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Understanding cam timing

Tfrasca

Tfrasca

Active member
Joined
Apr 20, 2015
Location
Ben Lomond, CA
My B23ft has ET pistons (9.3:1 static CR), a 19t, 4.30 diff gears, and an IPD turbo cam timed straight up. The turbo spools quicker than anything I've ever had. It's at 8 psi before 2500 rpm. Most of the time it feels like a 6 cylinder with near instant response.

Sometimes, though, if I floor it and allow boost to build from 2500-3000 rpm, the power will come on later than the boost. Does it make sense that if boost builds in the manifold but the car's not going, that the cam isn't letting the charge in?

I figure it's either that or LH2.4 pulling ignition timing. But mostly I want to understand more about the relationship between manifold pressure, cam timing, and power.
 
This guy is awesome:

https://www.youtube.com/watch?v=bnfdYc2GAz0

Are you sure the exhaust is good? Air's gotta go out before it can go in. Otherwise, you may want to consider advancing the cam timing. The IPD T-cam is known for not having a ton of bottom end. It's amazing what a degree's difference "at the cam" does with the power band.

-Ryan
 
VERY interesting you mention that Tyler. Since I swapped the 531 on, with larger valves, I've noted the same behavior on mine. I'll see 5-6psi but it feels like nothing really changed, then when LH finally goes rich, all that changes, and the car ****s and gets. Didn't notice that on the stock 530, but it was probably a little higher compression (minimal though, 530 with stock HG, 531 with tight squish). I've played with cam timing a bit, got a known baseline where the cam is actually straight up on my specific setup, so far a few degrees either way doesn't seem to change it.
 
That video looks interesting. I'll watch it when I have more time. Interesting you mention the exhaust. I have a nice 3" down pipe, but it bolts up to the stock 140 exhaust, which is 2". Obvious restriction there. The weird thing is that the car seems to want to keep pulling as long as I stay on it. I'd think the small exhaust would be nice for low end torque, but would run out of steam up top.

Ryan_R said:
This guy is awesome:

https://www.youtube.com/watch?v=bnfdYc2GAz0

Are you sure the exhaust is good? Air's gotta go out before it can go in. Otherwise, you may want to consider advancing the cam timing. The IPD T-cam is known for not having a ton of bottom end. It's amazing what a degree's difference "at the cam" does with the power band.

-Ryan
Click to expand...
 
gsellstr said:
VERY interesting you mention that Tyler. Since I swapped the 531 on, with larger valves, I've noted the same behavior on mine. I'll see 5-6psi but it feels like nothing really changed, then when LH finally goes rich, all that changes, and the car ****s and gets. Didn't notice that on the stock 530, but it was probably a little higher compression (minimal though, 530 with stock HG, 531 with tight squish). I've played with cam timing a bit, got a known baseline where the cam is actually straight up on my specific setup, so far a few degrees either way doesn't seem to change it.
Click to expand...

Hmm. That makes me think it's an LH thing. Like it's pulling timing until it can get some fuel in there?

For what it's worth, I have a stock 405 head and stock gasket.
 
Hmm, there goes the big valve idea then. Even on the 530 head it still felt peppier at lower boost, just ran out up top, when it was still going rich at around the same point.

Gotta see what it does if I ever get the MS installed.
 
Ahh, might change it a little. Funny though, even at 5-6psi, if it were just due to building up and lag, it should resolve itself, but mine doesn't. You can sit there at 5-6psi and it never changes much, until you hit a few psi more, LH goes rich, then it starts pulling strong. Didn't do that on the 530 head and that was the only change.

I suspect it's an airflow/2.4 quirk, plus pulling the AFR for torque vs. AFR for HP equation into play. If it richened up a little, torque would increase (in theory).
 
AFR for torque and horsepower are the same thing. If you're at some particular engine speed, torque is directly proportional to power. More torque, more power. More power, more torque.

Though I do agree that this is probably LH 2.4 doing something strange. I've never seen a car do something like that on a stock-tuned ECU, nor on a properly tuned aftermarket ECU.
 
Here's my guess about what's happening:

1) You floor it, which spins the turbo hard.
2) Turbo sucks a bunch of air through the MAF, causing LH to inject much more fuel.
3) Turbo finally moves enough air to actually increase manifold pressure.

Between 2) and 3), the engine will be running piss rich. When the boost finally makes its way to the intake valve, it leans out, and you make power.

I think the plenum volume is killing you, but not in the way that you think. There's no such thing as a momentum problem, because if you have manifold pressure, the air is already flowing in without a problem. The whole point of manifold pressure is to estimate the amount of atmosphere available to the intake valve, so momentum is a bogus argument.

What you're describing sounds like the computer is over-enriching on fast throttle increases, and the fuel is actually BEATING the air to the engine.

Also remember that a wideband will read lean when there is extra oxygen, including during a RICH misfire. So if the car briefly sees 6:1 AFR for a few cycles, the O2 might read off scale lean, then "richen up" once the engine actually leans out.


I'm running a similar setup, b230f+13c, but with a Megasquirt instead of LH, so I'm referenced off of MAP, not MAF. There's a delay between the MAF reading before the turbo, and the air actually arriving at the engine, but there's no such delay when measuring MAP.

(NB: there's a good reason that most if not all boosted modern cars use a hybrid MAP/MAF setup, instead of just MAF, while many NA cars still use only a MAF)
 
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Don't quite think that's the issue we're fighting. AFR's read stoich, no misfire, definitely not a rich misfire, and you can hold it at a set boost level, doesn't change. On top of that, 2.4 is known for a momentary lean condition rolling into boost, been a known issue around here for years, couple threads up here about it right now talking about the pinging at boost onset.

As for most, if not all modern cars using a hybrid MAP/MAF setup, I'm curious where you're finding that. Reason I ask, my job is to stay up on this stuff for every make sold in the states and I've NOT run across that on many cars at all.
 
Hi

This behaviour is totally camshaft related and nothing else.
This is why the T cam is stock in most Volvo redblock turbo cars.
I got the same "revelation" when I swapped the T cam with an A cam in an otherwise unchanged (and mechanically stock) 230FK. If lugged on low gear I managed to build 0,5 bar boost as early as 1500 rpm as long as it got plenty of fuel but it didn't produce any worthwhile power until say 1800 rpm where the A cam starts to get into harmony with the engine.

YS
Jaybee
 
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Hold on, by "the power will come on later than the boost", do we mean will come on later in time, or come on later in engine speed?

Because if it's the former, there's something strange going on with LH. If it's the latter, then it's absolutely the cam. My car will do ~6 PSI at 2500-3000, but comes up on the cam after about 2800-2900.

gsellstr:

Note that I said boosted cars. A MAP sensor is often added for engines that have turbos, but left off if it's NA, even if they're the same-ish engine. For example, the 2.0L direct injected Duratec found in recent Ford vehicles uses only a MAF sensor. The 2.0L Ecoboost, which is the same engine plus a turbo, has, in addition to a MAF, has two pressure/temperature sensors, one after the intercooler, and one in the intake manifold after the throttle.
 
I read what you wrote, understand the boosted thing (job requires that attention to detail, can't turn it off...lol). I'll check into the Ecoboost, but most that I've seen with a MAF only have a Baro for pressure, nothing downstream from the turbo. The few that do, reading up on system operation only use it for boost pressure, not for determining fuel/timing.

Back to the topic at hand...in my specific case, the only thing that changed was the head, same cam, timed the same way (verified cam timing on both heads), so it's not cam related in my case. The issue changed with airflow. Now, that said, I did change the ratio on the 531, stepping the exhaust about 10% closer in flow to the intake than the previous head, so cam timing in relation to that may have some effect.

The specific scenario...if you roll into boost at 2500rpm and hold the pedal steady, at up to 5-7psi of boost, it will maintain stoich, power doesn't increase while you hold the pedal steady. Run it up to around 8-9psi, AFR's drop to 12's, power picks up expontentially. That tells me it's LH related.
 
mrjaybeeze said:
Hi

This behaviour is totally camshaft related and nothing else.
This is why the T cam is stock in most Volvo redblock turbo cars.
I got the same "revelation" when I swapped the T cam with an A cam in an otherwise unchanged (and mechanically stock) 230FK. If lugged on low gear I managed to build 0,5 bar boost as early as 1500 rpm as long as it got plenty of fuel but it didn't produce any worthwhile power until say 1800 rpm where the A cam starts to get into harmony with the engine.

YS
Jaybee
Click to expand...

This is representative of my situation. It seems like the turbo is pushing air into the manifold, but the engine can't swallow it until later in the RPM range.

Unfortunately I don't have a wideband, so I'm not exactly sure what's happening with AFRs.
 
mck1117 said:
Hold on, by "the power will come on later than the boost", do we mean will come on later in time, or come on later in engine speed?

Because if it's the former, there's something strange going on with LH. If it's the latter, then it's absolutely the cam. My car will do ~6 PSI at 2500-3000, but comes up on the cam after about 2800-2900.
Click to expand...

It seems to be the latter, for me. I have Dale's timing gear, so I'll try 2 degrees advanced and see if that changes the boost/power curve.
 
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