Cams vs mileage - Team Camaro Tech
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post #1 of 14 (permalink) Old Nov 20th, 00, 12:32 PM Thread Starter
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Has anyone experienced a major change in gas mileage after changing a cam?
I went from a stock cam to a dual pattern 224/234 and 465 lift and seem to have lost about 4-5mpg.

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post #2 of 14 (permalink) Old Nov 20th, 00, 12:45 PM
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Depending on your engine, but a 295hp/350 has a -929 cam with about 195/202 duration. As you can see there's quite a difference in intake duration between the two for more fuel to get sucked in. And as we all know, that's how we get more power.

I would think your cam should be just as fuel efficient as a 350hp/327 -151 cam at 222/222 duration. IMHO. pdq67



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post #3 of 14 (permalink) Old Nov 21st, 00, 05:40 AM
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I've always kind of wondered about this. I had a stock 327 with a powerglide and 2.73 rear. I got around 17 MPG with this. I then switched to a TH 350, 3.42 rear, HEI ignition, larger valves, and a Crane 266 cam. I now get 19 MPG. Sure seams to me the gas mileage should have gone down when I switched but, it actually went up. Maybe it's in the tuning or HEI. Anyway a TH 350, 3.42 rear and about 345 hp is sure a lot more fun to drive.
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post #4 of 14 (permalink) Old Nov 21st, 00, 06:34 AM
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Rick, I had the same increase in gas mileage when I changed from 2.41's to 3.23's. The reason is that there is a better signal to the carb at cruise RPM with the gear change, and the total advance can be higher in the RPM range depending on how you have your ignition set up. Little tuning tricks can make a BIG difference in performance and mileage.

Crash, It's also been my experience that with no other changes than a cam swap, the mileage will drop off. The figure you stated seems about normal. You might be able to get some of that back from fine tuning.


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post #5 of 14 (permalink) Old Nov 21st, 00, 09:20 AM
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The porblem I have found with going to a bigger cam is that you keep sticking your foot into it just to hear the exhaust!

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Mark

1992 Firebird 355/Six Speed
1991 RS 350 / 700-R4
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post #6 of 14 (permalink) Old Nov 21st, 00, 12:39 PM Thread Starter
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Actually, I babied the last tankfull. I swear I didn't cut the Q-jet secondaries loose once just to see how much I could squeeze out of the tank and got 14 mpg.

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post #7 of 14 (permalink) Old Nov 22nd, 00, 02:59 AM
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My 73 bored 350 with a cam (dont know specs on cam) that was installed (in a 74 body)before I purchased the car gets me around 14-15 on the highway. Never better than that know matter how I baby it. I believe the rear end is a 3.23 with a 350 tranny and a shift kit. However around town, and mostly a foot in the floor, it drops to about 8-9. Still alot more fun to drive than my truck (99 F150)

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post #8 of 14 (permalink) Old Nov 22nd, 00, 05:30 AM
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how could you go a whole tank with out letting her strech a little .Iam luck if I get out of the garage without letting loose
then again I drive mine because I love it ,not because of the milage it gets. I don't even know what it gets .but it will pass just about anything.....but a gas station

just thinking outloud ,sorry!!

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post #9 of 14 (permalink) Old Nov 22nd, 00, 10:12 AM
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There is a lot to the gas mileage - cam comparison that is unknown. One good place to start though is to figure out what rpm you want to do at cruise speed. Example, if you want to cruise at 75mph, and you are geared such that the engine will be turning 2500 rpm (figuring with equations and such), then you would want to find a cam that also provides peak torque near that 2500 rpm cruising range. Theoretically, you would be providing the most grunt for the least amount of fuel usage in that range. Hopefully, I'll see how "theory" works when I'm doing 160mph for 90 miles at the 4000 rpm torque peak for my 496ci big block at the September SSCC in Nevada. Probably a whopping 6 mpg!

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post #10 of 14 (permalink) Old Nov 22nd, 00, 10:27 AM
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This has already been touched upon but your WILL notice a pretty substancial difference in gas mileage if you are not NORMALLY driving the car in it's most efficient RPM range.

For example, I had a BB with around a 220 degree duration CAM with .570 lift and the car got basically around 8-9 MPH (3.73 gearing TURBO 400). The most efficient RPM range for the motor was around 2600-2800 RPM and I most of the time would be in the 3400-3500 RPM range (75MPH).

I changed the cam out to one with 224/236 and .600 lift. I also installed a R700 with the same rear end (3.73). The car now normally opeartes in the 2600RPM (75MPH) range and I get 12-13 miles per gallon which is a substansial INCREASE in mileage with a decidedly more radical cam.

So the secret is to have the car the MAJORITY of the time in the power range of the selected cam. Conversely, if you are turning too low a RPM, that is bad also because the car will be lugging.
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post #11 of 14 (permalink) Old Nov 22nd, 00, 12:44 PM
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Sounds like mine is doing pretty good at 19 mpg with a TH 350 and 3.42 gears. I must have just fell into mine based on dumb luck. I never considered the mpg when assembling my combo. I will say, I think I got a really good combo and it is mostly based on what guys have told me on this board. The only thing I might have done a little different was, I used a Crane 266 cam mainly because everyone was recommending the Comp 268 and I wanted a little more torque. On hindsight I should have gone with the 260 I think it is the better streat cam.
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post #12 of 14 (permalink) Old Nov 23rd, 00, 03:59 AM
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sr71bb,
I'll have to agree with you, I should have said most of the time as you stated, that you want to "have the car the MAJORITY of the time in the power range." I did a test in my '56 Chev with the 302. I had 3.08:1 gears in the rearend (good highway ratio) with a TH350 tranny. I was pulling about 16 mpg which I thought I couldn't get much better. Then I had a friend who wanted to swap pumpkins in the rearend. I had wanted a little more off the line power with the 302 so I traded his 3.70:1 set for mine. It bumped my cruising speed into the noticeable torque curve of the cam, and cruising at 3500 rpm I pulled 20 mpg (no bull$#@%) on a road trip once. Another test was with a friend's '57 with a 406sb and 700R4. This engine is a torque monster at low rpm, and his car is geared such that he's running 1800rpm at cruise speed, logging 17mpg. So yes, depending on the motor, you want to be within the bounds of the working torque curve. The 302 probably has a peak somewhere near 5500rpm which certainly wouldn't be gas mileage friendly, but the 406 has a torque peak at much much lower rpm. And the cam choice has alot to do with that.
post #13 of 14 (permalink) Old Nov 23rd, 00, 06:57 AM
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You guys bring up a really good point. I had always thought I would go with a 700-R4 but since I'm getting 19 on the highway with a 327 does anyone think the 700R4 would be any better.
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post #14 of 14 (permalink) Old Nov 23rd, 00, 07:26 AM
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Rick,
It depends on your rear gear ratio and tire height. You have a tranny output gear of 1:1 with the TH350 and a rear 3.42:1 already, which is a pretty good highway gear, I dunno how tall a tire you have, but the equation for overall gear ratios to predict travelling rpm is: (mph x trans output ratio x rear gear ratio x 336)/tire dia in inches = rpm. If you had a rear tire height of 27 inches, at 70mph you are looking at 3000 rpm. If you go with the 700R4 at 22% overdrive (I think, 0.78:1) then it will put you at 2300 rpm. How does your 327 pull at 2300? Is it lugging or does it have some oomph? My 302 would be lugging it until about 3200 rpm. I believe it takes more power to turn a 700R4 than a TH350 with the extra clutches, so you might be happy with the TH350 at 19mpg. You would get a lower first gear with the 700R4 though, which'll get you outta the hole quicker.

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