Engine building discussion

[redgrandprix]

at idle this car doesn't sound like an f1 but it does sound pretty good on takeoff...

[skitchin]

Damn great examples there guys! Jarek that's a damn healthy sounding 86 - 4AG I presume

 

And holy fuck that 3800 sounds beast - sounds like it has a pretty lopey cam but jesus when he really lays into it... Hands down one of the sexiest sounding 3800's I ever heard. Wonder what revs he's pushing when it gets that glorious resonance. I've always thought of V6's as being inherently unbalanced

 

Edit: Also there's a 2nd video I found on his channel

 

[video=youtube;F7_-R3uhrbo]

[RobertISaar]

still sounds like a chainsaw to me?

[skitchin]

Ain't a bad thing so long as it sounds better than a typical ricer honda with a fart can

[redgrandprix]

apparently 8000rpm and high 8's in the 1/4

Edited July 26, 2013 by redgrandprix

[RobertISaar]

had an interesting relevation tonight: take two identical straws, cut one down to a stupid small length while keeping the other full length. notice any difference in the effort required to push the same amount of air through the longer straw? all else being equal, even just having a longer pipe is more restriction.

[RobertISaar]

Tail pipe length is

very important, it just takes a lot of testing to determine what is right. I know of a guy who played with

different length tail pipes and picked up 5-6 HP on a 190+ WHP engine.

 

that is a 3.2% increase.... not a LOT, but it is there. it would seem you want the shortest possible length that works well at the RPM you're optimizing for.

[skitchin]

that is a 3.2% increase.... not a LOT, but it is there. it would seem you want the shortest possible length that works well at the RPM you're optimizing for.

 

Yeah exhaust seems more an art than a science with all the different factors. Smaller/Longer pipe = better back pressure = better low end torque etc. Larger/Shorter pipe = less back pressure/less restriction = better top end performance.

 

Also I did some substitutions on the piston speed feet per minute formulas, for my own reference

 

FPM = STROKE * 2 * RPM / 12

STROKE = FPM / 2 / RPM * 12

RPM = STROKE / 2 / FPM * 12

 

Formula 1 values

1.563 stroke * 2 * 18000 redline / 12 = 4689 FPM

Edited August 17, 2013 by skitchin

[RobertISaar]

backpressure is always bad.

 

increasing pipe sizes and losing torque at lower RPMs is due to INCREASED backpressure happening at those speeds. unless you want to force a whole lot of EGR(both intentional and gasses that didn't win the fight to get out of the combustion chamber before the exhaust valve closed), in which case, pressure is your friend.

 

exhaust gasses are hot(duh), they cool down as they move further away from the engine, which is expected, but forcibly making them expand via larger sections of piping will cause them to cool down and condense REALLY quickly. what happens is that the big wall of cold, dense gas impedes the flow of hotter, less dense gas behind it, causing the entire stream of exhaust gasses to slow down(and even flow in reverse). if you limit the rate something can flow out of anything else, the result is pressure.

 

unless you can make exhaust that has variable area to always give the exhaust an optimal amount of tubing to slip through, it will always be a compromise between high or low speed. stepped tubing somewhat counteracts this, but not completely.

 

 

 

so, build your exhaust around a target RPM and either learn to deal with the RPM ranges where it suffers or come up with a way to have multiple flow areas. i've thought about that last part a lot.... the only somewhat reliable method that comes to mind would to be to have a slide valve that can close down and open up multiple sections of exhaust tubing. let's say they're all the equivalent of 3/4" diameter tubes and there are 6 paths for the exhaust to flow. at idle, 1 would likely be more than enough to keep the exhaust at an optimal velocity. a little bit of throttle and path #2 opens up, more and now there are 3 open, all the way up to 6 depending on airflow. the only "problem" i have with this thought is needing to impliment it pretty much from start to end and you would likely need an individual cat for each pipe for any chance of passing emissions testing.

 

 

 

compromises.

 

key word there. it isn't possible to make any one engine that excels everywhere, not for any kind of reasonable budget anyways.

[19Cutlass94]

However speaking about back pressure, I've read many articles from engine builders saying back pressure for a N/A motor should be between 1-6psi. You need some back pressure but not too much.

[RobertISaar]

my "professional" opinion is to optimize for whatever RPM range you're building an engine for. if you make more power(where you want it) with a 2" pipe than a 2.5" pipe, you're doing something that is helping the engine. if 3" turns out to be better, do that. since this thread seems to be themed more for building a full-effort race engine, emissions, fuel economy, drivability, etc, those aren't the primary goal, so if sacrifices to them must be made, so be it.

[skitchin]

Here's a sweet article I found about modifying a 302 to push higher revs. Looks like they spun it to about 7500 RPM

http://www.superchevy.com/technical/engines_drivetrain/completebuilds_testing/sucp_1004_high_rpm_302_engine_build/viewall.html

 

This graph represents that power curves from the stock and modified 302 Chevy. What is clearly evident from these curves is that the modifications resulted in not only a sizable hike, but also a serious shift in the torque curve. Where the stock DZ302 produced peak torque at 4,400 rpm, the roller-cammed, AFR-headed motor produced peak torque at 6,300 rpm. The significant shift in the torque curve resulted in a jump in peak power, from 357 hp at 6,700 rpm to 519 hp 7,400 rpm

 

[skitchin]

Found another interesting read. Not sure how accurate or recent some of this info is but makes me wonder. Their claims seem to be contrary to how everyone modifies their cars.

 

http://www.x-pipe.com/Scavenge.html

 

 

Urban Legend #6:

Long duration exhaust cams allow headers to make the most horsepower because headers flow better than stock cast iron exhaust manifolds.

The Truth of the matter:

Long duration exhaust cams are necessary with scavenge headers because such headers convert exhaust gas velocity into scavenge, slowing the rate at which exhaust gases can escape from the engine. Longer duration exhaust cams give the exhaust stroke more time to blow down exhaust gases. Longer duration exhaust cams give that unmistakable big cam (stumbling) idle, because at idle the intake vacuum is the greatest & scavenge is the weakest, allowing exhaust gases to be sucked into the intake manifold during valve overlap, diluting intake manifold mixture, which in the subsequent intake stroke, fills the combustion chamber with inert exhaust gases that don't burn, causing a misfire (stumble).

FACTS:

1. Exhaust blow down rate is usually limited by exhaust valve seat inside area. Exhaust gases can not exceed their speed of sound: 1400 ft/sec @ 1100 degrees f.

2. Headers can work better with long duration exhaust cams. But, not because headers flow better. Rather, headers flow poorly and need more time to flow the same amount of exhaust.

3. Long duration exhaust cams open exhaust valves earlier, dumping combustion energy into the exhaust system. The extra combustion energy and the extra exhaust valve open time (duration) allows the header to expel more exhaust gases than it otherwise could with the shorter duration stock cam.

4. Opening the exhaust valve earlier shortens the power stroke, drastically reducing thermodynamic efficiency of the engine.

5. Opening the exhaust valve earlier dumps hotter, still burning air/fuel into the exhaust port and header, making them hotter. Hotter exhaust gases shorten the life of exhaust valves, cook headers, and add heat load to the cylinder head.

6. If headers really flowed so well, a shorter duration exhaust cam, one more like stock, would allow a longer, more efficient, power stroke, kind of like a stroker crank vs. a stock crank. The longer stroke usually makes more power.

7. Hp = Work/sec (1 HP = 33,000 ft lbs./minute = 550 ft lbs./ second)

8. Work = Force X Distance (Work = (area of bore X combustion chamber pressure) X (power stroke length))

9. If all else were equal, reducing the power stroke will reduce Work done by each cylinder and total engine HP.

10. In a four stroke engine, the crankshaft rotates 720 degrees in order to complete all four of its strokes (4 X 180 degrees).

11. If one degree of duration is added to the exhaust stroke, one degree must be subtracted from one of the other three strokes (intake, compression, or power).

REASON:

Since the exhaust stroke generally can not be expanded into the intake stroke, due to the increased probability of valve to valve and valve to piston intersection during the valve overlap period, degrees are usually subtracted from the power stroke. If a long duration exhaust cam is, say, 20 degrees longer than stock, usually it also makes the power stroke become 20 degrees shorter than stock.

For example, a Ford 302 has a 3" stroke. If the stock exhaust cam forces the valve open at 70 degrees BBDC (before bottom dead center), the power stroke is 2.013". If the long duration exhaust cam opens the valve only 20 degrees earlier (in order to add 20 degrees of exhaust duration) the power stroke is reduced to 1.5". The long duration exhaust cam, with just 20 degrees more duration, has reduced the power stroke by .513"! That is the opposite effect of a 1/2" stroker kit!

People spend thousands to put even a 1/4" stroker crank in their engine, to make more power. You've just done the opposite. You've just cut your stroke by 25%. All else being equal, you can expect to lose about 25% of your engine's thermal efficiency (its ability to convert heat (thermal energy) into horsepower (mechanical energy)).

If a header really did flow better, wouldn't a person be smarter to leave the power stroke duration stock?

The reality is, a header does not flow very well at all, because it expands, cools and therefore slows the exiting gases. The long duration exhaust cam is really just a marketing band aid for the very poor flowing 4into1.

[RobertISaar]

bad headers can flow badly..... but even mediocre headers should outflow all but the best of cast exhaust manifolds.

 

i don't think they're taking into account the fact that headers are generally tuned for a specific frequency and they're measuring a range where the header is giving up flow compared to a stocker.

 

if they didn't work, F1 teams(and all other motorsports that don't have restrictions on it) would be using cast iron manifolds.... they don't.

[skitchin]

Listen to this 433ci LSX rev to 9,000 RPM. Over 1,000HP...NA

 

[video=youtube;lndmqwPty0M]

[jman093]

Listen to this 433ci LSX rev to 9,000 RPM. Over 1,000HP...NA

 

[video=youtube;lndmqwPty0M]

 

Several years ago (I forget what year exactly it changed) Nascar cup engines were running over 10,000 RPM and making over 900 hp from 358 ci max displacement and flat-tappet cams. Nascar then instituted mandatory rear axle ratios across the board to limit RPM and level the playing field because that's what Nascar does. It dampened perfoamce, but I think the engines now are back up around 9500-9800 RPM and making around 850hp. I've read that they could now push them over 11,000 RPM easy without the gear rule.

 

But there's also restrictions on pretty much every part. Restrictions on block dimensions, sizes, and spacing, part metals/materials, no roller cams, spec fuel systems, and on, and on, and on. It makes you wonder what they could do with a 350 ci pushrod engine without restriction. The things would be insane with exotic parts and engineering behind them. That's shit's way too cool for Nascar though.

Edited October 23, 2013 by jman093

[Imp558]

Cool as this discussion is I'm actually looking unsucessfully for an L36/L67 build thread to read through. Nothing so far, looking for advice on L36 bottom end with different cams and types of timing chains, any discussion about dual idler geardrive would be bitchin'. Anybody remember where any threads are?

[RobertISaar]

i have the solution:

 

 

install 6 more cylinders. 12 cylinder cars can't sound bad.

 

maybe of LQ1 w/ITBs and 13* exhaust retard variety.

 

and then to top off the insanity....... connect both engine's exhaust systems together.

[Garrett Powered]

here's what u want