Firstfives.org

I've been working on building a 3.5L M30 for fitment into my 528i.
I pulled a M30B35 from my 1991 535i parts car and thought I'd "freshen it up" before doing the swap. Yeah right, don't believe the PO when they say it runs good. The cam was shot along with all the rockers. Con rod bearings are worn, a stuck piston ring and water was getting into cylinder 6 through the headgasket. So this has turned into a lot more than when I first started.
That's ok, I always love a challenge and it's all going to be sorted.
But I am faced with a choice and I don't know which way to go. The stock M30B35 has a CR of 9:1. I'm told if I bolt on a B34 head the CR climbs to 9.87:1.
The tradeoff being the B34 head doesn't flow as well. The intake ports are quite a bit smaller and the intake valve is 1mm smaller. The exhaust I haven't really looked at side by side yet but I imagine there's a difference too.
So what's more desirable, higher CR or better flow? I have both engines here and can pick the best from both.

I'd be inclined to go for the lower comp option, especially if you think turbocharging is a possibility in the future.

Is there any data to back-up the flow differences between the heads?

I doubt I'd ever turbo the car.... I'm really not a fan of FI. Something about crisp throttle response and the sound of a tuned NA engine I enjoy.
I've been looking for data on flow differences on the M30 heads but haven't found any yet. One thing I did learn is there is nice room for improvement by doing some simple port matching between the head and intake. I was surprised how crudely the castings match.

In Michigan I have a friend that owns a shop that builds heads for NASCAR. I would say flow trumps compression since we are not speaking of huge differences in CR.

I think those pistons will fit without interference, but I'm not positive. But look at it this way. A stock B35 is rated at 211HP. A stock 10:1 euro B34 is rated at 218. And has higher emissions and is much more sensitive to octane. I'm as much a HP junkie as anyone, but for <7HP I don't think it's worth it.

With how you have to drive a euro motor on our crap gas, you'll never get the power out of it anyway. Go for the B35. I've got a 10:1 motor in my E28 M535i and I don't turn the radio on.

In your shoes, I would run the B35.

You can buy custom designed pistons for $170 ea. Maybe you should just get whatever CR you want and then go with the 35 for flow?

Pete,

Who is the outfit selling custom pistons?

Thanks.

I think I've seen on IE's website you can get custom forged pistons for something like $800 a set. And you can specify anything you like.

Thanks for all the input. I thought I'd ask. I guess bumping up the CR may create problems that out weigh the gains. I plan on running Motronic 1.3 with a Turner Motorsport chip. Messing with the CR will not complement the tune whereas improving flow will.

I've already got 2 days of porting work into this B35 head, I'll stick with it.

Luis A. wrote:Pete,

Who is the outfit selling custom pistons?

Thanks.

I think it's JE that supplies most of the custom pistons these days. I'm not sure if you buy direct, but I've seen several places sell them, I think TCD does, he's the guy who does the turbo kits.

http://www.turbochargingdynamics.com/pr ... on_Set.php