flat top or dished pistons

[Tom H]

I'm getting ready to order custom pistons for my Chevy Rod 1725 (converting to an 1820). I can have the pistons dished like the original, or maintining the same CR, I can have them flat topped and move the tops down 0.054 in (shorten the compression height). Any advantage one way or the other? Seems that flat is simpler but I thought I'd ask.

Tom H

 

[RootesRacer]

DONT move the compression height!

The combustion chambers in the rootes engines are bad enough without removing what little squish it has.

either have the pistons dished, or perform the holbay or vizard mod to drop your compression down to what you want.

 

[Tom H]

Jarrid,

I'm already doing the Vizard mod, but it only adds about 1.8 cc to the chamber. But what diff does it make if I set compression by dish in the piston or moving the whole piston top? Either way the compression is the same. If I do the dish, the actual top (at the center of the piston) is 0.24 inch down from the piston top. If we move the whole top down, it only needs to go 0.054 inches, I assume we would then also move the top ring down about 0.054 as well. I see no difference in the "combustion chamber" except its shape. But it seems these custom pistons would be stronger if flat topped. Or am I missing something here?

Thanks,

Tom

 

[RootesRacer]

Jarrid,

I'm already doing the Vizard mod, but it only adds about 1.8 cc to the chamber. But what diff does it make if I set compression by dish in the piston or moving the whole piston top? Either way the compression is the same. If I do the dish, the actual top (at the center of the piston) is 0.24 inch down from the piston top. If we move the whole top down, it only needs to go 0.054 inches, I assume we would then also move the top ring down about 0.054 as well. I see no difference in the "combustion chamber" except its shape. But it seems these custom pistons would be stronger if flat topped. Or am I missing something here?

Thanks,

Tom

The difference is squish, which has a profound impact on the ability to light off the mixture, and affects your ignition timing.

If you put the pistons farther down the bores at TDC the combustion chamber has far less turbulence and needs more ignition timing as a result.

Also if you are doing the vizard mod according to instructions, you will gain a lot more than 1.8 CC of chamber volume. 5 or 6cc IIRC.

IMO use flat tops and do the vizard mod and remove chamber material till you get to your desired CR.

On my motor I arrived at a 43 CC chamber volume, and I didnt go hog wild on the chamber mods, that is it didnt have the full kidney shape and depth that vizard intended because I wanted to keep as high of compression as I could get. I also set the compression height to just above the deck height (.005) to get even more squish.

 

[sunbeam74]

Tom,

Which ever way you choose you may want to check out Venolia Pistons as a source. They were very nice pistons and they weren't badly priced either.
The only catch is you had to use a slightly larger wrist pin and had to hone your old rods a few thou.

There may be still photos of the pistons on the old forum list, too.

Steve

 

[Tom H]

Thanks Steve,

In fact I am planning on using Venolia. I got in touch with them after seeing your coments and photos.

Jarrid,

I got it - "squish". I read up on it. Gotta get that turbulence from squish between the piston top and cylinder head bottom. Hmmm. I think that means dished tops. I do not see getting anything close to an extra 7 ccs (to replace the standard dish) out of the Vizard. My best estimate based on paper measurements and calculations shows Vizard adds 2.1 cc. But slightly less aggressive steps I've actually done so far yields about 1.8cc. I could polish it out to 2 or 2.2, but can't see more. And if I mill it 5 thou to flatten out some damage from the broken ring, I'll lose about 0.5 cc of that.

BUt I think I'll keep a good squish. Think 0.028 below deck for TDC is OK? I measure stock at 0.018.

Tom

 

[RootesRacer]

Thanks Steve,

BUt I think I'll keep a good squish. Think 0.028 below deck for TDC is OK? I measure stock at 0.018.

Tom

I'd keep the piston to head clearance of .035 or less. This will maximize your squish. Bear in mind the compressed HG is about .035.

I really dont know how you arent getting more than 2cc chamber gains, perhaps you arent cutting deep enough, or are you starting with a heavily milled head?

On a virgin never milled head, I started at a measured 38 CC, and ended at 42.5CC. Even so the chamber mods were not as aggressive as as the Vizard, and not even close to that of a full blown holbay. This because I wanted to conserve my compression ratio, and even had to push the pistons out the bores to get the compression I wanted.

Out of curiousity, what CR are you aiming for?

 

[Tom H]

Jarrid,
No, it's a virgin head. Here's how I estimated at first. I used two methods. In first estimate, looking at the Vizard Excel sheet I have, printed full size using the valve centerlines as a guide. You can estimate or appproximate that the added area (just the area, at the surface) is equal to adding 0.4 in dia or 0.2 in rad to about 37% of the "large circle" that represents the chamber area around the Intake valve. That calculates out to about 3.4 Sq cm area. I also took a cc scale and divided the added area into ten 1 cm long segments, measured each segment width, and totalled up all the pieces. It showed abot 3.5 sq cm area. So i am pretty confident that the area of the Vizard- at the surface- is about 3.5 sq cm. Now the total depth of the chamber - to the casting, not the valve, is a mean depth of about 0.5 in. And less deep on the side where the largest mods are made. IF the mods were then cut with straight vertical walls down from the indications noted above, the volume would be the area (3.5 Sq cm) times the depth (0.5 in or 1.3 cm). That would make 4.5 cc volume. BUT the Vizard drawings I have show the side wall mods all tapering linearly as they move into the casting. This would yield a volume exactly half as much as if you made the mods to the walls straight sided. So that makes it about 2.2 C. And if you take into account that the larger mods, in the edge opposite the plug holes, are the shallower part of the Combustion Chamber, it seems that 2.1 or 2.0 is more likely.

And when I actually made these mods I was careful to maintain as close as possible to a smooth, linear, not curved, taper. I actually achieved about a 1.8 cc increase. I could take some more material out and still maintain the right shape. I'll do that on final pass after I get the #4 chamber welded and fixed from the ring damage.

I assume a Holbay head has similar mods but I thought I saw that the mods had steeper sides to them, rather than the linear taper sides of the Vizard.

One more thing. In looking at the Vizard drawings I have it shows the changes at the surface and then two cross sections showing the taper. Actually only one of the cross sections show the depths of the mods, the other cross section shows the unchanged exhaust valve area. BUT, in looking closely at the cross section with the mod depth, it seems to show the larger changes are on the deeper side of the chamber. But this is wrong. The big changes are on the shallow side .

Thanks for all the thoughts.

Tom

 

[RootesRacer]

There must be different versions of the vizard mod out there...

The holbay is more volume loss for sure.

So whats your desired end CR?

Keep in mind that with the improved combustion chamber characteristics that the engine will be more tolerant to higher compression ratios without pinging.

A final CR of 9.6 will probably be at the same timing as the stock 9.2 due to the higher turbulence and and better thermal efficiency. That is 9.6/1 should be able to run on 92/93 octane fuel.

if you have access to better fuel yet, consider going as high as 10/1, which makes for a peppier engine and better mileage too.

 

[Tom H]

Hmmm. My plan was to maintain the same CR as original. Specs say 9.2 , but I calculate about 9.1. I run premium gas as obtained at local pumps, 93 ? octane, and have had no problems pinging. So plan is to maintain. If, as you say, I CAN run higher CR with the Vizard mods that would be fine. If I keep the dish and keep the deck clearance at 0.018, with a 3.248 bore and 3.35 stroke, I would have 9.33 CR. If I reduced the dish to 6.0 cc I would have CR of 9.53. If I used Flat tops, even with the deck clearance increased to 0.028 gives me a CR of 10.3. Sounds too high.

So you really are sure I could run 9.6 CR and keep using 92/93 gas, with the Vizard mod.

Thanks,

Tom

 

[LarryL]

I realize this is a little off from the thread of this topic but I have seen references to machining the block to unshrould the exhaust valve in conjunction with the Vizard mod. Does anyone have any details or patterns for this mod?

 

[RootesRacer]

Hmmm. My plan was to maintain the same CR as original. Specs say 9.2 , but I calculate about 9.1. I run premium gas as obtained at local pumps, 93 ? octane, and have had no problems pinging. So plan is to maintain. If, as you say, I CAN run higher CR with the Vizard mods that would be fine. If I keep the dish and keep the deck clearance at 0.018, with a 3.248 bore and 3.35 stroke, I would have 9.33 CR. If I reduced the dish to 6.0 cc I would have CR of 9.53. If I used Flat tops, even with the deck clearance increased to 0.028 gives me a CR of 10.3. Sounds too high.

So you really are sure I could run 9.6 CR and keep using 92/93 gas, with the Vizard mod.

Thanks,

Tom

I am pretty sure 9.6 is OK, the stock holbay is 9.6/1 it uses flat tops (no dishes) and plenty of people around run pump super unleaded fuel.

Jay Laifman runs a holbay H120 and uses presumably 91 octane fuel (socal max octane) perhaps he could chime in.

Earth to Jay, Earth to Jay...

 

[RootesRacer]

I realize this is a little off from the thread of this topic but I have seen references to machining the block to unshrould the exhaust valve in conjunction with the Vizard mod. Does anyone have any details or patterns for this mod?

I wouldnt do this unless the cam lift is really high and/or head has been skimmed so that the exhaust valve hits or comes close to hitting the deck at full lift.


If you need to do the work...

Put a thin layer of masking tape on the block, put the head in place and allow the exhaust valve to drop on the tape a few times to mark it.

After this, freehand grind (extra carefully)down about .050, at the angle the valve hits the deck.
You cant go much deeper because the top ring might catch the edge of the bore.

Repeat 4X then match the shape and volume on each of the 4 bores so the compression is consistant.