Fast355

Your truck is not 18,000 lbs either like that workhorse. It downshifts to 3rd on long grades because it has to if you want to keep speed up.

My 8.1 swap is getting a Marine HO cam and they are rated 425 HP @ 5,000 rpm. Same rotating assembly as a truck. At 4,000 the 8.1 is not even starting to sweat.

Fast355

One thing we are overlooking too is the OP said he has larger than stock diameter tires as well. That means his 2.80 gear ratio in OD just became even taller. If its 2" over stock that could effectively bring his overall gear ratio into the ~2.5:1 range in OD compared to stock.

Vindii

Your 350 makes its max torque at a MUCH higher RPM than a 8.1 does. Again its not even a similar comparison. Your not going to tow with your 350 at 6k are you?

My truck has 33" tires on it too. You can talk ratios and theory all you want but in the end you dont own or have experience towing with a 8.1 why offer advice about it?

Stock redline is at 4800 so cruising loaded at 4K is likely not a good plan.

Maybe my truck is an oddball. But is seems to tow well with larger tires and 3:73 gears. Must have got lucky!

Fast355

Actually the 350s powerband matches the 8.1s quite well. Peak torque is in the 3,000 rpm range with peak HP in the 4,600-4,800 rpm range.

I guess having tuned an 8.1 motorhome while riding along in it means I know nothing about one. I also happen to own an 8.1, in the process of acquiring psrts for the swap, and have done the research to know what it is capable of. Throttle cut/Fuel cut is in the 5,000 rpm range. It will run 4,000 rpm all day comfortably. When a 496 boat is on plane it cruises between 3,500 and 5,000 rpm for hours with the same engine.

Vindii

You can compare you 5.7 to the 8.1 all day long. If you really think they are the same motor that's good for you. Its a silly comparison if you ask me. But its the internet so everyone an expert.

Telling a guy he should run around in 3rd gear on this motor trans combo is silly.

Fast355

If you read carefully I said they have roughly the same power band/torque curve and that the 350 is making close to the same amount of torque as a STOCK 8.1. Tune the 8.1, put headers, a decent exhaust and intake on it and things change.

3rd gear is not silly when load and gearing require it. It is EASY to tell if it makes a difference. Get one of those scanner apps, look at instant fuel economy in 3rd vs 4th for the same conditions. The scanner will also tell you other valuable information such as knock retard, fuel trims, and other sensor readings.

1BadAction

iTrader: (10)

I guess GM are a bunch of ******* idiots then, because the owners manual for various 8.1 powered trucks say exactly what fast355 is trying to beat into your know it all thick skull. "If you're towing a heavy load or hills and its hunting in and out of OD, then shift to 3rd"



3rd gear locks the converter for a reason, and that's so you can tow in 3rd for long distances without a problem.

CaptCameltoe

iTrader: (3)

Vindii

Listen *******. He said it will get better gas mileage in 3rd than in 4th. It wont.

It will shift down at the bottom of the hill and shift back at the top. Trans temp never get too high. Nothing wrong with letting is downshift.

Yes a 5.7 may help to tow in 3rd as it will be shifting all the time. Not the same as this motor.

Atomic

iTrader: (42)

Keep it civil you two or you both get infractions.