I'm rebuilding a 390 truck engine. It will be used for some heavy towing.
Will the higher compression pistons cause any problems or should I stick
with the lower compression pistons because of the heavy loads??
Thanks, Duane.

 

I would stick with the lower compression, and run a cam to work with it. I would recommend the Crane 343971 for towing. You really want to avoid pinging when towing, thus use the 1.680" compression height stock 390 pistons.

Towing generates enough heat, you dont need to add to it through higher compression ratios. Leave that to the hi-po crowd.

 

I agree with the lower compression. 8.5:1 - 9:1 would be good. I would run a little bigger cam though. The crane 343901 would give you a little more power up top, about the same at the bottom and your mileage probably won't suffer.

 

I agree with the guys on the lower compression, I think you said occational towing? so I think 9 to 1 comp. and Like rat said Crane 901 cam will help it breath a little better for top end! and of course a nice aluminum intake and a set of headers will help it breath even more...

It all depends on your wallet..and how much torque for towing you want? and what your going to tow?

Keep ua posted and Good Luck, Russ

 

A camshaft change will alter things a little, though. I went from a stock cam to a Comp DEH255, with no other changes, and the octane requirement fell from 91 to 87.
Eric

 

It probably droped your compression, that the reason for the lower octane gas?

 

Well the 901 would probably work just fine. Either way I dont think you would go wrong. However, here's my reasoning for the 971:

1. It's a tow vehicle. You want as much low end torque to get things moving as possible.

2. On DD2k, with a stock 390 with stock heads and 8.5:1 compression, the 901 peaks on torque at about 2000rpm, which is just fine. The 971 makes 11ft-lbs more at 2000, but it peaks at some lower rpm which DD2k doesn't calculate. I would suspect it's much more torque at that rpm.

3. The 971 closes the intake valve sooner, which allows the engine to build more compression. This results in better fuel economy and torque with a lower static compression ratio.

I have no doubt that the 901 would allow it to make more top end horsepower. However, it's all about priorities. Dburgess will have to make his decision on what he wants it to do. Top end horsepower, or torqe way down low to get things moving. I really dont think he would go wrong either way, though.

 

Trying to get things moving. I need a lot of bottom end torque and horsepower.
Thanks for the info. Duane.

 

I do alot of VERY heavy towing with my 70 F250 4x4. Running 4.10's 35" tires and an NV4500 5 speed and a 390. The 390's got 9.5:1 compression, 901 cam, D2 heads, performer intake, 600 carb and DUI dizzy. I get 15 mpg cruising at 70 and while the mileage falls off to 10 with a 15,000 lb trailer over the snoqualmie pass in WA(steepest longest grade I've seen in NW), I'm still very impressed by what the little 390 will do. The engine does not ping whatsoever, although I fill the tank with premium when I plan on a heavy tow. I would not say the 901 is made for top end at all. It makes no power beyond 4500. Plenty of bottom end. Torque peak around 2500.

 

I just looked at the 971 again. I thought it was a small single pattern cam. It of course is not. It looks like a stock cam with about 10 degrees added. I think either the 971 or 901 would be an excellent choice.