Performance loss from headers to HP manifolds

-
M

MrMopar

Well-Known Member
Joined
Jul 7, 2004
Messages
119
Reaction score
0
I am going to instal the 68-70 340 high perf exhaust manifolds instead of the Mopar Performance headers that are currently on the car. The headers have been flattened out on the drivers side bottom 2 pipes as you all know can happen on the A body. But I am concerned about the HP loss when I do this. Putting any kind of header except for TTI's on the Dart is a real pain in the butt and am not looking forward to doing it all over again. I have the manifolds and would like to know how much power will be lost or is it going to be negligable and not really noticeable. Anyone out there do this "backwards" installation?
 
I think you`ll notice your low end torgue will improve, the upper mid range, and top end rpm will suffer, if your not racing you`ll probably not notice, I have fixed my kids twice now, by drilling a hole and using a dent puller, and plug weld the hole and by cutting and replacing sections, easier than pulling and replacing... for now anyways..
 
All depends on your exact combo. I've seen less than 10% difference in published dyno specs, but I think that has a lot to do with re-tuning after the swap to optimize the maniffolds. I feel more than that when I pull the cheapie headers. But, with the right cam, you can make similar power, just not as much.
 
Rick 5150 69 said:
I think you`ll notice your low end torgue will improve, the upper mid range, and top end rpm will suffer, if your not racing you`ll probably not notice, I have fixed my kids twice now, by drilling a hole and using a dent puller, and plug weld the hole and by cutting and replacing sections, easier than pulling and replacing... for now anyways..
Click to expand...

Long tube (under chassis) headers tend to improve the low end power so you will notice a loss through out the rpm range.
 
Now that moper mentions it, I remember reading a write up on a 340 years ago that went from the manifolds to 1-5/8 headers with a disapointing 25 HP gain.
Recently, (1 or 2 years ago....is that recent?) Mopar muscle did a thing on header types.
Comp Cams has a small line of cams for use in engines with exhaust manifolds. The duration specs are wildly long for the exhaust manifolds restriction.
 
Now I am not sure if I want to do the swap back to the old HP exhaust manifolds. It is hard to turn back once you have experienced the power before. Maybe a set of TTI's will have to be in order and sell the HP manifolds to someone who is looking for stock appearance and performance.
 
Mr. Mopar - just bite the bullet and buy the TTIs - they are considerably cheaper in Cdn funds now than when I bought mine - albiet still expensive.
You going to the swap meet in Reddeer on Sat 14th?
 
On a mild motor you will experience much less of a difference than on a built motor. It also depends on your purpose for the car. I'll stick with cast iron for mine.
 
I arranged to have the weekend off so I just may go to the swap meet on the 14th. Last time I went, there wasn't a whole lot of Mopar goodies around. Mostly Chevy and Ford. O.K., here is my set-up:

340 bored .030 using stock cast 10.3:1 pistons , block decked .010
J heads with a mild port and 2.02 intakes installed, milled .035
Edelbrock LD340 intake with Holley 3310 750 cfm vacuum sec. carb
Comp cams XE268 Extreme energy cam
Felpro .050 head gasket ( needed more thickness so the valves wouldn't hit, being cautious!)
Final comp ratio 9.8:1 The stock pistons are not near 10.3:1 as advertised.
Mopar performance headers
Sniper NOS 125 HP jets.
2/14" exhaust with H-pipe into Dynomax Super turbo mufflers and tailpipes.
3.21 rear gears
727 trans with TF2 shift kit and 2500 stall convertor.

Maybe I need the headers with this set-up, maybe the cast manifolds will choke it too much. More input needed please.
 
I think manifolds would be fine..And that 268 cam should clear the pistons by a mile, even with no valve releifs. But, running NOS thru it will really restrict things. If it were me, I'd run the manifolds without flinching a bit, but I'd upgrade to a 2.5" exh system. The XEs extra duration will help pump the exh thru the manifolds. I dont think you'll lose much unless you consider the choking from NOS use. And all that means is instead of 125 ish HP, the kit will make 100. Not a big deal IMO.
 
What Rick said. Fix it. Pull'em and cut out the flat section and weld in a new piece. Always a good idea to save 'old used' headers. Never know when they will come in handy. Easy fix...you can do it, and it isn't 700. bucks. Good luck, Terry.
 
Pulling the headers and then putting them back on is not an option, once they come out, they are staying out, the Mopar Performance headers fit like crap and were a real bear to get in the driver side. I guess I will put the manifolds on since I have them, and try it out. If it doesn't work out then it will be a toss up between TTI and Dougs headers. Thanks for all of your opinions.
 
I'd do the tti's (money is just falling out of mu pockets as well <jk>) and 2-1/2 exhaust pipe with that set up
 
This thread touches on many things that I`ve experienced with my car. I like cast manifolds , so I talked to the techs at Comp, told them what I wanted and now I`ve got a cam that`s designed for cast iron manifolds and high compression. It makes great power across a wide rpm band. I went through the header thing and (they are for whatever reason, leaks, bad fit ,noisy) a total pain in the *** and always have been. I happily went back to my cast iron manifolds after fighting a set of chassis headers in with a big block. I agree with others here that you`ll only see a great loss in potential HP in the upper rpm range and that the build of the engine will dictate the gain of power with headers. If you plan to race the car a lot ,then I`d fix the headers with a patch. If you want a nice street machine that will still smoke `em from stoplight to stoplight then go with a cast manifold, they`re a lot less problematic.
 
Not an exact HP loss number but..... I went from headers and 3", dump at the rear exhaust to 340 HP manifolds, TTI 2.5" x-pipe system. My best time with headers and slicks was a 11.78 @ 118. My best time with manifolds and E70-14 (Firestone Wide Oval, Redlines) was a 13.2 @ 105. The 13.2 pass was on a Ram clutch with at least one broken pin.

Some other changes between the two were.

SS springs to Mopar HD springs
Electric fan and water pump drive to stock set up
Battery in trunk to battery under the hood


original 340 and X heads, Ported and balanced
Crower solid street roller cam
Victor 340 with Holley 800 DP
 
1968FormulaS340 said:
Not an exact HP loss number but..... I went from headers and 3", dump at the rear exhaust to 340 HP manifolds, TTI 2.5" x-pipe system. My best time with headers and slicks was a 11.78 @ 118. My best time with manifolds and E70-14 (Firestone Wide Oval, Redlines) was a 13.2 @ 105. The 13.2 pass was on a Ram clutch with at least one broken pin.

Some other changes between the two were.

SS springs to Mopar HD springs
Electric fan and water pump drive to stock set up
Battery in trunk to battery under the hood


original 340 and X heads, Ported and balanced
Crower solid street roller cam
Victor 340 with Holley 800 DP
Click to expand...

I wouldnt call that apples to apples..lol..But the mph drop shows a loss of 115hp. I'd call that a little drop...but your cam I'm sure depends on scavenging effects from the headers that no longer exist, and the plenum is huge for the restrictive exh side too. So that drop doesnt surprise me at all.
 
MrMopar, a little off topic, but are you located in Daysland? If so, I'm 25 minutes South of you in Forestburg - small world. We'll have to get together over a cold one and swap some Mopar stories some day. :) PM me if you'd like.
 
Here is the interesting article comparing a variety of exhaust manifolds and headers that rumblefish360 mentioned. Each step shows small increases, but they are throughout the rpm range thus increasing the all-important "area under the curve".

I believe that a little more gear and converter, along with headers, would really help that engine, especially with the bigger valves and ported heads.
The TTI's work very well with my 360 Commando crate engine as well as on other Magnum 360's in the class.


Mopar Muscle Article:
http://www.moparmusclemagazine.com/techarticles/155_0307_mopar_crate_engine_exhaust/index.html

TTI Exhaust website:
http://www.ttiexhaust.com
 
I did have 3.91 gears in it and it really went well, to the tune of 12.30@115 mph. But I got tired of the high rpms crusin on the highway so the 3.21's went in it. I called the Comp Cams tech line about what kind of cam to use with the stock manifolds and he said that the XE268 has enough exhaust duration to do the trick and that it was basically designed with that in mind. He did not recommend another cam to use and even stated that it would be good with the NOS. So there you have it, something that I never thought about. This has been a real good thread as far as all opinions are expressed and the learning never stops. Any input is always appreciated. Thanks guys!
 
You must log in or register to reply here.
-

Back
Top