siggy_freud

Hey all. Just thought Id post my frustrations. I filled up my tank before driving the 63 miles or so between home, and my college home. I filled up again right when I arrived here just to get a feel for my mileage as my speedo currently isn't hooked up (the cable is too short for the new guage).
It took 8.5 gallons to refill the tank. That means that I used 8.5 gallons to drive about 63 miles. According to my rough head math, that comes to be around 7.5mpg. I will post my combo below and let me know if you think that its a bit obsurd.

66GMC 3/4 ton 2wheel drive
350 cu.in engine with a Tuned Port Injection system.
Trickflow 195 heads.
224/224@.050 - .528/.528 - 112lsa.
Th-350 Transmission.
4.10 rear
31" tires
Average cruise RPM was 3000 RPM (which puts me at about 70mph) for the drive.
Tailgate was down for the drive and tires had recommended amount of air in them.

My friends dad has an 89 GMC 1ton 4-wheel drive with a 454 and 4.10 gears that gets the same mileage.

Doesn't this seem a bit odd lol?

I have the equipment that allows me to modify the chips and was wondering what tuning aspects I should be looking at to help increase the MPG. Is it possible that by running at 3000RPM cruise I am actually being put into PE mode? I am basically looking for tuning information relative to 4.10 gears as I realize many parameters use RPM and TPS% as their base.

JPrevost

Sounds a little on the low side but not by much.
Things that are hurting your mileage;
1.) 1966 design
2.) Heavy vehicle
3.) Large heavy tires (mass = horsepower robber)
4.) Tires at recommended and not near max
5.) No overdrive to keep the engine at the rpm of lowest BSFC
6.) 4.10 gears also not helping but this goes along with 3.)-4.)
7.) Aerodynamics at 70mph is a gas robber, combined with 1.) and you're killing your mileage
8.) Cam mis-matched to heads/engine (single pattern cams are only good for engines that flow over ~85% of the intake) dual pattern cam would be best all around
9.) TPI at 3000rpm and your cam's intake duration doesn't work out too good at least I don't think it does
10.) You haven't really messed with the calibration meaning there could be a lot of mpg to be had

Get a scanner and hook it up. It'll tell you if you're evoking PE mode and you might very well be doing that at 70mph. Also, I'd bump the tire pressure up to 4-6psi below the max (4 if tires are hot, 6 if cold). Then get a dual pattern cam that better matchs your heads. The Lt4 hot cam would compliment the rest of your parts selection because of it's wide spread in duration. It'll wake up the TPI and allow the engine enough time to remove those exhaust gases through the rather crappy exhaust ports making for a more efficient engine (balanced). Other than that, try and keep the mph around 60. Going over that and you're just going to hurt your mpg, especially with the aerodynamics of that truck. I know, I've got a 66 c10 of my own only it's completely stock with column shift . Last thing to do is make certain there are NO exhaust leaks before the o2 sensor and no vac leaks. Leaks will destroy the ecm's perception of how well your engine is running. I strongly advise you to take extra steps to ensure you are mechanically functioning (look at the spark plugs and make sure they all look similar).
Good luck!

siggy_freud

Thanks for the input. I understand everything but your comment on the dual pattern cam. Why do you recommend this over the single pattern that is in there?

From what I've read, dual pattern tend to favor heads that are generally weaker on the exhaust side.

And what is it about 3000rpm and my cams intake duration that doesn't work out? I tried to match parts as best I could and I think I did a good job of it but if you have information that supports otherwise Id like to know.

The only other tires I have are 27.5 inch radials, but I figured they would increase my cruise RPM enough that their smaller mass wouldn't be substantial enough to help my MPG.

Thanks.

3.8TransAM

Couple notes I thought of in addition to what Jon has mentioned(did I get it right that time Jon? lol)

Mention TH-350 is your ecm seeing the correct vehicle speed?(mentions speedo but stops there)

Has any tune whatsoever been done on the engine.

MAF or SD, it is serioulsy out of whack with that cam and heads combo.

The lack of OD and high rpm's on a v-8 areent going to be easy to work around

later
Jeremy

siggy_freud

Its a MAF setup. I have done some tuning since the heads and cam but mostly its been for idle issues and whatnot. Part throttle cruising seemed to be okay as far as BLM's went.

It seems to idle and run great at all conditions. Nothing seems out of whack . . . the low MPG just concerns me a lot.

Grumpy

I don't see where, you've done anything that was done with mileage in mind, so why are you thinking you should get decent mileage?.

You have a big truck, with a relatively low torque engine (for the application), with huge tires. You have to use that much gear to even turn the tires, and then no overdrive.

FWIW, I have a 91 454, that I get 15 MPG, and 11 with a trailer. 4.10 gears, and properly tuned. High torque engine, fairly tall tires, stock engine.

Getting a scanner and optimizing the tune, might get you some better mileage, but the basic combo, doesn't look like it put together to be a good mileage combo, IMO.

siggy_freud

I realize this wasn't put together with MPG in mind. That said, 7.5 MPG seems VERY low for even a combination such as this.

When I drive a truck that weights considerably more, has over 100 cubic inches more (89 GMC 1 Ton 4x4 w/454) and that is getting 8+ MPG, I think there is cause for concern.

I have a set of smaller 27.5" radials, however I figured that they would increase the RPM's enough going down the highway that any MPG gained from the smaller rotating mass would be negated by the increased RPM.

Upon measuring the tires physical diameter they come to 29". I dont know why they are written at 31's. My other tires that are written as 27.5" are actually 27.5 inches.

yager

It might hurt your 1/4 mile times but try a set of 3.73 gears and drop to 60mph it will drop your rpms to ~2400... Might help some..


grumpy 31s aren't that big

Grumpy

My 285/40x17s aren't that big either, but take alot more HP to push down the road then a stock tire.

*that big* is a *relative thing*.

Around here when someone starts quoting tire diameters for a truck tire, it's because it's a *Mudder*. Great big block treads, are going to be just that, big, and heavy.

JPrevost

To put it bluntly; it takes x amount of energy to move a vehicle like yours y miles. It doesn't matter what engine is there, they all burn gas just some do it more efficiently than others. For this reason, you're really just looking at minimal gains. If I were you I'd go turbo diesel and never look back. Heck, if they made a diesel that fit nice in my engine bay I'd be working on how to wedge it in. Diesel = higher efficiency, turbo also reclaims energy that would otherwise be lost as heat. Seriously, either install a smaller cam with less overlap like the Lt4 hot or zz4, then add a nice turbo, then jack up the tire pressures and install an OD trans. THAT will get you better mpg .
At 3000rpm your TPI isn't doing it's job (at part throttle). It resonates at different RPM based on MAP pressure (speed of sound relative). So if you can, turn less RPM and get a cam that is better balanced for your heads. Nowhere, and I do mean nowhere, has a single pattern cam ever made more horsepower or been as fuel efficient than a dual pattern cam on ANY gen-I/II sbc. The LSX is a different story. They flow HUGE amounts of air on the exhaust thanks to the 18 and now 15 degree's. With 23 degrees we're pretty much limited to sub par exhaust flow so get a dual pattern cam and be happy.

siggy_freud

The "31's" I was mentioning aren't mudder tires. They are radials. I mentioned the diameter just for reference on the configuration as a whole. This isn't an off road truck by any means. Its 2wd for one, a bit lower than stock.

Here is a picture so you get a better idea of what I am working with.

Not that I dont believe you outright about what you say about cams, but I have never read anything to support your opinion and as such, hesitate to buy into the statement that dual pattern cams are always a better route. If my cam is mismatched to the heads, what type of profile would you recommend to match it together?

My main question in reference to this particular forum concerns the Prom tuning part of my better MPG quest. What parameters should I be looking at? Does Highway Mode offer some relief? Any power enrichment/accel enrichment values that will be of benefit?

JPrevost

I'm rather insulted. I think I've stated in both of my replies that I think the Lt4 hot cam or zz3/4 would work better.
Here's some "proof" if you don't believe me; 23 degree heads don't flow very well on the exhaust compared to the intake. What happens with single pattern cams is they tend to back up the engine. You're engine will run hotter and you won't make "better" power while the piston fights to push the exhaust out the small port. This is why lowering the valve angles almost always promotes a better in/ex ratio (greater than 80%). Your heads are right around 73% as in the ex flows about 73% of the intake. Infact your heads are very comparable to the Lt4 flow numbers and chamber design. Infact the flow ratio is 73% for Lt1's as well. I've yet to find a single pattern cam do very well on either of the Lt1/Lt4's let alone a hopped up gen-1. Granted the 73% is much better than the Vortec's pathetic 66% but it's over 90% of the time that if your head's flow ratio is below 80%, a dual pattern will work "best" and if you're in the upper 70's to 80's a single might work "best". It's just one of those rules of building a large power band. Sure the single might do well at one spot but when I say better I mean all around it's the hands down pro-s choice. Go into an LSX and you'll notice how well those exhaust ports flow... single pattern cam's and even larger intake duration can make those engines scream.
I don't have to prove to you anything other than that I've done my homework on the subject.

As for the highway mode, yes, almost always highway mode will improve your mpg so long as there isn't a lot of load changes and you don't lean it out too far. I've never noticed a n/a engine to not deal well with highway mode. Even a friend of mine with points (instead of HEI) handled cruising at nearly 16:1 and picked up 2mpg and that's without OD just like you.
AE is a tricky thing. I think Grumpy put it best when he said that you should only need enough AE to get you into PE, any more is a waste of gas. At least I think I remember him saying something to that regard but don't quote me on it. Basically saying you don't want nore do you need a rich AFR AE and even a lean spike of like 16-17:1 is fine so long as the engine isn't objecting.
My last input; nothing and I do mean NOTHING replaces trial & error. Answer some of these questions for yourself and report back to us what worked and what didn't.

BTW, truck.

Fast355

iTrader: (2)

I run a dual pattern cam(291/288 advertised, 224/224 @ .050, .450/.461" lift, .480/.490 w/ with 1.6:1 rockers, 115* lobe center, 4* advanced) with heavily heavily ported 305 TPI heads on a .040" over 305 with a very free flowing exhaust and TBI. With 3.73 gears, P295/ 50/R17s in the back and P275/60/R17s in the front, a 700r4, and the 305 I get 18-20 MPG at a steady 70 mph cruise. Cruise RPM at 70 is about 2,200 RPM. I found that highway mode helps MPG in the 40-60 MPH range, as speeds higher than that bring the MAP reading up too high to use it. Another thing to do is keep the ECM out of power enrichment, I noticed that I frequently got 50% TPS in the hills(what would you expect, small engine, relatively big cam, heavy truck, low cruise RPM, etc.) but rarely 60% My stock PE setting was around 35-40%. I moved that up to 55% and can now stay out of PE (Watch for knock counts and signs of detonation). Avoid tripping the knock sensor at all cost, that will give an instant timing retard, wasting gas. I also used the PE timing table to give some more advance under WOT and heavy loads. The timing table was a chore on mine. My particular combination was volumetric efficient enough to need very little timing under WOT, but yet the chambers burned slow enough at part throttle it wants alot of timing. Up to about 38* total @ 2,200 and 50 MAP. I have all my BLMs in the 124-128 range. Cruising along at 55-60 in OD the wideband I have indicates 16:1 A/F mixture. Roll into the gas, the A/F mixture richens to 14.7:1 and the engine comes alive, push harder and the mixture drops to 12.8-13.2 depending on engine RPM, PE timing kicks in, the converter unlocks, and the engine really wakes up.

siggy_freud

I did not mean to offend with my statement on the cam information. I am just very hesitant when it comes to new information, and like to have as much information as possible. I am a long term member of hotrodders.com and when someone comes in with new, "debatable" information, I try to always ask for backup and or evidence to support their information. Thank you for explaining it.

I am going to start by looking at my PE settings, and make sure I am not jumping in an out of it while cruising. I could just be near the TPS threshhold and bopping in and out of it.

I have heard arguements both on this forum and others for and against using highway mode. Does highway mode take it out of closed loop? Why is there a short time limiter for it? Does it jump back into that mode once it "re-sees" all the sensor readings are within the right parameters for Highway mode?

Grumpy

That's why I mentioned getting a scanner and seeing where exactly you are.

327_TPI_77_Maro

iTrader: (8)

Your gas mileage doesn't sound that out of whack for your combo. My 327 was getting 12 mpg cruising at 3000-3200 rpms with my old 4 speed trans but now gets 20 mpg cruising at 1600-1800 rpms with an 0.6x ratio O/D in my 5 spd.

thirdgen88

Yeah, I think an overdrive transmission is going to be the best bang for the buck as far as enhancing your mileage. Overdrive + Lockup would bring that beast up to 12-14mpg range or more..

JPrevost

Lt4 hot cam + OD + highway mode =

Z69

Going to a dual pattern cam adds overlap.
Which lowers part throttle efficiency. It's all about reversion.
When going to a wider lsa, it can reduce overlap even though you go to a dual pattern. But it will lower peak tq and raise the rpm it occurs at. As an example you can add 4 deg of duration to the intake and exh and spread the lsa 2 degrees and you'll have the exact same overlap #.
Yes a single pattern will/may lower peak hp. But it will add torque below the peak too. So it really depends on what you want. The wide lsa on efi cams helps the idle with lower reversion, and helps on the top end with a later closing intake.
I know of at least one "Pro" with 25+ years in cam design.
He recommends a single for under 5500 rpm.
Further proof- look on compcams site at the dyno charts for the various cams. The HE268 does quite nicely against the much newer XE262 & 268. Acutually beats the XE268 IIRC.
But I think that's because the XE268 has to much relative exh duration. The above mentioned Pro designed the HE268 btw.

That said- yes your cam is a little big for mpg.
But an OD trans would get you a lot of mpg back.
At least 13-15 I'd think with the proper tune.
You could go to a smaller cam too if mpg is more of a priority. But the od trans will help the most I think.

JPrevost

Going to a dual pattern cam adds overlap.
Which lowers part throttle efficiency. It's all about reversion.
**That isn't true. Just going to a dual pattern cam won't add overlap! Infact I could reduce overlap by going to a dual pattern cam, Lt1 cam, there, less overlap than the XE268.**
When going to a wider lsa, it can reduce overlap even though you go to a dual pattern. But it will lower peak tq and raise the rpm it occurs at. As an example you can add 4 deg of duration to the intake and exh and spread the lsa 2 degrees and you'll have the exact same overlap #.
**And this is why nobody is recommending going with MORE duration .**
Yes a single pattern will/may lower peak hp. But it will add torque below the peak too. So it really depends on what you want.
**I don't think you can say that just going to a dual pattern cam you'll loose low speed torque. Infact I think you're way off on that one. With less intake duration than he's running now he can utilize the long tubes to maximize the low RPM torque UNLIKE that of his current single pattern which raises the resonance tuning into the "hey, I'm the engine and I'm trying to breath here but I can't because of these small diameter non-tappered runners" RPM.**
The wide lsa on efi cams helps the idle with lower reversion, and helps on the top end with a later closing intake.
**That first part I can't disagree on but the second part about a wide LSA helping the top end just doesn't seem true. More often than not the top end is hurt from the wide LSA. What happens when you increase the LSA angle is this; You push the exhaust lobe into the "power stroke" bleeding off BMEP at the end of the stroke. Pushing the intake over does something similar by bleeding off compression with a later closing intake. All in all a high HP tight powerband cam is a single pattern WHEN matched to a set of heads that have an ex:in ration close to 80% or better.**
I know of at least one "Pro" with 25+ years in cam design.
He recommends a single for under 5500 rpm.
**See, that's so general that I can't see him as being a "Pro". A "Pro" would want to know the whole engine and would take everything into consideration like... oh, I don't know... the HEADS **
Further proof- look on compcams site at the dyno charts for the various cams. The HE268 does quite nicely against the much newer XE262 & 268. Acutually beats the XE268 IIRC.
But I think that's because the XE268 has to much relative exh duration. The above mentioned Pro designed the HE268 btw.
**I wouldn't call that further proof. They mention heads that I can't find data on. Does Dart even make "Dart S/R" heads? I thought that was Worlds Products!?!?! See, I don't know if I'd call that proof. GM and every engine builder that's winning races uses dual patterns on engines with weak exhaust flow numbers. Call up some engine builders that do R&D work, not just retail and I'll bet they'll all tell you what I'm telling you. Curious, do you have his name and number? I'd like to talk to him and see if he can help me with a cam selection. The "tech" phone guys are complete morons. I like talking to the guys that do the real work.**
That said- yes your cam is a little big for mpg.
But an OD trans would get you a lot of mpg back.
At least 13-15 I'd think with the proper tune.
You could go to a smaller cam too if mpg is more of a priority. But the od trans will help the most I think.
**I completely agree. The OD would help get that TPI working for you and keep those losses to a minimal. I still think a smaller intake duration would work wonders with that engine by giving it a better low end (LT TPI needs small duration to work well else mismatch). The added duration of the hot-cam's exhaust is minimal and it's resonance tuning is less influential to the powerband. The only other thing I can mention is that it isn't JUST the heads flow ratio but also intake and exhaust except the further you get from the valves the less influence they have.**

Teeleton

What ratio rockers are you running? As a test, you can easily run a mismatched set of rockers to put more ratio on the exhaust side to alter the intake to exhaust bias of the heads and cam combo to see how it does for you. It's not a perfect solution, but it works in a pinch. It helps to have a friend that wants to do the same thing in this sort of setup, where each of you buys a different set, and then you trade half of them.

I have a dual pattern in my TBI V6 (222/230@.050, .515/.531 lift), and I can get 18 mpg on the highway. I also have 3.73 gears, 26" tires, 700R4, and cruise at about 2500 RPM.

Teeleton

327_TPI_77_Maro

iTrader: (8)

I have a single pattern Isky 280 HL .485" lift, 232 @ 0.05 duration cam in my 327 and get 20 mpg highway with a Stealth Ram and a W/C T5, with 3.73 gears.

Z69

Your saying a dual pattern cam with it's corresponding increased overlap will help mpg when compared to a single pattern cam with the same intake lobe. I don't think so. Reversion is possible the entire time the exh & int valve are both open. So the sooner the exh valve closes. The less reversion. The less mixture contamination. Therefore more air/fuel in the cylinder.

Show your math- don't just disagree.

overlap is EC + IO. How can you add exhaust duration on the order of 8-12 degrees and not add overlap. And no they don't stick it all on the opening side. And if your thinking smaller intake and same exh. I'm not, nor do most people when specing a cam.
In this instance a smaller intake would help the mpg. But when comparing single and dual cams, it's customary to keep the intake duration the same or close. At least in the V8 world.
The LT1 cam is a smaller cam so it's not a true comparison.
A closer cam to the HE268 would have been the Hot cam since they have similar intake durations.

I'm well aware of whats happens to cyl pressure when moving the valve timing. Ask a builder what has more effect on power.
EO or overlap. They are intertwined I know. But you look at overlap more than you do EO. When you spread the lsa, what happens to peak torque or the torque curve in general?
It depends on how you spread the lsa too. All intake, all exhaust, or a combination. When I swapped to a smaller cam and tighter lsa, I ended up with the same overlap within 1 deg. All of the lsa movement was on the exh side.

Dart makes a 165cc port stock replacement style head. Look on their site.

.Your talking about 3rd hand info- I didn't bother to include any comments on the heads. But he does in fact take the heads into consideration. As did I when referencing his statement. You are making the general statement that a dual pattern is better than a single. As far as being a Pro, he was one of the founders of Compcams. Feel free to get another opinion on him. Email Doug Flynn and ask how knowledgeable Harold Brookshire is.
I can give you Harold's home # if you want to form your own opinion too. I spent hours on the phone with him. You'll enjoy it.

You keep letting HP get into the conversation. I never said a single would out peak HP a dual pattern with most heads. But if you looked at the comp #'s for the HE268, you'd notice it had a better power curve.
As long as the same heads were used it's a valid comparison.
Regardless of how good or bad the heads were or who made them.

At least we agree on that. I don't use the tech line for cam recs.
Or springs or much of anything else for that matter.

327_TPI_77_Maro has a similar or bigger adv dur cam with a tighter lsa and is claiming 20mpg. Granted the subject vehicle is a brick. But the TPI should help the mpg.
And the previous two posts both had OD. Which we both agreed would help his mileage. And a dual pattern 224 intake cam would get even worse mpg imop. It would make more hp more than likely though.

RednGold86Z

iTrader: (1)

When looking for economy, I wouldn't get too worried about the cam when speaking of normal streetable cams, as long as the cruise RPM isn't producing 90% EGR resulting in lots of misfire. Sure, get the most from your powerstroke (exhaust opening time, but there's a point of diminishing returns, and bigger isn't always better, and later isn't always better). Just remember that normal driving is part throttle, and half of the cam WOT theory gets skewed a lot.
What's more important is what's done with the air that comes in, not the amount of air - from one point of view you desire using less air, but getting the same power from it, or getting more air/cycle, but less air/mile.

Optimize spark timing is probably #1 on the list.
reduce amount going to cooling and exhaust system - iron heads, run higher thermostat, optimize spark timing/good combustion, high compression ratio.
Run most efficient air fuel ratio possible.
Get complete fuel burn - strong enough ignition, good combustion chamber design, high compression ratio, good mixture formation (now liquid fuel in chamber).
Reduce wasted load on engine - reduce RPM (frictional losses) - torque peak in no way indicates fuel efficiency. Increase throttle angle (see reduce RPM), reduce other restrictions (mufflers, etc).

Reduce wasted energy in the vehicle - lockup converter, no dragging brakes, put the tailgate UP (or so I've read is more aerodynamic than down), close windows, put smaller mirrors on, keep tire pressure up, get alignment straight, keep speeds in a range that matches the drivetrain.

Conservative driving helps. Lugging the engine a tad helps, but not quite possible with that transmission.

Just wanted to take the cam discussion out, since he's not likely to swap cams before trying other things.