Flat tops w/ large chamber vs. Dish w/ small chamber - LS1TECH - Camaro and Firebird Forum Discussion

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Flat tops w/ large chamber vs. Dish w/ small chamber

 
Old 03-25-2018, 01:02 AM
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Default Flat tops w/ large chamber vs. Dish w/ small chamber

Let's compare two hypothetical LQ 370ci engines and discuss the pros and cons of each for a fun daily driver.

​​​​​​assume we have a good 0.040 quench for each and they will both end up with the same static compression ratio.

flat tops with 68cc #241 heads gets 10.91cr

​​​​​​+6.5cc dish with 61.5cc #706 heads gets us the same 10.91cr

would the 2nd one be more responsive, make more tourqe, and or be more prone to detonation?
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Old 03-25-2018, 02:30 AM
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Chamber shape is more important then piston shape. The one with more head chamber will make more power.
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Old 03-25-2018, 07:37 AM
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Originally Posted by DietCoke View Post
Chamber shape is more important then piston shape. The one with more head chamber will make more power.
please elaborate further,
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Old 03-25-2018, 08:51 AM
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Originally Posted by DietCoke View Post
Chamber shape is more important then piston shape. The one with more head chamber will make more power.
Chamber shape unshrouds large valves and flows well
But smaller chambers (usually from flattening out the valve angle) have better BSFC numbers and therefore require less total
Timing to make peak power
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Old 03-25-2018, 09:18 AM
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Originally Posted by A.R. Shale Targa View Post
Chamber shape unshrouds large valves and flows well
But smaller chambers (usually from flattening out the valve angle) have better BSFC numbers and therefore require less total
Timing to make peak power
so are you saying that the 2nd engine with the smaller chamber and dish could possibly be more efficient than the flat top with the larger chamber? Remember both engines have the same compression ratio and would have the same cam timing and thus the same dcr.

The only downside I can figure is that with the dish there is less quench area utilized on the flat portion of the head (just get quench around the perimeter of the piston top) so it could possibly be more prone to detonation? and that would require less timing anyways?
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Old 03-25-2018, 11:08 AM
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Default Piston Cost = FT

Hi A4, I too agree with the above tech.

AS YOU ASK, I have the reason of LOWER COST with a FT piston cost being lower.
The FT WILL BE a LIGHTER piston, good to reduce engine balance costs.
The FT WILL BE a LOWER in surface area piston crown, good for a HP increase due to less heat transfer into the walls/oil
The FT WILL BE a STRONGER piston good for better top ring life.
The FT WILL BE a piston design that can allow for a higher top ring placement.
The FT WILL BE a piston design that allows for THICKER Ring Lands.
The FT WILL BE a piston design that allows for a better quench, good for Knock Resistance

Lance
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Old 03-25-2018, 03:07 PM
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Ok guys I'm sort of getting this. Thank you all for your input so far.

I think I can agree that the flat top piston is probably the best for most applications if the resulting compression permits. However, I arrived at these two engines while trying to figure out a cheap and fun motor for my stock converter 2.73 geared pump gas daily driver.


I'm trying to build this 370ci long block with stuff I have on my shelf or stuff that's already on the car so let's have some fun with it. It just needs the right pistons haha.

if I use true flat tops and the #706 heads with the same 0.040 quench I'm looking at a pretty exciting 11.89 static compression ratio. Is this really doable on decent year round pump gas with a late enough intake closing point?

I really want to use my 212/218 115lsa comp cam in this engine.
if I installed it -4 degrees on a 119 icl that closes the intake at 72 @0.006 for a dynamic ratio of 8.88.

is that really pushing the limit for high velocity and big compression with a small cam On the street?
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Old 03-25-2018, 11:21 PM
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A 243 head will have a smaller chamber than the 241 head which raises compression about .5 as well as a larger intake runner and D shaped exhaust runner.
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Old 03-28-2018, 08:34 AM
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Originally Posted by stockA4 View Post
Ok guys I'm sort of getting this. Thank you all for your input so far.

I think I can agree that the flat top piston is probably the best for most applications if the resulting compression permits. However, I arrived at these two engines while trying to figure out a cheap and fun motor for my stock converter 2.73 geared pump gas daily driver.


I'm trying to build this 370ci long block with stuff I have on my shelf or stuff that's already on the car so let's have some fun with it. It just needs the right pistons haha.

if I use true flat tops and the #706 heads with the same 0.040 quench I'm looking at a pretty exciting 11.89 static compression ratio. Is this really doable on decent year round pump gas with a late enough intake closing point?

I really want to use my 212/218 115lsa comp cam in this engine.
if I installed it -4 degrees on a 119 icl that closes the intake at 72 @0.006 for a dynamic ratio of 8.88.

is that really pushing the limit for high velocity and big compression with a small cam On the street?
well I just finished reading the new car craft.

​​​​​Looks like I'll be swapping everything, the entire heads, cam, and supporting valvetrain parts from my running and driving 4.8, directly on to an iron 370ish LQ with stock stylish dish pistons on stock rods. I just need to get the block machined first before I can order anything.

I've been wanting to do this in car shortblock swap for a while now and the Iron Maiden Series just confirmed that it would work out great for me. I bet my otherwise stock 4th gen will be faster than that orange peel thing too haha.
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Old 01-10-2019, 03:05 PM
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I thought I'd bring this back from the dead since I've made some progress on the build. I got some good used 853's for $75 complete with rockers so I'm going to use them on my 0.040 over 6.0 with some flat tops.

I still would love to see a dyno overlay of these two combos (dish with small chamber vs. flat top with big chamber) with the same cam timing though. Would just have to put a 2" valve in the 706 to make it fair I think
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