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Discussion Starter #1
Hi all,
I currently have my engine apart because of some head work(leaky exhaust valve). As this is being fixed I am wondering if I should advance the cam by 4 degrees or not.
The reason why is that my vacuum is a bit low (15 to 16 HG)which causes the brake booster to assist less driving slow and off idle response is also not very strong. I have already bumped the ignition timing to 17 degrees and shortened/quickened the advance curve to 36 degrees at 2700 rpm and played with the carb calibration and vac advance. It is ok now but I think there is more in it.

What I have in my '66 mustang:
’87 Roller 302 .020 over.
Edelbrock 500 cfm AVS with Performer intake
Stock exhaust manifolds
’68 302 heads with 60 cc chambers.
Flat top pistons with valve reliefs.
Deck height is stock i think, slightly higher then the piston itself at tdc
Cranking pressure is 148 PSI hot.
Compression was said to be 10:1 but have not calculated this myself.
2.79 stock gears
5.0 HO Camshaft ’86 / ’93 mustangs - Melling SYB-51 :
Specs @.050 Open Closes Duration Centerline [email protected]
INT. 11 ATC 41 ABC 210 115 .445
EXH. 40 BBC 9 BTC 211 115 .445
I created a degree wheel on the balancer and specs are definitely in range within a degree or 2.

Would it be worth it to advance this cam 4 degrees? Any experience doing this mod on a Mustang 5.0 HO engine?
 

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FWIW I would suggest that if you have low vacuum with a stock 5.0 camshaft, the problem is not with the cam. It has mild specs with a wide LSA designed to work with EFI that already has good low-mid range torque production.
 

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Leave it straight up. You have something else going on. I'm running a 91 or 92 5.0 cam. I degreed it when I installed it. When looking up the specs I got the impression from guys who raced on stock classes checking cams, the specs kinda depends on what day they were made do to loose quality control. Mine seemed to be pretty good. My LSA looked to be 114.5°.
 

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Discussion Starter #4
Thanks for your responses.
Yes, there is a little more going on. The exhaust valve on cyl 8 was not sealing up lately resulting in 100 PSI compression instead of the other cyls having around 148 PSI. This started recently but vacuum has always been a bit low with this engine(bought it freshly rebuilt). I also had one roller lifter that did not hold pressure very well(slight tick).
Other then that the engine is running with good power low/mid/high.
I was just looking to make a (small) improvement in advancing the cam for a little more low/mid power as this is the range i drive mostly.
If you say it won't bring me much i'll leave the cam straight up and see how it does with the fresh sealing valves and lifter.
 

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I'd also leave it straight up. I suspect that your static CR is nowhere near 10:1 but without knowing what compression height piston was used, who really knows. If you have stock (8.206") deck height, a 1.585"CH piston and 60.4cc chambers then your static CR is closer to 9.07:1 or, at the stock compression height of 1.600", 9.40:1.

I'd also be very deliberate in checking lifter pre-load to ensure you aren't setting the valves too tight. Do you have screw-in studs? Are they fully adjustable or positive stop?

When you have it all back together I'd be looking at your spark advance curve. You might be surprised that with your static CR and squish that total timing in excess of 40* may not be all that unheard of.... my '85HO runs 44* total.
 

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Actually 1 cylinder displaces 617.8 cc. Divide that by 60cc you end up with 10.3:1 now factor in head gasket, valve reliefs and how far down the piston is, I can see being low to mid 9's in the CR.
 

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Discussion Starter #7
Thanks for your replies again!
The engine builder told me it had flattops and 10:1 compression. I now realize that can't be the case. This is not the only thing they lied about but that's what it is. It has Speed Pro 654P pistions with 1.599 Compression Height and 4cc dish/reliefs. With your handy calculation this would be 4cc + 10cc(head gasket) + 60= 74 resulting in 8.3:1. Still have to scrape of the gaskets to check for deck height.
I have screw in studs and adjustable rockers. I also checked preload with the engine running and vac gauge. I found no drop in idle vacuum with 0 turns to 1 3/4 turn preload. After 2 turns it keeps the valve open and will start running bad. Normally i have it with 1/2 turn preload.
You are right it will take a lot of timing till it pings. I have had it dialed to 45 degrees all in to test and really couldn't tell if it was pinging and it seemed to be faster. It is a benefit that with advancing ignition timing you don't affect high rpm HP. When advancing cam timing i guess i will get some more power down low but this is at the expense of high rpm power..
 

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Richard, not familiar with that part number. But some pistons are short, some are tall, some have 4cc notches,and some have 8cc notches. What is a Speed Pro 654P ? I am not finding that in my memory,........LSG
 
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