350 TBI 2k-3k RPM woes

[shortboxchevy77]

I've been having this ongoing issue with my 77 chevy K1500. I put a low mile new GM 350 and 700R4 in the truck along with a painless harness. I've modified the the TB by shaving down the ridges, rebuilding it, and rasing the injectors along with shorty headers on true duals with no cats. Fuel pressure has been bumped to 15psi and timing is advanced 4degrees.

The thing runs like an absolute dream around town, idles beautifully, and seems to have plenty of grunt. Problem is that when it drops into OD on the highway it just disappoints the crap out of me, it feels like I'm dragging something almost in the 2-3k RPM range running 70mph. I replaced the dist. today (magnet cracked in the old one) and it runs even better but I'm still not happy with it. I feel that for what I've done to the engine it should pull harder in OD than it does.

I realize the thing is a brick with a flatbed, 31x10.5 tires, and a 2 inch lift but damn the thing in my mind outta run better than this. I'm not sure where to go with it now, I'm getting to the point of either dropping in a 454 or putting a supercharger on it. Any ideas or suggestions?

 

[HotRodPC]

Nooo, I would not be considering a 454. What I'm not understanding though, is what do you mean by dragging something at 2-3K? Are you saying the transmission is slipping? Brick with a flatbed? Not sure what that means either or what point you're trying to get across. Are the RPMS running at 2-3K and you want them less? Or are you saying it doesn't have any power in OD? Are you saying it's missing and running like crap in OD? Motor seems to be bogging down??? Maybe be a bit more specific. It's after 3am, maybe I'm just tired and not getting it.

So it runs good around town 1-3rd gear huh? What about in 3rd with the converter locked up? This is obviously a retrofit conversion since 77 didn't have TBI or the 700r4 as OEM powertrain. So do you know if you get lock up converter in both 3rd and 4th or just 4th? That'll depend on how you have it wired and if that model of 700r4 you're using has the 3rd gear pressure switch. I have a feeling what you're complaint might be and has to more to do with is your axle gear ratio. I'd be thinking it might have a tall 2.73 or 3.08 axle ratio, and that much weight and wind drag might be bogging down the motor when in OD and Converter locked up. But it that was the case, then you wouldn't be at 2-3K rpms unless you're doing 85 mpg or so +.

If you can be a bit more specific about exactly what your dissatifaction is, then maybe we can come up with some more guesses and get to the bottom of the issue.

I did notice the part where you said, you think it should PULL harder in OD. Do keep in mind, OD isn't intended to PULL anything, especially in a 700r4. All any OD gear is actually intended to do is, MAINTAIN your highway speed while allowing the engine rpms to be lower in hopes to have the engine RPM in a more favorable range for better mpg.
Of course that's just my opinion. I've never expected any noticeable horsepower or pull power in OD. You shouldn't really be expecting OD to have mega pull power anyway, especially a 700r4. They don't like working hard or being strained hard while in OD.

 

[89Suburban]

Assuming he converted to part time 4WD as well???


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[HotRodPC]

Assuming he converted to part time 4WD as well???


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Yup. Lots of variables here we don't know. But, I'm sure when he comes back, we'll be able to get to the bottom of it all.

 

[89Suburban]

I was thinking the same thing. Low gear ratios and the larger tires.


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[shortboxchevy77]

No it does not slip or miss in overdrive. The engine is operating perfectly aside from the no power. What I mean by it feels like its dragging something is it feels like I'm pulling a trailer at 2-3k RPM in OD and it just struggles. It should be able to gain speed reasonably well in OD in my eyes. It locks up in 3rd when it should and operates fine, I have the trans wired to automatically lockup in 4th, was tired of the TC always being unlocked to keep the truck at 75+mph.

Obviously the TBI is not factory, as stated in the original post it is being operated by a painless harness with the rest of the components being GM. It has been converted to part time 2wd and is using the NP208 Tcase. Gear ratio is 3:73. My beef is that in my eyes it should gain speed at a reasonable pace in OD with the TC locked up and it does not, it struggles on flat highway bucking a little headwind.

I've built this truck myself. I'm a life long gear head and mechanic by trade so I am certainly no novice. I am very particular about how things are done and the way they should operate, maybe I am just expecting too much from this engine. I am also looking at larger injectors. If none of this cures my issue it will get big blocked.

By "brick with a flatbed" I mean the thing has the aerodynamics of a refrigerator. Big square truck with a flatbed on it.

 

[Green79Scottsdale]

I would be looking at the cam specs. Most people think a good cam for a car is a good cam for a truck. That is not always the case. Don't know what your specs are.

I also would question locking the tranny right away in OD. To me, something doesn't jive with that situation. Maybe HR can chime in on this one, he knows way more than I do on this subject.

 

[Jims86]

Easiest way to accellerate the diagnosis process if there is a problem, is to get an ALDL cable and datalog a long drive, and see what the ECM is seeing.

 

[junkyardog05]

Do you know what gear ratios are in the 700R4 itself? Some transmissions are geared higher than others. 3.73 gears with 31x10.50's shouldnt be a problem though.

What is your cam size also that will make a world of difference.

 

[Jims86]

Do you know what gear ratios are in the 700R4 itself? Some transmissions are geared higher than others. 3.73 gears with 31x10.50's shouldnt be a problem though.

What is your cam size also that will work a world of difference.

Thats perfect for that tire size. I run 31s with 3.42s myself....almost too much leg.

 

[junkyardog05]

Thats perfect for that tire size. I run 31s with 3.42s myself....almost too much leg.

I have 32x11.50R15 with 3.73's in my 2000 Silverado and it does fine. I'm positive that it isn't the gearing.

 

[IROCmenace]

Overdrive is a .70 gear (0.696:1). Not intended for acceleration. Especially in a 200HP motor with swirl port heads that falls on it's face around 4500 with a small cam. Assuming you used the L05 K motor. Not knocking the motors, I love TBI, but it is a corporate, fuel economy motor to be used in wide production. Not a performance application.

I don't understand what you mean by the converter locking in 3rd unless you wired it to do that. The circuit is programmed to lock in 4th. I can put the 700R4 in my IROC in 3rd (D) and the converter does not lockup like it does in 4th.

I would honestly think the issue would be that you have it wired to lock always while in 4th.

When driving down the interstate the converter unlocks with a small % of pedal to gain RPM closer to the torque range. Then if you stab it, it will drop to 3rd depending on your gearing/governor etc.

My suggestion to the issue is to wire back to the ECM controlling lockup to see if you feel a difference. Or if you are still not happy try an aftermarket controller where you can dial in 4th gear lockup.

 

[Jims86]

Overdrive is a .70 gear (0.696:1). Not intended for acceleration. Especially in a 200HP motor with swirl port heads that falls on it's face around 4500 with a small cam. Assuming you used the L05 K motor. Not knocking the motors, I love TBI, but it is a corporate, fuel economy motor to be used in wide production. Not a performance application.

I don't understand what you mean by the converter locking in 3rd unless you wired it to do that. The circuit is programmed to lock in 4th. I can put the 700R4 in my IROC in 3rd (D) and the converter does not lockup like it does in 4th.

I would honestly think the issue would be that you have it wired to lock always while in 4th.

When driving down the interstate the converter unlocks with a small % of pedal to gain RPM closer to the torque range. Then if you stab it, it will drop to 3rd depending on your gearing/governor etc.

My suggestion to the issue is to wire back to the ECM controlling lockup to see if you feel a difference. Or if you are still not happy try an aftermarket controller where you can dial in 4th gear lockup.

All 700r4s in trucks from 87 and up locked up in 3rd from the factory.

 

[Jims86]

Overdrive is a .70 gear (0.696:1). Not intended for acceleration. Especially in a 200HP motor with swirl port heads that falls on it's face around 4500 with a small cam. Assuming you used the L05 K motor. Not knocking the motors, I love TBI, but it is a corporate, fuel economy motor to be used in wide production. Not a performance application.

I don't understand what you mean by the converter locking in 3rd unless you wired it to do that. The circuit is programmed to lock in 4th. I can put the 700R4 in my IROC in 3rd (D) and the converter does not lockup like it does in 4th.

I would honestly think the issue would be that you have it wired to lock always while in 4th.

When driving down the interstate the converter unlocks with a small % of pedal to gain RPM closer to the torque range. Then if you stab it, it will drop to 3rd depending on your gearing/governor etc.

My suggestion to the issue is to wire back to the ECM controlling lockup to see if you feel a difference. Or if you are still not happy try an aftermarket controller where you can dial in 4th gear lockup.

All 700r4s in trucks 87 and up locked up in 3rd from the factory.

 

[IROCmenace]

Gotcha, that would be why the ECM isn't commanding it in the IROC. My dad's 87 hasn't ran in so long I can't even remember driving it.

Did they set it up to lock in third for hauling? Since your are not supposed to in overdrive.

I still think his issue would be the constant lockup in 4th. What do you think Jim?