Author Topic: Thoughts on Engine Internals (Read 10429 times)

flyingbrickperformance · « **on:** April 08, 2011, 11:03:02 PM »

So I've been doing some research to see what variation of internals will work without modifying things to get a slightly than lower stock compression ratio (compared to the M42), with nothing more than swapping in a new combination of parts. Here is what I've found - would be interested in feedback. I know people have found versions to build, but for some reason, everything seems to need some sort of machine work - my goal was to eliminate this need, with some minor piston modifications for valve relief (which can be done DIY if you're careful). Here's what I have found.

Crank - M44 (83.5mm stroke)
Rod - M44/M42 (140mm)
Piston - M54/B30 (84mm bore, 28.32 comp. height)

If I compare basic length of the total assembly, I am finding that my combination is .83 shorter than a stock M42 -but the M54 piston has no compression dish, so the net effect should almost make this an equal exchange in terms of compression ratio.

Why hasn't this been done - am I missing something? My measurements seem to indicate that swapping in combination will get me back to the original compression, or slightly lower - which would be good if I later decide to abandon the NA approach.

pdxmotorhead · « **Reply #1 on:** April 09, 2011, 01:56:55 AM »

.83 is over 3/4 inch less, I think that going to have the piston way down in the bore?
Unless thats .083 Then ignore me

As long as the wrist pin location and skirt length are compatible with the block seems like it should work?

Dave

flyingbrickperformance · « **Reply #2 on:** April 09, 2011, 07:01:47 AM »

Quote from: pdxmotorhead;102904

.83 is over 3/4 inch less, I think that going to have the piston way down in the bore?
Unless thats .083 Then ignore me

As long as the wrist pin location and skirt length are compatible with the block seems like it should work?

Dave

Actually, that's not a measurement of inches (.83), that's less than a millimeter. Add to this the fact that there is a corresponding flat piston top compared to the dished compression volume of the M42 piston and they're almost the same in terms of relative compression characteristics. I'm actually worried it may increase cr - but was hoping someone would confirm/dispute this via this thread.

pdxmotorhead · « **Reply #3 on:** April 09, 2011, 04:01:43 PM »

Hmm. Wonder what the displacement of the dish is, .83mm is not much, I'd expect more compression with flat tops than dished unless the .83 adds up quicker..

You have 5540 cubic MM per mm of stroke. (3.14x84squared) so if you know the volume of the dish you'd know how far to lower the piston, You can measure the piston top with a piece of acrylic plastic and a calibrated Burete or large medical syringe.

JE and Wiseco both make custom pistons, You might be able to have the wrist pin holes bored higher on the piston when you ordered them so the part would be 90% stock, and a moderate cost for the custom pin bore? If they already build a piston, it may be cheaper than the BMW ones...

Dave

flyingbrickperformance · « **Reply #4 on:** April 09, 2011, 05:13:16 PM »

Sorry, I was comparing it to the M42 data to get to the .83mm - comparing it to the M44 (given the crank I'll be using), the difference is 2.08mm. Overall, I think I'm adding to the compression so it'll likely go up slightly - hmmm.

Maybe I'll start to look at dishing my used pistons a little to bring it back down. Luckily by the time I finish this engine I'll be at around $150 in costs, so if it blows up on me, it'll be an inexpensive lesson in engineering.

Quote from: pdxmotorhead;102910

Hmm. Wonder what the displacement of the dish is, .83mm is not much, I'd expect more compression with flat tops than dished unless the .83 adds up quicker..

You have 5540 cubic MM per mm of stroke. (3.14x84squared) so if you know the volume of the dish you'd know how far to lower the piston, You can measure the piston top with a piece of acrylic plastic and a calibrated Burete or large medical syringe.

JE and Wiseco both make custom pistons, You might be able to have the wrist pin holes bored higher on the piston when you ordered them so the part would be 90% stock, and a moderate cost for the custom pin bore? If they already build a piston, it may be cheaper than the BMW ones...

Dave

dsm2002 · « **Reply #5 on:** April 09, 2011, 09:25:25 PM »

Based on past threads on board, the M42 block height is 212mm and the pistons protrude 0.15mm - a total of 212.15mm. I make the effective total length of the crank, rod and piston combo as 210.07mm. This means it'd be short by 2.08mm.

Now, looking at the online compression calculator (www.csgnetwork.com/compcalc.html) using the above parameters and those listed below it comes back with a ratio of 9.5:1. With a thick MLS head gasket you could get comp ratio down to 8.2:1.

Head gasket bore: 85mm
Head gasket compressed measurement: 1.77mm (stock)
Combustion chamber: 33cc
Piston dome vol.: 0cc

You should possibly assemble one piston and rod in the block to confirm, check the figures I have used as they are from memory and check the calcs using other calculators to be sure.

NisseJ�rnet · « **Reply #6 on:** April 10, 2011, 05:28:09 AM »

I used 135mm nv M50 rods in one M42 low comp turbo build, works great!

flyingbrickperformance · « **Reply #7 on:** April 10, 2011, 06:10:50 AM »

If you use the stock components on this calculator, what do you come up with for compression ratio of the M42? Did you include the piston dish volume in your calcs, it is about 17.3 cc's - I'm assuming the 33cc's you've used is only for the head volume, not the piston volume. The M54 pistons have 0 cc's so although the compression height puts the piston 2.08mm lower in the block, the center of the piston is actually higher since it has no dish.

Quote from: dsm2002;102917

Based on past threads on board, the M42 block height is 212mm and the pistons protrude 0.15mm - a total of 212.15mm. I make the effective total length of the crank, rod and piston combo as 210.07mm. This means it'd be short by 2.08mm.

Now, looking at the online compression calculator (www.csgnetwork.com/compcalc.html) using the above parameters and those listed below it comes back with a ratio of 9.5:1. With a thick MLS head gasket you could get comp ratio down to 8.2:1.

Head gasket bore: 85mm
Head gasket compressed measurement: 1.77mm (stock)
Combustion chamber: 33cc
Piston dome vol.: 0cc

You should possibly assemble one piston and rod in the block to confirm, check the figures I have used as they are from memory and check the calcs using other calculators to be sure.

flyingbrickperformance · « **Reply #8 on:** April 10, 2011, 06:25:31 AM »

I recalculated your info using what I've collected and I'm coming up with 9.61:1 for compression ratio. I would have thought it would have been a bit higher given the piston dish volume of the M42 and M44 pistons.

That's not too bad for a midly boosted engine if I'm running high octane fuel.

I'll let you know how it looks once assembled - I should have the pistons shortly.

flyingbrickperformance · « **Reply #9 on:** April 13, 2011, 09:13:27 PM »

So I received my set of M54 pistons and the setup looks just as I've described it. I'm going to take a few measurements this week, and post some pics of the difference between the M42 piston and M54 piston at TDC using the 140mm rods and M44 crank. The m42 piston is slightly above the deck, and them M54 just below the deck.

Finally feel like I'm ready to build the engine.

wannam42 · « **Reply #10 on:** April 27, 2011, 08:13:59 AM »

Interesting thread. Progress updates?

flyingbrickperformance · « **Reply #11 on:** April 27, 2011, 08:22:37 PM »

So I've added the M54 pistons with the M44 crank, and here's what it looks like. Just at the height in the block calculated in this thread - so far, things are looking right where I want them. Aside from maybe having to cut in some minor depth for valve relief reasons, this may actually work without any effort at all.

dsm2002 · « **Reply #12 on:** April 27, 2011, 09:24:56 PM »

Sounds good.

pdxmotorhead · « **Reply #13 on:** April 29, 2011, 02:47:45 AM »

If you can find one, there is/was a tool that you put in the bore with the piston down about 20MM and it will allow you to cut the valve reliefs deeper, I think its a MAC tool. Have not seen one for years.. But it may still be out there... The cutter was powered by a drill. It had a 4 blade cutter that woudl allow you to relieve the piston cuts depth and radius.

Dave

wannam42 · « **Reply #14 on:** April 21, 2012, 11:36:39 PM »

Update?