hello all,

 I have a 355 with 7.8 to 1 compression and edelbrock rpm heads with a roller comp cam with dur,224 in & 236 ex @ .050 with lift of .502 in & .520 ex. w/ 1.6 ratio roller rockers. My cam is set at 108.0 intake center line. i have a msd dist., coil, and 6btm box for the ignition stuff. I am running low comp. because i have a procharger p1sc producing around 18 pounds of boost.

my problem is that when i set my timing, it loves 15 to 20 degree's at an idle, and when i run down the it starts to pop thru the exhaust at certain rpm's above 3000. when i set the timing around 10 degree's at an idle it sounds slughish and not as respondsive as 20 degree's. this is the first blown motor i built for a super charger with low compression so this problem is new to me. i have asked a few machine shops and they think that it is unburnt fuel, because of the low compression. any help would be greatfully apreciated.

thanks, micky

with my buds blower motor we used an msd boost timing master and set the timing at idle was 15 or 16 and full advance was 32,  with the btm you can make it retard 1-3 degrees for every pound of boost up to 15 degrees,  I think you may need something like that for all that boost...I hope your cylinders are o-ringed  thats alotta boost.  From my experience your cam may have too much "overlap" when the intake and exhaust valves are open at the same time.  With a blower I believe your lsa should be 112 or higher and your exhaust duration 12* more than the intake( which i see you have)  what holley told me when I had bout the same problem was that with the overlap it was pushing the boost out the tail pipes.  

Blower motors need less advance as boost goes up. The increasing mixture density with RPM burns faster. The end result is a blower motor will like a lot of advance at idle because of the low compression ratio and lack of cylinder filling till the blower comes up, then it wants less advance.

You have to go back into the distributor and not only rework the advance curve rate with springs and weights, but the amount of advance the flyweights can put in by virtue of the distance they move will have to be reduced so that the amount of reduction degrees can be added to the base setting.

The problem you have now is that the centrifugal advance can put in too many degrees. When you time the base for best idle and low speed performance, the centrifugal advance puts in too many degrees on top of the base setting and over advances the engine. The result is power surge and blowing back through the intake. If this was a positive displacement blower like a Roots, you'd do some serious damage to the engine, blower of both.

You may also have to go to a pressure retard system. This looks a lot like a vacuum advance except instead of using manifold vacuum to advance the spark, it uses manifold pressure to retard the spark.

The blower manufacturer or sales point of the unit should offer techncial help that you can use as a guide to a successful installation.

Bogie 

this is the second cam i have tried and the 1st cam was a 114 lobe separation, the new cam is a 113 lope separation. I do have a 6btm on my car but my main problem is when its cruzin at a constant speed and its not making any boost. If you give it any gas to speed up, it goes away.

I will try to adjust my total advance in my dist. to see if that works. I think this will do it because when i retard the timing i can get the popping to go away.

 Thanks for your input.

micky;

My 1+ year old F1 procharger is at ATI [procharger] for warranty consideration. I agree with what others have said here so far, but there are a lot of variables to consider.

I'm still tuning on my setup too, it likes a lot of timing at idle and cruise, less under boost.

Is your set up intercooled? what octane fuel do you use? surge valve? Do you know your air/fuel ratios?

My combo blows 15psi @ 6200rpm thru an intercooled, EFI, 8.9 to 1 383sb, 255/262 @.050, .613lift & 114 LS solid roller.

So far its made 712 rwHP with 27 degrees timing @6200, on the Westech chassis dyno.  At the track, best ET is 10.50 @ 130mph .

NOTE! On pump gas I had to run even less timing or detonation was a problem under boost.

So tell me more about your set up. Maybe we can compare tuning notes and ideas here, then oldBogie can keep us out of trouble.

 Good luck  Bill. 

I am running a 670 cfm holley and yes i am running a 2 core intercooler.

I have been using 91 octane gas and not yet have i had any detonation problems. I just installed new springs in my dist. so my advance kicks in sooner and reset my timing to 9 degree's at a idle.

Surge valve? I do have a blow off valve that came with the procharger.

I have been considering a fuel injection unit of some type instead of a carb.

As for a dyno or et at the drag strip, very soon.

Thanks Micky

micky;  What do you think your total ignition timing advance is ?

I had mine at 27 total , and was starting to increase it one or two degrees each pass, when the procharger died. Sucks.

I've been using VP100 unleaded , but might need 103 to get best power with [ i'm  guessing]  about 32 degrees total ignition advance?

Also do you know what your AFR [air/fuel ratio] is? I'm in the low 11's to 11.5  That might be a bit rich, but I'm not sure?

   Bill.