304 Where is my bottleneck (who is my car so slow)

[Mr DOHC]

hey all, I need some help. This may be a bit wordy so excuse me if this doesn't make much sense.

I have myself a VP ute with a mild 355 and auto, it was built as a torquey streeter, never a race car. But it is just not as fast as I believe it should be even with the basic mods it has:
355
10:1
284 cam with YT rollers
HSV headers, HSV twin 2.25" dumped before the diff (y piece after headers is only 2.25")
215i injectors
Kalmaker
Stock auto with corvette servo

I took it to the track and without stalling it up (starts to spin at 1600) I left from idle and ran 14.88 @90 on a 31 degree day.
The car feels almost as quick overtaking at 1/2 throttle as it does at full throttle.

Tuning issue:
I've tuned it as it was a dead-set daily and tuned mildly. I cannot give more than 20 degrees timing above 3600 rpm without it detonating and reducing power significantly. This doesn't sit well with me as this is a massive drop from what I have been told it should/could run. Or is it the cam/355/stock manifold creating such big cylinder pressure, combined with the shitty headers at 3600-4400 and it just causing it to choke?

I don't have cash to just throw at a car at the moment to hope it fixes ****. If I can get an idea as to what any of you guys think it might be so I can have something to look at

 

[2INIL8]

What's your afr's at full throttle? 1000-6500 rpm?
Whats your base timing at in diagnostic mode?

 

[Mr DOHC]

What's your afr's at full throttle? 1000-6500 rpm?
Whats your base timing at in diagnostic mode?

I have the ecu set to chase a 12.5:1 it doesn't rev past 5500, cam is only good to 4800.

Base timing is 8-10

 

[EYY]

Have you had a wideband sensor on it? Are you doing the tuning and if so have you had much tuning experience?

When you mentioned the exhaust y piece, do you mean that its not actually a twin exhaust? A single 2.25 exhaust won't be doing you any favours.

You should be looking at a ~2400 rpm converter for your combo. With the cam I would've gone with the 286, but the 284 should still produce good results. Did you degree the cam, or was it ust fitted and the dots lined up?

What's been done to the heads and what valve springs are you using?

Lastly the intake will really be choking it in the upper rpm range as you already know. If you want to keep it looking stock you can source a starr manifold (extrude honed with a bigger elbow).

 

[Mr DOHC]

No WBo2, seat of pants and not much experience.

The exhaust has the factory HSV headers to a 2.25 y piece that then splits off to a twin 2.25 system.

The engine was professionally built by a performance guy. It will have been timed/degrees 100%

I'd rather not get a converter as the bottom end is quite nice. It's the top end that is lacking. As I mentioned it gets around very well, just that it's almost as fast at 1/2 throttle as it is at 100%

I have played with the WOT AFRs all the way to 10:1 at 3600+ and it doesn't make a difference.
I would love to grab a WBo2 and road tune each fuel percentage/load point but I can't find anyone in Bris who has one and can help me.

 

[EYY]

Look at buying an innovate mtx-l. They can be had from $200-300 depending on where you buy and the exchange rate. It will be the BEST tool to help you understand AFR's. It's a small investment compared to the risk you run of running lean or running too rich and causing engine damage.

The ECU cannot tell what AFR it's at - the narrowband sensors are really only good at telling the ecu whether it's running lean or rich either side of stoich (14.7:1). So in fact when it's shooting for 10:1 it may not be anywhere near it (I highly doubt it's running anywhere near the correct AFR's). Are you able to do live tuning with kalmaker? And are you able to view the map with bubble tracing (small indicator that tells you what the injection timings are at each rpm/throttle position etc. when running/driving)? I don't know much about kalmaker software so not sure how it works. I'd even go as far as to suggest that you don't even drive it until you know what its doing.

What Y piece are you using? I have 2 factory Y pieces here and they're both 2.5". One is off a s3 VS which is mandrel bent, and the other is from a VQ which is press bent. Single 2.25 is far too small for a mild 355. Are you still running the stock cat?

 

[markalan1two]

What is you diff ratio? what gear and rpm are you crossing the line??

 

[Immortality]

Can you confirm the exhaust? it it goes down to a single 2.25" pipe before splitting into a twin system that will be a major problem.

I've tuned a mild 355 with pacy headers to single 3" system and it will take 28 degrees of timing no worries at all.

 

[Mr DOHC]

Yes the Kalmaker can be tuned live but it only has a gauges screen, it doesn't have the bubble that moves across a map. It's easy enough to use.
I didn't realise the stock o2 was that bad.

Definitely a single 2.25 to twin 2.25, no cat.

Standard 3.08, crossed the line about 4100-4200

 

[Immortality]

Your first problem and probably the biggest one is the single 2.25" exhaust pipe, once that is sorted then you can start thinking about tuning it properly.

The factory O2 sensor is a narrow band sensor and is only really designed to measure around the stoichiometric ratio (14.7:1), although it can be used as a guide (as it shows rich/lean) you can't rely on it with any accuracy for WOT tuning.