Engine build questions & suggestions?

[e30 gangsta]

Hi guys well my bottom end finally called it quits, I think the rings are toast. I am currently in the planning stage of building my bottom end. I know that I want to run an m52 crank and use 135 rods with 86mm pistons. My question is primarily on piston design and cr. currently my motor has a completely rebuilt head with HD rockers and a new billet 272 cam.

Now are my questions.

How high of a cr should I run with the current head setup in a N/A format?

How high of a cr would you go if I planned to boost the car months down the road?

I would like to be able to drive the car on pump gas (93 octane in Florida).

What piston design should I go with? Stock type dome and valve reliefs with just raised cr, or a flat top styled piston?

Lastly, what kind of rings do you guys suggest? I was thinking of running the total seal topless rings on the top instead of the secondary total seal rings. Do you guys have any preference or real life experience on this option? Or any insight/opinions on totalseal rings vs conventional piston rings?

Obviously this piston is going to have to be custom/forged. If anyone has any tips or tricks and or specs that they can advise me to go with on the pistons id appreciate it.

Thanks

- G

[digger]

Hi guys well my bottom end finally called it quits, I think the rings are toast. I am currently in the planning stage of building my bottom end. I know that I want to run an m52 crank and use 135 rods with 86mm pistons. My question is primarily on piston design and cr. currently my motor has a completely rebuilt head with HD rockers and a new billet 272 cam.
Now are my questions.
How high of a cr should I run with the current head setup in a N/A format?
How high of a cr would you go if I planned to boost the car months down the road?
I would like to be able to drive the car on pump gas (93 octane in Florida).
What piston design should I go with? Stock type dome and valve reliefs with just raised cr, or a flat top styled piston?
Lastly, what kind of rings do you guys suggest? I was thinking of running the total seal topless rings on the top instead of the secondary total seal rings. Do you guys have any preference or real life experience on this option? Or any insight/opinions on totalseal rings vs conventional piston rings?
Obviously this piston is going to have to be custom/forged. If anyone has any tips or tricks and or specs that they can advise me to go with on the pistons id appreciate it.
Thanks
- G

Naturally aspirated and 272 id be looking at 10-10.5:1 static CR this will give you a DCR of 7.8 to 8.2 based on it being similar timing to schrick 272. This is the right ballpark for a street engine on 93PON.

I’d look to either JE OEM dish dome (top end do these) or contact IE for Ross dish dome for the 885 head. I would avoid a flat top type design these suit a 200/731 style head better, people have used them but IMO the piston shape and style are not matched as well as they could be.

If you get a 86mm piston with compression height reduced to account for longer stroke and stock b25 8.8 dome then you end up with static CR of ~10.2.

The stock comp height is 34.2mm (206.7-135-(75/2)) = 34.2mm

With 84mm stroke comp height would need to be 29.7mm (reduced by 9mm/2=4.5mm )

you would probably get something slightly smaller to allow for some material to be taken off the top f the block to get everything square and true.

Set squish clearance between head and piston to be 1mm when assembling and you will have a nice strong engine

You need to make sure you have proper method to custom tune ignition timing and fuel etc not budget generic chip swaps IMO.

If you want gapless (not topless) then do the top ring not the second ring. gapless cost extra from memory compared to a conventional ring which has also worked fine for decades. From what I seen there is still no definitive consensus amongst professional guys it comes down to preference so it seems like an area to save a few $$$ and go for conventional rings but upto you.

[ForcedFirebird]

You need to make sure you have proper method to custom tune ignition timing and fuel etc not budget generic chip swaps IMO.

It will be tuned properly. ;)

[HennaE30]

What he said ^^^ lol.


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[TobyB]

topless rings

My experience with those unfettered rings was that they gave great leakdown numbers.
For a long time.

Whether that made any REAL power, I'm less sure, but they held up well.

t

[digger]

My experience with those unfettered rings was that they gave great leakdown numbers.
For a long time.

Whether that made any REAL power, I'm less sure, but they held up well.

t

yeah leakdown tells you next to nothing about what is happening in a reasonably healthy engine that is actually "running".

http://www.thefoat.com/DartMachinery/video/Dart+University+Master%27s+Series+-+Leakage+and+Compression/video_id-Yx1Zj0Ij3xsg121sg7uo5HLuIZNA%3D%3D

[e30 gangsta]

Thank you very much for your information digger, I’m going to call top end performance get some specs on the pistons and post them up before buying. I’m extremely nervous I’m going to buy custom pistons and mess something up because I know nothing about these specs. All this is new to me.

So far I’ve decided against an 89.6mm crank due to machining costs and worse rod ratio than the m52 crank. So I’ll be sticking with an m52 crank, 135mm eagle rods, and custom pistons. I just need to make sure I get these piston specs dead on before ordering.

[digger]

honestly i would not worry about rod ratio if that was a major concern, all else equal you will be leaving alot of bottom end and midrange on the table using a smaller crank and all because of the spread of mostly BS on the internet and things being taken out of context.

http://www.thefoat.com/DartMachinery/video/Dart+University+Master%27s+Series+-+Rod+Ratio/video_id-xKfzB7DY%2BkYYrMY6xkNwAw%3D%3D

the S52 has the same rod ratio and bmw/alpina have made much worse in fact the most powerful production version of the M50 style engine was a 305bhp S52 blocked engine in the Alpina B3S . VAC sell stroker cranks that are much worse in rod ratio but no problems....dont let ths be a deciding factor. fair enough if you dont have a crank but ......

there isnt a lot of extra issues fitting the bigger crank.
-crank nose (same issue you need a sleeve for both )
-block clearance (probably better with 89.6mm crank due to smaller counterweights)
-intermediate shaft (might need to be machined slightly but this is cheap and easy for anyone with access to a lathe)
-rod clearance to bore (molnar rods are a nice slim lightweight design and am pretty sure will clear without chamfering the bottom of bore)

[e30 gangsta]

Digger if i purchase the 89.6mm crank do you think the block will clear with eagle h beam rods that are 135mm? And if not I can just run bigger bored pistons 86mm I suppose. Making it a 3.2l m20 correct?

If I do go that route will the 272 cam suffice? What about the stock sized valves?

[digger]

Digger if i purchase the 89.6mm crank do you think the block will clear with eagle h beam rods that are 135mm? And if not I can just run bigger bored pistons 86mm I suppose. Making it a 3.2l m20 correct?

If I do go that route will the 272 cam suffice? What about the stock sized valves?

i'd say you would need to chamfer/notch the bottom of the bore for the rod shoulder transition area but i have not mocked it up myself. you need to remove a smidge with M52/S52 rods and the big crank and most H-beam have less clearance than the OEM I-beams from M52/S52.

an 86mm bore will need a smidge less clearance but not enough the save some extra effort is my guess.

having said that there should be enough meat from what ive seen and ive seen pics where alot was removed to allow the rod to clear. i like the molnar rods because they are economical, compact and light perfect for a NA engine.

a 272 will be a torque monster and fun to drive but it wont make impressive numbers on paper but then again without extensive headwork no m20 will

read this thread

http://www.r3vlimited.com/board/showthread.php?t=265471

better results were found with the MM sport cam over a schrick 284/272 though im not a fan of the split cam in general especially that much.