Exhaust headers gain or loss

Ant · #1 (**permalink**) 07-12-2009, 05:41 AM

We have built a set of custom exhausts that were spec'd in US and came with real nice fluted collectors etc that go down to 3" and then up to 3.5", the fabricator here has built many exhaust systems for championship winning cars, but the job on my car was made more difficult with the rear cylinders being tight with the chassis tubes etc, but we were still able to use standard U-bends and retain the correct diameter and being well within 2" of equal length.

Dyno results the engine made 610bhp with 373 cubes on 10.2 compression which I think is pretty good for pump gas on a SBF, we ran leaded AVGAS for the dyno runs which is rated at 100 octane, there maybe some hp gain if I used unleaded 98 fuel as the compression ratio isnt that high with this engine and the molecular chain and slower burning of the avgas maybe costing me hp compared to the faster burning 98, as I have experienced with engines of less than 10 . 1 comp AVGAS can take the edge right off.

The engine produced 675 bhp when first built, that was on a similar dyno to the one we used, with a slightly leaner mixture and a carb trumpet, and different fuel, granted there are other variables and I am not questioning the validity of the dyno engine builder, my question really is can a custom exhaust system, which is definitely not a back yard built system loose 40 - 60bhp over a dyno set of exhausts, or good headers.

With the second dyno sheets we did all the hp and torque readings were down proportionately across the range compared to the first dyno sheets, so having each rear cylinder a bit tighter in the bend than all the other cylinders or a different design compared to the original dyno headers, to my way of thinking would show up higher up in the rev range more so above 5000rpm where any restriction would inhibit higher rpms, and the lower range starting at 4000rpms would be closer.

Having said all that how long is a bit of string and unless I ship the engine back to the builder with my headers and then compare dynos/exhausts I wont really know!

Jerry Clayton · #2 (**permalink**) 07-12-2009, 10:15 AM

Instead of shipping the engine---borrow the engine builders headers--they will be easier to ship--

Did the engine builder use belt driven water pump and alternator???

Ant · #3 (**permalink**) 07-12-2009, 03:41 PM

Jerry,

Used belt driven WP and oil pump as we did here. I guess this is a hard one or doesnt really matter as I have hp to burn as my car is lightweight with all alloy running gear. Obviously there is a 65bhp difference between dyno's and the different exhausts would make some difference, but that might only be 10-30bhp.
I guess 15 years ago if you had 600+bhp from a 6litre engine that was good and they used to need high compression to achieve that where this has nice flowing heads etc.