Fun is in the eye of the beholder... I just got home from work a little while ago, it's now 3:43 am... I had to be at the studio at 7 am yesterday... And have to be out on location in the So. Cal. desert at 9 am... A mere 5 hours from now...

Now you know why I bury myself in this car when I have the time to do it...

 

That's quite a job even in a "no frills" racer.

I found a low mileage M112 engine from a crashed CLS with an auto box attached - £2K was too steep, particularly as our Crossfire (mostly the wife's) is likely to remain stock. With a free hand and time I would love to strip all the extras off, fit throttle bodies on the top (don't know if there is clearance) and run a manual box, in a stripped out Crossfire.

Attached is a picture of some bike bodies someone is attaching to a Mazda/Ford 1.8L V6 - no not what I would want to see in a Crossfire engine bay but you get the idea.

 

This is something I have been looking at for over a year but it will have to wait until I have got my other project back on the road as a daily driver to free up the Crossfire and gararge space. I have CADed up manifolds for some stubbie Jenveys (there is just enough height) and plan on junking the trick plenum arrangement. I'll probably start it next spring. The bike throttle bodies (Triumph triples are readily available apparently) and Megasquirt setup is starting to get quite popular these days; I know of several early 911's running it successfully.

 

Adding a slightly larger injector to the N/A engine might give you a little more when in WOT conditions, but will do nothing in closed loop as the O2 sensors command the AFR.

Does not surprise me to see such larger injectors on the SRT-6, once you go into boost, it is common for the forced induction SRT vehicles to run a nasty rich 10.5 to 11 AFR, hence using a large injector.

There are no manifold ported regulators, the pulse widths for the injectors are written into the fuel tables in the PCM. That is how they run such bigger injectors in the same displacement engine when not in boost.

If you pop the SRT-6 injectors into an N/A crossfire, you will be lucky if it even starts, and there could be a big risk of flooding the cylinders with fuel and possible hydrolock.

Looks like the SRT-6 injectors are more than three times the size of the N/A engine, I bet if you checked injector pulse widths, you would find the SRT-6 to be around 30 - 40% of the N/A engine pulse width.

Just because the MB version uses a different injector does not mean the PCM program is the same and that somehow Daimler ripped us off. But, would you think Daimler would give the Chrysler Badge the same HP/TQ as their flagship Mercedes product? Don't think so.

The adaptives in the PCM can usually handle either plus 20-25% or minus 20-25% injector variance, meaning it could figure out how to run on a 30-40% bigger injector, but it would be ugly while it learns.

But, also (always the down side) if you added say a 10% bigger injector, the short term fuel trim would handle that pretty quick, and you would see a bit more umph in WOT. For awhile. The short term trims eventually bleed over into the long term trims, which then affect the WOT tables, so after a while, you would be asking where the extra umph went?

If playing with a MegaSquirt, the best way to do it without a PCM flash, is trick the O2 sensors when in part throttle to run richer, intercept the injectors pulse after the PCM and before the injector and modify it so the PCM does not see it!

Cars are far too smart these days for the Hot Rod at heart.

Mr MoPar

 

The post all made sense - what do you make of the fact we seem to need 100% duty to make 215 hp? (with the stock injectors). Doesn't seem a normal design principle for a road car.

 

I'll have to second that. Sure doesn't seem right. M-Benz engineers motors for long-term reliability and running injectors that hard (especially in a stick-shift sports car application where WOT is far from unusual...) would seemingly result in a lot of warranty claims !

Slim

 

That's WOT at near redline... not just WOT guys... please keep in mind that the duty cycle gets closer to maximized as the RPMs climb.

At idle and power band area, the 100% duty cycle would not quite be reached.

At redline and above, it would be 100% duty cycle (from what I understand of HDDP's calculations).

Also, if the PCM is only in closed-loop mode during 'cruise' mode, then the idea of putting even slightly larger injectors should yield a performance boost. I do agree with YellowBee that the best idea is to add something like the MegaSquirt system where it intercepts the stock pulses and modifies them without the PCM knowing about it and alter the O2 sensor data slightly.

 

OK - but all your saying if full power is only made at WOT and with revs which isn't really news! My point was simply to get 215 hp (engine) you need to flow enough fuel that the injectors are at 100% duty cycle, that implies that without a change to injectors or fuel pressure (which is already quite high) you are stuck at 215 hp.

New exhausts air boxes and the rest can't improve the amount of power for fuel burnt (brake specific consumption) they can only allow more air to flow - which needs more fuel.

Megasquirt is a fun bit of kit but I don't like piggy back systems and the "cleverness" of the factory ECU means you are likely to throw faults up when the AF deviates to an increased richness etc.

 

NO, what I'm trying to say is that the 100% duty cycle does not occur at peak horsepower. The camshafts determine when this peak occurs.

After peak horsepower and at or just after redline THEN the injectors would experience 100% duty cycle.

Simply replacing these with 10% to 20% bigger injectors SHOULD yield some additional permenant power as the injectors would be able to give more fuel at top end and around peak horsepower. Fuel trim or not, if the injectors are reaching 100% duty cycle at or just after redline, then having them a bit larger may yield some higher RPM gains. Everything below peak power would be higher power until the fuel trims compensated.

Cheers!

 

seems your going to make more power with bigger injectors, I think the larger injectors will allow the timing to increase all the way up the powerband from 4,000 - 6,200 rpm. It would make a great excuse for someone to do another dyno run with base crossfire using stock vs srt injectors. probable only gain about 5-8 hp above stock power output. unless you have a more aggressive tune.

 

Sounds like we're starting to reach a consensus here; going to an injector set with a MODERATELY LARGER flow rate should slightly richen things up and improve Open-loop high load (hard acceleration) power particularly in conjunction with increased airflow (bigger cams, intake, filters, bigger air boxes, headers, higher flow/removed/gutted cats, free flowing exhaust etc).

So here's the $ 64,000 question...what is the OEM Mercedes or Bosch part number for a compatible injector which will deliver a 15%-20% flow rate increase ?

Do the injectors in some of the Mercedes Models equipped with the 3.2 SOHC motor flow more?

And isn't there a 3.5 version of the SOHC V6 used in SUVs ? I bet those injectors would work and deliver a little extra fuel!

Let me know what to look for and I'll try to find a working "junkyard sourced" set to try.

Cincinnati Slim

 

I would just stick with the SRT6 injectors, but I guess if you want to go nuts you could try the SLK55 injectors, or maybe the 4 liter v8 benz injectors.

 

This is slightly off topic, but does anyone know the size of the fuel schrader size? Like AN equivalent or what not?

 

EXACTLY ! but what our non-SRT 3.2 motors need is a SLIGHT upgrade from 17 lbs./Hr. injectors to perhaps 20-24 lbs./Hr. units.

The SRT6 injectors are going to be WAY TOO BIG.

When I was working with FoMoCo 4.6 SOHC "Mod-Motors" Stock injectors were 17 lbs./Hr. Upgrade injectors for better head/intake/cam/exhaust combos were typically 24 lbs./Hr. SUPERCHARGED applications went with 30 or 36 lbs./Hr units.

So here's what I think we need; An OEM Mercedes/Bosch injector rated @ 22-24 lbs./Hr!!!

With the widely used Bosch/Speed Innovations/Upsolute/Powerchip ECU download "flash", a lower restriction exhaust and 10-15% more fuel delivery under WOT conditions we might expect to see 245-250 Horsepower.

Now THAT would be nice !

Slim

 

Don't forget about the upgrade cams, they say they make another 20 hp, but it's not totally confirmed by dyno. Then you can throw on a larger airbox from the v8 benz.
then some underdrive pulleys, good for 3-8 hp gain.

 

So come on somebody, let's plug in a set of injectors from a junkyard MB112 engine in place of the lower flow stock Crossfire injectors and let's see what happens. This might be a painless way to pick-up 10-20 Horsepower !

Slim

 

OK Can't find good figures for the 3.5 MB V6, lots of MB seem to use 18 lbs / hr but no indication of pressure. 3.6l V6 using 0-280-155-758 should be 19 lbs/hr at 43.5 psi so bit more, perhaps 21.5 lbs/hr at 55 psi.

Base line for injector flows is usually 43.5 psi (3 BAR) and the Crossfire NA runs 15.1 lbs/hr at that. You need HIGH impedance injectors (very important or you will fry an ECU).

BMW 6 series 2.8l supposedly has Bosch 0-280-150-440 High impedance which would run about 25 lbs/hr at 55 psi. BMW look the best bet for close but a little bigger, but the info available is sketchy on most models. 3l and 3.5l might also be OK but the base or operating pressure is missing.

Most important thing is impedance is high (usually 12 to 16 ohms).

 

OK, I follow what your saying but peak HP is normally near the red line - not many dyno curves for the M112 engine but it feels that way to me. Don't know where the cams come into this...but never mind, unless it this, the power drop that occurs at high rpms is because the VE (volumetric efficiency) of the engine declines at higher rpms, but the decrease in VE means less air is going in (per rev), less air means less fuel required, so duty cycle doesn't keep increasing in a linear manner with rpm?

During acceleration the AF needs to drop from 14:1 and that acceleration enrichment will increase the duty cycle even if the rpm hasn't maxed you might get to 100%. Say it goes from 14 to 12 that's a 16% increase in fuel.

Basic point is to get 215 hp you need to put enough fuel in (and the figures are usually for 14:1), that makes the injector cycle 100%'ish for 215 hp, - end of.

Power in the mid band, on acceleration might be improved with a small increase in injector size, I think tip in would be adversely affected by SRT injectors, if the engine started at all.

 

My two dyno runs that I did totally stock had the HP dropping off a just under 5,500 to 5,700 RPM... Now, it was a warm day and I had not upgraded the intercooler pump or system at that time (Yet to be done).

But I do understand what you are saying when peak is usually near redline. I'd have to sit down with my calculator to figure out how close to 100% duty cycle I was at 5,500 to 5,700 rpm. I don't know what the dyno results are for an unleashed N/A Crossfire, but I would expect the HP curve to be similar.

I digress... either way I think slightly larger injectors would be a benefit to both the N/A and Supercharged Crossfires.

 

I forgot you are in an SRT6, lucky you, they are way too expensive to be worth while here in the UK.

Given the risks of going lean with boost I hope you are down at 80% max, with the much greater flow of the stock SRT6 injectors and conservative power increase over stock you should have a bit in hand.

I don't suppose you had Air/Fuel ratio measurement on the dyno run, that would be interesting and critical to knowing how much fuel was running at 5,700 rpm, you would also need to know air consumption (engine is MAF not MAP? so that might be easy'ish) and boost.

I think the SRT6 runs a fixed fuel pressure, the large possible flow rate of the injectors is likely to be in part due to the reduction in possible flow with boost pressure against the injector outlets, at full boost the max possible flow will not actually be the same as 100% duty and no boost, another reason why if you put those injectors in a NA and they will be way over size.

Just wish I had more time to play!