Advance or Retard cam question?

[cgarb]

I have an engine I'm helping my stepbrother put together. He's just getting into racing and is on a pretty tight budget. Its a 350 that is a garage cleanout special. It's a STD bore motor with forged single valve relief trw pistons. Scat replacement rods and an Auto Zone crank kit. The heads came off a dirt car, they are ported smog castings with 2.02 1.60's. Not the best choice but the price was right. I converted them to screw in studs and did a valvejob on them for him. I plan to use the .015 shim head gaskets to get the most compression we can get and I had .025" milled off the heads. I would guess the compression would be around 9.5-1. Its getting the smallest flat tappet cam I had laying around in the shop and its 250ish at .050 like .530/540 lift. I do remember its on a 106. The motor is going in a really light Mazda RX-7. Probably finished weight under 2500 for sure maybe even under 2400. Powerglide with a 10" 3800-4000 stall speed. 4.10 rear with 29" tires. My question is if I have lower static compression than desired would it help to install the cam adavnced or retard to try and raise the dynamic compression and which way would I want to go? He has a timing set that will do 4deg plus or minus. My dad always told me advancing the cam will help with low end torque, so if that is true that probably would be the way I would want to go. Any advice would be helpful. Thanks

[wyrmrider]

What intake centerline is on the cam card?
If the LCA is 106 then intake Centerline would be 106 for straight up
(if this a symmetrical lobe, if asymmetrical how does the cam grinder say you should determine the Intake centerline? (ICL)
It may be ground 4 degrees advanced from the timing mark on a 0 degree gear or it might be anywhere the camgrinder could get the lobe on the core
you have to check
You've run this before?
With no additional information measure down the .050 down from max lift on both flanks and find the Centerline
and
measure up .050 from the base circle and find the centerline
a symmetrical lobe will be the same
Instal at 106 ICL from the top down method and check your VP clearance before and after TDC- find the closest point and write it down
advance the cam check again
run the car
you have so many variables that you need a baseline
The cam (timing) may not (and most likely will not) be the first thing that needs tuning

[cgarb]

I have the camcard at the shop, I just dont have a photographic memory. Yes its one of my old ones, a Reed that was ground for my car. The 106 was the lobe sep. I was just asking in general if advancing the cam makes the cranking compression increase or decrease.

[wyrmrider]

increase- up to a point
https://home.comcast.net/~ingramenginee ... te/?/home/
may have info on your reed cam
he also may be able to tweak it if needed
but you have to know where you are to know which way to go

[F-BIRD'88]

I would degree it and install on a 100 to 102deg intake c/L based on the .050" timing points that you found
by actually degreeing it. The 4deg keyway probabily won't be 4deg. So check after ya move it.
Advancing the cam in the motor moves the intake valve closer to the piston.
So check VTP clearance. You can monkey with the valve lash a bit +/- at the track.
But start with it rather tight.

You won;t make any real HP with smoggers so you might as well max the torque.
You need that with a glide. Needs more gear or a shorter tire.
You need aggresive gearing with a glide.

[cgarb]

This is not a motor we are expecting to set the world on fire with. I just want to give it as much of a chance as I can. It will be his first time down the track in something other than a pickup. This will be a good motor to make rookie mistakes on, its an old loose bore and he got it cheap. I remember being excited and forgetting to turn the fan and waterpump on only to look down on the return road to find the temp at 240. I didnt build something too nice because of that and keep it slowed down some. We chose the 4.11 on growth factor. Put a health 383 in that car and a 4.11 will fit the bill pretty decent.

[rally]

More torque, adavnce, more top end, retard. Very simple. You just have to try it and make sure to check valve clearence.

[Cubic_Cleveland]

To answer you question in your first post, advancing the cam will gain you "dynamic" compression as you are closing the intake valve sooner. Like F-Bird said, re-check intake P to V as it will decrease the more you advance the cam.

[cgarb]

Thank you Cubic_Cleveland.

[Force Fed Mopar]

Seems like a lot of stall for that engine...

[cgarb]

Well i know its not the converter summit racings converter selecter recomended but i think it will be ok. We are going to find out labor day weekend provided no major problems show up.

[wyrmrider]

good luck labor day
let us know the density altitude etc

[jeff swisher]

I retarded a 292H and lost topend and lost low and midrange also.
straight up it ran hard to 7500 rpm,, retarded 4 degrees it quit pulling at 7000..
lost 2-3 tenths in the 1/4.
Theory does not always play out once you put it to the test.

[cgarb]

I will let you know what I can...I don't monitor the weather when I race. I just watch the cars, ones I know and dial mine to what they are trending. We will be at Mason Dixon Dragway in Hagerstown MD. Its at around 900ft elevation if I remember correctly. I know mostly my car is around 1-2 tenths slower there than Maryland International Raceway. That track in my opinion is the fastest on the east coast. Right around sea level.

[MadBill]

Jeff makes a good point: The majority of engines run best with ~ 4° advance, so lacking any knowledge to the contrary, it's a good setting to go with but the only way to know is by testing. A friend of mine built a 604" Hemi using a custom cam by a very well respected 'boutique' grinder and installed it 4° A as directed. It was plenty stout (I forget the actual numbers) but he decided to try 2° less advance and it picked up almost 15 HP. A further 2° R gained almost 10 more, with no torque loss above the converter stall...

PS: a 1,600 cc Formula Ford single seater I wrenched on in the seventies had to use a stock cam. For our RPM range, it ran best with 8° retard...