechnic]

Depends what you set your initial timing at and how many degree's your advance can will pull in and how much your motor can tolerate. I ran a stock 350 and with 8 degree's initial timing and a stock HEI on manifold vacuum. I have idea how much that can pulled in and no idea what the total timing was since there was no timing tape, but it never pinged and has a good sounding idle so I wasn't worried.

By comparison, if you have an aftermarket cam that likes around 34 total and 13-14 degree's initial, you need a crane cams brand limiter plate and adjustable vacuum can and limit it to 10-12 degree on the can, then use the internal screw in the can to adjust the rate.

If that Q-Jet has a connector on the top front of the carb, it's electronic. Not going to work. It's a mechanical carb, first verify that you have a 4 hole spacer under it. Second, start finding out what that carb's original application was and go from there. If it's a V6 carb, you can swap in primary metering rods and jets for a stock 350 application.

Alternatively, an Edelbrock 650 or Holley 650 will match up to your intake and have readily available tuning parts.