NixVegaGT Nicolas Wiederhold Minneapolis, MN (659 posts) Registered: 10/16/2007 05:30AM Main British Car: '73 Vega GT 4.9L Rover/Buick Stroker |
Re: cam option
I don't suppose you know a part number or something? I'll post it on my website just in case it's useful to someone.
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BlownMGB-V8 Jim Blackwood 9406 Gunpowder Rd., Florence, KY 41042 (6468 posts) Registered: 10/23/2007 12:59PM Main British Car: 1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS |
Re: cam option
Just got done degreeing my cam and the specs are a bit different from what I expected to see.I tried to check it as accurately as possible using an 18" wheel and a .050" sweep 1" dial indicator but found I was fighting 5* of backlash in the new double roller chain, compliments of the align bore (About a quarter inch of slack). I used a pair of solid lifters swapped in for checking. Anyway the actual specs are .424/.424 lift, intake open at 5* atdc (all at .050") closed at 52* abdc, exhaust open at 38* bbdc and closed at 7* atdc. Specs were .436/.436, -4btdc, 36*abdc, 40*bbdc and -8*atdc. The biggest discrepancies were closing of the intake (16* later than spec) and closing of exhaust (15* later than spec).
My measurement for lobe centers was 105 for exhaust measured at max lift vs 114 specified and I had a little trouble getting a good read on the intake, first measurement was 115 but it didn't take account of the chain slop. 2nd measurement was 121, vs. spec of 110. Lobe separation was specified at 112. I don't understand how you calculate the lobe separation angle so I wasn't able to come up with that measurement. This is the first time I've used a degree wheel, could some of you more experienced individuals perhaps throw me a bone here and tell me what this all means? It sounds to me like the grind doesn't match the spec card and everything is lagging by almost 15 degrees except the opening points. As it stands I'm at a real loss as to whether the cam should be installed straight up or perhaps advanced some and if so how much. This is a high compression (10.6:1) blower engine with low boost. We were shooting for economy and extended rpm range, not maximum power output. I suspect Charles may have extended the duration to enable the higher speed operation, but how much is that going to affect the economy? Jim |
NixVegaGT Nicolas Wiederhold Minneapolis, MN (659 posts) Registered: 10/16/2007 05:30AM Main British Car: '73 Vega GT 4.9L Rover/Buick Stroker |
Re: cam option
Did you mean to say -5º ATDC? If it's 5º ATDC that seems strange. the difference between 36º and 52º is insane. WOW.
I was going to say you could advance the cam a bit to balance the numbers. Now that I look at it I really don't get -8º atdc for the exhaust opening from the adv. specs. And the intake opening at -4º btdc? This would equate to both valves being closed where there is overlap. Maybe it means -4º ATDC for intake opening and 8º ATDC for the exhaust. That would make more sense to me. Then again this is at 0.050" so maybe there is a tiny bit of adv overlap. Is this because it's for a superchager? |
BlownMGB-V8 Jim Blackwood 9406 Gunpowder Rd., Florence, KY 41042 (6468 posts) Registered: 10/23/2007 12:59PM Main British Car: 1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS |
Re: cam option
No typo Nick, at least not on my end. -4btdc would be 4atdc, off 1* from what I measured (5). The .050" measuring point does mean there is some overlap, but maybe not all that much. Charles *was* grinding it for a blower app. TA says in their catalog that all stock timing chain sets have 4* advance built in, which would (should) shift the numbers a little if I use the silent chain set. Maybe I can use the V6 chain tensioner with that. I'm looking into it. That assumes my double roller set is "straight up" and it may not be.
Jim |
MGBV8 Carl Floyd Kingsport, TN (4511 posts) Registered: 10/23/2007 11:32PM Main British Car: 1979 MGB Buick 215 |
Re: cam option
"TA says in their catalog that all stock timing chain sets have 4* advance built in"
That's so when it stretches breaking in, it will be closer to correct. ;) |
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BlownMGB-V8 Jim Blackwood 9406 Gunpowder Rd., Florence, KY 41042 (6468 posts) Registered: 10/23/2007 12:59PM Main British Car: 1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS |
Re: cam option
Maybe so. These numbers are at the stock location. Guess I'll run it like that and see, just need to get an oversize timing set, looks like probably .008" over. Anybody know of a good place to get that?
Jim |