If you were starting from scratch - blank page, purely conceptual...

Let's say you intend to build for high compression, over 14.1. Would you cut way back on chamber size, or would you run large domes on your pistons? What would drive your decision?

 

Up over 14, to you’d have to cut the mess out of the chamber which hurts flow to a degree, as we all know. So personally I’d go with domes and some chamber work to keep my domes as small as possible. The smaller the dome, the better flame travel across the piston.
Having said that, in my circle track days, we shot for 15:1. We started with 49cc chamber casting and cut over .250 off of it, angle milled of course. The valves stuck down further than the deck, when layed on a table. I still have a set here. I wish I had showed them to you Darth, but time wasn’t being friendly that day. Pushrod channels were custom milled. It was tough, but end result was torque.

 

I can "listen" to this kind of stuff all day.
Please continue!

 

As you know I went with a smaller dome and a tiny combustion chamber, mainly because the smaller dome pistons were cheaper, and I had a set of 862�a with some work done to them already. Well, couple things I�d like to point out that I kinda regret.

the smaller dome pistons weren�t actually cheaper. They didn�t come with rings, by the time I bought rings I might as well have bought the 12cc Weisco pistons.

That in itself isn�t a big deal. Where I feel
I screwed up is, obviously the heads on my build are the weak point, if I would have went with the 12cc dome variants I could have switched to almost any aftermarket head and kept my compression. Now, since I opted for the smaller dome and smaller chamber, I�ll lose a ton of compression if I go to an aftermarket casting.

i guess I�ll save that for the offset ground crank build.

 

I would use both, how much of each would depend on CID as a large engine would only need a small dome to reach 14

 

Big domes can also impact flow during overlap in addition to messing with the flame front.

Domes can add weight to the piston.

Why do small chambers hurt flow? I've heard this too - but why?

If the chambers are opened up to the bore size the valves should be un-shrouded. I think it might be the difference between cutting down a typical 70cc chamber to 50cc vs designing a chamber from scratch.

 

I do not know if small chambers hurt flow or if it is more the milling process hurting flow. The LSR head is cast with 54cc chamber. Maybe that is not the same as milling a 70 to a 63?

Edit - that was my thought process on my current heads, anyway - to avoid milling to protect flow.

 

It isn’t that small chambers hurt flow, if the chamber was designed that way. What hurts flow is cutting a chamber down significantly in such a way that you disturb the Venturi effect. The area of the port 1” above the valve seat and the the area 1” below the valve seat (into the chamber) is what makes up this Venturi area. This area is designed and tuned by the manufacturer, so in some cases milling will hurt performance. And in other cases, unless it’s a high end casting, a nicely done CNC port and chamber blend job really wakes up the casting. Cutting the chamber down by milling enough material away to disturb this Venturi will hurt flow. But you’d have to cut the head down significantly as in like .125” to make a noticeable difference that you’d feel or see on a dyno. Will show up on a flowbench, but what’s on a flowbench doesn’t always equate to a dyno or track, with either losses OR gains.
Of course there’s a trade off here, as in every aspect of performance mods. The smaller chamber (via milling) equates to more compression (given all parameters are equal) which can often make up for the flow “defficiency”. Compression makes torque and often times the increase in compression more than makes up for the ever so slight flow losses that are present.
Here’s a random pic I found for reference of Venturi area...compliments of Darrin Morgan of course...

 

Dome pistons can limit flame travel but milling heads too much can shroud the valves

that being said, most people will limit their combo elsewhere rather than miss a few cfm from their heads

 

Interesting, thanks for that.

 

I would think the slower burn would make the engine more prone to detonation. If this is true, you'd want domes to be pretty small and flat. Think of a hemi with domes. The residual volume looks like a walnut shell or orange skin.

 

I think the answer is different for each engine design.

Whether it be milling heads or big domes or a combo of both eventually there has to be a point of diminishing return. I would say a good approach is asking what�s right for each engine build.

Questions such as are you losing power by milling the heads too far? Does changing the entire flow characteristics of a head just to hit a certain number for compression ratio really make sense or are we losing power with this approach?

 

No insights here just a few questions.

At ~14 to 1 compression is it definitely time for a spark plug with the ground strap cut back to help promote flame travel?

What sort of plug gap and coil should be used?
Good application for the 1GN-A1 coils?

 

I would agree with 70velle venturi post.

here is something Mr Maux told me on his forum.
we had a discussion on my 250cc buret, which turned out was a great buy.
heads or runner always use a quality class A buret

SuperStock Racer from, Canada
who purchased the worst 100CC Buret + Stand Kit ... like i had also did ,... from PMS in San Antonio, Texas
it was off 3.5 CC per 100 CC poured + the included plexiglass plates were warped 0.003" in the middle

so i ported this Racer a pair of killer Chrysler Heads/Manifold
shipped them thru Cutoms ... he calls me up after receiving them , and says
they have great Flow Numbers , but you did not mill them far enough , so i milled them another 0.020" off to get them closer to NHRA minimum .

i asked him where did you purchase your Buret ??? ... he said PMS in San Antonio, Texas

i told him you just possibly ruined your Heads

i told him go double-check your Chamber CC volume with another Buret or Shop or Pharmacy that you trust and call me back with results

he calls back and says "you're right they are too small now and illegal"

Ship them back thru Customs ... i had to sink the Valve Job + redo some expoxy in Bowls + re-Flow test all 8 Ports again
ship them back thru Customs ....
end result ... he set the NHRA Record ... so it ended up great !



milling or sinking the valve job will change flow, i wouldn't say you cant get the flow back, but time consuming forsure, I purposely milled some oem LS heads to see if i could gain some flow.
Was eventually my best flow #s of that model head yet. I havnt tried to match it with unmilled heads yet.

 

Glad this got brought up. Would love to know what you found regarding flow numbers after valves were sunk to gain chamber volume. Typically murders low-lift numbers. Was a camshaft change necessary to get back what you had?

 

Since you didn't get an answer -- yes, great application for the IGN1A. As to plug gap, I would personally end up starting tight and seeing if it could tolerate loosening. I'd also upgrade the wiring to the coils and set them up to just shy of 8-ms dwell, as those coils can sustain that.

I wonder at what point you sacrifice detonation vs flow. What I mean is, sometimes you can gain by slightly reducing compression so you can push more timing. Other times, you gain by bumping compression and sacrificing timing. A lot has to do with flame speed I'm sure. Faster burn is less prone to detonate. .

Also, sometimes you see people say, "Well if i have an issue, I'll just run X-fuel". Is it better to start with the fuel and use that to determine compression, which is what I find myself doing?

 

To many variables, L Meaux thought it was important enough to retest/flow and fix heads mentioned above. .All you can do is reflowtest and see what happens after a
shave or sinking the valves.

 

Big domes on BBCs with 119cc + chambers needed a lot of timing, like 36-40*,to get max power, very archaic and very inefficient. The very first post in this thread kinda said it, it’s going to be a combination of chamber size and and some positive displacement on the piston crown or lots of stroke to make compression.
FWIW, some NASCAR cup engine builders use tiny cone shaped combustion chambers (48-50cc) and dished pistons to make 11:1 static, the dished piston takes a bigger gulp of air/ mixture in theory.
It seems like domes on pistons are a sacrificial trade off and boost is the new king of power enhancers.

 

What I tried to do on mine was preserve as much of the chamber as possible. Ended up with a net 1.3cc dome (AFTER valve reliefs figured in), so it's like a little flat "mesa" on top of the piston. but that got me 12.7. Not 14. And it did make the pistons heavier. I noticed that while shopping. I could get dished pistons in the 450g range all day long, but domes were in the 490-500g range all day long.

Then heftier pistons require beefier rods, and it spirals.

What I have in my head for a future build is sort of like this. Another big bore motor. Stroke as yet undetermined. Something based off the LSR casting, starting at 52cc and then running a 5cc dish to get compression into the 14-15 range. Seems like the best way to do it. Or even more extreme, a C5R bare casting in the right hands with a 30cc chamber, no milling, but then needing a 20cc dish to get into the 16:1 range.

But it's diminishing returns, and boost becomes much easier, so I see your point for sure.

 

most dome pistons are a solid dome so they end up heavy. Approximately half of that extra weight can be milled off the underside in a na engine. Piston dish volume is limited to an extent by the compression height needed so we are back to a compromise between the two in my book