Sherman M4A2 Detroit Injectors

[Malcolm Towrie]

We’ve got the twin 6-71 Detroit’s out of Bart, one of our M4A2 Sherman’s. After replacing a bunch of seals and gaskets to fix horrible oil leakage, we test ran one of the engines on a stand and got lots of fuel knock and rough running. The injectors on Detroit 2-strokes are activated by additional lobes on the cam that operate rocker arms that depress a plunger in each cylinder injector that generates the 2000-3000 psi pressure required to spray atomized fuel into each cylinder at TDC.
There’s a good test you can do on these engines where you use a screw driver with the engine running to hold each injector plunger down away from the rocker arm so it’s doesn’t move up and down to inject fuel. It’s exactly like pulling a plug wire off a gas engine to monitor the rpm drop at idle.
We did that test and found 2 injectors that made virtually no difference to idle rpm when held down. And a third that made little difference, in other words, there was a problem with those injectors.
We are lucky enough to have a Detroit injector tester tool here, so we tested the worst of the injectors. It was pathetic. Instead of spraying an intense jet of atomized diesel out of each of the six tiny orifices in the tip, it squirted an anaemic solid jet out of each. I suspect that is why we get a lot of fuel knock from this engine as it’s warming up.
Even the injector that gave the most rpm drop was not atomizing the fuel like a new injector did.
I have a bunch of spare parts from N65 injectors but I’m not sure what would cause this. Worn plunger? Plugged tip orifices?
This is the needle-type injector with the needle in the tip that pops open about 2500 psi.
Malcolm

[Andrew Rowe]

Hi Malcolm,
Are you running these N65's currently or are you using the older type injector?
It is a really good idea to have a fuel pressure gauge inline so you know exactly the fuel pressure going to the injectors and also what your by-pass cuts in at for the fuel pump. Also very critical to have your return line with the .080 thou,
(2mm ) restrictor hole on the end of the return line from the injectors as this holds the pressure for the injector, Cheers Andrew.

[Kirk Armitage]

A drop of water in the fuel in the can do wonders on these injectors , a good teardown and clean will probably get you going. Great that you can check the spray after assembly, lots of pieces and timing involved. They may be coming back as I saw one on a table beside a Viper fighter, In Batttlestar Galactica.
Post some pics if you get a chance .

If you running the older type , the inj body is narrower and the tip set up is different as seen in the pic.

Regards K Armitage

[Perry Kitson]

If you want the best performance from the engines, don't waste your time on the worn injectors. Exchange the old ones for the GM Reliabuilt factory reconditioned ones. A little pricey, but you wont regret it.

[Malcolm Towrie]

Originally Posted by Perry Kitson

If you want the best performance from the engines, don't waste your time on the worn injectors. Exchange the old ones for the GM Reliabuilt factory reconditioned ones. A little pricey, but you wont regret it.

Perry, we replaced the bad #1 injector with a Reliabuilt injector we (miraculously) found in stock. And then I replaced the bad #3 with one I rebuilt from parts. Both worked well. And the interesting thing is I found nothing obviously wrong with either of the removed ones except I saw a lot of debris when back flushing the injector inlet filters. I pulled the filter on the #3 injector and found lots of flaky debris that was way bigger than it should be, considering we have a fuel filter after the pump. I suspect a collapsed main filter or a total lack of main filter at some point in its life. So I think our main problem is plugged inlet filters. We need to check the filters in the other poorly-performing injectors.
We’re always a bit strapped for cash so just replacing injectors at around $300 a piece for the measly 5 miles a year we put on this tank is a hard option to sell!
Malcolm,

[Malcolm Towrie]

For those that don’t have a tester, I’ve seen a neat way of doing a quick check on an injector. With the injector in your hand, fill the inlet fitting with WD40 using the red straw. Stroke the follower up and down by hand. When it goes solid on you, give the follower a rap with a rubber mallet. You should see a misting spray from all the spray nozzles.
Of the 6 spare tips I have, I found 3 had at least one one nozzle plugged. The N65 tips have 8 nozzles. These nozzles are only about 0.006” diameter so I don’t have anything to poke them clear with. Detroit had a tool that I guess had a 0.005” wire attached to a handle but it must have been very fragile. And anything driven into the nozzle under 2000 psi pressure ain’t going to move very easily.
Malcolm

[Malcolm Towrie]

Originally Posted by Kirk Armitage

A drop of water in the fuel in the can do wonders on these injectors , a good teardown and clean will probably get you going. Great that you can check the spray after assembly, lots of pieces and timing involved. They may be coming back as I saw one on a table beside a Viper fighter, In Batttlestar Galactica.
Post some pics if you get a chance .

If you running the older type , the inj body is narrower and the tip set up is different as seen in the pic.

Regards K Armitage

Kirk, I notice from the Detroit manual there are two type on injectors, crown and needle. We have the needle type which I think is the later type. The needle valve pops open only after the injector pressure has reached a high value and gives a better spray pattern, I think.
Talking of spray pattern, I noticed something with our tester. The atomization of the 8 spray tip nozzles depends a lot on how quickly you bang the popping lever down on the injector follower. The faster you bang it down, the more atomization occurs. Considering the speed the injector is opened on a running engine, the atomization is probably better than I am seeing on the tester.
As you suggest, I built an injector from the best of the various parts we have and replaced the bad #3 injector with it. It worked far better and gave a good rpm drop when “shorted”.
Malcolm

[Malcolm Towrie]

Originally Posted by Andrew Rowe

Hi Malcolm,
Are you running these N65's currently or are you using the older type injector?
It is a really good idea to have a fuel pressure gauge inline so you know exactly the fuel pressure going to the injectors and also what your by-pass cuts in at for the fuel pump. Also very critical to have your return line with the .080 thou,
(2mm ) restrictor hole on the end of the return line from the injectors as this holds the pressure for the injector, Cheers Andrew.

Hi, Andrew, yes, these engines both have the brown N65 tag on the injectors. I’m not sure what the “brown” signifies but it seems to be important.
I don’t have a pressure gauge installed on the engine but I did run the fuel pump on the bench and confirmed that the pressure regulator opened at 75 psi, which is in the spec range.
I pulled the elbow fitting on the return gallery and confirmed it does have a restriction orifice. Looks more like the 0.055” orifice listed as one of the options rather than the 0.080” one.
Malcolm

[Tony Smith]

Originally Posted by Malcolm Towrie

Hi, Andrew, yes, these engines both have the brown N65 tag on the injectors. I’m not sure what the “brown” signifies but it seems to be important.

Malcolm

Is an N65 a big enough injector size?

I have a few Injectors from Australian Grant Tanks, and these were the earlier type, but a Number 80. I realise that you can improve the performance, and efficiency (and emissions!) by upgrading to later versions of injectors and also the Cylinder liners, but would a N65 size Injector give equivalent power and performance to an earlier 80? I understand the earlier Injector has a different nozzle pattern that gave less efficient atomisation, but fuel volume per injection cycle is what makes power.

[Malcolm Towrie]

Originally Posted by Tony Smith

Is an N65 a big enough injector size?

I have a few Injectors from Australian Grant Tanks, and these were the earlier type, but a Number 80. I realise that you can improve the performance, and efficiency (and emissions!) by upgrading to later versions of injectors and also the Cylinder liners, but would a N65 size Injector give equivalent power and performance to an earlier 80? I understand the earlier Injector has a different nozzle pattern that gave less efficient atomisation, but fuel volume per injection cycle is what makes power.

Tony, neat photos!
I checked through the manuals I have but I couldn’t confirm what injectors these tanks came with originally. All I know is the Detroit manual for the 71 series of engines lists the N65 as an option.
I found out today that one of the injectors I removed from this engine was actually an N60. It was the only one with the ID tag on the body missing. Somebody must have installed it hoping it was close enough! The part numbers stamped on the injector tip and the plunger gave it away.

The engine is running much more smoothly, with easier starting and much less vibration, smoke, and eye-watering fumes after replacing these two injectors. #6 is the only one left to be replaced as it doesn’t cause any significant rpm drop when shorted.

Our other Sherman has N95 injectors in both engines. That size isn’t even listed as an option in the Detroit manual! The manual is a lot more modern than the tank so probably emission concerns left it off the option list. A previous member here must have decided, to hell with the smoke, let’s get some real power.
Ironically, it’s no more powerful than this tank, but of course there are a multitude of reasons why that may be so.
Malcolm

[Andrew Rowe]

From what I have discovered, it is not always that simple to just keep putting bigger injectors in. From the old HV series to N series, things like injector rack have to be changed and compression ratio and type of liners have to all have consideration , as there are several options available, Cheers Andrew.