chevy milage best option, transmission, heads or rear end
Just a quick couple of questions relating to milage on a chevy.
Car is 68-69 327 chev, 3608 pound car, 350 turbo trans, very mild cam, Barry Grant Demon Jnr Carb 625, Edlebrook Performer RPM manifold, crappy 882 heads (75cc? low compression?) running either 95 or 98 octane, HEI, recently reconditioned, running full manifold vacume advance.
I have been trying to tune it myself for the last few months, and recently took it to a dyno to see if I was in the ballpark. They were happy with the carb tuning, and couldn't improve the timing curve, so I though I did ok. The motor has loads of torque at low revs, about 850nm, but it drops off rapidly so its only about 170 rwhp.
The end result is that there is no further room for improving the MPG. At present it is achieving 14 MPG, and it doesnt seem to matter too much whether it is highway or city, always returns about the same MPG.
So, seeing as I can't get any more improvements thru tuning, I am wondering what the best option would be to try to get a few more MPG, would love to get to 18 or 20.
Three options I can think of, wondering which would have the biggest effect on milage.
1. REplacing the th350 trans with a 700r4 (very costly but option 1 still.)
2. Changing rear end. At present has a 3.08, I believe there is a 2.78:1 ratio, I am guessing but woud this drop the revs about 300 rpm? Would the torque at low revs motor still be able to get this thing moving ok?
3. 3rd option, would fitting smaller chamber more modern heads help the MPG by increasing compression ratio etc?
4. Just thought of this, would changing the cam to one that produces torque at a higher rev range have any positive effect on the milage? On the assumption that higher horsepower engine requires less effort to get the car moving or keep it going?
So thats all I can think of. I drive like a nana, not after power or speed, more just a car I can afford to drive!
I would be very grateful for any ideas or options you can think of to try to get another 2-4 (or better) MPG, and any comments or suggestions on the 4 options above would be much apreciated.
Well it sounds like a fairly nice combo for the street. :cool:
Obviously from your statement it really is not what you want though. :confused:
Going to an overdrive trans will only benefit if your normal driving is at freeway speeds. :(
Depending on how you drive, a standard shift trans might help in the MPG area. :)
Going to a higher (lower numerically) rear gear ratio than what you currently have will once again primarily only help at freeway speeds. Around town in stop and go traffic the performance will be so sluggish that your mileage will actually go down. :(
Swapping the heads for higher compression will not help. As it is you are running an octane higher than what your combo should require. Your engine should run fine on 87 octane and that is less expensive than the octane you are presently running of 98. Why are you using that octane of gas with the low compression heads? :(
Swapping the cam to one closer to the stock factory specs might help by 1-2 MPG and will decrease your overall performance. :rolleyes:
Pull the V-8 and swap in a straight six and you should gain a few MPG. ;)
Best bet for you is to sell that car and buy a cheap compact 4 cylinder grocery getter. :D
hi frisco, thanks very much for your reply.
I have implemented plan A, and am now saving myself 5 cents a litre by using 91 octance gas. I have asked several people about this, and everyone, including the dyno chap said it would run like $^% on a lower octane. They were wrong. So excellent suggestion thanks.
I have priced a second hand 700r4, and thats clearly not going to happen. Just over $2k, more if its ackward to fit.
The reason I mentioned the diff is that mine has been on the way out for the last 7 years, so I thought if I have to change it I might go for 2.78:1 diff. Am I right in assuming the revs at 100ks would come down by 300rpm? I understand what you mean about losing some performance but does the fact the engine produces torque down low on the rev range mean it would find it reasonably easy to turn a lower ratio diff?
At present its reving at 2350 rpm at 100ks. I assume the reason it is not 3080 rpms is the different rear wheel size, 17" rather than 14" or 13". I figure if the revs come down to around 2000 that surely has to make a milage difference?
Oh and i do have a 4 cyl jappa for work etc, this is my weekend car, meant for long trips up country, so any savings in gas means I have more to spend on beer when I get there :-)
Thanks again for your help
Actually just found this post below somewhere. My rear tyres are 24" which seems to me to indicate it already has a 2.78 diff instead of the 3.08 I had thought.
Submissions by Jacks:
For what it's worth. Below is a formula to use that might save you $$$$
To get M.P.H. divide K.P.H. by 1.6 example (96KPH=60MPH)
At 60 MPH, x Axle ratio x 336/Tyre Diameter = R.P.M.@60 M.P.H
THEREFORE 60 X 2.18 X 336 / 24 = 1831 R.P.M.
RATIO: Equals tail shaft turns, to one revolution of wheel.
DIAMETER = Measure tyre height from top to ground with tape.
Listed below, examples of R.P.M @ 60 M.P.H (96K.P.H)
Diff Ratio vs Tyre Diameter Diff Ratio
24" 26" 28" 30" 32" 34" 36" 38" 40" 42"
2.18 1831 1690 1570 1465 1373 1293 1221 1157 1099 1046
2.50 2100 1938 1800 1680 1575 1482 1400 1326 1269 1200
2.74 2301 2124 1973 1841 1726 1625 1534 1454 1381 1351
3.08 2587 2388 2218 2070 1940 1826 1725 1634 1552 1478
3.23 2713 2504 2326 2170 2035 1951 1809 1714 1628 1550
3.50 2940 2714 2520 2350 2205 2075 1960 1857 1764 1680
3.73 3133 2892 2686 2507 2349 2212 2089 1979 1880 1790
3.90 3276 3024 2808 2621 2457 2312 2184 2069 1966 1872
4.10 3444 3179 2952 2755 2583 2431 2296 2175 2068 1698
4.56 3830 3536 3283 3064 2873 2704 2554 2419 2298 2189
4.88 4099 3784 3513 3279 3074 2894 2733 2589 2460 2342
Might be something for all to use.
It will give you a guide as to what your Revs will be.
Knowing Diff Ratio & Tyre Size, at a given speed.
Without taking your vehicle for a run & no tachometer required. Hope it helps
You didn't say what cam you have, but it sounds like you might not get much more from a cam change. Something in the 204/214 (or even milder) range would probably be a good choice.
I think the simplest (but not cheapest) way to increase your fuel mileage might be to use one of the simple TBI fuel injection systems like the FAST EZ-EFI product. It will cost you about 2k if you also buy the high pressure fuel pump (which it needs), but the computer control will do a lot to help with your gas mileage.
Fuel injection may not win out over carburetors for horsepower, but it is a lot more predictable for fuel mileage.
Better flowing heads will help, as will a low restriction exhaust system. Using a clutch fan can help a bit. Inflate the tires to the max allowed, coast when able, use brakes sparingly, start off slow, etc. There's a member who mentioned a site where the members trade ideas and such for extending their MPG but I don't recall the name. Google will likely show something.
One of the biggest improvements can be found by decreasing the accelerator pump shot to the bare minimum required. This will be found to be too lean until the engine is up to operating temp., but will net you an increase if you do a lot of stop and go, on and off the throttle driving. This may also require you to use a "custom" reduced-travel pump cam and/or a smaller than normally used shooter.
The gasoline where you are is figured differently than it is here in the US. We use MON (motor octane number) plus RON (research octane number) divided by 2 to get a "anti-knock index" number. But we still refer to it as being the "octane" of the fuel.
You guys use the RON number, which alone is higher than our rating by about 5 numbers, i.e. your 95 octane is our 90 octane, give or take.
The 700R4 would help everywhere. Not only would you get overdrive for the highway but you would get a much lower first gear making the car easier to move around town, especially with those 3.08's. But like you said.. they can be pricey $$
Hi Guys thanks for alll the replys :-)
75gmck25 I would love the idea of that fuel injected system, looked it up on the interweb, definately I grouse bit of kit. I think though if I was going down that route I would prolly look at getting a later model engine, more power, more efficiency etc. Just a paid with the computerand loom etc. However I have it in mind for my next project, for which I will be questing for more cubes. But thanks for the idea anyway, I was not aware those systems existed.
cobalt327 (always good to hear from someone with 327 in their name! All the driving pointers suggested I already try to use, full tyres etc and I drive off a vacume gauge. I don't think I can get much more there.
When you say accelerator pump, is this the powervalve in holly type carbs, or the cam on the throttle plates? I have already lowered my vacume activated powervalve to a point where it doesn't come in too early and waste fuel, but nor is it too late, meaning you have to give it the welly to get anyware.
Haven't considered shooter size yet, will do some reaserch on that and get back to you.
With regard to better flowing heads do you mean have mine ported etc, or get a more modern design with smaller chambers? If I get smaller chambers/higher compression, would I need to use higher octane fuel?
Finally, your information on octance numbers etc are interesting. I have noted on this site some people run their chev on 87 octane. Our lowest is 91, which is roughly 4 higher than 87, so it seems I am running on the lowest (cheapest) type possible. I am getting a little ping justoff idle both excellerating andf decelerating. I believe this is caused by the vac advance coming in to quick or too much to early. I am trying to taylor it, but will probably just retard timing by a few degrees, as our 91 gas is 5c cheaper than the 95 octane.
So cobalt thanks for your suggestions :- )
And last but by no means least, Mr Nothing's Easy (I agree by the way, but soldier on we must). Am soooo keen on a 700r, as you say better gearing for the street in low gears, then the overdrive on highway. Price here is beyond a simple cheap swap, I guess there is not soo many old ones over hear. I might loook into getting on over from aussie, if I can souce it cheap enough. What are they worth over you way, either for a runner, or one in need of a recon? Judt out of interest, cos I figure if I can get one, I will be able to use in all my future old school V'8s, of which I intend there to be many :-)
Thanks again for your replies I will try the various things and get back to you with result.
As delivered, the white cam has the least amount of volume but a cam can be modeled to provide less than this amount. If you have a selection of pump cams, I'd suggest you use the white cam as a template to scribe an outline on a "larger" cam (a cam that you presumably wouldn't be using) to use as a starting point for making your own custom cam.
This might take some trial and error but might result in some savings.
The whole thing w/using a bare minimum accelerator pump volume and shooter size is that the engine will need to be up to operating temp to run correctly (w/o any flat spot off idle). This may or may not be a concern, depending. You wouldn't want to just sit there while the engine was completely warmed up before pulling away, but if you didn't have a lot of stop and go as soon as you pull out of the driveway, it might not matter nearly as much.
That said, I'm willing to bet that the engine will run fine w/less than the white pump and possibly w/less than a 0.025" shooter.
IMO, there's little to be gained by fully porting the 882's- but if that's going to be what you run, pocket porting and a 3-angle valve job will help flow. A 30º backcut on the valves will improve flow as well.
In any event, don't take any advance away from the engine if you can tailor the vacuum advance better to avoid the transient ping you described.
Here are some parts that might be of interest to you:
The ACCEL #31035 is an adjustable vacuum advance can for GM HEI that allows infinite adjustment to BOTH the amount and rate of advance. Comes w/instructions and tool.
To limit the amount of advance w/an OEM or some adjustable (for tip-in only- NOT total advance applied) cans like the Crane can, you will need to physically limit the vac can's travel w/a VACUUM ADVANCE LIMITER PLATE- Crane #99619-1, #99619-1 INSTRUCTIONS.
ADJUSTABLE VACUUM ADVANCE CAN KIT- Crane #99600-1, #99600-1 INSTRUCTIONS.
I guess that means step on the gas harder?...welly being slang for Wellington?...LOL
700R4 core price
We bought a 700R4 core from JY (autorecycler) for $250 CAD. It was a clean unit that had been rebuilt within 40Kkm. We plan to use it behind a 350hp 327 so we are going thru it and upgrade some key parts before we subject to abuse.
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