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  #1 (permalink)  
Old 06-01-2005, 04:25 PM
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Camshaft - how does it affect MPG/HP/TORQUE

Hey guys, we (my dad and I) are rebuilding our sbc 350 and were thinking of replacing our cam with a more lumpy one that would make the idle sound real mean, rough and uneven.

The thing is, I was just curious about how a cam like that would affect the MPG of the car. When the car used to run (it still runs now just very poorly) it had a 4 barrel rebuilt carb on there with frozen secondaries (the reason the engine is getting rebuilt) with a Turbo350 tranny along with headers, HEI Ignition, bored block (.010 over I think) about 9.5-10:1 CR ran on regular pump gas. With that setup and a good tune job on the carb it got about 18MPG though at it's peak performance it ran 20MPG.

~ Note - (I don't know if that's the exact specs of the engine, it's something of that spec though. Not sure on the CR or the bore of the block.)

My question is, with a new cam that provided a 'lumpier' idle, how would it affect the MPG of the car. My dad thinks it would drop to like 14MPG but I thought it wouldn't drop much if at all... Note that we are going to replace that old carberator with a new Qjet carb. The old rebuilt carb that had the secondaries frozen was only for a short time, my dad then realized they were frozen and un-froze them, but the damage was already done. Plus I've heard with Qjet carbs you get the performance with the MPG too, that's why i'm asking.

Please also to note that I am not a motorhead so if i'm talking out of my butt on some of this stuff please correct me/criticize me, I am still learning and have a LOT to learn. I've just got the basics on how an engine works (or so I think).

Thanks guys

Edit - Almost forgot, the car is a 1973 Chevrolet Chevelle Malibu 4-door sedan.

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Old 06-01-2005, 04:54 PM
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Yes, it will...
One thing to note that if this is a daily driver the lumpy cam is really may not be the best thing for the car. Normally to get that sound you have to go with a big cam.
Cams work within rpms ranges the lumpy at idle cams normally are for those engines that won't power to come in later over 3000rpms. The bigger cams for for those going for yop end power. The smaller cams provide more low end TQ and power but runs out higher in the range (above 6000rpms)
With a car like yours which is pretty heavy I don't think your going to like a big cam. It will want to be a dog when flooring it. You would want more power lower in the range to get that car moving.
The idea is that if you want an engine to run well and drive it on the street all the time you want something that will idle well. That does not mean you have to go with a small cam...however you want something within reason. You want a cam that will give you power in the area you will use it.

A rule of thumb at this site is to NOT buy a cam. for the sound but for power.

Gas mileage is simple enough in that if your engine does not run as well as it should or you have to rev it to 3000 rpms before it clears it throat your going to use more gas...

Call comp, crane, whatever company you like and ask them to suggest you a cam. Other then that try to get a good understanding of what the cam will do. What else you might have to switch (headers, TQ con.)

Chris
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Old 06-01-2005, 06:27 PM
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I agree 100% with 'strikingthe match', forget about sounding cool unless you have a lightweight weekend toy, buy a cam that is designed for your intended usage and vehicle you'll be MUCH happier when it's all said and done. Back in the day I saw a lot of my friends ruin a really good running car by swapping in a "big" cam just so it would sound cool. Yeah it sounded cool but it ran like butt cheese.

Something like the Edelbrock Performer cam would work really well in your application and might actaully improve mileage a little but it will not have the lumpy idle, it will give the exhaust sound a little "burble" but it will still idle smoothly and will wake up your 350 making it a lot more fun to drive. A Comp Cams 268HE will give it a little lope and still be drivable but like 'strikingthematch' said a real "ground shaker" will make your car a complete dog under 3,000 without a whole lot of other changes. And as much of a fan as I am of the "Forgotten Years", '73 to '77 GM A Bodies, a '73 Malibu 4 door isn't something I'd spend that kind of money on for performance parts. I'm not saying don't fix up the 4 door but keep it a nice clean driver, sure put some goodies on it but make it stuff that will help it be a good highway car and cruiser. Get the Performer manifold and cam, some small tube headers and dual exhaust and maybe later swap in an OD automatic and some 3.73 gears. It will run a lot better and won't break you at the gas pump and you can drive it anywhere, anytime. You'll enjoy it a lot more.
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Old 06-01-2005, 06:34 PM
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Thank you so much for the info match. I was just talking to my dad and he's going to try and find out what cam he has in there right now.

The car sounds pretty mean already, not real lumpy but not like a stalker either.

With the setup currently he had the car at 135MPH on a long straight coming back from the coast, he had to slow down because he ran out of road

It sounds like we have a small to medium cam in there right now, becase the car was no dog, it got right up and went. We don't have a tachometer in there now so I don't know the redline, but i'm going to put one in there after it's rebuilt.

We'll be putting a freshly rebuilt Turbo350 tranny with a shift-kit in it along with the rebuilt engine. Hopefully this'll be able to beat some of the 'ricers' out there. I'm hoping to get 350-400HP out of the engine, can you get that with a small/medium cam?
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Old 06-01-2005, 06:39 PM
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Quote:
Originally Posted by Hippie
I agree 100% with 'strikingthe match', forget about sounding cool unless you have a lightweight weekend toy, buy a cam that is designed for your intended usage and vehicle you'll be MUCH happier when it's all said and done. Back in the day I saw a lot of my friends ruin a really good running car by swapping in a "big" cam just so it would sound cool. Yeah it sounded cool but it ran like butt cheese.

Something like the Edelbrock Performer cam would work really well in your application and might actaully improve mileage a little but it will not have the lumpy idle, it will give the exhaust sound a little "burble" but it will still idle smoothly and will wake up your 350 making it a lot more fun to drive. A Comp Cams 268HE will give it a little lope and still be drivable but like 'strikingthematch' said a real "ground shaker" will make your car a complete dog under 3,000 without a whole lot of other changes. And as much of a fan as I am of the "Forgotten Years", '73 to '77 GM A Bodies, a '73 Malibu 4 door isn't something I'd spend that kind of money on for performance parts. I'm not saying don't fix up the 4 door but keep it a nice clean driver, sure put some goodies on it but make it stuff that will help it be a good highway car and cruiser. Get the Performer manifold and cam, some small tube headers and dual exhaust and maybe later swap in an OD automatic and some 3.73 gears. It will run a lot better and won't break you at the gas pump and you can drive it anywhere, anytime. You'll enjoy it a lot more.
Yeah, we have a 700R4 tranny out of a '89 Chevy Caprice wagon that we're going to get rebuilt and put in there. My we're just trying to get the car rebuilt so I can drive it to school before it's out (June 15th).

My dad's friend has a '71 Camaro that he might sell us if he gets this other RS Camaro, so if that's the case then we're going to take the engine out of this malibu and stick it in there.

To reply to what's been said about big cams: I guess that's why people would buy a big cam and couple it with a 2500-3000RPM stall converter so they would already reach those rpm's and get the HP/Torque. Just a thought..
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Old 06-01-2005, 06:40 PM
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Sounds to me like you've already got a pretty good setup for the application, Ol' Dad must know a little something he ain't sharing just yet.
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Old 06-01-2005, 06:43 PM
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Quote:
Originally Posted by Hippie
Sounds to me like you've already got a pretty good setup for the application, Ol' Dad must know a little something he ain't sharing just yet.
Maybe I'm so excited, I cannot wait to get behind the wheel of this thing. I hope it performs awesome I'm driving a '83 buick skylark now so anything's better
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Old 06-01-2005, 07:13 PM
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Your on a good track here. What has been said is very true... with a heavier car you want that power lower in the RPM range. MPH really won't tell you want cam. you have but more so what gears. 2.73 or the like are highway gears and will take you up to 100+ (in time) without redline. If you later got something closer to the good ol 3.73 you'll redline quicker but from start you'll love the feeling or like hippie said get the OD trans. and still be able to hit the highway.

On another note... about building the car it is true that the car may not be worth as much and therefore putting money into the car can be kinda pointless in the sense that you could never get all your money back then again howmany true hotrodders do you know could get all the money they put into a project back out
Point being though is that putting money in the CAR may not get you much BUT putting money in the ENGINE that you can always enjoy. An engine can be swapped in and out of things (sticking with same maker) no problem. So you can build an engine and later swap that engine in.
All in all it should be lots of fun and if you can get a really good combo together then you'll have a blast!

FOR HP
You may want to look at getting something like the edelbrock perf. package if your doing a full rebuild on the top end and such. They have proven combo there for 400+ hp. New heads, cam, intake, etc.

Chris
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Old 06-01-2005, 10:38 PM
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sounds like your on the right path....and from waht i understand the bigger the cam the more that things gonna suck gas man....i have a hearse with a lumpy cam that got about 12 mpg on a good day so i switched it out for a small comp and that made a world of diffrence also made it alot more fun to drive(well that is if you like dead sleds lol) that setup is a caddy 472 and matched to a turbo 400 .. ever seen a 64 caddilac hearse lay a black strip? lol
later
GOTH
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Old 06-02-2005, 08:48 AM
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There is some real world info on what cam swapping can do. The problem is that you will have people tell you "no, no go way bigger then that" "I used to have a huge cam and it worked fine!" but then you have to ask... ok what else did you have done!
Guys that run huge cams (and liked them) would have their high stalls or have their idle screw set up... all that may make it sound cool and such but I don't want to have to tap the gas when I come to a stop light just to keep the car running...
It is in my mind that a smaller cam is good for "most" people that drive street or STREET/strip cars because they spend so much of their time in the 1000 - 4000 rpm band. Going down the road at normal speed (depending on gears) your going to be at say 2500. SO it would be a good idea that you have a cam that pulls that entire range. THat wway when your at 2500 your cam is workin great makin the car run great and you still have plenty of cam left to go.
Again call a cam. company and they can suggest for you the ones hippie told you are some great std. cams for a 350.


Chris
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Old 06-02-2005, 11:55 AM
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i've got about a 1/2 dozen cams but the smallest one i have is one of the best sounding along with 1.6 rockers that is which does make it bigger.if you have a cam now you may want to try that first.mine is a wolverine/blue racer right before cranr took them over,280/290,214/224 @ .050",and .443/.465 but using the the 1.6 rockers it changes the duration a couple of points,they say1-3 more.the lift .472/496,i have friends with much bigger cam and they just love the sound of this one.make sure you use adequate valve springs.
i just know i'm going to here about the duration facts.
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Old 06-02-2005, 12:16 PM
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Quote:
Originally Posted by GoneNova/406
i've got about a 1/2 dozen cams but the smallest one i have is one of the best sounding along with 1.6 rockers that is which does make it bigger.if you have a cam now you may want to try that first.mine is a wolverine/blue racer right before cranr took them over,280/290,214/224 @ .050",and .443/.465 but using the the 1.6 rockers it changes the duration a couple of points,they say1-3 more.the lift .472/496,i have friends with much bigger cam and they just love the sound of this one.make sure you use adequate valve springs.
i just know i'm going to here about the duration facts.
That cam works pretty decent in a medium weight car in a 350 and has a nice lope to the idle. That would be a good choice if you need to replace your cam. My buddy put one in a stock 305 in a '63 Bel Air with a 200-4R and stock 3.08 gears and it even worked pretty well in that. It's available from many different companies including Summit, SSI, TRW, Speed Pro, Elgin, etc.

You are correct about the effect of 1.6 rockers on the duration other than it's actually "degrees" not points but we know what you meant, otherwise you shouldn't hear about it.
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Old 06-02-2005, 05:24 PM
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it'll do better than that.i had mine in 406 and 3/4 ton van with 2.73 gears and it did great.i advanced it at the cam 4* and it ran fantastic.i had guys with much bigger cams drooling over the sound of mine.i have a comp cams magnum 280 in it now and it doesn't sound near as good. i've built another 406 just like the other one accept this one had the c.r. dropped from 10.5 forged pistons to 9.4 hypereutectic because of gas price's but that won't make much difference.i had the comp cam in the other motor to and the small cam wins out for sound.it performs well too.it comes with fast bleed lifters but i went with the good chevy lifters instead.they have the extra hard face on them.that made some difference not being fast bleed.
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Old 06-12-2005, 12:44 AM
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So, do larger cams generally go with big block engines?
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Old 06-12-2005, 07:44 AM
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Quote:
Originally Posted by Malibu73
So, do larger cams generally go with big block engines?
"Larger" cams are intended for higher RPM usage however the same cam will work in a different RPM range in different displacement engines. What would be a fairly lumpy mid range to top end cam in a 283 would be tamer in a 350 and operate in a little lower RPM range and in a 406 might make a good low end and mid range cam. A cam that made a 283 or 305 have a very noticeable lope at idle might idle like a stock cam in a 406. That is a bit of an over simplification but basically how it works.
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