Degreeing a Cam and its Effect on Manifold Vacuum

I installed a TA-20 with 110 LSA cam in my 425. I felt it was the closest I could get to the stock dual quad cam. Manifold Vacuum is low around 13 at stock idle. TA Perf says there should be no problem with idle vac for brake booster but Schneider who made the cam says 14" is all I can expect and that's not enough for a car loaded with accessory vacuum lines. Tom Mooney and Tom Telesco have been gracious assisting in the troubleshooting and process of elimination and its narrowed down to the cam as the root cause.

I did not degree the cam because I wasn't worried about squeezing every ounce of performance out of the engine and did not think degreeing could impact idle vacuum so much, which I understand now is probably an incorrect assumption.

I've spent hours reading about cams and degreeing but haven't seen a case where someone had low vacuum, degreed the cam to spec and it was corrected. Its always relative to performance (HP)

So I am looking for anyone who may know how many degrees a cam would have to be off from the spec to cause a problem with idle vacuum for power brakes? Or better yet actually installed a TA-20 with 110 LSA and what their vacuum is/was.
I have the engine stripped down and plan to check and see how far off the cam is in another day or so. I'd like to keep the TA-20 because I feel the idle lope is very close to the stock dual quad cam but I don't need any nuisance problems or want to run a 900-1000 rpm idle speed so will go with the TA112 cam which is the next step down if it means guaranteeing good idle vacuum.

 

ive been a shade tree mech. sinceage 7 . much to my parents grief , in the early years!! ive never degreed a cam thinking the factory didnt , and if they didnt need to , why did i .ive put a couple cams in ,with 2 degree adv , to get more low end torque , but if that realy works , and 2 retarded gives more high end , it would seem there is leadway . just my thoughts . sure not an educated guess

 

I'm going to subscribe to this thread- I've degreed all the cams in the engines I've built, but the cams have been difficult to get to match the cam card since I am so dense. Overthinking and under following proceedure. I've got a 68 400 with a TA Stage 1 cam that I'm not impressed with- feels like it may be retarded and the idle vacuum is low. If I get into it, it will be degreed in. I am afraid of the "While I'm At It" syndrome on a running car.
I purchased a cam from Russ Martin that is a copy of the infamous 091 cam.. Have not used it yet

 

Ted,
What was Russ' reference number on that cam? There is a chance its the same grind I have. Nobody makes the exact grind of the 091. It sure would make many of our lives easier if we could buy an exact 091 replica. As we know the 091 was 109 LSA which might make it seem even more aggressive than the 110 LSA TA-20 but apparently duration at .050 lift probably has a lot to do with the overlap rating and idle vacuum but I am learning here to. For the record Schneider told me the TA-20 110 LSA has 57 degrees running overlap and that was the reason for the lower idle vac rating. I will guess the Schneider rep is using data vs hands on nailhead experience to arrive at the expected vacuum number so there could be a margin or error either way in real world application.

TA Perf website lists the TA-20 as having 112 LSA but when I bought it about 5 years ago, they only had it available in 110 LSA and a recent conversation with TA (if I understood correctly) indicated the TA-20 110 LSA has been standard for a good while (years)

 

Jason,


As Russ mentioned ANY aftermarket cam needs to be "Degreed In". They are NOT produced like a factory cam which was MORE ACCURATELY ground than ANY aftermarket cam can ever be. Don't forget that the ORIGINAL 091 cam was ground with a built in 4* retard also. Unfortunately you will spend MANY hours learning how to do this. One of the things I've learned to make it easier is to take measurements at .050" before & .050" after MAX lobe lift rather than making complete rotations.
Hope this helps.

 

Here is the cam card for the one I got from Russ.

 

There's quite a few things that can affect vacuum a few inches, maybe even add some up on you if variables are stacked up just right.
Ignition timing the most obvious, you need more initial timing with most aftermarket cams (Not looking closely at the differences here...).
I would start with that and shorten up the total advance if it helps.
Too much throttle opening for idle would go right along with it.
Was the diagnosis in person, or just phone and internet?
As sharp as help as you have had, are you sure EVERYTHING else is perfect?
Any testing to the rings or valve job done?
Did the vac gauge needle show any shake either at steady or changing throttle conditions?
Is it a responsive enough gauge?
What I'm getting at is that everything is fair game and up to question.

It should not be necessary to change cams just to achieve a couple inches of vacuum.
Your cam is tiny for a big engine. Vacuum should be a bit higher.
Advancing it 'could' easily solve the problem.

A few degrees cam timing can indeed affect vacuum, along with slight differences in both static and trapped compression, from the intake valve closing sooner or later.
You can see A to B changes from head work alone, valve jobs, geometry and especially any porting, even mild.
Many times, just good porting work can increase vacuum, even if it seems that more CFM's would be counterproductive to that.
Anything that affects mixture motion prior to combustion can change vacuum (swirl, tumble, atomization...even carb type changes).
Often times a little bit more overlap can help vacuum...if the top end wants those cam specs.
A person would have to really look closely at every machined and blueprint spec to determine the differences, meaning it isn't likely that anything more than guessing would figure out your issue unless you have a glaringly obvious error.
Ex., more cam overlap combined with cam timing way off and an unsuitable exhaust system, blocked off carb heat... .080" quench clearance...the list goes on.
Someone's bound to guess right, I suppose you'll let us know the effect of things after you've tried them?

I do a LOT of top end work. Head work, flow bench, cams, etc. and see issues like these on a daily basis, with all types of engines.
I can see a change in vacuum with as little as different VJ geometry along with mild porting.
Vacuum can be viewed as a net measurement of how well things are working together.

Curious to follow this one.

[An example is that I put together a 406" @ 10.8-1 comp. with 270* @ .050" cam, 105* LSA, going through a well ported single plane manifold and 310 cfm heads (2.050" valve) with a worked 850 DP on top that showed 12" vac @ 900 rpms and 14" @ 1000-1100rpms.
The 328 CFM 2.080" valve heads showed only 8" vacuum and wouldn't idle below 1350 rpms. The heads CSA was greater leading to a higher peak torque rpm (regardless of the cam) thus changing it's power band.
The ports also measured less swirl, which seemed to affect idle quality, as the chambers were exactly the same fast-burn type, as indicated by the burn patterns]

 

4-6 degrees off can make a substantial difference, even in the bigger 455 motors. The smaller motor will actually be a bit more sensitive to it.

Follow the directions, don't over think it, it's not that hard..

http://www.v8buick.com/showthread.php?220945-Camshaft-degreeing-101


Your Nail will have different looking parts, but the procedure is exactly the same.

Having them ground so far retarded that I have to advance the timing chain a full tooth, is not as unusual as you would hope.

Doing a Crower cam right now that is exactly that way.. nothing wrong with the grind, it;s just indexed incorrectly.

Correct it with the chain, and the engine does not know the difference.

JW

 

you have to look at the advertise duration at .006. the ta-20 has a high 280* duration. that is one reason for low vacuum. the ta-112 is a lot closer to the 091 cam. ta-112 has only 255* intake. advertise duration, much better idle and vacuum. 255* is what is listed in there catalog. a nice copy of the idle of 091 cam would be 210*-210* on a 109 LSA and a 109 centerline, but with more lift say 470-475 lift for better all around performance. why no lift on russ cam card ?

 

Thanks for the replies everyone. Attached my cam info. Maybe I am misunderstanding but it says 0 overlap yet on the slightly less aggressive TA-112 that Ted listed above, it has 42.5deg overlap. The Schneider representative said the TA-20/110 grind has 56 degrees overlap which seems more logical after looking at the TA-112 card. Is my card in error or?

Ted,
Its interesting the "Degree Intake Lobe To" figure is overwritten on your card. I can't read the number for certain.

 

Ok that explains it then. Too many similar numbers/variation for a novice to cypher. Initially I could not understand how I could have low idle vac with 0 overlap as stated on the card.

I'm right at 12 degrees initial at idle. Advancing more than that really didn't seem to change the vacuum but retarding it lowered vacuum.
Its closer to 12" of vac actually. See short vid here. Idle was around 650rpm. When it reached 19-20" rpm was about 1900.
https://www.youtube.com/watch?v=ju_J2Vy32qQ

 

If you have a dial indicator you can do a quick cam timing check just by removing the valve cover and rocker arm assembly over cylinder#1.
Place the indicator on the end of #1 intake pushrod when the lifter is all the way down and zero the dial.
Turn the crank clockwise till the dial reads .050"
The timing mark on the crank should read 3 degrees BTDC if the cam is timed correctly.

 

is your vacuum advance line going to the right side of carb, the lower non ported side. it should be, this will help your vacuum.

 

I thought about getting another gauge but since the booster had no boost until I started to raise rpms slightly close to 900-1000, I felt it was pretty close. Within +/- 1" anyway.

The heads are off and I am having them evaluated just to make sure nothing was missed. I do not want to take it apart again.

 

Vac advance is ported and the temp line you can see in video was plugged. For reference the gauge was connected directly to the intake manifold port. All other accessory vacuum was disconnected. Though connected in the video the brake booster was also disconnected and result was same.

 

Couldn't find anywhere in shop manual that listed min vacuum requirement for the brake booster. The 66 has the smaller diameter booster vs the larger 67 and up units. I suspect the smaller dia would require a higher minimum.

Checking the mainstream forums like Chevelle, Camaro, etc where this has been discussed more I am seeing 16" come up as the recommended minimum. I could check with Booster Dewey and see what he says. He rebuilt the booster.

Just called Booster Dewey - They say 14 is the min for that booster. He said I could install a reservoir which would solve problem but that is not an option on this authentic project

 

Kinda off subject, but I must say those carbs. I rebuilt for you look good.