Long rod 4A or how to destroke a 7A

[bermy]

Here's an article I wrote on a local forum. It's a special brew of A engine that doesn't have a lot of info on. Hope I'm not selling someone's secret but here is how I was able to build one with a little more than a standard rebuild budget, all with standard OEM parts readily available and cheap.

So here we go

First of all, it's not rocket science, the 4A engine has a pretty low rod to stroke ratio in stock form, 122mm rods for a stroke of 77mm which gives a rod/stroke ratio or 1.58:1. It lead to higher acceleration forces and lower piston dwell time at TDC as well as pretty steep rod angularity. All of this leads to more strain on the rod journal and higher load of the major trust side of the cylinder wall which limits the rev potential of such engine. What builders had to do to create such beast as the F/A engines what to relocate the piston pin 9mm higher than stock, right under the oil ring and use a 9mm longer rod to get the rod to stroke ratio to 1.70:1.

Well, luckily Toyota was kind enough to create a stroked version of the 4A called the 7A and in order for the rod/stroke ratio to not fall too low, they casted a 15mm taller engine block to be able to fit longer rods to allow a 85.5mm stroke. As a lot of people know, the 7As have their own short coming when used in power applications. The crank, with its 6 bolt holding the flywheel, is responsible a lot of the time for catastrophic failure, the oil system has the main journal drilled offset from the center so oil holes that feed the rod journal are not spaced 180° apart... not optimum for oil flow. Worst, the rod journals are fed through a single oil hole that is drilled at an angle to meet the main journal... the 4A crank is different since it was created as a performance engine and not a grocery getter, both main and rod journals are drilled all the way through so lubrication of the rods is way better on a 4A-GE crank than a 7A-FE crank.

7A-FE crank above, 4A-GE bluetop crank below




The 7A's 1.56:1 r/s ratio combined with longer stroke will limit it rev potential. The 7A will normally put out a little more torque than a similar 4A because of the added 200cc of deplacement, but it will also tend to have a lower rev limits which will limit overall power in N/A form to that of a decently build 4A.

For someone who is looking at creating a very rev happy 4A under budget, the 7A block seems like a nice starting platform because of the taller deck. But you need to get rid of the internals. All you need to do next is drop a 4A crank inside this 7A engine block and get some 15mm longer rods and you end up with a 1.78:1 rod/stroke ratio. But custom rods are expensive aren't they?

Well here are some nissan E15ET/E16 rods out of early Nissan sentra and pulsar.

7A-FE stock rod and piston, E15ET/E16 rod, 4AG (bluetop) rod and piston.



Notice how longer the E16 rod is. Actually 15mm longer.



Also notice the bigend dia... and how the bluetop bearing fits in there like a glove?

So I bolted a bluetop crank (with 40mm rod journal) inside a 7A and test fitted the thing



Big end side clearance



I also got some smallport (redtop) piston up top and here is the deck clearance with crank at TDC





Stock is 0.015"

[bermy]

Well it's almost bolt-on. 2 things need to be adressed

1) E16 rod small end is supposed to take a 19mm press in piston pin. 4A-GE piston either use 18mm press in or 20mm floating pins. The most cost effective thing to go is to get a machine shop to oversize the small end to use redtop or 20v piston with there respective 20mm pins. You could either bore the end, press a brass bushing, make an oil hole in the small end and use the 20mm pin as a floater like all late 4A-GE engine or just oversize the small end and leave 0.003" interference and press the 20mm pin which leaves more rod material around the small end and is also cheaper.

2) E16 rod bearing are a little too thick to use on a 4A crank since the crank pin is slightly undersize on the E16. 4A-GE small rod bearing (bluetop) fit the rod pretty well but the oil hole on the E16 rod is on the other side. In order to direct oil up under the piston, you can simply drill the bearing and chamfer the hole or make a "cosworth notch" on the side of the big end like the blacktop has. I decided to make the notch since the bearing already has a hole (that is now blind) I didn't want to reduce the bearing surface any further.

Here are the modded rods to fit

Bored small end to press the toyota brass bushing and drilled an oil hole. Now the setup is full floating and uses the stock 20mm floating piston pin that are on every late 4AG, smallport, 20v and GZE



Notch on the big end, 2mm wide, 1.5mm deep



Symmetrically on both sides for good piston cooling and pin lubrification



And all that is what's needed to turn this into a long rod 4AG



Final piston rod assembly, weighting in at 950.8g, compared to the 909g or the stock bluetop assembly and 920g or the 7A. The rods with ARP bolt weigh in at 482.2g, from what I was able to gather, blacktop rods weigh in at 485g.



Throw all that in a block...



And you end up with a destroked 7A-G or a bigblock 4A-G... or like a couple guys on a local forum started calling it, a Tom-AGE

So the parts list goes as follows

- 7A engine block

- 4A-GE bluetop small journal (40mm) crankshaft (early TRD crank with 40mm journal could also be used if such a thing can still be found and a 4A-FE crank would work but the 6 bolt flywheel and half weighted crank plus the crappy oil pass won't)

- 4 connecting rods form either a E16 or E15ET (very important not to use non-turbo E15 rods since they are longer)

- 4A-GE head of your choice, bigport, smallport, 20v

- Intake that goes with the head

- 4 pistons that goes with the head also. For all 16v head, use smallport highcomp piston (for the 20mm piston pin), for 20v head uses any 20v piston, for F/I use GZE piston.

- Any A engine crank bearing : 48mm crank journal are all the same

- 4A-GE bluetop small (40mm) rod bearing (4A-FE would work since they are the same)

- 7A or late 4A-GE oil pump (19036) found on the smallport and 20v engine. Both oil pump have the late sinus gear design but the 7A has less volume than highcomp or 20v. If fiting oil squirter in the 7A engine block use the smallport/20v pump for increase flow.

- Any 4A water pump to suit your configuration, RWD, FWD, electric fan, etc... search it's the same deal as any other 4A build

-Porsche 944 timing belt if using 16v head and I don't remember the one for 20v but refer to any 7A-GE build for that one, it's no different.

- Maybe forgetting some stuff

Keep in mind I'm still in the assembling phase. Engine as not run yet so power figures are just a guessing game. All we know is that the part fits so nothing should keep it from being succesfull. I'm pretty confident that the recipe is right and shouldn't create any problem but I'll need to report back with proof as soon as the thing is up and running.

Oh and here are 3 graphs I made to compare rod angle, piston speed and piston acceleration... note that it's taken at 7700rpm (not important for the rod angle graph though...)

Red = 7A
Blue = 4A
Green = Tom-AGE

[trdee]

Cool stuff and some lateral thinking going on there, but I am wondering what gains are you expecting to see in the real world, or is this more just a thought experiment? A half decently built standard 4A will happily rev well north of 9000rpm for a long long time so unless you are building a formula atlantic engine I dont see much point?

[bermy]

Cool stuff and some lateral thinking going on there, but I am wondering what gains are you expecting to see in the real world, or is this more just a thought experiment? A half decently built standard 4A will happily rev well north of 9000rpm for a long long time so unless you are building a formula atlantic engine I dont see much point?

Why would it be pointless in anything else than a 10,000rpm revving monster? Take any decently build 9000rpm 4AG and and sit it on a bottom end that is more efficient at making power up top and you'll gain every time. Plus as a bonus the engine will be under less stress at the same RPM levels...

Lets take a very basic 4AG that falls on it's face at 6600rpm making lets say 100ft/lbs, that's 126hp. Sit the same engine on a bottom end that is more efficient up top and falls on it's face 1000rpm higher at the same level of torque : 100ft/lbs @ 7600rpm... 145hp the same goes for any other 4AG you can think about.

It's not a new idea, Honda did exactly that with the B16B, taking hte taller B18 block and dropping a B16 crank in there with longer rods, stretching the r/s ratio from 1.73:1 to 1.84:1 Link for the whole B16A to B16B story

I've seen guys take old 2T engines and fit 3/4Y rods to rev them upwards of 9000rpm sustain for really long periods.

I mean if you'd have the chance to build 2 setup, with one stretching it's torque over higher RPM over the other one while having less stress on the internals at the same RPM level, would you really build the one with the lower r/s ratio...? Discuss

[fixeruperer]

Been doing this stuff with my bikes.
Smoother and higher revving, longer dwell time at tdc, less bore side load all equals a better more reliable engine.
That's what building an engine is about, not just slapping parts together.

[trdee]

I get that, just wondering how much of a difference it would make to a 4A in the real world rather than just saying "its better" will be interested to see what results you end up with once it is up and running, this is something not really seen in the 4A world in aus to be honest so I am quite keen to see how it turns out

[bermy]

Honestly, I don't think it's something that is normally done anywhere in the world and that's why I gave it a go. I have heard of people running destroked 7A in racing circles (heard mainly about NZ) but is it real or myth? I have no idea. Info on those setup is very very limited and I've never heard real account of actually building one, only such things like "a guy I know... you know..."

Been doing this stuff with my bikes.
Smoother and higher revving, longer dwell time at tdc, less bore side load all equals a better more reliable engine.
That's what building an engine is about, not just slapping parts together.

Wow, I couldn't have said it better... I always say that an engine's performance is never equal to the sum of all it's parts but how the interact with each other.

[trdee]

I dont disagree, there is a reason why F1 engines run rod stroke ratios north of 2:1... but one must also take into account the fact that they rev to 20,000rpm Not sure at what point the gains would be worthwile on this engine. 7000rpm? 8000rpm? 10000rpm? Will be interesting to see.

edit; the fact that you can do this with all more or less off the shelf parts does help with making it worthwile of course! Amazing that those nissan rods fit more or less perfectly haha

[Sam_Q]

I like your work, now here's an interesting question for you. Do you think you may of compromised some of force reduction with a weaker crank that doesn't have a proper full counter balance such as the later 4age engines? I know I am not wording that well, in short is the early 4age crank much weaker and subject to greater vibrations to start with?

[bermy]

Well, yes it is a very interesting question indeed.

If the question is about compromise then yes I did compromise. I didn't find any rod suitable to the late 42mm crank pins, if I had I would've used them just for piece of mind. I have no idea about the difference in vibration and harmonic between both cranks. For strength, the larger crank pin and higher journal overlap of the late crank makes it a stronger crank their is no doubt about it.

Will the 40mm prove sufficient? I can't say yet. If it is then that compromise will turn into an advantage. Smaller journal mean lower journal speed for any given RPM which result in lower friction and lower oil temperature. Also the smaller crank is a little lighter, combined with the fact that the 45mm ID rod big end will also be heavier than a 43mm ID big end. All that leads to lower rotational inertia... and a little less strain on the crank. Will it offset the fact that the crank is weaker? Don't know... Will the crank be strong enough or brake in half on one of the journal? Don't know either, we'll have to wait and see for that one.

I've balanced all the internals to the 0.1g, got rid of the stock oil pump in favor of a gimler driven external unit so crank harmonics will have to work harder to brake something... I did all I could to make it work.

[Sam_Q]

The smaller journal can indeed reduce losses, one of the three reasons why the formula atlantic pistons have a 19mm small end pin. I was more referring to how the early cranks have only a half counterweight on one side while the later cranks look more like the 7afe model up there with two full equal counter-weights. However with the better rod to stroke ratio you would have less vibrations to absorb in the first place, but then there is some extra reciprocating weight to offset so who knows eh? Someone in another forum commented on a way to measure the harmonic vibrations of the crank I will ask more about it.

Just curious are aftermarket rods available for the nissan model? I know that takes away from your cost effective approach but still.

Don't misunderstand me though I do like what you have done and I would like to see further updates.

[bermy]

Well for the weight of the reciprocating parts, I've heard that blacktop rods are 485g while mine are 482.2g after balancing... drop blacktop pistons, rings and pin on top plus the smaller 40mm bearing and you end up with a lighter assembly than the lightest among the big crank 4A-GE (if indeed stock blacktop rods weight in at 485g)

http://wholesale-racing.com/index.ph...mm-19-0mm.html

If you buy a boat load I guess that would be even more cost effective than the stock ones... still need to mod the small end and notch the big end though.

Oh and please report back on the harmonics calculation method, I'm really interested

*edit*

Here's a way to buy smaller quantities... just don't know how much they want for them... good thing is that they already come with ARP fasteners

http://www.maxspeedingrods.com/parts...%20Conrods.htm

[Adsport]

I have wondered about this a few times myself and I have just never had the resources at hand to see what rods would make compatible substitutes. While I can't provide any technical additions I am certainly subscribing.

Thank you so much for sharing, Toymods delivers again !

[PrettyCoolWagon]

Interesting thread, hear mixed reviews about maxspeedingrods turbos, ..if their rods are anything like spool rods they may still require significant machining to get the big and small ends up to spec which would mean their not so cost effective afterall.

If this works I can see people jumping on it.

subscribing.

[Sam_Q]

I know first hand 3 different people who have used spool 4age rods off the shelf and where right on spec ready to be used for the engine