Big cubes, cast iron exhaust manifolds, and cam specs
 

I'm used to using high flow manifolds on small displacement and headers on larger ones (or just always headers). So this area is new to me. I'm hoping someone has experimented in this area to really understand the effects.

Let's say you setup your engine to work in a certain torque band. Now you go from headers to cast iron exhaust manifolds. For the sake of arguement, no other alterations to the exhaust system are made. Is there an adjustment you want to make that will maximize scavaging without significantly moving the torque band?

Let's assume a custom cam. If I keep my intake specs the same, what (if any) alteration to the exhaust side help without significantly changing the rpm band? More duration and higher lift? Same duration, but earlier opening? Later opening? If same duration but higher lift was possible, would that help since the real restriction is in the manifold instead of the head? I'm trying to understand the dynamics of it.

Thanks.

I've gone from 3-tube to 4-tube to Headers to Long Branch with no-diff in 60foots or MPH. But the Cam was over-advanced.

To answer your question directly, open EXH a few degrees earlier than optimal with Headers to blow-down the Primary&Collect so that momentum will create a powerful vacuum toward Valve Closing. Typical thought is you want to slam the Exhaust shut before Reflection-reversion occurs (Headers) or other Exhaust Pulse occurs(Manifolds).
The vacuum from the Exhaust is paramount to off-idle 60foot power (that my present car presently lacks).

Observation 1: do everything necesary to optimize idle Vac to maximize Intake Manifold Inrush at WOT.
Observation 2: do everything necessary to non-reversion'ing overlap.


Definitely fill crossovers with aluminum.

I've read the tea leaves on the Exhaust crossover and figure there is merit to a CYLs 3&6 "Torque-Tube" that flows well across/under the Intake manifold to REMOVE a nasty 8+4 pulse that offends CYLs' 6 Exhaust Bowl. Also, a nasty 5+7 pulse offends CYL's 3 Exhaust Bowl.

There needs to be an interested student that will evaluate a 5-to-4 torque tube to see if that REMOVES the nasty pusle combining that occurs at 6&3's valvebowls.

To add impact to these observations: I recently port-matched my Long Branch manifolds using the House Vac attached to the Collect Flange while grinding a flange opening, with other flange openings Tapedshut. In ALL CASES the other Flange openings were caught significant iron filings onto the tape. This shows a remarkable ability for heavy iron filings to travel up 3 dead "stand pipes". Recon with this.

Rhoads lifters help to broaded the benefit.

now that is an very interesting phenom! that is what? about 14" jump from common back to port? are you sure the filings didnt fall when you moved the header upside down?

Oh, I am darn sure Manifold was mostly upright; Manifold Outlet alway on table top sealed&taped to vac hose, while Long Branch body rested against a CYL head for steady, while flanges were up-in-the-light for me to see clearly while grinding.

Happened to BOTH DS & PS manifolds.

House Vac is strong but we're talking fairly constant "pull' through a port path while grinding THEN discovering the blocked ports "feature" many iron shards stuck to the tape. That means All the shards travel down a Primary leg, most hit the outlet, yet Many ( about half) found ability to travel-back-up remianing 3 dead legs to stab the tape & stick.
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Tis prompts me to re-open my notes on exhaust pulse timing & Crossover torque-tube concept. World rather have many set of eyes doing this so I don't have to. HIS

HIS,
I have noticed over the years on my cars (iron Qjet manifold setup) that if I could punch the throttle with a un-blocked exhaust crossover , it responded/hit harder and seemed quicker as long as it was still cold .. but obviously the window of opportunity to hit it at the starting line is quite narrow. The block heatriser on the same manifold always felt a bit flat on the punch but outperforms a hot open one though... Hard to get best of both.. would be an interested test if a connection could be made with a square 1" steel tube welded to an iron intake to function but not touch the casting where the intake runners are at?

Hmmm, sounds like an "Exhaust crossover cutout" should be entered in the Produce suggestion area of this site. :rolleyes: Right next to the new and improved "hair splitter"! :D

Maybe something to the RamIV/HO intake with separate exhaust crossover? I blocked the x-over on my car when changing from T1 to iron intake. I was surprised that there doesn't seem to be a big gain on the bottom end but noticeable loss on the top end (still have a lot of carb tuning to do which could have a large effect due to the less open plenum). Next step would be 180 degree headers, but good luck stuffing them under a t/a.

The first thing you need to make a distinction about is whether you just put a sheet of tin to block off the heat riser, or actually make a pour and shape the exhaust ports. I just cannot see how streamlining the exhaust port would hurt power output at any part of the RPM range.

The 4/6 and 3/5 exhaust ports are a disaster with the gigantic holes allowing all four cylinder exhaust ports to cross talk to each other. We just pulled down my friends 455 that suffered a cracked cylinder wall in #4 cylinder. All four cylinders (3,4,5,6) showed the steam cleaning effect of the introduced water in only #4. Now if they they share gulping the water mixture out of #4 exhaust, then there is WAY too much activity going on in the crossover system. Cylinders 1,2,7,8 did not show any signs of steam cleaning so I'm pretty sure it was caused by the blended crossover system.