Denting exhaust parts for clearance

[mrk1]

Pretty good video to watch, I've dented parts for years now and always believed it made zero difference. Here is proof.

https://www.carthrottle.com/post/wha...ifolds-dented/

[Cole]

Very interesting watch. But would we see the same results in an FA20? I guess that even further down the line, closer to the exit, it wouldn't matter at all. Why is it not making a difference? Is it because the pipes are too big of an ID, so that efficiency isn't being affected because there's still tons of room to evacuate the exhaust?

[strat61caster]

Depends on what your values are, in some instances 1hp can be the difference between 1st and 3rd place.

Originally Posted by Cole

Why is it not making a difference? Is it because the pipes are too big of an ID, so that efficiency isn't being affected because there's still tons of room to evacuate the exhaust?

Well the fact that they had gains when they first smashed it up (decreasing the diameter) kind of points in that direction and only had losses when they really smashed it I'd agree, their headers were oversized to begin with. There's a telling comment he makes at the end that the difference between 1 5/8" and 1 7/8" makes a difference and you should pay attention to that. Maybe if they started with a smaller diameter they would have seen those losses on the first round of hits.

I also think a big displacement V8 would be less sensitive to exhaust restrictions as they are generally designed further from the limits of efficiency than a low displacement 4-banger like the FA20 where any choke point is noticeable.

If I was doing what roadkill does? Big engine swaps on a budget with little heed to competitive uses or maximum performance, yeah, take a hammer to it and make it fit if the clearance can't be managed.

[mrk1]

Originally Posted by Cole

Very interesting watch. But would we see the same results in an FA20? I guess that even further down the line, closer to the exit, it wouldn't matter at all. Why is it not making a difference? Is it because the pipes are too big of an ID, so that efficiency isn't being affected because there's still tons of room to evacuate the exhaust?

Engine is an engine don't see why a FA would be any different. Pipe diameter to dent ratio probably does come into play, obviously if you dent it enough to collapse the pipe thats a problem but thats not the discussion here. We are talking making clearance.

A good 86 example would be denting the over pipe for some clearance to the sub frame where its tight. I have had to do this on 3" over pipes and always believe a 3" with a dent in it is still the best flowing option. To me dents for clearance and an extra bend here or there for better fit are really over looked.

To many manufactures say there design is straight for "best flow", is it for best flow or your too lazy to make it contour properly. Look at front pipes for example, some out there have 1 maybe 2 bends while others have 3 or 4. Ill always take the one that doesn't rattle like a can of marbles over the "straight" path.

[Spartarus]

Consider me enlightened. It's fascinating. That doesn't necessarily hold true for all engines though...

I'd wonder if it makes a bigger difference on a motor that flows better than that one. As volumetric efficiency approaches and exceeds 100%, the sensitivity of the motor to airflow changes increases exponentially. (In theory.)

That is a 2-valve OHV 6.5 Liter motor (400 CI) making 560 HP, so we already know it's running far below 100% VE.

I can see on the dyno screen that BSFC is .51, which at 560 HP, means an approximate fuel flow of 285.6 PPH. The WEGO AFR on the screen is showing 10:1, from those 2 points we can determine mass airflow, and extrapolate volumetric airflow.

Sparing you the rest of the math, that means the engine is flowing approximately 622 SCFM which, at 5800 peak power RPM, equals 92.6 % volumetric efficiency. That's in a standard atmosphere, and they aren't testing in one; their values are corrected for standard though. So, to make my numbers good, you have to undo their 1.066:1 STP correction.

Thus, you arrive at 86.87% VE. An engine that flows that (comparatively) poorly probably isn't oversensitive to minor exhaust system changes. I wonder how much difference that would make on a 4-valve-per-cylinder engine running close to, or over 100% VE.

That would be useful and enlightening information.

On an engine running at a VE that low, and an AFR that rich, it's probably on fixed ignition timing as well. It's apples and oranges at this point. Comparing that test to a small-displacement, High VE, variable ignition and valve timing engine that IS sensitive to small changes in airflow is inappropriate. I would like to see that same test conducted on a motor similar to ours, and see what happens. You may be surprised how different the results are. That engine is also carbureted.

Just a side note, if all that banging reduced the flow number on those pipes by even 10 CFM, which is huge, VE in that engine would change less than 1%. This is because of its large displacement. Try a 10 CFM change on a 2-liter.