stockA4

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?

DietCoke

iTrader: (18)

Chamber shape is more important then piston shape. The one with more head chamber will make more power.

stockA4

please elaborate further,

A.R. Shale Targa

iTrader: (2)

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

stockA4

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?

pantera_efi

iTrader: (1)

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

stockA4

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?

wannafbody

iTrader: (2)

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.

stockA4

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.

stockA4

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

stockA4

I thought I would update this crazy old thread I was wondering what if a few years back, because I recently read a magazine article that Mr Holdener wrote where he took an old lq4 bottom end (dished pistons) and box stock 706 heads (small Chambers and 1.89 intake valves) and it made like 487hp at 6100 with 1-7/8 headers and a truck intake with a mild comp 212/218-115lsa

I built myself the other engine, flat tops with a larger chamber because I believe it is better and will make a little more power everywhere and so I used the same camshaft in mine so although mine also has a few other advantages over Holdener's 6.0 (12 more ci, 0.6-0.7 more compression, 2" intake valve, and LS6 intake) I think it is a great counterpoint to that engine. I have it installed in my car already so it will have to be trap speeds and Dyno's # for comparison. I've put almost 4k miles on it delivering pizza with it since December and it runs great so hopefully the quarantine won't keep me from getting it to the dragstrip and the Dyno this season! The stock injectors are maxed out with the stock exhaust manifolds still on it but I'll probably just put headers on it anyways. The car has a 255 drop in pump already so I don't think it's starving it's just not optimized yet but it will get there my budget just sucks, it feels really good though the torque off idle is absolutely delightful