Muad'Dib 4.7L stroker build

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Muad'Dib
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Re: Muad'Dib 4.7L stroker build

Post by Muad'Dib »

Ok guys i need some help!

I have been studying my engine outputs with my newly purchased MT2500. One thing that concerns me is my Short Term Fuel Trim (STFT).

From the research i have been doing, the RENIX STFT ranges in value in closed loop mode from 0-256 with 128 being the median. 128 appears to be the ideal meaning basically that you have as much room as possible to enrichen or lean out the mixture when necessary to do so. If you drop below 128 then this indicates a rich mixture and the computer is pulling fuel to maintain stoich. When above 128, then you are running lean and the computer is having to add fuel to maintain stoich.

My dilemma is that according to the MT2500 my Stroker is running rich and the computer is pulling fuel to maintain stoich. My STFT values at idle jump around between 10-15ish to 30-45ish. When using my AFPR if i increase the fuel pressure from stock then the ECU appears to fault and stay in open loop. At this time the MT2500 shows near/at 0 readings for STFT. If i adjust my FPR to decrease the fuel pressure as far as i can go then the STFT values go back up, but not much more than what i see at idle. What this seems to indicate to me is that my Fuel Injectors are too large.

Correct me if im wrong, but in my mind this could potentially cause the lean tip in issues i have been experiencing. If the ECU is pulling fuel to maintain stoich and is running with a STFT value of ~20, then it really only has 20 steps to use (to bottom out at 0) to be able to increase fuel and force AFR's back up to stoich when needed. Is it possible that what im thinking here is true? On the surface when you have too big of injectors you would think that you would have plenty of room to add more fuel when needed, but if the computer does this based on STFT values and is already close to maxed out.. then the computer doesnt really have any more room to add more fuel when needed.

What really has me confused is my current injector choice. As you may or may not know, i started with 26lb injectors that were too large. Now im running brand new 23lb 280155703's. Most Stroker builders use 24# injectors and fare just fine, but for me it appears that even 23# injectors are too large!

Any insight into my post and thinking would be greatly appreciated! Thanks!
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Muad'Dib
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Re: Muad'Dib 4.7L stroker build

Post by Muad'Dib »

As an update to all of this if anybody is out there...


I got some feedback via the RENIX power group and they also concluded that its likely that my Injectors are too large.

As a test, today i swapped in my stock injectors. They are Orange Top 19# ford mustang injectors. The part number is BOSCH 280-150-943. From the flow rates i have found, these appear to flow 18.25 @ 43.5. Just as an FYI, they had been sitting out of the Jeep for quite some time and were stored incorrectly (just loose). This week i soaked them in WD-40 for a couple days, and then gasoline the past day or two. I was lucky as when i swapped them in they worked fine.

Anyway, monitoring STFT i increased the fuel pressure with my AFPR until i achieved close to a value of 128. I came as close as possible... it tends to jump around. My AFPR was almost maxed out to increase pressure.

After doing this, i hooked up my fuel pressure gauge. Measuring fuel pressure with vacuum connected it was 40psi. With the vacuum disconnected it was 48psi. This tells me that the injectors are flowing 19.2lb/hr at this pressure.

After making these changes and getting my STFT at idle as close to the median (128) as possible, i took the Jeep for a short drive. So far i can tell you that the ECU is much happier. Before my AFR's were going high (lean) on tip-in and low (rich) on decel; and would generally jump around when i wasnt idling or coasting. Now, my afr's seem much more steady and tend to stay around stoich (14.7). Lean tip-in doesnt seem to be as bad (short drive so time will tell) and im not going way rich after a decel wasting gas. Additionally, i never got stuck in open loop like what has been happening to me lately.

To recap, here are the issues i was having:

1.) Lean tip-in.. mainly at stop sign, or light
2.) Rich on decel.
3.) Goes along with 2, but when rich on decel, it would try to bring the afr's back up to stoich and when doing so drop the STFT to 0. This would cause a fault and the it would drop into open loop; rinse repeat. Eventually after doing this a few times it would completely fault and stay in open loop all the time; or at least until KAM was reset (Key off\Engine off).
4.) Weird coasting issue. Without getting into too much detail, this could also be described as an RPM flutter.

So far after the injector change, everything appears to have been fixed. Again i went on a short drive, so time will tell.. but im very happy with the immediate results.

The only thing that concears me now is the injector size. Everyone else that builds a stroker uses 24lb.. but im basically running 19# injectors. Either my engine is crazy super effecient, or im making crap for power.. something is definitly up there.

As always comments are appreciated.
If it feels right, then STROKE it!
You're lucky that hundred shot of CAPS LOCK didn't blow the welds on the forum!!
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Muad'Dib
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Re: Muad'Dib 4.7L stroker build

Post by Muad'Dib »

Changed oil again today.. nothing new to report. Now have 9247 miles on it. Considering switching to Synthetic since i will be over 10,000 miles on my next oil change.

Ever since i got my STFT right with the correct size injectors, i no longer feel the need to go crazy tuning this thing. Its awefully close on its own without a piggyback. I may do it in the future.. but its not a high priority.
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Re: Muad'Dib 4.7L stroker build

Post by CobraMarty »

"As a test, today i swapped in my stock injectors. They are Orange Top 19# ford mustang injectors."

But those are not 'stock Jeep' injectors. What are the correct stock injectors for your year renix? If they were closer to 20.5-21lb then you could use them and normal stock FP.
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Muad'Dib
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Re: Muad'Dib 4.7L stroker build

Post by Muad'Dib »

Stock RENIX Injectors flow 18.6lb hr @ 3bar.. which is very similar to the Orange Top ford mustang injectors which flow 18.25# @ 43.5psi.

As stated earlier, the Orange Tops work great with my 4.7 with the fuel pressure cranked up to 48psi which effectively makes them flow at 19.2#hr.

Im not too worried about the increased pressure.. if anything it forces better atomization. My main concearn was how im able to get away with running 19.2# injectors when most stroker engines require 23-24# injectors.
If it feels right, then STROKE it!
You're lucky that hundred shot of CAPS LOCK didn't blow the welds on the forum!!
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Muad'Dib
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Re: Muad'Dib 4.7L stroker build

Post by Muad'Dib »

Another Oil Change today with Valvoline VR1 10w30. Still haven't made the cut over to Synthetic (too chicken maybe?)

11,417 Miles on the Stroker. Nothing else new to report.
If it feels right, then STROKE it!
You're lucky that hundred shot of CAPS LOCK didn't blow the welds on the forum!!
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Muad'Dib
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Re: Muad'Dib 4.7L stroker build

Post by Muad'Dib »

Finally converted to Synthetic today. Shell Rotella T6 5w-40. 14,971 miles on the 4.7 and so far so good.

Also, I switched injectors about a month ago to 280-155-712. Up until then I was still running the Orange Top mustang injectors with the fuel pressure turned up. These new ones are the newer style and work great @ closer to stock fuel pressure. I can give more details on them if anyone wants it.
If it feels right, then STROKE it!
You're lucky that hundred shot of CAPS LOCK didn't blow the welds on the forum!!
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Muad'Dib
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Re: Muad'Dib 4.7L stroker build

Post by Muad'Dib »

Talking to myself again.. for future reference proper belt length is 80.25 in. AKA 4060802 for a Goodyear Gatorback.
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Re: Muad'Dib 4.7L stroker build

Post by nicpaige »

Muad'Dib wrote:Finally converted to Synthetic today. Shell Rotella T6 5w-40. 14,971 miles on the 4.7 and so far so good.

Also, I switched injectors about a month ago to 280-155-712. Up until then I was still running the Orange Top mustang injectors with the fuel pressure turned up. These new ones are the newer style and work great @ closer to stock fuel pressure. I can give more details on them if anyone wants it.
I would like more details on those injectors.
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Muad'Dib
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Re: Muad'Dib 4.7L stroker build

Post by Muad'Dib »

You can look them up, but essentially they flow 19.4# @ 3 bar. I believe they are mainly used in a V6 Saab. They flow about 18.4# on stock 39# rail. I have an AFPR and have fine tuned the pressure just a tad above stock to make the fuel trims correct on my RENIX. So far i am about the only person I know that has a Stroker that doesn't seem to need 24# injectors. I am closer to a stock injector size.
If it feels right, then STROKE it!
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Re: Muad'Dib 4.7L stroker build

Post by jeepxj3 »

? closer to a stock injector size = ? close to stock power

What are your short term and long term fuel trims?
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Muad'Dib
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Re: Muad'Dib 4.7L stroker build

Post by Muad'Dib »

LTFT doesn't appear to change on the RENIX, but STFT I have adjusted to the median. IIRC it ranges from 0-256 128 being the median. I adjust my fuel pressure to make sure that I run as close to 128 as possible so that it has the range to cut \ add fuel when necessary. Definitely nothing close to stock power in a 4.7.... but yet to dyno the thing. I can tell you I get up to 60 in 6 seconds... but that doesn't mean much.
If it feels right, then STROKE it!
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Re: Muad'Dib 4.7L stroker build

Post by jeepxj3 »

Thanks, I forgot that you adjust your FPR to higher pressure for more fuel out of the injectors.
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Muad'Dib
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Re: Muad'Dib 4.7L stroker build

Post by Muad'Dib »

jeepxj3 wrote:Thanks, I forgot that you adjust your FPR to higher pressure for more fuel out of the injectors.
Well.. yes I do.. but the pressure isn't raised much over the stock 39#'s.
If it feels right, then STROKE it!
You're lucky that hundred shot of CAPS LOCK didn't blow the welds on the forum!!
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Muad'Dib
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Posts: 1497
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Re: Muad'Dib 4.7L stroker build

Post by Muad'Dib »

Updated main post with new Stroker Compression Ratio Calculator PDF. Reflects 7.77 DCR @ my altitude. 8.47 at sea level.
If it feels right, then STROKE it!
You're lucky that hundred shot of CAPS LOCK didn't blow the welds on the forum!!
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