I want to thank everyone for their replies. I just received an email from Extrude Hone and this is what they said for performace gains on exhaust manifolds: ""Rick says that after processing the average airflow increase is 15% with an approximate 15 to 20 horsepower increase. The cost to Extrude Hone process your exhaust manifolds is $675.00 (per pair) plus shipping. The turn around time will be approximately two weeks""
I just wanted to pass on the information.
1970 YO7 A66 [Canadian Export] F8 Challenger 340 (Currently in shop for stroker assy.)