Synchronizing thermal efficiency and the Torque Curve.

[sfair74]

Hi I am looking to rebuild a 1996 ford 300 six. The engine comes from an F-150 and has EEC-V with MAF. I have the full donor truck and intend to build and tune the engine in the donor before swapping to a 1997 F-250 HD that currently has a 351 Windsor EEC-IV and SD. A switch preswap to Mega Squirt may be in the cards. I am a computer geek by trade so building and programming a Mega Squirt sound like fun to me.

I have done some research on thermal efficiency. The basic idea is that if your piston is moving too slow a lot of heat is lost to the cylinder head, walls, and piston. Alternatively if your mean piston speed is too fast the friction losses greatly increase.

Based on research the optimum thermal efficiency should be between 16.4 and 19.8 FPS Mean Piston Speed. This calculated out to 1484 to 1791 RPM. With 3.55 gears an E4OD and stock 235/85/16 tires this should be excellent at desired cruse speeds.

I understand that the desired mean piston speed is a generic number and there are a lot of factors but I would like to aim for the 1500 to 1800 window for peak thermal efficiency. Since the 300 is an almost square engine this should be a fairly accurate range.

The old single carb engines reached peak torque at around 1600 RPM. The EFI engine has peak torque listed at 2000 RPM. I understand intake runner length is a large factor in peak torque due to the ramming effect. My goal is to try to move peak torque into the same range as peak thermal efficiency.

I have done the math and 3.55 gears E4OD with stock tires should work great for keeping the 300 in this optimum thermal efficiency. Since Mean Piston Speed is a function of stroke I can’t really change that variable.

My overall goal is to place ideal thermal efficiency and ideal volumetric efficiency at cruising RPMs. The goal is mainly drive ability and maybe a little MPG to boot.

My question is would it be worth it to move peak torque to the same range as peak thermal efficiency? Since with the current gearing peak thermal efficiency will fall in the ideal cruising MPH range to me it makes sense to adjust peak torque to that range.

Listed below are the speeds corresponding to gears and keeping within the 16.4 to 19.8 FPS:
Descriptor RPM
Minimum 1484
Average 1637.5
Maximum 1791

Descriptor / Gear MPH
Ideal min 1st 14.56
ideal avg 1st 16.07
ideal max 1st 17.57

ideal min 2nd 25.62
ideal avg 2nd 28.27
ideal max 2nd 30.92

ideal min 3rd 39.46
ideal avg 3rd 43.54
ideal max 3rd 47.62

ideal min 4th 55.42
ideal avg 4th 61.15
ideal max 4th 66.88


Would it be desierable to move peak torque to around 1650 RPM?

Is there an EFI friendly CAM that would work to drop the torque peak?

If going to a Mega Squirt does that allow more cam options that are currently labeled as carb only?

Thanks,
Shane

 

[Harte3]

viewforum.php?f=5 That is the Hard-Core Tech Forum...try posting your Topic there too.

 

[sfair74]

Thanks Harte3,

IF a mod stumbles on to this post could you please move this post to the Hard Core Tech forum?

Thanks in advance.

 

[80broncoman]

Thanks Harte3,

IF a mod stumbles on to this post could you please move this post to the Hard Core Tech forum?

Thanks in advance.

Done.

 

[StrangeRanger]

Couple of thoughts:
1) Unless you have run it on a dyno, do not assume the torque peak is at 2000 RPM. Advertised HP and torque numbers are largely merde. Even if the torque peak is 2000, the difference between 1600 and 2000 may be 1 or 2 foot-lbs. the 300 doesn't have a torque curve, it has a torque plateau.

2) The torque curve is a function of the variations in volumetric efficiency over the RPM range. If you believe Ricardo's findings apply to the 300, and I'm pretty sure they do, the VE as limited by the velocity through the intake valves/ports is maximized at about 2900 RPM and is in the efficient range from approx 825-5000, in other words, the entire working range of the engine from less than idle to more revs than you probably want to use.

3) To more the torque curve down by 400 RPM is possible probably by simply advancing the cam by 4° but you really need to do before and after dyno runs.

 

[xctasy]

Good stuff.


As mch as I admire your work, the issue is that the working range of a 300 engine is based on its road load, and the whole set up interplays with that.

EFI raises the torque peak rpm because its not sucking through one 1.6875" hole at a gall point in the intake manifold, its sucking in air via six 1.5" ports to two 1.85" throttle bodies which is very efficient. So the disparity between EFI and 1-BBL carb is efficiency related.

You can raise and lower the torque curve to suit via cam retard, but an EFI makes more off idle torque everywhere compared to a 1-bbl six, so 1600 vs 2000 rpm is just a peak toque figure. Plenty of people have seen variable valve timing, dual tract intake 24 valve twin cam 261 hp 243 cubic inch Falcon Barra engines with 283 lb-ft peak torque at 3200 rpm, that doesn't mean it has less torque at 1600 rpm than the old OHV 250 engine.

Every 80's and 90's I6 or V6 gains torque peak rpm when converted from 1 or 2-bbl carb to port EFI, it means nothing. A carb was and remains just a dopey leaker which gets tuned via Dearborns smartest engineers, but even so, its still like trying to link six malteds to one hungrey kid via six straws to one mouth. EFI is just a more efficent porting arrangment than a carb, and the peak torque rpm point goes up.

Or better still, its like trying to blow up six 20 foot containers with an ideal 16.2:1 air to fuel ratio for lean burn maximum economy, or 14.7:1 for cleanest burn, or 12.5:1 for best power. Each cylinder event starts from the centre and how the heck due you get the same boomb from each container if the outer ones are further from the inners from one fuel source in the centre. EFI raises power and torque, the abiliy to do work is increased, and a low peak toque rpm is a failure not an advancement.

 

[80broncoman]

Couple of thoughts:
1) Unless you have run it on a dyno, do not assume the torque peak is at 2000 RPM. Advertised HP and torque numbers are largely merde. Even if the torque peak is 2000, the difference between 1600 and 2000 may be 1 or 2 foot-lbs. the 300 doesn't have a torque curve, it has a torque plateau.
.......
3) To more the torque curve down by 400 RPM is possible probably by simply advancing the cam by 4° but you really need to do before and after dyno runs.

VERY good point on the "Plateau"
I've seen more than a few youg guys driveing Stock F150 4X4 in stock class mud pit races. All most allways THey complain that the 300EFI feels like it makes "no power at any RPM" I usually tell them that 300 makes torque over a quite broad range and that is just the way they are, and that that makes the 300 better suited to using one gear than say a 302 (like most of them tell me they want to install).

 

[Firepower354]

Would thermal barrier coatings, reducing the rate of heat loss, then alter the equation?

With varied runner lengths, the carb manifold likely isn't optimized at any one RPM, widening the "plateau" but compromising total figures all along the plane.

With the goal of maximum torque at a specific RPM, I'd think that a set of runners working on 2nd or 3rd order would be ideal, maybe?

 

[80broncoman]

Would thermal barrier coatings, reducing the rate of heat loss, then alter the equation?

With varied runner lengths, the carb manifold likely isn't optimized at any one RPM, widening the "plateau" but compromising total figures all along the plane.

With the goal of maximum torque at a specific RPM, I'd think that a set of runners working on 2nd or 3rd order would be ideal, maybe?

on your first question I'm not sure. I've never seen where they hurt anything.

on your second I think on the stock 1 bbl intake It looks like it has one runner length and coming off one log.

on your third Q I admit i have no idea.

 

[Firepower354]

Q1, it seems that if piston speed/heat loss matrix was altered, similar or greater efficiency at lower revs would occur.
I didn't look hard or long (heehee), but found a diesel application: http://www.google.com/url?sa=t&rct=j&q= ... 4380,d.eWU