effect of cam timinmg on fuel system

Hi go from guys, we run a 532 BBC with HH SSI 10=71 approx 30lb boost, methanol. Last year we had the tune up abolut right but found the cam was 6 degrees retarded. We fixed this and the only other change was to move the blower back another 2 1/4". When we raced this weekend we had to go from a 95 main to a 125 main to get some colour in the plugs. Our data logger wasnt working so I dont know what boost we had but the car was also 20mph down and .5 sec slower. What effect would you expect from having a cam retarded in this manner?

 

cam timing

Your Cam and Cam timing is everything to your engine did you put it together and check it the first time? How did you degree it in? I think most blower guys will tell you not to look at lobe center its where the intake opens and closes that counts.

 

Cam timing

Hi, He is right cam timing is extremely important.

` By changing it your changing how much air is entering and exiting the motor via moving the intake and exhaust cycles. By advancing it for example you may see a 2lb drop in your boost which means the engine is now most likely using 2lbs more of air and would need to be fattened up to match. In some cases depending on cam design it just wasted the air and pushed some out on overlap. I don't remember where the normal cam timing points are for a blown alky BBC so if someone can chime in that may help but I would start around 34 degrees advanced at .050 lift and play with it +/- 1 degree from there watching performance after adjusting the fuel system. Remember to check valve to piston clearance each time.

I have been told the better the blower the more you can advance this to an extent, with a smaller blower it may end up at 32 degrees or so.

The other thing you could do is call the cam manufacturer with the part number or serial number and give them your combo...see what they recommend.

your setback will change the fuel distribution some as well.

I know its a bit vague but I hope it helps a little,

Trevor Sherwood
727-364-4368

 

cam timing

thanks guys, we used the intake opening at 050 will check everything over the weekend . all the best

 

How did you miss you number by 6 degrees, was your TDC mark in the wrong place ?

 

BBC timing

YES the TDC marker was off which screwed up everything. That combined with a brain fade on my part !!. While I have your attention, what would you expect to see (fuel requirements wise) if you had your ignition timing retarded?

 

I can't answer you fuel question, but I can ask you some other questions ---- like since you moved the blower and have no data or boost readings --- are you 100% sure you don't have a boost leak ?

They don't look like much, usually a grey goo looking small trail on the manifold somewhere---

Do you set your TDC mark with a dial indicator setup ? Or a hard stop on top of the bore ?

 

BBC timing

hi again, I cant find any evidence of a manifold leak but will have the data logger running again for next time. We set TDC using dial indicator on top of the piston. However the reference marker worked loose and moved without us realising

 

If you set top dead center with a hard stop it will be correct--- there's room for error using a dial indicator -----use a hard stop ---

 

What are your cam specs, gearing, weight, converter or clutch, etc.? This will help determine at what point your valve events should occur. You are sure that the cam was in the retarded position when you discovered that the pointer had moved? Too little timing will make the engine a little on the lazy side and a telltale would be high egts with no appreciable color on the plugs. If you leaned your engine way down to get color on the plugs and the timing is way retarded and you are planning on adding some timing back in, add some fuel back as well. To me it is pointless to run the car without the data recorder and make changes because you don't know how the changes are effecting the car. Did you check your piston to valve after moving the cam 6 degrees? Can you post or pm the valve events @ .050 for each scenario?