Alfa Romeo Forum banner

1 - 20 of 38 Posts

·
Registered
Joined
·
3,000 Posts
Discussion Starter #1
Anyone know - can you use the dished 8v pistons with 16v heads.

I am looking to use the 8 valve pistons - the ones with a fair bit scolloped out from the middle, I think they are the 9.0 compression ratio ones from a carb engine - and use them with 16v heads. Is there any interference?

I could have put some photos of the pistons on here - but they are away with the block being carefully installed into the block.

Anyone done this before?
 

·
Registered
Joined
·
78 Posts
I once cut some pistons in half...16v on the left, 1700 8v on the right. it will give very low compression I think, probably lower than the 9:1 of the 8v.
 

Attachments

·
Registered
Joined
·
3,000 Posts
Discussion Starter #3
Bert,

it is low compression that I am after. It is for my supercharger project.

In fact a compression ratio of about 8.5 would be good.

What I am hoping is that when the valve is fully open the dish in the top of the 8v piston is the same or more as the valve cut out in the 16v piston. It is a pity your saw cut did not go through the valve cut out - as that would have been an excellent comparison.

what I may try and do is do a visual comparison between an 8v and a 16v piston and try and see if the pockets are covered by the dish in the 8v piston. Not very scientific I know - but it may be obvious. What I am nervous about is the valve hitting the edge of the piston.
 

·
Registered
Joined
·
1,465 Posts
Hi Ed

I'd be concerned that you are going to get both. I think compression will be lower than 8.5 and I think the valve will hit the edge of the piston or its radius.

Sorry

rsfruitbat
 

·
Registered
Joined
·
2,828 Posts
I too think the valves lower edge will hit the pistons - but you could get the pistons "notched" for suitable valve clearance. You should assemble the heads on the block with no cams then see what lift you can get on the valves with a piston at tdc then have that the valve lift less the clearance (plus a bit!) machined out of the piston. It would then need checking carefully again with plastercine on the piston with the cams - rotating the crank by hand.
 

·
Registered
Joined
·
12,386 Posts
8v pistons will not work in the 16v engine..... valves will hit pistons.
 

·
Registered
Joined
·
4,084 Posts
Edward there might be better pistons out there for this set up..

I always sit down with piston books and find a suitable off the shelf replacement when doing this type of job.

my first turbo clio used another piston from the renault range that had loads of deck height and was machined down to fit the clio perfectly..

i,m now using a honda 100mm forged lightweight 24mm proven 14,000 rpm unit in the alfa 3.7 build.. and cheap too

takes some work but can usually find some thing of the shelf that does the job..

though if the compression is correct machining valve pockets is easy enough
 

·
Registered
Joined
·
3,000 Posts
Discussion Starter #8
http://i997.photobucket.com/albums/af95/edwardgurhy1/P1280353_zpsysyk3bgv.jpg

Dickyb very kindly sent me this picture. It explains it all really. When you see the 2 side by side it really gives you a clear picture of where the interference is going to be. Going to have to do a bit of thinking before I progress to the next stage. Also a bit concerned now about the eventual CR. Taking Maxiboy Wales comments into account - if I can just machine in a couple of small 'knicks' into the rim of the piston with the hollowed out middle then I may get to where I want to be - but will no doubt need to speak to the engine shop that has the other parts. Slow progress.... :rolleyes:
 

·
Registered
Joined
·
3,000 Posts
Discussion Starter #9
My next thought is the comparative volumes in the head from 8v to 16v. I presume due to the positioning of the valve in the 16v head there is probably greater volume in the 16v head. I presume this could be what Bert308 was alluding to in his post?? :(
 

·
Registered
Joined
·
3,000 Posts
Discussion Starter #10
Hmmm.... now starting to think about sticking with 16v pistons.

Clearly these are going to work well with the valve cut outs, with the valve throw matching the piston. Not only the pockets in the crown of the pistons but the head chamber shape as well.

As this is for my supercharger project, I was looking to reduce the compression ratio down a bit toward 9:1 or even a bit lower. As I understand it higher boost with a lower CR works better that higher CR with low boost.

Saying that it may well be worth having the normal driving characteristics of a standard 16v engine when not on boost.

The other thought was to keep the 16v pistons - but to have some of the crown machined down a bit, to lower compression - without removing too much that it would weaken the piston.

I keep looking at Bert308''s picture above and wondering if there is any safe areas in that cross section to remove material.

Anyone have any thoughts....

The engine builder has the block, crank, and piston rings ready to go - just not sure which pistons to give him - and whether to tell him to keep them standard - or stick them in the lathe....

Anyone have any thoughts?
 

·
Registered
Joined
·
2,613 Posts
For what you're trying to achieve you're going to have to make stuff. But thicker head gaskets should be easy to have laser cut from (I imagine) stainless steel? I think you would use a std HG either side of the stainless plate, but a quick search should give you some ideas.
 

·
Registered
Joined
·
3,000 Posts
Discussion Starter #14
Ok, interesting.

What is going around my head now is that having googled the compression ratio for a modern Mini,the CR is 8.3:1! As I am using what is effectively a mini supercharger and am going to copy the pulley size ratio I start to feel that perhaps I should try and reasonably match the compression ratio.

The problem I have is that the pistons I was intending using are 9.0:1 but lack the small valve cutouts. Not a great problem as I could get them cut in at the engineering shop. The valve cutouts would only just eat into the raised piston rim edge. The other problem is that the head chamber volume for 16v versus 8v is different. With a 16v head and pistons the CR is 10:1. I think the cut outs in the piston for the valves has a negligible affect on volume - but I don't know about the possible increase (in volume) due to the 16v head. :confused:

If the volume is greater it is not a problem unless it starts to drop me below 8.3:1. Hmmmm!

I think I may have to take a chance on some things. :confused::confused::(
 

·
Registered
Joined
·
2,828 Posts
I would think that the 16V head has a greater combustion chamber volume then the 8V one hence the reduced volume of the 16v piston crown. You can measure head volume with a perspex plate across the head face and fill the chamber from a graduated burrette full of paraffin or similar fluid.
 

·
Registered
Joined
·
3,000 Posts
Discussion Starter #16
Yes I was thinking that. Although if I know the volume of the chamber in the 16v head I would then need to compare it with the volume of a 8v head - which I don't have.

What would be really good is to bolt everything together and drop paraffin through a spark plug hole, measure and strip down. Followed by an 8v piston and do the same. Part of the reason for this is - I am not sure if the pin position in both pistons is at the same location. What is the reference point at the top of any piston to compare one with another, for how far they reach at the top of their travel at TDC. If pin locations are different and piston top shapes are different how do you compare one with another :confused:. From memory 16v pistons to the flat surface pretty much come level with the block - is that surface comparable with the rim on the 8v piston. The reason for needing to understand this is that there are 2 volumes at play here - the head chamber volume and the volume not occupied by the top of the piston at TDC. Hmmm confused.

I wonder somewhere if there is a chart that measures combustion chamber volumes. There probably is somewhere.....
 

·
Registered
Joined
·
78 Posts
If you look at Bert308's picture above - it looks as if the gudgeon pin is at differing heights in the piston. That is really going to mess with any calc when the piston at TDC.
No, the pins are exactly the same height. I have the halves here with me, when connected sharing a pin, the ring grooves are all three at same height, also the pistons tops and skirts just like in the picture.
 

·
Registered
Joined
·
78 Posts
If you could determine the difference in cc's that fit "in' the piston tops, you can calculate your CR. If the regular 16v has 10:1 CR:
CR= piston displacement+compression room divided by compression room where compression room is cc's in piston top plus cc's in cylinder between piston and head including head-gasket thickness plus cc's in head. Piston displacement 87 bore 72,2 stroke is 8.7x8.7xpi/4x7.22 is 429.2 cc. Compression room is then 429/9 is 47.69.
Add to this the extra cc's the 8v piston has, let's say 5: compression room would be 52.69.
New compression 9,14:1...
I have plenty 16v and 8v pistons around but wouldn't know how to measure cc's that fit in the piston top...maybe open my picture in Photoshop, scale to real size 87 mm with 10 pixels/mm, add a grid 1x1mm, and count the number of square mm that are in the piston, then calculate cc's...
 

·
Registered
Joined
·
3,000 Posts
Discussion Starter #20
I have just been looking all over the web for those calculations - pop back here for a moment and Bert308 comes up trumps.

So, having read through that and got the basics (need to read it a few more times to fully understand / take in) presumably a simple way to measure the volume in the top of an 8v piston is to cover it with a perspex lid, with a small hole in the lid and fill with a syringe / burette. That would give me the piston crown space volume.

Unfortunately, my pistons are away at the moment with the engine builder. Even so, even if it were 10cc that would only bring the CR down to say 8.3.

I am thinking that any figure for CR in the range of say 8.3:1 to 9.0:1 would be good.

I will try and find out that volume - but what I think is the way forward - is to use the 8v pistons and have cut-outs made in the rim to take the valve projection when open. This I can just copy from a 16v piston.

Would you agree?
 
1 - 20 of 38 Posts
Top