HP Exhaust for '68 300

Cazman

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This is described as '67 Imperial exhaust. I am looking for '68 300 HP exhausts and this looks different. Not HP,I guess. But, are the exhausts different per model and year?

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They look cool those HP manifolds and I have them but from what I have read and been told they don't in themselves add much power. But they look much better than logs and worth getting in my opinion.
 
There
They look cool those HP manifolds and I have them but from what I have read and been told they don't in themselves add much power. But they look much better than logs and worth getting in my opinion.


Manifolds themselves are good for 7-10 more horses depending on what you put them on. Don't help much on low compression smog motors, more effective on the older high compression models.

Dave
 
Manifolds themselves are good for 7-10 more horses depending on what you put them on. Don't help much on low compression smog motors, more effective on the older high compression models.

Yes that is about what I have read 5 to 10 hp. So not that much,,on ANY motor,,but they do look cool.
 
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I have seen these listed mostly for mid-60s B-bodies, don't know exact year, but I don't think they came on any C-bodies. Perhaps an early 60s Imp, but can't confirm/deny.
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From experience,the HP's do look sexy but retain heat something awful and getting to plugs #7 and #8 are a *****.
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Nope. Summit distributor with a recurve in it so not needed.
Wrong your killing your piston rings at part throttle cruise.
Part throttle = molecules further apart no matter the compression ratio of the engine. Molecules further apart equals slower burning flame front, in order to get as many burned as possible you have to light it earlier (more spark advance), unburned fuel waste fuel (bad mpg) and dilutes your oil causing more wear. Running anything but a dedicated race car (track only) without a vacuum advance is foolish $
 
Are they better than what is on my stock 68 300?
Probably not. They don't have significantly larger passages, and they all dump into a single log, unlike the logs of most C-bodies.

Our firing order is 18436572, with 5 and 7 beign right next to each other in the order, and physically also. So when #5 exhasuts and heads down the manifold, #7 is coming out also, as #5 has further to travel, so they're right there together at possibly the same time. Maybe it doesn't matter, I dunno.

But from 65 thru perhaps 74 all C-body manifolds had #7 having a small 180deg passage before it dumped into the common area of the manifold, which dumped down between #5 and #7. Both HP and standard manifolds are this way.

Some might say 'oh, they did that for routing around the steering shaft' and perhaps that was a benefit too.

But the next piece of evidence I'll pose is that the B and E-body HP manifolds dump near the rear, and those have a funny #7 curve to them also, and it would have been much simpler to dump #7 in straight. So IMO there was a reason the engineers did that.
 
Can you give us details?
We interrupt this thread......

With the cam I am running Comp 268HE with Eddy intake and 750 carb. Orange box.
HP manifolds with TTi 2-1/2 H pipe system.
Initial timing is a 14 degrees and 34 all in at 3000 RPM. Vacuum is 16 inches at idle.
Try that with a stock non-modified distributor! Points initial timing was 5 degrees BTDC.
The Summit dizzy has a slower advance curve so i can set more initial timing without getting too much total advance at cruising speed which means no ping under load or part throttle. Especially with today's corn filled swill we call gasoline.
Mopar engines like a lot of timing nowadays.
Light stall 2250/2500 shift kit and 2.94 gears with 2357015 28 inch tall tires.
This thing hauls ***! Built to run on the highway and does it very well.
I get 15.4 MPG when cruising the speed limit. But I cannot keep my foot out of it so 13 on a good day,LOL

Sorry for hijacking the thread,back to the regularly scheduled program..
 
Wrong your killing your piston rings at part throttle cruise.
Part throttle = molecules further apart no matter the compression ratio of the engine. Molecules further apart equals slower burning flame front, in order to get as many burned as possible you have to light it earlier (more spark advance), unburned fuel waste fuel (bad mpg) and dilutes your oil causing more wear. Running anything but a dedicated race car (track only) without a vacuum advance is foolish $
Will hook it up when she comes out of storage in the spring
 
Will hook it up when she comes out of storage in the spring
You may need to slow the VA rate or more importantly make it drop out faster when you add throttle at cruise to prevent pinging. Adjustment is done with 3/32nd allen wrench through the vacuum nipple.
You will only gain from having it connected no loss in any way.
 
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