Denting your exhaust - AKA beating hookers with a hammer

[hybrid]

I bet most of us have done this at some stage if we've been around cars long enough.
I found this video so interesting. We're all so anal about header size, flow etc so this was a bit eye opener for me.

I would love to make a little more (couple of mm) clearance on my headers in certain parts. Now I have no issues at all doing it.

[xpconnor]

Interesting. It makes you wonder how much difference it really makes when you pay the extra for the tuned length and whizz bang super dooper headers over a cheaper set. Is the cost to hp benefit really worth it?

[hybrid]

Hard to say. You could argue that it's once the design is good, it can handle some rough spots.

[boofhead]

It is interesting. The key in my mind has always been selecting the right primary size first then length (followed by other factors). So if you have the right primary size it does show that a dent here and there is not a big influence on output. Good to know. They did confirm at the end that if you select smaller diameter primaries then it will have a major affect on the output. With the choice of wiz bang tuned length etc etc headers - yep it costs a lot more per extra HP so for a street car - not worth going to to much trouble. Get the right size long tube primary that fits your car (or close enough) then do not sweat over a touch up here and there to make it fit.

Good post Hybrid.

[gbx78]

Thats a pretty cool vid. Ive seen that guy from another show somewhere but cant think where.

[mert]

They did way more than a "a dent here and there"!!!! They effectively crushed every tube... basically no difference...

Would be interesting to see the same test with an exhaust system attached to show if the common tribal lore that 3" systems "make" huge HP over a stock 2" are true (I'm thinking the results would be similar, not as big a deal as many think, or the vendor sell on...

Do the same test with a stock 2", then 2 1/4" 2 1/2 and 3 and see if it really makes much difference.

[boofhead]

The power reduction from ideal falls further and faster if the pipe is to small rather than being to large. The one main issue I have with the vid was that they did not tell use what primary tube they were using for the test (unless I missed it). If the tube was larger than the ideal size (which I believe was the case) then I agree a dent here and there (first few test) can increase the power. The vid showed this to be the case. This is because the exhaust can better maintain velocity toward the over lap period so it can draw extra fuel/air as the scavenging is likely better. To large and the velocity is lower at the same time the exhaust is expelled to early so no pull on the intake when it starts to open during the start of cam overlap.

Later they start to close up the tubes further and start to see a drop off in power. All the rules are the same with the full exhaust system. The results will be dependent on so many factors - to be a decent study you would need lots of different engines, different cams, headers, sizes, etc etc etc. There are basic volume verses engine size, verses RPM ranges to allow pipe sizes to be determines that are ball park reasonable for purpose. Often they can be boiled down to power ranges need X sized exhaust tubes (single or dual). Again a little bigger is better than being smaller. As far as return (HP) verses cost ($$) that is a log curve. You get to the point where the $$ sure do grow for smaller returns in HP.

[hybrid]

Some engineers were commenting in the comments I think.
They were suggesting that the valve causes more restriction than the headers would, which is why it doesn't make much difference.

I'm not sure the headers were bigger than would normally be recommended. That is a 560HP engine.. its no baby, and the headers don't look all that big to me.

Of course it depends how they were crushed too. If it oval'd as they crushed it, then the resulting area may not be much different. The dents they did with the sharper end of that hammer though... that wouldn't have made them oval too much I wouldn't think.

I wonder where that crossover point is where bigger becomes better though. Smaller dents made more power (possibly because of velocity, but to be fair could also be environmental), but at no point did the crushing really make much difference, even at the big end.
If their graph is to be believed, power stays above 550HP all the way to 6500.
Going higher than that is useless for 99.9% of cars anyway.

It would have been nice for them to really show multiple angles of those headers so you could get a good idea of what they were left with.

[xpconnor]

Just to argue a small point I would say they see no change in hp over all the engine pulls they try. At roughly 560hp then to say they see 1hp or even a 6hp change I would argue is not really a change at all. If they did ten dyno pulls in a row over a few hours of the day I would almost guarantee there would be more than a 1% variation over the runs. Not really that important in terms of the discussion as has been mentioned there are endless other things they could have done to make the test more conclusive but the scientist in me thought it was worth mentioning.

I do think now though as has also been mentioned there are far more cost effective ways to gain some hp rather than shelling out a grand or more for top of the line headers. Good topic though. I like seeing different peoples interpretation of what it means.

[boofhead]

Some engineers were commenting in the comments I think.
They were suggesting that the valve causes more restriction than the headers would, which is why it doesn't make much difference.

I did not see the comments - maybe I should. Any way, the valve only provides a restriction for a very short period of time - e.g. from closed to a specific lift point of the valve (which can be calculated). The valve curtain area and hence flow potential at a lift point can (and normally does with decent lift cams) become a larger area than the port it is controlling. Hence the valve being a restriction for the bulk of the time the valve is open is not true as the port becomes the restriction (unless the header pipe is a restriction).

A small calculated restriction is actually wanted as you want to be able to complete expelling the exhaust at the start of the overlap. This then means a vacuum is generated by the weight of the moving gasses which pulls more fuel/air into the intake valve. If it expels early, then you have less vacuum or if it expels late then exhaust gets mixed into the intake charge this diluting it. Obviously, the Cam timing events, port sizes and RPM (Torque peek) need to be co-ordinated. The tuned length headers then help further with the wave pluses pulling on the fuel/air as well.

I do enjoy these discussions as well. Always something to learn.

[boofhead]

Here is a dyno test on various long tube headers of different primary sizes and lengths as well as coatings and collector lengths. Everything came out as I would have expected.

[69candy]

Interesting. But now I am gonna pull my 428 (in winter) and put some dents in my $800 super comps to make them fit!! I was thinking after trial fitting I had wasted my money as there was no way to make them fit. I won't have to bash them in as much as they did though!! Will still be better than stock manifolds as general consensus is there is 40hp to be gained on a c/jet be fitting the supercomps!!

[boofhead]

40 HP is a big difference though I have no idea from where your starting. There will be a difference for sure in my view.

[hybrid]

That dyno testing is on a pretty powerful 427 big block engine, so doesn't relate all that well with what most of us are running.

IMO what it shows is that what works depends very much in the entire package (which fits for most components of an engine really). I have posted info from a header manufacturer in the past that showed going bigger on headers can hurt (street) usable power.
But obviously this big block needs to breathe.

I wonder how these guys reconcile the results in the denting video versus the header change video.

I think it just says more about the header design being the important part - not whether the headers have a few dents.

Edit: By the way - the collector length was the most surprising part to me. Some easy gains to be had there.

Edit2: Summit video on primary size


This guy gets a bit weird talking about molecules, but the theory is OK.

For 350CI, corresponding peak torque RPM vs primary size

1 5/8" -> 4175 RPM
1 3/4" -> 4415 RPM
1 7/8" -> 5100 RPM

[boofhead]

Look at the two formula - the top one then the one below on his board. The formula rewrite is wrong - assuming the top one is correct the one below is incorrectly written.

It should be

PPA = Peek_TQ_RPM x 1CylindarDisplacementCI / 88200

In any case, we all make mistakes. I agree the primary pipe sizes sounds about right for a around the 350 sized engine.
As I have always said most people building 347s talk about 450 HP or so so peek TQ would be mid 4s so 1 3/4 is the correct primary pipe size.