Can my K27HF be too small ?

[hobieboy]

I have rebuilt & upgraded the engine since last year and now I experience a LOT more lag than before.

Here's the background - engine before: stock 3.3 internals & CIS with Garretson intercooler, GHL headers + straight pipe.
Now, its a 3.4 with ported intake port (41.5mm) and a Bell full bay intercooler + EFI.

Good news is: there's lots more power and the intercooler works beautifully controlling temp.

Bad news is: lag is HUGE now compared to before. Just to be sure, when I say lag, its the time you punch the throttle till you see boost.

So, can this be that the turbo is too small & can't supply air fast enough? Or, where else should I look?

thanks.

[WydRyd]

Usually the other way around - if the turbo is too small, it will spool very fast, however, it will run out of breathe up top and begin to drop in power as it'll be out of it's efficiency range. I think you must have some issues with your tune, or perhaps vacuum leak somewhere or a suspect wastegate.

[hobieboy]

Thanks for the comment Merv...

I thought about the tune as well, but how would that affect lag?

According to my LM-1, I'm running good AFR at max boost (around 12.0:1). Do I need to adjust the transient AFR (leaner? or is it richer?) to help?

[WydRyd]

The state of tune on an engine can adversely affect the responsiveness of the motor & turbo.

However, if your AF's are in check, then it's time to look elsewhere. Is your wastegate functioning properly? It could be venting exhaust gases before the preset boost level is reached, thus affecting spool-up due to loss of exhaust energy to the turbine.

[mb911]

you need to be closer to 11.25-11.5 AFR for a bit more fuel. This helped mine spool quicker as more fuel to burn plus protect my engine..

[hobieboy]

Originally posted by mb911
you need to be closer to 11.25-11.5 AFR for a bit more fuel. This helped mine spool quicker as more fuel to burn plus protect my engine..

Wow, that is rich!!! I thought with EFI and better ignition timing control, we don't need to be so pig rich?

Maybe I need to improve the acceleration enrichment part as I step on the throttle to speed up the response?

[RarlyL8]

Something's not right, that is for sure.
You say the (stock) intake is ported, are the heads ported as well? If not the turbo may be too big.
How much wheel HP is being produced? Can you post a dyno graph?
What does your boost gauge tell you during a run?
What has been done with the ignition? Ignition timing? Cams? Cam timing?
Lots involved that can contribute to lag.

[Craig 930 RS]

Switch back to CIS. I hear it is now an 'upgrade'

[hobieboy]

Originally posted by RarlyL8
Something's not right, that is for sure.
You say the (stock) intake is ported, are the heads ported as well? If not the turbo may be too big.
How much wheel HP is being produced? Can you post a dyno graph?
What does your boost gauge tell you during a run?
What has been done with the ignition? Ignition timing? Cams? Cam timing?
Lots involved that can contribute to lag.

Yes - the head is ported as well (I'm using a 3.2 intake).

I have no idea how much power I'm making - it certainly feels a lot more than before. I am not a big fan of dyno, and at this stage of my tune, I know I'm not ready yet.

And I am also considering ignition timing and fueling as potential contributor to the problem, but not sure where to start

[Jim2]

Just did an injector size change and re-map. Thought my AFRs were OK under about 2500 then I tossed an overall percentage at my AFR maps and noticed my bottom end torque and turbo response picked up immediately, surprised me.

I'd also highly suspect ignition timing as Brian said.

Sounds like your turbo, headers, and camshaft profile remain unchanged, so I'd be looking more closely at AFRs and ignition timing since these two important factors have been completely changed. Try to emulate the stock timing curve to narrow down the hunt, or at least inspect your timing values more closely.

[beepbeep]

1st: what people generally call "turbo lag" has actually at least two compontents: boost treshold (influenced by capacity, turbo sizing and plethora of other deatils) and inertia lag itself (influenced mainly by shaft weight)

2:nd: What you now feel as 'lag' might be the headroom you feel after updates. What I'm trying to say is: before the upgrades your power jumped from approx. 120hp off-boost to 300hp on-boost. Now you installed bigger ports (=less air velocity at low revs=less torque off-boost) and your power probably jumps from something like 100hp to 370hp (yes, it's a crude approximation but you know what I mean).

So yes, you both gained high end and somewhat decreased the torque at off-boost low-end due to bigger ports. Add the possibility that your ignition timing might not be optimal and yeah, you might feel increased lag.

As I said before, if you measured the lag period before/after It would probably be the same but as you now get a whole lot more it feels like lag is bigger.

There a couple of ways to decrease the lag. You could try a more modern turbo to start with. Garrett GT35R is a splendid design. Shorter headers help as well. You can also retard off-boost ignition and shorten the spool-up time (but you will pay it with less torque off-boost). Everything is a tradeoff.

Most extreme method is by using WRC style anti-lag. It can give you no-lag full boost all the time but it won't last long

[Porschephd]

If you have more lag with the setup you have now, look at your tuning. You will have some delay compared to the orginal setup becuase of port size etc, BUT you can tune that out with EFI.

[hobieboy]

Thanks for the comments guys...

As Stephen & Goran pointed out, the 1st thing I suspect is also the port size/flow that the engine have now, as well as the much bigger intercooler (takes longer to fill). Hence, thought if I need a bigger pump (turbo) in front to fill it faster.

Sounds like I should go back to look at my ignition timing more closely & see if I can dial some of the lag out.

Question - I thought retarding timing will slow the engine rev up response; hence more lag?

[Jim2]

Yes, retarded timing causes the mixture to burn later in the cycle. It produces less power and increases EGT, when compared to optimum timing.

Don't read too much into port size, intercooler volume, and turbo. None of these items will induce noticable lag unless taken to extreams.

Unknowns aside, abnormal lag is more likely timing and tuning related.

[Craig 930 RS]

Turbo has a huge impact on lag, gotta objectively disagree with ya Jim.

[RarlyL8]

My money is on tuning.

I have personally seen lag dissapear from both EFI and CIS engines through meticulous tuning. When you get the cam timing, ignition timing and the A/F ratios perfect the whole world changes.

Another moving target, there are turbos designed specifically for EFI applications and turbos designed specifically for CIS. Your K27HF turbo probably does not have the same characteristics now that you run EFI. Stephen is an expert in that area.

[Jim2]

Topic "can my K27hf be too small"

Craig, what I'm trying to get across here is his turbo is not subject, it did not change, but other things did. I don't disagree with you, that in theory the turbo can be poorly matched causing severe lag.

Any good news for us hobbieboy?

[Craig 930 RS]

Now if you are speaking of midrange throttle response - too rich of AFR will certainly give away 30 lb ft tq as well as deaden response - perceived as lag.

Keep in mind the two are different and manifest themselves as similar.
Turbo lag is one thing....

With your setup it could be a number of things -
------------------
"Don't read too much into port size, intercooler volume, and turbo. None of these items will induce noticable lag.."

Also what I am saying, is this: The 'lag' / response of a K27HF vs. a K26........night - n- day. And the delay from a larger intercooler + one with a non-optimal design will be perceived as lag - just not 'turbo lag'.

[WERK I]

Are you running any advance at all? Remember, at low engine speeds, you are more than likely running more in vacuum than boost until the turbo builds up energy. I think you have to recurve your ignition to run in no-boost/boost conditions.

[hobieboy]

I change my map slightly to improve off boost response as well as to address my not-so-stable idle AFR issue. So far, she drives A LOT better.

I was at the track yesterday but didn't have a chance to datalog (instructing with a student in car). Still have more lag than I like but seem a little (very little) better, so I think all of you are right - it will likely be a tuning issue.

I also noticed that now it takes a heavier foot for boost to build - when it builds, it VERY fast. But what used to be "minute throttle movement" to build boost is now like I need to floor it to activate that initiate boost build up.

On the ignition timing front, yes, I will try to dig up the stock curve again to compare. I'm running quite a lot of advance (I think) in no boost situation (up to 30 deg then back to 16deg @ full boost/6000 rpm).

After speaking with Gerhard @ Bell Intercooler, he also suggested that tuning - he pointed out that with the CFM that the engine/turbo can flow, even if the intercooler is way too big by 1 cu-ft, it will only take ~.1 sec longer to fill (i.e. .1 sec more lag)....