Nitrous for street truck?

[Tucson38]

I have an '85 GMC 3/4 ton 4wd 350 all stock, 790 cfm Edelbrock Q-jet. When towing my boat through the mountains I'm struggling up some of the hills in 2nd gear which limits my speed to about 35 mph with the SM 465 trans. Can I safely utilize an Edelbrock or similar nitrous system w/50 hp jets to us on these hills?

I considered a tranny swap to a six-speed for better spread between gears but wasn't able to come up with anything that would work. I'd lose my granny gear and in most cases have a trans with 2 overdrive ratios so I'm looking at something to give me a momentary boost.

Appreciate hearing you opinions on this project.

[wilkin250r]

Define "momentary boost".

Nitrous doesn't do well for sustained boost. As the nitrous depletes from the bottle, the bottle temperature drops, which means nitrous pressure drops, which means nitrous AMOUNT decreases over time. However, because fuel is fed from a fuel pump, the amount of fuel being fed into the engine remains constant.

Since nitrous is the oxygen source, and you have a rapidly decreasing oxygen supply, you are rapidly getting a richer and richer mixture.

Larger bottles can supply longer periods of time, and I've never TESTED this theory, but I have run nitrous before. I would be surprised if you could hold the button for a full minute before you start actually loosing power due to your fuel/air mixture being overly rich, possibly fouling your plugs.

[Tucson38]

I wondered about the length of time you could run on it. It takes from 2 to 8 minutes to run some of the hills so it looks like this concept won't work for me.

Appreciate you input, I'm not that well versed on NO2 but your info changes the whole story for my application. I'll have to come up with another solution.

[Gmc.guy]

Maybe a rear gear or tire diameter change might help. What's your current ratio?

[geezer#99]

Maybe just a timing change will restore your lost power!
What are you running for initial timing?

[68Timber]

How much does your boat weigh?

[Gmc.guy]

^^ what he said. Make sure you are tuned for best running(carb too) and see if anything changes. It sounds like you need more permanent reliable power If you looked into trans swap and a nitrous setup, you probably have a budget that is fairly decent and you have a specific reason/use to upgrade, I would look into options for a bit more torque, my advice would be to upgrade the 350 until it's "good enough" or upgrade to a stock big block.


Then you can gun it in 3rd and do 52 mph

[Tucson38]

Originally Posted by Gmc.guy

Maybe a rear gear or tire diameter change might help. What's your current ratio?

Rear is 3.73, the 350 pulls well from off-idle to around 3500. Truck is a HD 3/4 so it's basic design is for a tow truck application.

[Tucson38]

Originally Posted by Gmc.guy

^^ what he said. Make sure you are tuned for best running(carb too) and see if anything changes. It sounds like you need more permanent reliable power If you looked into trans swap and a nitrous setup, you probably have a budget that is fairly decent and you have a specific reason/use to upgrade, I would look into options for a bit more torque, my advice would be to upgrade the 350 until it's "good enough" or upgrade to a stock big block.


Then you can gun it in 3rd and do 52 mph

18 ft aluminum Boat & outboard weighs around 1400 lbs. trailer is pretty heavy duty, had a 2nd axle added so I'd have 4 tires on the ground, couldn't guess the weight.


I think your'e right on the engine upgrade. Considering stroking to 383 for a little more torque, maybe an upgraded intake manifold. Don't have the specs on the cam (290 horse basic Goodwrench 350) but it's seems ideal for my application, pulls strong up to 3500 rpm or so. I've been driving it stock for 32 years now so I don't have any engine upgrade experience.

[Gmc.guy]

Cool that you've had it that long. Maybe a compression test would help you choose. It might just need a re-fresh to restore compression and get it back to where it's happy.

[Dead Parrot]

What altitude are you normally at vs the mountains you are having trouble with?

Might just need to tune(usually leaner) the carb for the higher altitude. Bumping the timing once at altitude will help some as well.

One advantage of the Edelbrock 140x series is the ability to quickly change the metering rods. Last time I made a trip to the middle of Colorado, at around 5000ft or so I swapped to a leaner set of rods, about a 5 minute process. Truck had much better performance then the first time I had made the same trip. End destination was about 9000ft. Normal OK altitude is about 1300ft.

[wilkin250r]

Are you running a stock tire size?

I agree with everybody else, I think this is an engine tuning/power problem. I could see 2nd gear if you were running 3.08 or something, but you should be able to pull 3rd gear with 3.73 gearing, unless they're really steep hills.

[andrewmp6]

Only thing i can add is a gear vendors overdrive will bolt on the end of your t case and give you a high and low for every gear you got.You would have to shorten the rear driveshaft but everything else can stay as is.

[Tucson38]

Thanks to everyone who responded, appreciate your input.
The state of tune of the truck doesn't seem to be a problem. Plenty of carb, new distributor last year, pulls well etc. I think the biggest problem is the wide gap between gears. An extra gear between 3rd and 4th would probably do the trick but that doesn't seem doable. I've essentially got a 3-speed trans with a creeper gear. The tires are stock height and the elevations I'm driving are from 2000 to 3600 feet so I don't see an aspiration problem.

[geezer#99]

Originally Posted by Tucson38

I don't see an aspiration problem.

Wanna bet!!
At elevation you want maximum timing and leaner jetting.
What's your initial timing?
How much mechanical in the distributor?

[wilkin250r]

Originally Posted by Tucson38

I think the biggest problem is the wide gap between gears.

With a little bit of research, I think I agree with you. I never really played with the SM465 before, I only knew the basics (I knew the low first gear, but I never looked at the other ratios).

You're correct, there is a huge gap between gears. Looking at the ratios, I see it very possible that you've got plenty of engine to pull 2nd gear and then some, but not quite enough to pull 3rd because of the huge gap between the gears.

It's looking more and more like your best bet really would be an aftermarket overdrive unit to give you more gear options, help you split the difference.

[Tucson38]

Thanks Wilkin. You put your finger right on the problem. I've looked at gear vendors O/D but there seem to be a lot of limitations on its use, based on what I've read on this site and others. I once had a '66 CJ5 with a Warn all-range overdrive that would be just perfect for this application but it's no longer available. Richmond Gear made a real nice 6-speed overdrive with a single overdrive gear but it's no longer listed.

Geezer - thanks for the input. I live in Tucson @ 2300' elevation so a 3500' hill is not that far out of my normal driving range. It really comes down to limited gear selection.

[Dead Parrot]

Any chance you could borrow a set of smaller tires already on rims? If the smaller tires let your existing 3rd gear work better, then the wide gear split probably is the problem. If your best speed stays about 35mph, then you might need a motor refresh or the boat is heavier then you thought. Be sure to use a GPS to measure speed as the speedometer will be off.

Thinking swapping mounted tires is easier then swapping transmissions or gear sets.

[wilkin250r]

I was thinking about this thread just the other day, I was towing a small car on a trailer with my Ford. Slightly smaller engine (4.9) and manual transmission, and 3.73 gears. I couldn't quite tackle the hills in 3rd, but I could easily do it in 2nd. However, I had an advantage because I have a true 5-speed manual, not a 3-speed with a granny gear. Your second gear ratio is very close to my 1st gear.

Unfortunately, it didn't lead me to any revelations on how to solve your problem. I just reaffirmed that there really isn't an easy solution, but several possible solutions that have already been mentioned.

[AnotherWs6]

An all stock 85 350 isn't making too much power. A cam and intake? That and heads? I'd at least throw a Edelbrock Performer or something similar to it on it. You can probably do higher ratio rockers. Exhaust...what's going on there? You need more power in general, nitrous is not the way to go.

[68Timber]

Originally Posted by AnotherWs6

An all stock 85 350 isn't making too much power. A cam and intake? That and heads? I'd at least throw a Edelbrock Performer or something similar to it on it. You can probably do higher ratio rockers. Exhaust...what's going on there? You need more power in general, nitrous is not the way to go.

That's what I was wondering, the SM465 is wide-ratio, but it sounds like lack of torque. A cam and intake swap and some headers should wake it up, but if you're seeing elevation changes like that you might be better served with one of the fuel injection kits first.

[friscobob]

Consider buying an aluminum trailer to lower weight you are pulling. Less down time with your truck too.

[lime1982]

I'm thinking gears in rear would be best option. I know when I re geared my Jeep it was like a new vehicle it will pull anything with the 4.88s.