Dyno results from “Bonneville” motor, 765 WHP, 900 CHP
05-25-2011, 08:09 AM
#211
As I don't know the thickness of the metal in the exhaust,
has the exhaust been tested in relation to long runs? The headers look like coated mild steel. With that much power they may have some trouble with the high levels of heat. The Nationwide cars run minimum 304 stainless with many running 321.
has the exhaust been tested in relation to long runs? The headers look like coated mild steel. With that much power they may have some trouble with the high levels of heat. The Nationwide cars run minimum 304 stainless with many running 321.
Here are some photos of the new exhaust system we just finished for the Bonneville setup.
The primary goal was reducing crankcase pressure caused by exhaust back-pressure, and we succeeded at that.
Last month we were still were running last year's exhaust system; the same 3.0" dual exhaust I ran on last years' 650 HP 16v motor. On this 900 HP motor, we noticed crank pressure go up, so we knew we had too small of an exhaust system for this motor.
Here are the stats from the two exhausts on the same engine:
3.0 dual exhaust: 18 psi in the intake plenum, 2.8 psi in the crankcase at 6800 rpm, and 4 psi of exhaust back-pressure at 6800 rpm.
3.5" dual exhaust: 15 psi in the intake plenum, 2.0 psi in the crankcase at 6800 rpm, and 1 psi of back-pressure at 6800 rpm.
Note that the exhaust was the only change. We ran the same pulleys, supercharger, etc. The difference in intake pressure between the 3.0" and 3.5" exhaust shows that the 3.0" setup was so restrictive at this level (900 HP) that it was backing up thru the cylinders and lowering our volumetric efficiency.
Now the supercharger does not have to work so hard to overcome the back-pressure, the parasitic losses are less, and the engine breathes better.
Made from all 304 SS, .065 wall mandrel elbows, .049 wall straights. V-band clamps instead of header flanges.
Some photos represent work-in-progress. Thats why only the last photo shows the balance tube section.
The primary goal was reducing crankcase pressure caused by exhaust back-pressure, and we succeeded at that.
Last month we were still were running last year's exhaust system; the same 3.0" dual exhaust I ran on last years' 650 HP 16v motor. On this 900 HP motor, we noticed crank pressure go up, so we knew we had too small of an exhaust system for this motor.
Here are the stats from the two exhausts on the same engine:
3.0 dual exhaust: 18 psi in the intake plenum, 2.8 psi in the crankcase at 6800 rpm, and 4 psi of exhaust back-pressure at 6800 rpm.
3.5" dual exhaust: 15 psi in the intake plenum, 2.0 psi in the crankcase at 6800 rpm, and 1 psi of back-pressure at 6800 rpm.
Note that the exhaust was the only change. We ran the same pulleys, supercharger, etc. The difference in intake pressure between the 3.0" and 3.5" exhaust shows that the 3.0" setup was so restrictive at this level (900 HP) that it was backing up thru the cylinders and lowering our volumetric efficiency.
Now the supercharger does not have to work so hard to overcome the back-pressure, the parasitic losses are less, and the engine breathes better.
Made from all 304 SS, .065 wall mandrel elbows, .049 wall straights. V-band clamps instead of header flanges.
Some photos represent work-in-progress. Thats why only the last photo shows the balance tube section.
05-25-2011, 10:48 AM
#212
Developer
Thread Starter
Maybe a foot of 2.5" in the center on each side would help street bottom end perf.
Fortunately, the drop in bottom end performance was minimized because we left the primaries the size they were, we just changed the collectors and back.
05-25-2011, 10:52 AM
#213
Developer
Thread Starter
Carl, at post 204 it looks like you have your front 'axle' stands underneath the thin-walled box section that simply provides a bit of stiffening to the floor pan. Maybe you have stiffened this section or done some other modification (maybe U-section steel supports atop axle stands? - hard to tell from the photos) to make this an OK lifting place.....??
http://www.928motorsports.com/parts/...jackpoints.php
05-25-2011, 10:56 AM
#214
Developer
Thread Starter
As I don't know the thickness of the metal in the exhaust,
has the exhaust been tested in relation to long runs? The headers look like coated mild steel. With that much power they may have some trouble with the high levels of heat. The Nationwide cars run minimum 304 stainless with many running 321.
has the exhaust been tested in relation to long runs? The headers look like coated mild steel. With that much power they may have some trouble with the high levels of heat. The Nationwide cars run minimum 304 stainless with many running 321.
05-25-2011, 11:17 AM
#215
Developer
Thread Starter
Amongst engine builders it is commonly accepted that the right size of tailpipe for a 750 hp engine is 5" or dual 3,5". For a 1000+ hp engine the tailpipe size need to be increased to dual 4".
You should have seen how much fun we had getting the big tube primaries on the headers around the steering column!
In the end,the fix was elegant and simple, but getting there was not.Now that we have measured the back-pressure to be 1 psi at 7000 rpm, I know that the dual 3.5's were the right choice.
Last edited by Carl Fausett; 05-25-2011 at 06:42 PM.
