what is so bad about gen ii engines?

[Superdave]

Overall dimensions are the same between the 2 generations.

The exhaust port spacing is identical, bolts are the same size/location (I've used Gen 2 logs on gen 3 heads). The exhaust port shape is different (D vs O).

Intake ports have different shapes but are in a similar location. The LIM bolts are all in the same place for a Gen 3 VS Gen 2, but there is no way the ports will line up. Coolant ports are the same. Combustion chambers are smaller but the same design, valves are at the same angle but the intake valves are bigger in the Gen 3 heads IIRC.


Here are some pictures..

This is the top side of a ported 3500 LIM vs an un-ported Gen 2 LIM:


Here you can see the exhaust ports on a Gen 2 head:


VS Gen 3:



I'll dig up some intake port pictures... I know I have some. If not i've got gaskets from both generations on the wall so it's easy enough to get an idea of port shapes from that.

[Superdave]

Remember, SFI is Sequential Fuel Injection.. just an idle/low RPM mode that made the Gen 3 engines idle smoother and save a little bit of fuel.

After around 2K RPM they revert back to MPFI (Batch fire) just like the Gen 2's were and the later Gen 1's. 80% of the swaps end up running the Gen 3/4 engines in MPFI mode with either OBD1 ECM's or aftermarket like Megasquirt.

[SwiftDrift]

i take it i should stop referring to the 3100/3400 as the "sfi motors" lol and thanks for the pics, i never knew how similar they were. i made a post in pushrod general about the 3400 and 3500, no one responded. :/ maybe u got pics of those too? i like the 3400 sfi, but i am starting to feel like the 3500 makes it obsolete, and i would be better to get a 3500. but how much bigger can the ports be?

[Superdave]

Not that much bigger... but better is the key. The intake ports were raised and re-shaped plus have a few small changes that promote great flow stock. Exhaust ports are a little wider but the inside is quite a bit different.

here is a 3500 LIM gasket on a 3400 head:
You can see how much they raised the 3500's intake port.


Same gasket on a 3400 LIM:

[SwiftDrift]

could i use the 3400 plenum on the 3500 manifold without major loss in performance? basically i just want a cam, injectors, and a tune. but i want a somewhat mean cam so i will have to do porting.

[Guest]

could i use the 3400 plenum on the 3500 manifold without major loss in performance? basically i just want a cam, injectors, and a tune. but i want a somewhat mean cam so i will have to do porting.

Why would you do that? Why not use the matching 3500 UIM?

People swap the 3500 UIM onto their 3400 for the additional plenum volume, so why would you go the other way?

[Superdave]

It's really pretty simple... For Gen 3 engines 3500 is the best. 3400 would come in 2nd and 2000 or newer 3100 comes in 3rd...distant 4th is the small port 3100. Mixing/matching is done all the time but it's usually putting 3500 heads on a 3400 or 3500 UIM on a 3100/3400. Like The Raven said, there are gains with the 3500 UIM on the older models so don't go backwards.

If you want the most possible power from a combination, start with a 3500 and put in whatever cam you want. Even without porting it will still perform great. There aren't that many people that really know the 3500 heads, If you have the funds to get them professionally ported then talk to WOT-Tech. If not then I wouldn't touch them. Not the best head to learn on.

[19Cutlass94]

For an N/A motor, I take it ported 3500 heads would out perform ported 3400 heads?


Sent from my iPhone

[Superdave]

In my experience yes, some others may disagree but I have yet to see a N/A 3400 run anywhere close to what my N/A 3500 did.

[1988GTU]

For an N/A motor, I take it ported 3500 heads would out perform ported 3400 heads?


Sent from my iPhone

For Boost, it's no question a ported 3500 WILL out do the 3400 ported

[SappySE107]

For Boost, it's no question a ported 3500 WILL out do the 3400 ported

3400 can flow more so there is absolutely a question until its proven.

[1988GTU]

3400 can flow more so there is absolutely a question until its proven.

With the same port work effort done to the 3500 that would have been used on a 3400? That I question.

[SappySE107]

Who do you think I am? You can question anything you like, but its not up for debate as far as I am concerned.

[IDCavalier]

Remember, SFI is Sequential Fuel Injection.. just an idle/low RPM mode that made the Gen 3 engines idle smoother and save a little bit of fuel.

After around 2K RPM they revert back to MPFI (Batch fire) just like the Gen 2's were and the later Gen 1's. 80% of the swaps end up running the Gen 3/4 engines in MPFI mode with either OBD1 ECM's or aftermarket like Megasquirt.

So, sorry to thread jack, but is the 3500 NOT considered a genIII? It's a kind-of fourth gen, right? Called the High Value engine?

Just wondering. I want to get the nomenclature right, so when I start another build, I can at least know what I am asking! LOL

Thanks again Dave for your sage advice!

[1988GTU]

3400 can flow more so there is absolutely a question until its proven.

Who do you think I am? You can question anything you like, but its not up for debate as far as I am concerned.

Stating that a 3400 can flow more is implying comparative testing has taken place to be proof against the 3500.

I don't get why the hype of a 3500 LX9 is typically downplayed when it is the best bang for the buck to most of us that didn't start out with a 3100 or 3400 as a factory engine. The 3500 just happen to catch the 3100/3400 aftermarket in its midst of a possible growth spurt snuffing interest of the already niche crowd is all I can relate as to the "why" .