Questions about a mild build for a 350 in a Jeep

Seems like a decent rpm range.

What would the objective behind going up or down a little bit of rpm at cruise speed?

 

I'm an engineer. Always trying to optimize the equation. That and 2700 RPM does seem to be a little higher than I'm used to.

But if it's "in the right ballpark", I'm good with it.

 

More specifically, are you trying to influence mileage or something?

High rpm compared to what? Motorcycles and sleds cruise at a fairly high rpm.

 

I'd like to see the engine in it's powerband so that it doesn't have to downshift all the time when driving around the mile high foothills (primary concern). But I also want to make sure I'm not spinning the engine faster than it really needs to spin because that hurts mileage (secondary concern). I'm just trying to nail down the "best freeway cruising RPM" target for the engine. Then I can decide whether I need/want to pick different size tires and/or swap out the axle gears.

 

As you probably understand...
Fuel consumption doesn't graph like a 2 dimensional shape.
It isn't "rpms" and "cubic inches" on either side of a rectangle to show a net result in consumption.
Adding the readings from a manifold vacuum gauge now makes the rectangle a cube.
The higher the vacuum, the thinner the slice.
It math's out to have a great influence on the mass flow through the engine.
A few inches higher vacuum has a greater influence than a couple hundred rpm's.
If the taller (or heavier) tires cause the vac gauge to show reduced vacuum you can be confident that there is more consumption.
There are many circumstances where a little bit of an increase in rpm will result in better mileage.
Being that you are an engineer, the pathway through these calculations should fascinate you.

Are you asking about mileage loaded or empty?
Either could be influenced greatly with your rpm concerns.

 

I'm familiar with the ideas behind displacement/speed/load/vacuum effects on fuel economy and how they're shaped by the design of the engine from the carburetor, through the manifold, into the heads (via the cam action on the valves) with the shape of the combustion chamber as well as both static and dynamic compression ratio, not to mention ignition timing and even the design of the exhaust. At least at a basic level. And each engine design will have an optimum range where the brake specific fuel consumption is lowest.

And it is fascinating stuff. I just rarely have time to learn as much about it as I want. Which is why I'm here asking questions.

Curb weight of the vehicle in stock form is about 3,800 pounds. I don't know if that's dry weight or not. Add the driver and maybe a tank of gas and it's stepping to or over a full 2 tons. I'm modifying a couple of things like putting on bigger tires (with rotating mass not being exactly equivalent to static load), and some accessories. I figure 4,200 pounds in running trim is a fair estimate. Load it up with camping gear, basic off road recovery gear, tools, and spare parts and I'll be pushing 4,500 pounds or more for my trips up into the mountains. I don't own a trailer now but it's a possibility.

I bet 90% or more of my driving will be unloaded around town so that's probably my target. I will be hitting the freeways in town and in places they have a good healthy grade.

And that's more where I want to focus. I'd like to be able to hit those grades in town (when the traffic is lighter) and not have to downshift very often to hold speed. Sure, as I add weight, the point at which I need to downshift will hit sooner and I can live with that. Certainly, heading up the steep grades into the mountains will have me running in 2nd gear for a lot of the distance (if not 1st in some cases). And that's expected.

But being able to hold speed, mostly unloaded, at altitude (mile high), on moderate grades, in town, without having to downshift out of 3rd very often is where I'd like to be. If it makes sense to run a little higher than that threshold, that's fine. But I don't want to get too far beyond that threshold because it starts negatively affecting mileage.

And while I'll never expect to get more than teens for mileage in what amounts to a brick on wheels with the aerodynamics of a barn, I'd still like to do what little I can to make it "not as bad" rather than "close to as good as I can get", given the above criteria for power to pull hills with as little downshifting as is practical.

 

I suspected this.

I wouldn't assume you are close to that point.
This is largely determined by the induction strategy, you can alter that greatly if needed.
(Unfortunately, I don't think you are going to get the answers you are looking for from a forum.
I don't think too many know where that threshold is, or would be willing to share.
Mileage seems to be a hot spot on forums.) Sorry if I offended anyone uzzled:.

You are a long ways away from rpm itself negatively affecting mileage.
As you mentioned, aero is bad with this combo. At freeway speeds, drag will be a much greater loss than any slight difference in rpm's.
If you can get it to see higher mpg's loaded, you should improve around town.
Cruise isn't so hard to tune for mileage as much as acceleration time.
Disproportionate amounts of torque really help combo's like this.
Piston speed can be your friend.

 

Thanks for all the info.

 

Hi there,

I have had a bunch of Buick 350 combos in heavy cars, and got mileage ranging from 14 MPG to 21 MPG... I have also had lots of chevy trucks with various combos of gearing, tire size etc.

If this were my vehicle here is what I would do:

Build the short block doing only as much work as needed to get it back within factory spec, but spend a bit of money on a forged piston with a custom pin height to get a zero deck without cutting the deck of the block. Autotech or diamond make these pistons for us.

Wait just a little bit while TA performance finishes up and releases the alum 350 heads, I bet that out of the box they will flow better than most ported iron versions.

Use a dual plane TA intake, with a TB EFI system that can adjust for the elevation changes when your driving in the mountains. TB systems are no better than carbs in many ways but they are simple, low cost, and self tuning... The ability for them to correct to elevation changes is hug for you.

For the trans either keep that good old reliable th400 or if it needs work look into the 4L80E trans at least is shares the same output shaft spline count un like other trans options.... If you go that route look into leaving the t case in position and moving the engine forward slightly to take into account for the longer trans.... It is a huge pain in the but to go through moving the t case around and getting both new driveshafts etc. If you can live with a manual shifted trans there is a manual valvebody for the 80E trans and you also get the lockup torque converter. or they make really nice trans controllers for them too.

I am doing some testing in my daily driver 2005 chevy lifted 4x4 truck to see the fuel mileage difference between locked up torque converter and un-locked as a have it manually toggle switch operated... I can say for sure there is an improvement of between 1 and 2.5 MPG when the lockup is engaged. This may or may not matter to you, but it is interesting.

The way I see it the only way I would re-gear the axles is if you are adding huge tires, or adding an overdrive trans... Adding an overdrive without going to a 4.10 gear or something you will find the overdrive gear not of much use I would think.

My current truck with a hopped up 5.3 and an overdrive trans with 3.42 gear and 33" tall tires the overdrive gear does not really help my fuel mileage much because the RPM are too low on the highway unless you are goign super fast... I see not much difference in fuel mileage at 60 MPH in either 3rd or 4th gear... Over 70 MPH the 4th gear is better and over 80 MPH it really shines... Just food for thought, I know I would enjoy the truck more with a 4.11 gear.

Based on the intended use of the vehicle I like the reliability of the th400 and I like the 2700 RPM at 70 as it is now is not too bad and can help you get up the hills without gearing down as often as it would with less gear. There is a lot to be said for leaving the driveline alone, it would save thousands and thousands of dollars that I would rather put into the engine to make it both more powerful and more fuel efficient.

 

:gp:

I'm all for aftermarket support and the improvements they offer!
Would there be any guess as to the TA's port flow characteristics regarding swirl or the chamber's ability to enhance the mixture motion?
The stock Buick head does a really nice job of this, maybe perfect for the application with a bit of bowl porting.
I'd hate to see raw flow traded for quality flow.

 

The TH400 has a Dana 20 transfer case behind it and I'm swapping in an NV242 from a newer Jeep. So I have to swapping I the tail shaft no matter what. If I go with an overdrive, it'll be a 700r4 because it doesn't require any electronics and it has more aftermarket support.

I think the 3.31s will be "close enough" for the TH400 with the way I'll be driving it. And that saves me from having to drop cash on replacing the gears.

 

Extra cubes would come in handy for you application its sounding like the more I read what you want to achieve. 20 to 30 more cubes won't kill your fuel consumption, I think it would be just what you're looking for to increase the low end torque using the SBC-5 for that just a bit extra that Gary explained.

If you're planning on using a good piston like the AutoTec forged ones anyway might as well get a set of the nascar take out rods so you can have the crank stroked to force more low end torque out of that beast. o No:

The take out rods are usually less expensive than having stock rods reconned with new bolts and if the crank needs to be reground anyway then its only around $150 more to have the crank offset ground while its there. We're not talking breaking the bank, and if you cam the engine for low end torque it will make more with more cubes! GL






Derek

 

We don't know yet how the new heads will perform, and I agree that well ported iron heads or even mild ported iron heads would be fine for this application. Once we get the new heads in hand we can get the skinny on how much they help the performance. I am doing dyno testing to see what the real world deal is and also having a pro port the alum heads to again see the gain there... My prediction is that the alum heads will give a huge gain but thats a guess.

I still agree with the other comments that say a turbo setup properly sized is the best way to gain massive amounts of torque, ride along and see the pretty much stock 350 that shreds two 20" wide slicks:

https://www.youtube.com/watch?v=qmdHBrNNOpQ

 

I know they aren't tested yet.
Curious on what was talked about, planned or intended.
To what demographics are they intended for?
CFM #'s off a flow bench are near meaningless to my query.
I suspect the castings will have plenty on meat for the porter to work some magic, as always.
Keep up the good work, the market need these.

20 to 30 more cubes will likely help mileage.

 

The heads are intended to be street/strip heads, better than stock out of the box and lots of room to grow. They are a closed chamber design so we need our pistons at zero deck to get the quench. So they are working hard to create a better head for us... Fully ported our iron heads work well but it's a lot of $ to put into an old iron head that's prone to cracking.... Like you said there will be more meat for porting. Scott brown is offering a CNC porting option for $1100 and I think that's a lot of bang for the buck but we will see...

Sorry to sidetrack the thread... Back to cool old jeeps with turbo 350 engines lol!!!

 

No worries on the hijacking. I am open to using the new heads so knowing more about them is a good thing.