Going with a 3.4 DOHC Twin boosted

[Identity Crisis]

Well guys I have been gone for a while. A lot has happened. Recovering from my operation, dealing with life in the Army, bought my house, and sold my 85 GT, picked up two TIGs and one MIG, and bought a new 2014 Dodge Ram 1500 Sport.

But I did finally decide on what engine I am building. 3.4 DOHC. I want to start it out NA then move into twin superchargers. Why Twin Superchargers? Why not? I have two M90s sitting in my garage and I have never seen anyone do one, and after seeing that marine engine with twins on it I liked it.
Also I used to have a 96 GTP with a 3.4DOHC and I loved the way it performed. Well almost loved it. But the car IMHO was too heavy for that V6

So out the window goes the N* and the 3500V6, and in comes the 3.4DOHC for my 87 GT.

I already have two complete sets of heads, carriers and intakes and two front covers and a bunch of other parts. I pick up the block and transmission next sunday.

So over the next while I will be picking your brains for info.

So far here is my thoughts.

Overbore 30thou
New pistons and rods
Head work
Custom Grind on the cams
Home built headers

Thoughts? I know you guys will have some ideas to give me and opinions.

Cheers
BULL

[CincinnatiFiero]

It is a neat idea, if you are going for looks not power. M90s are not very efficient and hit a ceiling fairly quickly on 3800s. If you are doing it for power you would be much better served by a turbo, you would not need a foot tall trunk scoop.

Post pictures if you do it.

[crzyone]

Heavy engine + two heavy superchargers and more complexity. No sir I don't like it.

[Shaun41178(2)]

can't wait to see the belt routing on this and no decklid. My thoughts? Skip the eatons

[Identity Crisis]

can't wait to see the belt routing on this and no decklid. My thoughts? Skip the eatons

If I found a Turbo locally for a good price I might go that route, but I have the engine and the superchargers already and they all cost me nothing. As well I now have a TIG and want to play with it and some Aluminium.
But I am going to install a DOHC.

[crzyone]

What's a good price too you? Sell the M90s and buy a turbo. You can get a brand new journal bearing turbo for fairly cheap. Something properly sized for the amount of power you want to make.

Not sure the desire to hotrod a Fiero. There is no purpose to twin M90s. Make it heavy, higher center of gravity... There are no Pros to this project, only Negatives.

[Series8217]

I have no idea how you're going to pack two superchargers into the engine bay of a DOHC Fiero.

That said, I love my 3.4 DOHC despite it's flaws.

Get good pistons, shotpeen the factory rods, install ARP rod bolts, remove all casting flash from the block, overbore and hone with a torque plate and the mains bolted in and torqued to spec (the block will move), balance the rotating assembly, replace the intermediate shaft bearings, and get good grooved main bearings to improve the rod oiling.

Throw in some iridium plugs so you never have to change them.

Also look up the top end oiling TSB if you don't have a later motor.

[The Dark Side of Will]

Boost is the M90's enemy. At high boost, their inefficiency makes a huge amount of heat. A single M90 will provide enough air to satisfy a TDC/DOHC engine in the 12-15 psi range pretty easily, which is about the most boost you EVER want to run with an M90. Therefore, if you had 2 on the same engine, you'd have to turn them half as fast in order to keep the boost low enough to keep the whole shebang from dying of its own waste heat.

3800's that make power with the M90 do it because they flow air (ported heads, big cams, etc.), not because they make a lot of boost.

If you want a COOL way to supercharge the TDC/DOHC, I have a 1.5 liter Autorotor twin screw unit for sale. It's comparable in size to an M90, but more efficient.

[CincinnatiFiero]

There used to be a good thread on 3800pro with some dyno numbers where someone had a chart of charge temps, and WHP as "boost" was increased. I can't find it though.

I think stock is 6psi with a 3.8" pulley. And the standard is a 3.4" pulley to get you 10psi. The gain there is pretty good. Especially since it only costs $100. At 3.2" pulley without rockers and headers you start getting too much KR. But anyway, the motor in question had the supporting mods to run however much boost he wanted and going to 3.0, 2.8, and think as low as 2.6" pulley the diminishing returns were substantial. In some cars there was no gain going from a small pulley to a smaller pulley because the intake temps got so high and there was so much KR.

For a stock supercharger and to do a slight pulley upgrade, I rather like the M90, its easy to replace the coupler and the bearings, and makes respectable power for a bolt on car. But, if you're trying to go nuts it just becomes a furnace.

I think for a long time the record for an M90 on a 3800SC2 was like 450whp, and that probably was intercooled, which is not cheap on a 3800. With a turbo on similar boost people were making over 500HP.

Is the autorotor similar to the whipple? people used to make big power on whipple swapped 3800s.

note, all my info is hugely out of date lol. I built my 3800SC was when I was 17, and haven't driven it in years, nor have I been on 3800pro in years.

[The Dark Side of Will]

Is the autorotor similar to the whipple? people used to make big power on whipple swapped 3800s.

AutoRotor is very similar to a Whipple.

[Aaron]

Ya I'm with all the rest. Uniqueness is cool, but uniqueness and slow sucks. And I don't think a DOHC turbo is un unique, I can't think of more than 10.

The great thing about the turbo is it exaggerates everything that is so great about the DOHC, that huge rush come 4,000rpm. And it still makes enough down low that it's easy to drive around in, making more low end torque than my daily driver.

[Identity Crisis]

Turbo hey? Ok.. What type do you think? T4 w/A/R .70 64 TRIM 400 or should I go bigger like a T66?

http://www.ebay.ca/itm/NEW-HUGE-GT45-T4 ... c7&vxp=mtr

Still thinking about going straight NA but if I can sell my M90s they would pay for a Turbo setup.

[Shaun41178(2)]

What's your power goal?

[Identity Crisis]

300-400 whp

[Aaron]

I'm using a GT35R with a T4 hot side and it's perfect for my setup, but my motor is more modified than most, and my build is completely biased for over 4,000rpm. I'd go a bit smaller if you only want 3-400.

[Identity Crisis]

well those Turbos go for a decent price on EBAY too. Might consider that size.

[Aaron]

I didn't think $2000 was "reasonable," but sure.

Instead of trusting some random guy on the internet, how about you run the compressor maps and determine the proper size turbo for your engine, not the one I said was proper for my engine, which is quite different from yours?

[Shaun41178(2)]

I'll say 400 for your goal as 300 is fairly low but easily doable. If you want to spend money ulyou can get anything in the billet wheel range between 54mm and 58mm that will easily support that power.

If you want to go a bit cheaper and old school t3/4 50 trim and even the 57 trim will get you there no problem. I think you can pick up a new 57 ttim for about 650 or so. The billets will prob be closer to 8-900.

[Identity Crisis]

Rods, Pistons and lifters? Where can I buy new ones and are there any performance orientated ones available?

[Series8217]

Rods, Pistons and lifters? Where can I buy new ones and are there any performance orientated ones available?

Lightweight lifters: http://wot-tech.com/shop/gm-60v6/91-96- ... d_276.html

Stock rods are fine. Have them shot peened and resized. Install ARP rod bolts.

No serious performance pistons are available, but some of the stock replacement stuff has coated skirts. Order custom from Wiseco.