Going back to the street.
Well, I have had a change of heart. I have posted several times about how to make the best power and save the most time on my old 1983 Silverado but, alas, the drag strip has been closed down permanently and I don’t want all of the work I’ve done to my old truck to go to waste. I have decided to go back to driving it on the street more often and doing a little body work on it to spiff it up a little. My question is do you guys think I could make a few minor changes to perhaps get a little more gas mileage out of this truck just for cruising without sacrificing much power at WOT and too much time in the ¼?
My engine is still plenty mild enough to be driven on the street fairly easy. Here is my current set up: standard bore 350 with dish pistons, RHS Pro Action 180cc heads 2.02/1.60 valves 64cc combustion chambers. Lunati Bracket Master II 230/230 292/292 480” lift cam, Comp Cams Ultra Gold 1.5 roller rocker arms. Edelbrock air gap intake with an Edelbrock 750 carb. Headman 1 5/8 headers. For the drive train I have a B&M Hole Shot 3000 torque converter, TCI Super Street Fighter TH350 transmission and 4.10 posi rear end. I am running 315/65/15 Mickey Thompson ET Street Radials right now, but I am going to get a shorter tire for the street.
This truck gets terrible gas mileage. Do you have any suggestions on what I could do to maybe save a little gas without hurting too much of its potential. I still plan to pull it to the drag strip some, but I live at least a 2 hour drive from one now so, I can’t go much.
Thanks for your help in advance.
The obvious answer is an OD trans like the 4L60/700R4.
Put a Gearvendors overdrive behind your current tranny. It will hold up to the abuse and give you better fuel economy.That is what I put in my coupe.Not finished yet so I can't give you any numbers,but according to their chart it will make my 4.11 rear the equivalent to 3.20.
I've never tested it 100% accurate but, I'm pretty sure this truck gets around 5 miles to the gallon (maybe 7 if I keep my foot out of it completely). Does that sound right? That seems worse than I would expect. Could it be the carb is too big, or maybe just running rich?
That's about what my Firebird got with the old carb on the 454 BBC. I rebuild a different carb of the same model, dialed in the timing, and spent hours tuning it and I now get 11 MPG's average.. And that's with a very healthy 454 pumping out probably just above 500 HP/TQ. So to me your small block only getting 5 MPG's doesn't seem right. And I am also using a 750 CFM carb, but its a Holley with vacuum secondaries.
You can cut the effect of the 4.10 axle by going to larger tires. A 33" tire should fit on your truck without any mods. Something like a 285/70/17 or 285/75/16 would work. A very standard truck size is 235/85/16, but its a little shorter (32"). The cheapest choice is usually to use 16" wheels, since they were offered stock on many trucks, but the stock GM wheels are only 6" wide which is is really too narrow for anything more than about 255 or 265 width tires.
You could swap an Edelbrock 1406 600 cfm carb and maybe pick up a little mileage. You would need to tune it for your cam, since the stock jetting is probably too lean. The stock step-up springs will also create an incredible bog with a high lift cam because it will transition from cruise to power mode way too late.
I have a very mild 350/TH350 in my '75 GMC truck, and I can't get much more than about 10 mpg if I drive it on the highway at about 55-60. If I kick the speed up to 70, my 4.10 gears drop the gas mileage down even further. These trucks are built like a brick, and its hard to overcome the drag. I'm running stock height 31" tires, but I haven't wanted to spend money on trying any other combination.
The swap to a TH700R4 is fairly easy and would help highway mileage, but you will have to spend money on building a good one if you plan to still race it.
I don`t like Edelbrock carbs, not a big fan of Holley`s either. Most holley`s come with straight leg boosters, these aren`t as bad as down leg boosters however, as dog legs are completely worthless.
A annular booster carb is perferred. It gives it better throttle response and real nice manners. However, annular booster carbs are more expensive.
I perfer a Quadrajet carb and they are pretty much all I use.
Not to say a holley won`t work for you as it will. So if you want to go holley by all means do so. The only cheap carb with annular boosters I know of is the summit racing carb, however you have to get the carb that`s a mechanical secondary which would work fine for what your doing.
How to improve mileage, first off, with a 3000 stall and the low gear good mileage isn`t to be expected, however you should be getting better than 5 miles to the gallon. What`s the timing set at? What plug wires are in use?
First off, if you haven`t already, get the process started by packing the front wheel bearings with synthetic grease. Then fill the rear with royal purple or other synthetic lube if you haven`t already. If your not running synthetic oil swap over to it. Lowering resistance is the first step in the right direction. To get decent mileage you have to keep the revs down which is difficult with your rear gears. You could somewhat defang the engine by going with a smaller cam such as the comp 268H which will still give it a lopey idle, but it`s duration figures are on the mild side. With the heads and combo you have it would still have good power. Then you could switch to a lower stall converter.
the carb I have now is the Edelbrock 1407. I have the calibration kit for it. Do you think I could calibrate it to get better MPG?
I thinkthe best thing would be to install an Ait Fuel gage (AFR gage) These run a couple hundred bucks. Innovative motorsports is very popular and they have an active forum. This will help you with the tuning. I get 15-16 mpg with the blower so you should be able to get the same or better....this is on 87 gas too. Also install a vacuum gage. These are a great load sensor....low vac means low mpg.
Next you will have to ditch the gears. Hoosier makes a Pro street tire in the 31 inch range. I have these and they have 18k miles on them now. They work great in the rain...even in the light snow last week. With a good suspension they will hook quite well too. These run about $700 a pair and work tubless. I'd gear for about 2500 rpm at 60 mph +/- a little what ever it takes to get this.
An OD trans is shaky at best....JMHO I cruise with over 150 guys and there are a lot of OD's in the group. There are a lot of them in the shop for repairs too. You will spend $1500-1800 for anything that will hold up and even that is shaky at best. If you had factory EFI the story might be better. Bottom line is any OD is going to be relatively expensive. I like my T-350 without a kickdown. I just manually drop it down a gear if I really want to charge on the highway otherwise I just step on it.
[QUOTE=bentwings]I thinkthe best thing would be to install an Ait Fuel gage (AFR gage) These run a couple hundred bucks. Innovative motorsports is very popular and they have an active forum. This will help you with the tuning. I get 15-16 mpg with the blower so you should be able to get the same or better....this is on 87 gas too. Also install a vacuum gage. These are a great load sensor....low vac means low mpg.
I'm sorry it has taken me so long to reply. I have been looking at some air/fuel ratio gauges. That sounds like that would be great for tuning. It would definitely help for calibrating the carb wouldn't it? I would think it would be pretty easy to do just basic tuning with one wouldn't it? Is the gauge and the sending unit all I would need?
In order to make the AFR work or better yet to use it properly, you must have a very good understanding of how the carb works. You need to know how the idle circuit, transition power and cruise circuits affect the AFR.
Best bet is to get the Holley book with the cut away view of a 4500 on the cover. I don't have mine handy but just check Amazon.
Above all don't get "drill happy" before you understand what you are doing.
The AFR gage simply tells you what is going on. It can be misleading if you don't understand the carb. It's not hard learning you just have to put your mind to it.
IMO, the things that are killin your MPG are:
* the 230 deg cam
* the tq converter
* the 315's wide tires on the rear
* the 750 carb
I would start with the cam. Get something with around 220* at .050, and 112 LS. This may wake up the motor because you will have a compression increase. When you do the cam, also install a 2 piece timing cover (much easier to swap out the cam later on)
Next I would replace that TQ converter to match your new cam. Get something with a low stall. As much as I like that Gear Vendors Unit, I could never pay that much. Maybe one day I'll find a cheap one on ebay.
You will need at least a 31" to 33" rear tire to offset the 4.11's. Just keep it around 11" wide
I would sell that 750 carb and get a rebuilt 625-650 cfm carb. The smaller carb will "work" better with the air velocity increase & the new cam. You can find great used carb deals all day long on line. The good news is that your 750 carb has a good resale factor, so you might make out good on that
The o2 meter kit is a good idea. If you cant afford to get one now, at least order an O2 bung & cap from Summit when you get the cam - like 7 bucks
With the cam & torque converter change, you are looking at around $450.
You can also find a good used vac advance dist on-line (vac advance = MPG)
Not counting the tires, the changes above should give you about 12-13 MPG
the edelbrock #1407 750 only requires a pri metering rod and or pri jet swap to improve the fuel mileage.
Its factory calibrated a bit rich for power.
lean it down on the primary cruise. The secondary jetting is fine and does not effect mileage.
Its not too big and its very tunable.
You don;t need a new carb. Just tune the 750.
A AFR gauge will help you dial in the jetting.
A cheaper narrow band type (with heated 02sensor) is good enough for this.
Its a real eye opener.
The 230° cam has to go if you want mileage.
The duration and LSA(overlap) is too high for mileage.
A cam with 208 to 214° @.050" intake duration and a wider 112+ LSA will help a lot. A single or a dual pattern cam is fine.
it will still go like hell.
I've used the 214-224 .442 .465 112 cam with good results.
* summit, crane blue racer etc*
Recurve the distributer mechanical and vacuum advance curves.
They both need optimizing.
The 230 cam will need lots of initial base timing at idle 24-26deg at idle
vacuum advance also needs to be dialed in.
The stock-default distributor curve will be all wrong.
The jetting and distributor curves make a huge difference.
The high stall converter is not as bad as you think as long as you can keep your foot out of it.
If you are heavy on the foot you are going to burn gas.
How you drive has more to do with it.
I used a 214-224 cam and a 3500stall converter and 4.10's and it was reasonably good on gas. Again, if you rug it a lot, you are going to use fuel.
get the jetting and timing curve sorted out and see where you are at.
then find a milder cam.
This is what your 230° cam needs:
start with 24-26deg initial 10-12deg mechanical (34-36 total advance)
shorten limit the mechanical advance travel limit stop.
+ 10-12deg vacuum advance that comes in at hiway cruise.
Ported or full manifold vacuum source no rules, don't care, use what works.
swap the primary jet to a .110" pri metering rod .075"x.047"
4" power piston springs yellow
This will lean down the part throttle cruise and insure the power piston stays down-lean at idle.
This and the ignition recurve will really clean up the idle and low speed and mileage. Get new plugs with the correct heat range.
A smaller voodoo cam will help #60100 or 60101
Lunati #301A2LUN or 301A4LUN are other good choices.
Anything mild like this is good.
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