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IvanTheTerrible

And now for something a little bit different.

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Guest xyzzy frobozz

Rev limiters don't cause the exhaust to backfire like that, its because the mixture isn't properly adjusted for the advance timing of the valves at high rpm. People seem to think this sound is cool, but its just the side effects of a poor tune.

You're wrong.

 

Rev limiters will cause a highly tuned car to backfire at high revs due to the simple fact that, having cut ignition (which is what a rev limiter does), the fuel then gets exhausted into the hot manifold, causing it to "backfire".

 

Where else does the fuel go?

Edited by xyzzy frobozz

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Guest xyzzy frobozz

Well, maybe I should have said "lack of timing advance at high revs"....regardless of that, with an ECM you can lean the mixture to prevent the banging. I still say its a poor tune.

 

Lot of nonces around.

Again, you're wrong. I don't mean to sound nasty, but it isn't the "nonces" who are wrong in this case.

 

Ignition timing has very little to do with valve actuation. Timing refers to the timing of the spark (or ignition) as related to crank position - specifically to "Top Dead Centre" - where you advance toward TDC or retard from TDC. The fact that the transition from mechanical ignition to electronic ignition eschewed the distributor for a crank angle sensor should be a pretty good hint!

 

With regards to mixture, in a performance car you'll always run the mixture slightly richer than stoichiometric as the unburned fuel can help cool the piston crowns. This is particularly true in turbocharged petrol engines. As you compress air, it heats up, if you run the engine too lean the mixture will pre-ignite and stuff your engine in pretty short order. Pistons heading upward into combustion equals BOOM! sometime soon. Having hot piston crowns exacerbates this problem.

 

EDIT: I realise the engine doesn't have variable valve timing...and this is the problem

How exactly are you going to have VVT in a single cam pushrod V8???

 

the revs increase, the speed of the flow of mixture doesn't increase, thus the valves come out of sync with the ignition of the mixture in the cylinder. This was one of the innovations of VVT which fixed that problem., though it can be mitigated by leaning the mixture, thus causing a slower burn.

Gas (air) will always move from an area of high pressure to an area of low pressure. Although the effect is not linear, the fact that the piston moves downward faster at high revs means that gas moves into the cylinder faster than it does at low revs to fill the vacuum the cylinder creates. No doubt, variable valve timing has an advantage in this respect (being able to change cam phasing at higher revs), but usually they're tuned for better torque at low to middling revs where a road car's engine spends most of it's time, rather than power at high revs.

 

In any event, all of that is a moot point in a single cam pushrod V8 as shown in the video.

 

That engine sounded perfectly well tuned to me, and it looked to me like he cracked off a pretty good quarter which could be bettered with a better second to third change next time round....

 

Either that thing is very heavily modified for no good reason, or you're talking out your neck. What's your source?

Easy. Look at the parts that are unchangeable. Windscreen, doors.

 

It's a Jalpa, albeit a modified one.

Edited by xyzzy frobozz

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I'm not a car person, but to me this truly honestly looks like either:

a) A kit car.. (It may well have been a Jalpa to begin with.. But now? It's got so much chopped off and tacked on.. Wellllllll.... Jalpa is a very generous phrase.. Unless you look at it like a grandfathers axe. New handle, new head. Same axe)

 

b) Like those douche's that are very common around Canberra. Buy a Porsche 911 and put a fully sick exhaust on it. (That's gotta add 2,000,000,000 HP I'm sure of it). No other mods. Just the exhaust.

 

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So many self-appointed experts.

 

It doesn't sound to have a lumpy cam, in fact it could well be stock. Watch the end of the second video especially, and also the owners comments on what he's done to it. I'm sure if he'd put another camshaft in, he'd have been sure to mention it.

 

Rev limiting on modern GM V8s isn't necessarily cutting the fuel off only. It can be a mix of fuel/ignition off and intermittently cutting fuel to certain cylinders as well as backing off the throttle valve (remembering that many engines are now "drive by wire")

 

Ingestion of air or air/fuel into the cylinder during/after exhaust. Contrary to popular belief, it's not exclusively due to vacuum caused by the cylinder going down and creating a void. The high velocity of the exhaust gases exiting the cylinder creates both a vacuum and scavenging effect which helps draw in the next load, even before the cylinder reaches TDC on the exhaust stroke.

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Guest xyzzy frobozz

So many self-appointed experts.

Welcome to the club!

 

It doesn't sound to have a lumpy cam, in fact it could well be stock. Watch the end of the second video especially, and also the owners comments on what he's done to it. I'm sure if he'd put another camshaft in, he'd have been sure to mention it.

 

Rev limiting on modern GM V8s isn't necessarily cutting the fuel off only. It can be a mix of fuel/ignition off and intermittently cutting fuel to certain cylinders as well as backing off the throttle valve (remembering that many engines are now "drive by wire")

 

Ingestion of air or air/fuel into the cylinder during/after exhaust. Contrary to popular belief, it's not exclusively due to vacuum caused by the cylinder going down and creating a void. The high velocity of the exhaust gases exiting the cylinder creates both a vacuum and scavenging effect which helps draw in the next load, even before the cylinder reaches TDC on the exhaust stroke.

All true. Although I'd doubt the car in the vid has a drive by wire setup...

Edited by xyzzy frobozz

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