Carb SBC vs TBI SBC vs Gen 3 LS reliability

[L31MaxExpress]

Mostly, no. Shorties are compromise headers. Our trucks aren't hurting for room for headers, so your debate is really stock manifolds versus long tubes. If you go with headers and you're not building a max hp engine, go with 1 5/8" primaries and a 2.5" collector.

Also Tri-Ys if you can find them. At one point Thorley made them so they can still be found at times.

 

[Scooterwrench]

I have heard that shorty headers work better for lower rpm, and long tubes work better in the higher rpm’s. Is that what you meant? In the post you had them switched.

Nope,other way round. Long tubes for lower end power,short tubes for top end power.

 

[Scooterwrench]

Also Tri-Ys if you can find them. At one point Thorley made them so they can still be found at times.

I've never run those but I've heard you can build a torque monster with them. Always too pricey for me. I tend to pinch a penny until Lincoln sh*ts.

 

[Erik the Awful]

Also Tri-Ys if you can find them.

100% agreed. If it's not a race build, tri-Ys are the best option, trumping stock manifolds, shorties, and long tubes. But wanna be racer-bois want 4-into-1s, so all the header manufacturers quit building tri-Ys.

I handbuilt a set of tri-Ys for my Cadillac 500 in my Jaguar, mostly because that's the only design I could fit. It was a major pain in the butt, but I had to jet the carburetor way richer versus the Sanderson shorties that had been on the engine. They flowed a crazy amount of air for being such a frankensteined set of headers.

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[slow_c1500]

Man, those tri y’s are expensive! Has anyone had any noticeable gains by porting the stock manifolds?

 

[Pinger]

Negative pressure,not negative pulse. I think what you are thinking is reversion when the pressure wave moves back up the pipe.

Yes - proper exhaust tuning as with 2-strokes!

The negative pressure is easy to prove. Take a water hose and blow down through it then quickly stop up the hose with your tongue. You will feel the suction on your tongue caused by the negative pressure. You will also feel the reversion. It's kind of like the air is bouncing on a rubber band but it's actually the air compressing and contracting. Air has weight so moving air has inertia. The inertia of the air moving through a pipe is what creates the negative pressure(vacuum)behind it. The inertia of moving air is also what helps fill the cylinder during the intake stroke.

But that doesn't quite explain why the length of the primaries is so important. Tuning a 2-stroke exhaust the length of the sections is crucial - but that's harnessing the sonic pulses (which have no inertia). If 4-stroke exhaust tuning is about forces generated by the gas flow, then a long primary should work at all rpms.

 

[Scooterwrench]

Yes - proper exhaust tuning as with 2-strokes!



But that doesn't quite explain why the length of the primaries is so important. Tuning a 2-stroke exhaust the length of the sections is crucial - but that's harnessing the sonic pulses (which have no inertia). If 4-stroke exhaust tuning is about forces generated by the gas flow, then a long primary should work at all rpms.

And sonic pulses move how fast? Uh,,,,1,125fps. Sonic waves are moving air,that's how they work. That's why there is no sound in a vacuum.
Remember the ad quotation for the movie Aliens. "Because in space,no one can hear you scream" No air,no sound! If you know 2 strokers than you have seen how short the headpipe is. That's because the exhaust pulses are twice as frequent as a 4 stroke engine and their being tuned to make power at higher RPM's. They usually get up into the pipe around 3000 RPM depending on application.

 

[Scooterwrench]

Maybe this will explain it better for you;

How To Size Headers - Popular Hot Rodding Magazine

If you need guidance sizing your headers or if you're simply having difficulties trying to find the right header size or collector form, check out what this month's tech article has to offer! We took a set of Doug's headers, tried out different collector styles and hooked it up to the dyno...

www.motortrend.com

 

[Pinger]

Maybe this will explain it better for you;

How To Size Headers - Popular Hot Rodding Magazine

If you need guidance sizing your headers or if you're simply having difficulties trying to find the right header size or collector form, check out what this month's tech article has to offer! We took a set of Doug's headers, tried out different collector styles and hooked it up to the dyno...

www.motortrend.com

It confirms my initial assertion that 4-stroke exhaust tuning is about the sonic pulse energy and not the inertia of the moving gas column.

But, run the maths and it doesn't really compute.
I work with the speed of the pulse as 500 m/s (1600 ft/s) which is higher than you quote but despite this my case isn't at all weakened.
Take 3000 rpm as the target rpm. Each revolution takes 0.02s, 180 CAD (crank angle degrees) takes half that (0.01s) and using that figure gives from opening of the exhaust valve to BDC for the pulse to be established in the exhaust port. 180 CAD from there takes us to TDC and the valve overlap period - where the returning negative pulse supposedly arrives to do its work.
V = d/t where V is velocity, d is distance, t is time.

d = V x t = 1600ft/s x 0.01s = 16ft = 192''. The pulse has to traverse the primary twice so the required length of the primary is half that ie 192/2 = 96'' which is double the longest mentioned primary length of 48''.

Alternatively, we can take a 40'' primary and calculate where in the exhaust event it will arrive.
t = d/V in this case d = 2 x 40'' = 80''
t = 80''/1600ft/s = 80/(1600 x 12)''/s = 0.004s. Each revolution at 3000 rpm takes 0.02s so
(0.004/0.02) x 360 CAD = 72 CAD.
72CAD from BDC is nowhere near the valve overlap event and I've been generous in giving the pulse until BDC to be established and generous with its speed. Even calculating for 6000 rpm gives a primary length of 48'' - which is regarded as a long primary more attuned to low rpm operation.

The article mentions secondary pipe length as being crucial and perhaps/possibly that makes the difference though it is far from obvious how (unless we assume there are two returned pulses with the second one arriving at around TDC but that would imply a secondary length similar to the primary length) but the point here is that that primaries alone at the usual lengths do not return a scavenging pulse to coincide with the valve overlap period.

If any sees a flaw (or flaws!) in my calculations please correct me. But this is the methodology I apply to to 2-stroke pipes (hence my choice of slightly higher pulse speed) and it's always been accurate.

 

[L31MaxExpress]

When my 83 G20 van was small block powered it had the old design tri-ys. The collector is nearly even with the 700r4 crossmember. Very noticable low-mid range torque gains even at part-throttle. 1-5/8" to 2" to 2.5" pipes.

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