Thread: Boosting High Compression vs Low Compression M90 Setup

Hey guys,

I know this question comes up often on various forums (High Compression/Low Boost vs Low Compression/High Boost) but I'd like to limit the discussion to the Eaton M90 on the 3800SII.

If I am putting together an engine build, say a 350-400bhp setup, and I am limiting myself to using the M90... does the L36 make more sense than the L67? (I'm not really talking about a simple top swap, but rather, building an engine from the ground up.)

My understanding is, both blocks can handle this horsepower range as long as detonation is prevented. Since the L36 can make more power with the same amount of boost as the L67 (assuming no knock), I would assume that you can run closer to the stock pulley than you could with an l67.

So rather than running a tiny pulley, which creates more drag on the crank, and generates more heat... why not just run a bottom end with more compression, and a bigger pulley? Seems like a much more efficient setup.

From what I understand zzp uses a l36 in their setup so I think what you are saying makes sense. In theory you can make more power with less boost.

I like the dished pistons on the L67 though, only reason why I would stay with a lower compression block.


Another way to look at it is, you don't always have to run the really small pulley. You can use timing to your advantage if using a 93 octance + meth and or E85 fuel, then you could run a 3.0ish pulley with extra timing to be at the same level as a 2.8ish pulley.

typically you'll run out of pulley options with the L67 block a lot sooner, but setup right both are capable of spinning the m90 way out of it's efficiency range.

Originally Posted by Bio248

typically you'll run out of pulley options with the L67 block a lot sooner, but setup right both are capable of spinning the m90 way out of it's efficiency range.

Exactly, if your going for maximum power and plan on running enough octane to support it high compression will get you further. Once the m90 is out of steam you have to make power somewhere else, at that point more compression and better airflow are your only options. Or nitrous that works too I guess.

the L-36 and L-26 Pistons are less dished for higher compression, this makes it a very common practice to do a "Bottom end swap." this allows the some 9:1 higher compression ratio of the N/A 3800s... HOWEVER consider this, the connecting rods on the L-36 and even the L-26 are not as heavy duty as the L-67 L-32 engines. if you start pushing 500HP or 350 with high revs you could be dealing with broken rods or spun bearings!

my advice if you really want too push power, go with a L-67or L-32 bottom end for a budget, or "race prep." your own, you can get high strength connecting rods and ceramic coated pistons through ZZP, and you can get the crank and the rest of the assembly balanced so then you can have the higher power rating and revs and not worry about spinning a rod, then you can compensate the compression for higher boost.

But the cost of machine time would be crazy. I doubt the N/A bottom ends are a that much weaker than SC cars.

most likely you'd chip a piston before bending a rod. just about every time.

Next motor I'm building is going to have a stock L36 bottom end

Either motor will work fine as long as you build it accordingly.

For a high power build, it's foolish to do anything but turbo it.

For a strictly M90 blower setup though, you're going to need a big lump stick (with supporting mods) and ported heads/matching LIM.

Stick with a "larger" pulley (3.0-3.2) and run a bunch of timing. If you are able to get away with 24+ degrees or so with no knock, start dropping pulley sizes.

you sir have the same mindset I do. rather than running out of pulley options, go L36 bottom end and keep the blower much happier and have options for modding in the future. and as stated above, build it right and eliminate knock and you will not have issues

I have a l26 bottom end in my car, they are cheep as dirt and almost always have low miles too.

timing is harder to run consistently versus more boost due to fluctuations in weather and octane of fuel.

however, boost > timing is a turbo mentality, too.

i'm a firm believer if that i can't roll up to a gas station and be forced to run 91 and not worry about destroying it then i didn't build it big enough.

i love ethanol, methanol, and race gasses, but i'm just not a dedicated race car fan so being able to street drive it and fill up wherever i end up is a key thing to me about any car i put together.

This is what I'm talking about, here is where I'm at. Maxed m90 (2.7 on a genV), maxed timing (22-24*), and I still have plenty of headroom for KR on E85 but no way to use it. That 9.5:1 sure sounds good about now. You will max out an M90 very easily and still be left looking for more power. At this point there is nowhere to make more power with the system without rebuilding the valve train with a bigger cam and expensive ported heads.

l26 rods will snap if you look at em wrong, l36 is the only way to go unless yer considering spray, then is a l67/32

if ya want more compression from the 67/32 then just use two layers of the MLS HG and bump it up .4

L32 L67 blocks are hard to find. They also want stupid money for blocks with 150,000


I put in a L26 block.