TheSamba.com :: View topic - 1957 Euro Semaphore/Ragtop Vintage Speed Project

Thu Nov 18, 2010 12:17 pm

I guess in every engine build you need to decide on the compression ratio. This Engine Calculator has been very handy.

http://www.cbperformance.com/enginecalc.html

Fri Nov 19, 2010 11:27 am

Here is the closest tolerance in my engine .088. This with 74mm stroke so it looks possible to increase stroke another 2mm or so. Not sure it would be worth the money>

Fri Nov 19, 2010 11:53 am

One of my goals i wanted to accomplish on this engine build is not having to use any shimms between the cylinders and the case to get the compression ratio i want. I have decided on a CR of 9 to 1

With the use of some Porsche 356 1500 pistons i was able to mock up the engine with my cylinders . The Porsche pistons had a deck height of .004

A VW stock piston in comparison

Fri Nov 19, 2010 12:27 pm

So i sent the piston off to have an copy made less .036 in compression height and no valve cut out.
My new pistons should give me a deck height of .040

With a deck height of .040 and 41 CC's in my heads the compression ratio should be on the money with out using cylinder spacers.

My stock Okrasa heads are 46 CC's so they will not require much boring at all maybe .030 or .040 i will let my head guy do the math on that.

Fri Nov 19, 2010 12:35 pm

Hello-

Quote: I sent the piston off to have a copy made

Who's doing your pistons?

What type of rings are you installing?

Thanks,

Paul

Fri Nov 19, 2010 1:28 pm

in fact of the chamber of an okrasa head you could told your piston guy just to cut the enge so that you can use the little dome . maybe you will need no or just less cutting on the heads. with a dome 1mm and 60mm diameter what will fit you are at 8,7:1 with a dome that fit in the camber you are at 9:1

and you messured not deck height...you messured dome to cylinder edge that is easy to see on the pic "piston into cylinder"

if you let made some pistons do some really modern ones. small modern rings and much less in weight + a less weight pin...short pin i would say all together pistons+rings+pin arround 350g. that is the best what you can do for your crank, mearings and also the case.

buuut. with such an engine it would be the same if you just cutting the top end of your existing pistons. your engine will not need the other expensive stuff. just a beer for the local guy on the mill or on the lathe.
you will not hurt you pistons by doing cut a 1mm ring on them.

my 2 cent

Fri Nov 19, 2010 4:00 pm

Grueni

You are the second person to tell me that i could shave the top of the Porsche pistons down. I have never done that and didnt quite believe that they would hold up. Have you done that? If so i will do just that. I have a lathe and it would be easy to cut them down 1mm or .036.
Just measure the cc's in the valve cut out and cut heads accordingly.

Thanks for your input

PS. I just mailed a piston off today to have it measured and the new ones priced. so i can use the ones i have if it is safe to shave them.

Fri Nov 19, 2010 4:04 pm

Paul
My son used to be involed in motorcycle racing . And i have a inside conection to Ross Racing Pistons However if it pans out to shave the Porsche pistons down i may just use what i have.

Fri Nov 19, 2010 4:13 pm

Grunei

I mesured from the top of the dome to the top of the cylinder. I had the studs in the case with a deck height tool torqued to specs when i took the measurements and from the top of the dome to the top of the cylinder it is .004 The picture was taken before i did the measuring without the tools in place

Fri Nov 19, 2010 5:25 pm

Rwoodshvac wrote: Grunei

I mesured from the top of the dome to the top of the cylinder. I had the studs in the case with a deck height tool torqued to specs when i took the measurements and from the top of the dome to the top of the cylinder it is .004 The picture was taken before i did the measuring without the tools in place

but its the same...this pistons have domes. dome is not compr. height of a piston. this is just for understanding in your case it is ok to do it how you do.

i did it in to engines. my 36er with about 100hp and spin 7400rpm all day with boost of up to 0,9bar. and a turbopiston for another aircooled 1200.
but this pistons was with chamber so not comparable. but that 1mm in a 1cm ring doesn't hurt your piston in any way. the thickness of a piston top should be about xxx x diameter)don't have the factor in head). this is more importand in the middle of the piston.
look at your piston, messure it out than you will see that with that machining your piston will have its thinnest area still in anoter area of the piston top.

what i mean is to machine then just like tom did. and don't care about the valve pocket.
https://www.thesamba.com/vw/gallery/pix/549132.jpg
i did it also on a mill with dividing head on the end of this page...
http://www.thesamba.com/vw/forum/viewtopic.php?t=3...p;start=20

Fri Nov 19, 2010 9:11 pm

Ok very nice. You have opened my thinking. I will go with what i have. I weighed the pistons and they are 314g without pins. I think with Titanium pins and machining them to fit my heads and cc ratio i could have a total weight of 355 to 360 g per piston.

And like you said with less money spent
And less machining on the heads
Thanks for your input

Sat Nov 20, 2010 12:49 pm

the samba is to help.
but forgett about titan pin. titan is in that case not for longlife.
but you can machine the original pin with cones on the end.
in your engine it is not needed to have superlight pistons, with cour cam and co it will not spin that high.

keep on posting.

wishes from germany

Sat Nov 20, 2010 1:28 pm

flattop porsche pistons, no problem, to machine em down wit a lathe..

did it on mine 0,6mm had no picture availible at the moment, webserver crashed down... tomorow i think i'm able to find that picture and post it up

Mon Nov 22, 2010 10:41 pm

Tue Nov 23, 2010 7:59 am

Oval 56
Wow that is a very nice job. How long have you been running those pistons? What kind of HP?
I will do the same to mine.

Thanks for sharing
Rick

Tue Nov 23, 2010 8:39 am

After reading a few posts on how to CC heads I built myself a Head CCing kit. It takes only four items to get good results.

Plexiglas disc with hole to insert fluid
Syringe in ml a 50 to 60 ml works great for VW heads
Sparkplug that you are going to run in the head
White grease

To make the Plexiglas disk use one of your cylinders for a sizing tool. After marking the Plexiglas with a sharpie I used a band saw to cut it out.
For best results the placement of the fill hole should be at the extreme edge See photo. I beveled the hole but it is not necessary
Put a very thin layer of white grease on the ledge in the head where the disc will rest while measuring this will make a seal
Do two or three measurements for accuracy.
Any fluid will work I like antifreeze

I found thru trial and error to tilt the head at a 45 degree angle with the fill hole at the top gave the most consistent results.

Tue Nov 23, 2010 1:03 pm

Rwoodshvac wrote:
Oval 56
Wow that is a very nice job. How long have you been running those pistons? What kind of HP?
I will do the same to mine.

Thanks for sharing
Rick

10000km ~ 6300mls 88ps ~88hp 1582ccm

without any problems running....

Tue Nov 23, 2010 1:06 pm

88 HP I bet that is a blast to drive. Are those 82.5s Have you had any sealing issues cylinder to case?

Thanks again

Tue Nov 23, 2010 1:06 pm

oval56 wrote: Rwoodshvac wrote:
Oval 56
Wow that is a very nice job. How long have you been running those pistons? What kind of HP?
I will do the same to mine.

Thanks for sharing
Rick

10000km ~ 6300mls 88ps ~88hp 1582ccm

without any problems running....

:wink:

Tue Nov 23, 2010 1:09 pm

Rwoodshvac wrote: 88 HP I bet that is a blast to drive. Are those 82.5s Have you had any sealing issues cylinder to case?

Thanks again

real fun to drive :-) i enjoy every minute...

yes 82.5's

i machined the botom of the zylinders down to 88mm and modified the case, no problem, works fine, the only oilleak i have is trough the oilfillertube :wink: