Camshaft Question - Impala Tech
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post #1 of 12 (permalink) Old 08-31-2019, 11:45 AM Thread Starter
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Is there any way to know whether or not a camshaft is power-brake friendly in terms of vacuum?

496 BBC

Intake
Lift: 0.635"
Duration: 288.6

Exhaust
Lift: 0.640"
Duration: 294.6

Lobe Separation: 112
Intake C/L: 108

I haven't touched the brake system since replacing the engine and transmission, and since I recently started driving the car again, I noticed that when coming to a stop the brake peddle starts to get stiff right before stopping like I'm losing the assist in the power brake system. No vacuum leaks anywhere. I'm also trying to figure out if this cam is "lopey" or I just haven't gotten the EFI tuned in correctly.

Thanks for any replies.

1968 Impala SS
496 Stroker
T56 Conversion
3.73 Posi-Trac

Last edited by 68WASAGOODYEAR; 08-31-2019 at 11:34 PM.
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post #2 of 12 (permalink) Old 08-31-2019, 01:28 PM
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Power Steering is not dependent upon engine vacuum it is belt driven. By any chance are you talking about Power Assist brakes?

The ZZ502 has a hydraulic roller cam with duration at .050" tappet lift (intake/exhaust) of 224/234on a 110 LSA. It has a very noticeable lope at idle, but works with power assist brakes. I don't know how your EFI will do with your cam. Chevrolet engineers designed their EFI tolerant "hot" cam (used in Gen III LS engines) to feature 0.577" intake and exhaust lift with 228 degrees intake duration and 248 degrees exhaust duration @ 0.050" of lift on a 116.5 degree centerline.

EFI unlike a carb can not withstand a lot of overlap. That is why except where required by the rules most sportsman class cars still run a carburetor to make more power than an EFI car. EFI was used by the factories because of emission requirements. EFI costs at least $300 more than a similar sized carb and provides no benefit to the racer at all.

If you want to retain EFI then you will need an EFI cam that is a lot tamer than what you have now, and that doesn't even address the issue of a lot of overlap to make the rumpity rump noise people like killing your manifold vacuum needed for power brakes.

The only reason I would run EFI is if it where on a boosted application because electronic fuel injection beats mechanical fuel injection on the street.

Big Dave
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post #3 of 12 (permalink) Old 08-31-2019, 01:41 PM Thread Starter
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My mistake Dave. Yes, was referring to power brakes.

Would "running out" of vacuum be symptomatic of a cam that is too aggressive or is it a situation where it either makes enough vacuum or it doesn't?

Thanks again.

1968 Impala SS
496 Stroker
T56 Conversion
3.73 Posi-Trac
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post #4 of 12 (permalink) Old 08-31-2019, 02:03 PM
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Greatest vacuum is made with throttle blades closed (at idle). I run a vacuum gauge to know when I am wide open under load. If I am not near zero on my gauge with my foot on the mat then I know the throttle cable has slipped again and time to adjust it.

At idle your engine should evacuate the power booster canister. Depending upon driving style and cam that should hold on the road. In city traffic it will run out of vacuum so you need a supplemental vacuum pump or an accumulator (big tank to hold all of your nothing: aka vacuum). Failing in that you can run a HydroBoost system off of a light truck with a diesel engine.

Big Dave
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post #5 of 12 (permalink) Old 09-01-2019, 01:04 AM
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With a lobe separation of 112, I would have expected you to have decent vacuum and with .635+ lift I would expect the cam is going to have some lope. (that's a good amount of lift)

the missing number is critical though, the "net" duration, or duration as measured at .050" lift. Usually in the 230-260 range. I've not had that large of an engine but I would suspect that anything over 244 degrees at .050" lift is going to have decent lope in a 496 engine.

Agree with Dave though, you should see what you got with a vacuum gauge. It's a helpful in multiple ways.

HOW A NOVICE REBUILDS A 66 IMPALA CONVERTIBLE:
http://www.impalas.net/forums/blog.php?u=1432
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post #6 of 12 (permalink) Old 09-01-2019, 03:00 PM
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Lift has no affect on lope. Which is why I can get away with the custom cams I grind with about fifty percent more lift than found on an off the shelf cam (really hard on springs and valve train which is why I run bigger diameter push rods than stock, and PAC alloy springs with Chromemoly spring retainers).

Two things that affect lope is a narrower LSA and more duration that what you actually need. Duration is an indication of the engine's ability to fill the cylinder at high RPM. It does this by leaving the valve open for a longer time to allow air to flow. You can counter the effect of a longer duration (overlap) by spreading the LSA. A larger LSA means you are physically moving the lobe on the cam shaft further apart so that the valve can actually spend some time in the closed position. This also improves bottom end torque for the street were we live. Narrowing the LSA makes more top end horse power for those who like to race engine dynos.

Down side is you can not make power with the valve open and it decreases the effective displacement of your engine. Ask most people if they want a bigger engine they would say yes, but they all want a radical cam that idles at 2,400 RPM just like in a Top Fuel race car.

Top fuel cars don't normally drive on the street (though Leah Pritchett, and Matt Hagan both raced on Woodward ave after the cops closed it down and put up retention barriers to keep and car parts away from spectators). Both complained that the raw asphalt ripped their slick apart as they got too good a bite (the slider clutches and big tires are dependent upon controlling wheel speed, which the extra bite really played with the computers mind).

If you have a hydraulic roller (that weighs nearly twice as much as the weight of a hollow solid roller) you need a stiffer spring to control the valves (prevent valve float). Problem is a 300 pound spring is the upper limit of a hydraulic lifter. Any more spring pressure (like the 570 pounds that I run with my solid rollers) will collapse the lifter. So you are effectively limited to 5,800 RPM before your valves are bouncing off their seats (while I can rev a nearly 600 cube BBC to 7,800 RPM all day long)..

If you are running a BBC then springs really mater because the canted valves mean it needs even more spring pressure to control harmonics (valves move through x. y. and z plane when opening and closing but at different rates of acceleration which is where you get into harmonics. This is why a BBC eats valve springs for breakfast. You have to use a quality alloy steel and check the springs frequently for pressure if you want to rev the engine (a low spring pressure indicates it is about to break; just like bending a coat hanger until it fails it gets progressively easier to bend it just before it snaps).

Big Dave
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post #7 of 12 (permalink) Old 09-01-2019, 05:42 PM Thread Starter
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Thanks for all the replies. I have a vacuum gage now and will take some readings at idle. I have a long way to go toward understanding cams. Especially thankful for all the knowledge on this site.

1968 Impala SS
496 Stroker
T56 Conversion
3.73 Posi-Trac
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post #8 of 12 (permalink) Old 09-01-2019, 07:48 PM
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Dave, I wasn't trying to make a direct correlation to Lift and Lope sound.
It was more of a point 'by proxy' with certain assumptions made for typical cams.

There is a typical range of expected lift for a given (small) range of duration noted at .050" lift. We both know that this Net duration is a significant factor for a lopey sound. So, if a cam has as much as .630-.640 lift at Gross duration, there is a typical small range of what would be expected Net duration at the .050" lift. That was the jump from A > C that I was making with my statement.

HOW A NOVICE REBUILDS A 66 IMPALA CONVERTIBLE:
http://www.impalas.net/forums/blog.php?u=1432
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post #9 of 12 (permalink) Old 09-04-2019, 11:23 PM
 
 
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I always ask a customer: where do you intend to drive?? drag strip or highways and cruise nights? Then I tell them to consider drivability. Since you are concerned with P/B vacuum I assume you like to drive and enjoy your car. Go with an RV grind cam. Keep it simple.
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post #10 of 12 (permalink) Old 09-06-2019, 09:11 PM
 
 
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It could be the EFI or it could be the timing too. When I ran a "too big" cam my power brakes went to the floor and acted like there wasn't enough peddle to stop.

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post #11 of 12 (permalink) Old 09-10-2019, 12:02 AM
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For a few hours of fun, you could load this app onto a PC and see where the power band falls for your cam and your cubic-inches.
CamQuest ? Select The Perfect Camshaft & Valve Train Components


or


Desktop Dyno (I used to use this, thought it was pretty neat 25 years ago)
I dont' think it shows vacuum, by once again by standard assumptions, you could make a commen-sense guess-timate of the vacuum range by checking out your HP/TQ curves. eg. if that cam and 496 cubic inches has HP peak 7000RPM, probably not ideal idle vacuum.

https://www.summitracing.com/parts/CCA-186011/
https://download.cnet.com/s/desktop-dyno/
DeskTop Dyno5 Main Page

HOW A NOVICE REBUILDS A 66 IMPALA CONVERTIBLE:
http://www.impalas.net/forums/blog.php?u=1432
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post #12 of 12 (permalink) Old 09-17-2019, 08:59 PM Thread Starter
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UPDATE: I measured the vacuum at idle and got right at 10 inHg, which is in the range of suggested values (8-13 inHg) for using a vacuum resevoir. I added a Summit resevoir and it solved the issue, which is nice. I needed an easy win.
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1968 Impala SS
496 Stroker
T56 Conversion
3.73 Posi-Trac
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