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How much HP is this exhaust costing my 408W?

10K views 44 replies 18 participants last post by  Jeff351w 
#1 · (Edited)
Hey guys.

Just curious how much this exhaust is robbing HP from my 408W. I have Hooker LT headers and the 2 1/4" pipes are crimped bent over the axles and mufflers down to about 2" in several places. Ive just picked up a Magnaflow 2 1/2" kit that will be fitted next month.

My last dyno run was a tad disappointing @ 352 HP, with AFR heads, custom cam and Performer RPM manifold I would have expected better numbers. The only component that I can see lacking is the exhaust.

Any guesses on what kind of HP gains I might expect to see with the new system?

Cheers!
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#2 ·
Do you have any sort of crossover piping in place. Like an H or X? Have to admit, some of those bends do look pretty restrictive. But adding a crossover might help more than nicer bends would. Either way, might be a good time for a new set of pipes.
 
#5 ·
A couple Q's (starting at the top):

What carb are you using?
Which AFRs?
Is the "custom" cam matched to your heads/intake? If it is a 7000rpm cam your intake side may need adjustment.

Assuming these numbers are on a chassis dyno, but what type (Superdyne or Mustang)?

What transmission are you using?

You should pick up some HP with a 2.5/3.0 inch exhaust but I wouldn't expect a 50 hp gain.
 
#7 ·
This empirical data shows the effect of some restriction is not the +75Hp some claim by bolting on a set of headers or exhaust....:smile2:


 
#13 ·
This empirical data shows the effect of some restriction is not the +75Hp some claim by bolting on a set of headers or exhaust....:smile2:


https://www.youtube.com/watch?v=azPKIjxmmdU
This exact video came to mind for me as well. I wonder if restrictions downstream are actually worse than in the headers as they have less velocity to push the gases through.
 
#8 ·
I have to admit I'm surprised by that number to be honest and can understand why you would be disappointed. You aren't going to find the difference in exhaust I don't think personally. There has to be something else farther upstream not quite working. Maybe time to start looking at timing curves and jetting? It doesn't sound like any of your "hard" parts are the bottle neck. Unless you have some sort of mismatch where you have unusually low compression or something else funky going on. It could also just be a heartbreaker dyno, some of em' just read that way.

Just thinking out loud. The exhaust certainly isn't helping, but I don;t think it's the silver bullet here either.
 
G
#12 ·
I'm glad you will have it Dyno'd again to see the results. Not many have ventured a guess :) Here's my thoughts . . . on a Dyno your WOT hp is going to go up by 18hp-22hp. I think that amount will be fairly consistent from about 2500 RPMs up. Can't wait to see the results.
 
#14 · (Edited)
This should be interesting. I wonder how much is lost on the outlet of the muffler having the tail pipe slip inside? You have that sharp transition. I'd like to have it slip over the muffler's exit. On bang Shift, one member is a retired Chrysler exhaust engineer. He said the rolled lip common on today's tail pipe tips cost a few HP too.

On the dyno, was it a load bearing or inertia? Load bearing will show lower reading. A few years ago Hot Rod did a article on chassis dynos. They took 1 car and went to a half dozen different places in one day. What they found was everyone had did different results and the more runs you did on anyone dyno, the HP would climb each run with everything being the same
 
#15 ·
No doubt the exhaust is a restriction. Not knowing the compression and cam, the following are my thoughts. AFR225 are big, I have AFR205's on a 418W. I suggest a Holly 950. The performer RPM is restricting. I ported and flow balanced mine and it made a big difference. Alan



Hey guys.

Just curious how much this exhaust is robbing HP from my 408W. I have Hooker LT headers and the 2 1/4" pipes are crimped bent over the axles and mufflers down to about 2" in several places. Ive just picked up a Magnaflow 2 1/2" kit that will be fitted next month.

My last dyno run was a tad disappointing @ 352 HP, with AFR heads, custom cam and Performer RPM manifold I would have expected better numbers. The only component that I can see lacking is the exhaust.

Any guesses on what kind of HP gains I might expect to see with the new system?

Cheers!
 
#19 ·
I had a similar exhaust setup on my old 306 combo. I replaced the front half with 2 1/2 pipes and free flowing mufflers and even with the crimped up 2 1/4 tailpipes there was a noticeable increase in power. About 3/10ths quicker at the track in the 1/4 mile and a couple mph. I agree the exhaust,intake and carb are all holding you back some. Track mph would be a better gauge though like was mentioned earlier. I have a buddy who's mustang dyno'd at around 390hp on a chassis dyno but was more like 600hp at the flywheel.
 
#23 ·
Yes, those pipes are not optimal. But, how much it's costing 10-15? I'd start with getting a mandrel bend system, then check the following:
What are the AFR numbers from 2500 to your cutoff point?
Have you re-curved the distributor?
You don't mention the intended use of this build? Street, street strip?
The torque numbers look good though.
Good Luck....
 
#27 · (Edited)
The AFR205's are OK and good to at least 600HP at the motor. (I've been 10.49@128+ in the quarter with them on a 427w NA with a 3550lb car street/strip car.) Intake and exhaust are everything for them (as in other good heads.) I used a mildly modified Vic Jr (angle milled for hood clearance), 1 3/4" primary tube headers and 3" exhaust tubes, headers, and mufflers to run that fast. This setup can fit under the stock hood with a few other mods, like a drop base air filter and lowered motor mounts. I did that for years and years.

Street driveability is awesome and it can pull away at 1800rpm's in 4th gear when rolling. I've been told by Ed Curtis and other cam designers that there was more in it and specifically that the 1 3/4" headers were killing it . . . . These are the old Outlaw 205's . . . .

I'm not trying to make the OP a drag racer, but merely giving a comparison of what power is lost when compromises are made. Just jumping on the bandwagon that the intake and exhaust are essential to make good power. To relate this back to the thread topic, don't discount what the proper exhaust can do for a properly built motor.

The photo in my sig is actually an old one from the 10.49 run with the above combo using the same basic equipment that I used under a stock hood. The description is of the new motor being built.
 
#29 ·
My "guess" is that the max torque will move higher in the RPM range. You might loose some low end grunt, might get a little on the top if you are spinning beyond 6500. That's just a guess. That said, I got rid of my 2 1/4 muffler shop bent exhaust for my Performer RPM 351 and built a mandrel bent just because I needed to re-route my exhaust around my new Watts Link. I could'nt bring myself to do anything but mandrel bent just because... It didn't remarkably change the personality of my car but I'm happier when I look at it!

I tend to do these projects over the winters here in upstate so by spring my mustang feels like greased lightning after 5 months driving a 5 cylinder Volvo and a 6 cylinder jeep, probably would still feel staggeringly fast with 2 spark plug wires shorted ;o) Frankly, even sitting in the Recaros in the garage dreaming of spring feels faster than the volvo/jeep.

Be interesting to see how you fare...
 
#30 ·
The performer RPM is a very good manifold out of the box. If the runners are worked on to increase flow and balance, you will love it on the street with your 408. My Performer RPM runners flow around 285 CFM @ 28 in pressure which is pretty mild, it had a notable performance increase and even sounded different.
 
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