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Cam Timing Specs, 1961 145
Posted by: wittsend ()
Date: November 08, 2019 02:25PM

I ran the 145 "Turbo Air" (80 HP) service manual cam timing specs through a calculator Wallace Racing (image below) and came out with a 316 Intake / 336 exhaust duration. This seemed enormous especially since this is a low HP motor.

So, thinking it might be a very slow ramp cam I put the cam on V-blocks, zeroed out the dial indicator and checked the duration at .050. It still calculates in the mid 290's for duration. The LSA is 107 degrees and sure enough the overlap specs in at a large 112 degrees. With that much overlap I'd think the dynamic compression ratio is tiny (I assume 8.0 listed is static).

This is a very low lift cam at .314 intake / .344 exhaust. Is what seems like an immense amount of duration a compensation for that? I'd appreciate input to understanding this. It seems contrary to the typical cam specs for a low HP motor. Am I missing something here? Maybe dividing by 2 (cam vs crank rotation) gives 158 intake / 168 exhaust duration and that seems equally low as the other method is high.

Second question:
The car is a '61 Lakewood with a Powerglide. This car will likely see no more than 100 miles a year and 1,000 miles in my lifetime. It is simply a 'keep it running' Survivor car.
It will probably be running taller tires than stock (14" in the 70 series). Hence low end torque is desirable. Would a 95 HP or a 110 HP cam be of any benefit? This is a Below Broke Budget Build. So, I'd be looking at a used cam (new lifters of course). This is no race car but if a free or cheap cam came my way that would help I'd entertain the idea. Otherwise I'll stay with what I have.



Edited 1 time(s). Last edit at 11/08/2019 02:26PM by wittsend.

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Re: Cam Timing Specs, 1961 145
Posted by: MattNall ()
Date: November 08, 2019 02:43PM

GM cams of the era did not use the @ 0.050"

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Re: Cam Timing Specs, 1961 145
Posted by: joelsplace ()
Date: November 08, 2019 02:47PM

"and checked the duration at .050. It still calculates in the mid 290's for duration"

Joel
Northlake, TX
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Re: Cam Timing Specs, 1961 145
Posted by: JimBrandberg ()
Date: November 09, 2019 07:04AM

Something seems wrong with the numbers. Intake opens should be way closer to TDC than 43. On 8o HP PG Intake centerline should be more like 104 than 115. The lift at .314 multiplied by 1.5 rocker arm is .471 which is HUGE.

I'm not talented enough to set a cam on blocks and know where TDC is and multiply it all out due to cam running at half speed. I brought a cam to Berry Cams and Scott was talented enough to do it. I think Ray Sedman can do it. I've just never tried it.

I prefer to assemble the crankshaft and camshaft in the crankcase with a dial indicator on a special lifter with a bolt epoxied in and a degree wheel on the crankshaft. I will use the timing tab on the rear cover for TDC reference for a crude measurement and verify TDC with #1 piston installed and a deck dial indicator. That way I'm actually measuring what is happening at the lifter and measuring crankshaft degrees which is what we are communicating.

I don't mean to question your measuring abilities and such but the numbers just don't seem right to me. They would have to seem right for me to comment. I wish I was better at it but I have to stick to what I feel safe with.

Jim Brandberg
Isanti, MN
CorvairRepair.com



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Re: Cam Timing Specs, 1961 145
Posted by: nirvairna ()
Date: November 09, 2019 07:58AM

I don't have a service manual handy, but Bob Helt's book lists much different numbers for an 80 hp cam.

intake is 1 btdc, 27 abdc, exhaust is 44 bbdc, 6 atdc
intake duration is 208, exhaust is 230, overlap is 7

specs at .050 are:

intake is 21 after tdc, 3 abdc, exhaust is 23 bbdc, 18 before tdc
intake duration is 162, exhaust is 185, overlap is negative 39

HTH

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Re: Cam Timing Specs, 1961 145
Posted by: wittsend ()
Date: November 09, 2019 10:54AM

Jim Brandberg - I've included a picture of the numbers I used taken right from the 1961 Corvair Factory Service Manual. I agree the numbers seem strange and that is why I was asking.

As to the lift, as you can see in the manual the tappet lift and the valve lift are stated. So, yes the .314 and the .344 are factoring in the 1.5 rocker arm ratio. A very low lift cam. And in the next post nirvairna indicates a very short duration too.

As to my measurements (at .050) I simply zeroed the dial indicator on the base circle and marked the point (on the timing gear - with a stationary pointer) where the cam rose to .050. I then went over the nose of the cam until I passed .050 on the back side and then backed up to it. I then used a protractor to measure the difference between the two marks and subtracted that for 360 to get the 'at .050 duration." I wasn't looking for a dead on number but I'm sure I was within a few degrees and close enough for this comparison.

nirvairna - Thank you for those specifications. I greatly appreciate it. They seem far closer to what the cam numbers really should be. I'm a little lost as to why the manual stated them as they did although as you can see in the picture they did state "(Theoretical)".

Given the minimal specs on this cam, and as I asked above, would a 95 HP or a 110 cam (I believe it is the 889 cam) be a reasonable step upward? Again this is a 100 mile a year, driving it on city streets, budget build car. It is just that if a "next step up" cam fell into my lap is it going to offer anything or is the 80 HP motor just what it is?

Thank you all again for the replies.



Edited 2 time(s). Last edit at 11/09/2019 11:00AM by wittsend.

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Re: Cam Timing Specs, 1961 145
Posted by: nirvairna ()
Date: November 09, 2019 12:16PM

from Bob Helt's book: "...the Cheverolet Corvair cam specs are crazy. They used both the zero lift points (for their "including ramps" specs) and a combination of differing low-lift points on the ramps (for their "excluding ramps" specs), instead of the SAE recommended .006" lift points. These official Chevrolet cam specs make it impossible to make comparisons with other cams."

the numbers I posted earlier are at "approximate specs @ .010" as per Bob's book.

the book is The Classic Corvair and I highly recommend it.

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Re: Cam Timing Specs, 1961 145
Posted by: 63turbo ()
Date: November 09, 2019 03:00PM

For the reasons above, you can do a lot better by NOT using a "stock" grind... the Otto OT-10 is a better grunt cam than the stockers are. It is the excessively slow ramps on the stock cams that hold them back.

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Re: Cam Timing Specs, 1961 145
Posted by: JimBrandberg ()
Date: November 10, 2019 06:34AM

How do you know where TDC is with a cam on blocks? I've never done it. It must be off the keyway but how do you extend a line accurately, a few degrees off is A LOT.

95 HP LM used the 889 cam; 110 HP used the 891. I don't think the 891 would be an advantage with 8/1 compression ratio engine.

I don't even remember what cam is in a '61 80 HP PG. I usually rebuild LM engines. It costs just as much to rebuild EM and most folks like the idea of "free" displacement afforded by the stroker crankshaft.

The Service Manuals sometimes have wrong information. I don't know if that is the case here.

I'm not sure about using a LM cam in EM. The LM has narrower lobes to clear the stroker crankshaft and rods which doesn't matter. The EM bellhousing has a silver dollar sized relief in it where the cam nose lives. I imagine that a LM cam has a little shorter nose so it would be okay but I'm just not sure.

'61 heads may not have enough valve spring pressure to control a more aggressive cam. 80 HP engines can be sweet running and purr like a kitten. Sort of low demand on the parts.

I would be inclined to use '62 and up heads and lose the choke cable stuff.

Jim Brandberg
Isanti, MN
CorvairRepair.com



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Re: Cam Timing Specs, 1961 145
Posted by: wittsend ()
Date: November 10, 2019 04:23PM

JimBrandberg Wrote:
-------------------------------------------------------
"How do you know where TDC is with a cam on blocks? I've never done it. It must be off the keyway but how do you extend a line accurately, a few degrees off is A LOT."


I'm not setting up the cam in the motor. I'm simply measuring the characteristics of the lobe. I'm not looking for the LSA, Overlap etc., just lift and lobe duration. So TDC is irrelevant. This works on any of the lobes.

1. I simply zero the dial indicator on the base circle of the lobe.

2. I use a stiff piece of wire that is anchored and point it at a position on the face of the cam gear.

3. I rotate the cam in the V blocks until the indicator reads .050 and put a mark on the cam gear where the indicator pointer is.

4. I rotate over the nose of the cam, back down the other side until I read .050 on the other side of the lobe, marking the gear again.

5. I this case I simply held a protractor and measured the degrees between the two marks on the gear (this would be the smaller angle) and I subtract it from 360. This would be the 'at .050 duration.'

No, it is not dead on exact. I was simply trying to see if the GM published numbers in the service manual (which calculated large) and the 'at .050 numbers' seemed to follow "typical" number differences of other cams. Example: I have have a 270 advertised cam in a Valiant I own but the 'at .050' is 206. That is what I was was looking for, just an approximate.

When I degree a cam in I use a degree wheel and a piston stop to locate TDC with a dial indicator on the edge of the lifter to confirm or correct the characteristic timings the manufacturer has spec-ed.



Edited 1 time(s). Last edit at 11/10/2019 04:24PM by wittsend.

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Re: Cam Timing Specs, 1961 145
Posted by: JimBrandberg ()
Date: November 10, 2019 07:08PM

I thought by reading your first post that you were measuring the cam and coming up with timing points. It would seem that you're coming up with the timing points from the Service Manual that you posted in the second post.

The numbers that nirvairna posted from the Bob Helt book seem more in line with what I would expect, especially the Intake centerline in the 104 neighborhood.

I think I measured 889 and 891 cams and compared my findings to the Bob Helt specs and found them pretty close.

I hope I didn't run your thread off the rails.

Jim Brandberg
Isanti, MN
CorvairRepair.com



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