X pipe H pipe or side by side

I ran my mazda for about 3 minutes without a headpipe, straight cast 4 into 2 "double barrel" manifold and I could barely get the thing rolling with a stick, NO torque. After I put the full exhaust back on, back to normal stump puller. I did the same with a 318, no manifolds...no restriction, right? Na, didnt run at all. Could have burned the exhaust valves too. I remember seeing a Pontiac gasser with open headers out the front wheel wells and a long a$$ H pipe not 3 inches before the dumps tying both sides together. Looked like a 2nd straight axle.