68-70 Hi po manifolds vs quality headers

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tom340

tom340

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Like this topic hasnt been brought up before, LOL. Specifically, has anyone tried using them( factory manifolds) on a hi perf stroked smallblock street/strip application making between 400-500 hp. In your opinion-based on EXPERIENCE- do you feel it significantly hurt you with a solid "in the pants" feel? How about in the upper RPM range say over 4600 rpm and higher. I have them on the car now-new motor dyno'd at 430 peak HP but we had to use undersized 1 5/8" dented up shop headers since the manifolds would not fit the application. No ones fault, just how it happened. I can only hazard a guess it could potentially be hurting peak HP by 20 to as high as 50 hp. The recommended exhaust for 372" and up is 1 3/4" or 1 7/8" header. Im also dealing with a converter issue/ stall speed - Im thinking the stock manifolds are hurting my upper rpm pulling power as well. Im sure a lot of questions will be asked, to be certain YES everything else is fine, timing-compression etc. carb is tuned perfectly for current setup. Running tti 2 1/2" exhaust, H pipe and the best flowing Dynomax mufflers. The car has recieved high praise at some big shows already this year especially since the manifolds are in place and I disguised the engine to appear "stock" best we could. MoPar engine bays are a work of art. I may have to ruin the look a little next year if these things are killing me.. PS, no I aint selling them sorry
 
Tom, Cam specs are a big factor in how much the manifolds are killing. A tight lsa will loose more power then a wider 112* lsa.

Sounds like you built your new motor the way I want to build the Dart up.
 
Thats what my Mikey said, AND Jeff told me today. Mike designed a camshaft with the manifolds in mind to be a bit more forgiving. These suckers dont scavange well. Mike saw a little "black" inside the intake runners, potentially getting some reversion? Would have been worse with more aggressive cam like Greg has in his R/T.
 
Headers don't really come into play until you reach 5000rpm. The horsepower gained is slight unless you have too big of a cam to start with.
 
The dragstrip has many variables involved. That guy also changed many things and it is not fair to blame the exhaust only. Just to start, his cam is probably too big. I got my information from watching my husband run engines on the dyno at his machine shop. I will say it again. The exhaust system is not that important at lower engine rpms on a stock or mild performance engine. Now if I was able to put stock maxwedge manifolds on his 8 second superstocker, of course that would be a different story.
 
Mopar Muscle did a exhaust comparison on a mild 360 (300hp) crate engine.

Results on 340 Hi-Po man vs TTi's

340 man......4600 rpm......315 hp
TTi's...........4600 rpm......327 hp

340............3700 rpm.......409.1 fpt.
TTi's..........3700 rpm........425.5 fpt.

Not sure if this helps,but thought I would pass it along.
 
Hughes engineering also post results on their website: Two motors, stroker based on a 360 block and stroker based on factory 340 block. ALL stock iron ported and extrude honed. the 1st 360 test based 409 peak HP, 429.7 TQ with manifolds. Test 2 360 added the tti headers: 471.6 peak HP, 498 peak TQ. To note, at 2600 rpm there was a 42 hp increase and an amazing 82 ft lbs tq increase headers only.
The 340 stroker included Hughes stage II ported heads, extrude honed intake and factory 68-70 hi-po manifolds. With manifolds 447HP, 471 peak TQ. Added exact same tti headers only: 478 peak HP, 507 peak tq.
 
I'm a rookie, however, aren't manifolds the better choice for street usage since they do pretty well at lower RPM's? However for pure performance isn't it pretty much a given that headers are the way to go?
 
Manifolds are nice because they are thick so they dont warp and they are quieter. The proper set of headers will boost power over the entire rpm range, not just the upper rpms. If you loose torque with a set of headers the Primaries are to big, The collector is to be or to short or a combination.

Headers should boost efficiency resulting in a gain in power and milage.
 
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