-:Piston Decision (I Gotz Da Rhymez):-

[whiplash473]

I'm trying to figure out what to do in terms of piston selection..this would normally be a simple decision for me but i've decided to keep the compression ratio down a bit just in case i decide to supercharge this motor again.......boost is not my world..building a motor to make power is.......so with me knowing about as much about boost as Paris Hilton knows about aluminum driveshafts i'm having a problem figuring out what compression bump i can go up to with the piston top without having a problem if i were to charge it

Bottom line - i'm getting pistons..i'm upping the dome on the pistons..what can i get away with before boost wouldn't be an option?

 

[wash]

Stick to flat tops or plan on swapping pistons.

With pump gas and normally sized combustion chambers, domed pistons aren't very practical. Even without boost, flat tops with a zero deck, ~0.040" head gasket, reasonable valve reliefs and chamber can still get around 10-1 compression which in my opinion is too much for boost on pump gas.

The cam also has a lot to do with it, specifically the timing of the intake valve closing. Do a search for "dynamic compression ratio" which is always lower than static compression and the bigger the cam, the lower the dcr (which is why stock motors with huge cams lose the low end torque and why stock engines with small cams usually have low static compression ratios).

If you don't run a cam that is too big, dropping a point on your static compression ratio only hurts your power a few percentage points which is a good trade if you can run cheaper gas without fear of detonation or add a little boost.

 

[whiplash473]

Nice to talk to someone else that actually knows their shit......anyways the heads I've got now are 61cc and I'm running stock pistons..so instead of bumping up the pistons I'll just replace them with stock spec since I'm running a smaller than stock combustion chamber?


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[wash]

With standard connecting rods, the general procedure is to zero deck the block and use the head gasket thickness to set piston to head clearance so your machining and blueprint job are the critical details, then just make sure your valve reliefs give you enough piston to valve clearance and most small block Ford combos will be OK where you end up (as long as your cam is suitable).

 

[whiplash473]

Agreed..also my cam is @ .499/.510 so I'll be ok......I found a set of DSS pistons that'll give me (when I put all my numbers together) about 9.4:1..which I think will be fine if I felt like charging it down the road..thoughts?


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[wash]

That's good until you add boost, then its going to limit you a bit.

At that point you can switch to a slightly thicker head gasket and nice aluminum heads with larger chambers (the improved heat transfer of aluminum combined with carefully designed chambers can increase detonation resistance even if the compression ratio remains the same).

One other thing, valve relief clearance has more to do with camshaft duration and valve diameter than maximum lift. Unless your timing chain breaks the piston is never near the valve when maximum lift happens. Things get close when the exhaust valve is closing and when the intake valve is opening.

 

[whiplash473]

Valve float shouldn't be an issue with my setup right now but if it were supercharged that could be a different story......anyways the pistons I was looking at had valve reliefs deeper than normal..I don't remember the exact spec......also I really don't beat my car up like many others..I'll probably stand on it a few times throughout the season but that's about it..that's the reasoning why I was thinking 9.4:1 would be any problem.......I might not even charge this motor..but if I do I just don't feel like swapping pistons out for it


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[whiplash473]

The heads I'll be throwing on in a few weeks are 61cc..I'll throw a mic on the gasket and see what I'm dealing with


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[rz5.0]

The stock heads vary on the CC so the compression ratio with 61 might actually come down a bit with stock spec pistons. Most the time the 58cc heads will get you closer to a stock comp..
Agree on allot of what was already said except using a thicker gasket to bring compression down the road if going super charged later. With aluminum heads @ 9.5 is a very reasonable ratio. Boost will have to be lower than if he was running lower comp ratio but it will have a better lower end responce.
By the time it really becomes an issue he will pushing on the ol block limits anyways..
Wash brought up alot of good points and its nice to read responces like that on this forum

 

[whiplash473]

Yeah no kidding.. Like I said before - nice to talk to someone that really knows their stuff.......I'll look into pistons again 2mor and see what I come up with


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[wash]

Again, your machine work has a lot to do with it.

Ideally you would want an index ground crank to equalize stroke and make sure there is no twist, blueprinted rods, a decked block, good pistons that don't vary in compression height, a balance job and allign honed main journals (if necessary).

If you are buying pistons, look at a set of forged I-beam connecting rods. They may cost only a few dollars more than blueprinting and ARP rod bolts on your old rods but also be set up with bronze bushings on the small end for floating wrist pins which is a nice feature for pistons if you are buying new.

If your crank needs an index grind, then the cheap stroker kits start to look good.

If you are going to spend $1,000+ on a rebuild to gain a few percentage points in power, you should look at how much more a stroker costs that will raise that percentage another 10-15%.

 

[rz5.0]

That's why my main recommendation is to talk to the machinist...

 

[whiplash473]

yeah my old man's buddy has a speed shop so i'm gonna run this by him..just for shits and giggles tho i'm thinking about these so what's the thoughts?

http://www.summitracing.com/parts/dms-3000psx-std/overview/make/ford

 

[wash]

Those are standard bore, it doesn't mention what size valve relief, compatible heads, etc. and it sounds like it has a 3 cc dome.

If you have to ask you should probably ask your machinist to order pistons for you.

A lot of people wait to finish the block machine work so that they can order exactly the right custom pistons.

I'm planning on a budget of around $6-700 when I buy pistons (probably CPs) but they will have all the bells and whistles.

 

[whiplash473]

Yeah I should've deleted that post..I did more liking around after I posted that and found some that suited my needs better..anyways I'll talk to the speed shop 2mor and get some opinions on what I'm looking for..thanks a lot for the help


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[whiplash473]

for what it's worth i figure i'd update this..DSS Racing is making my pistons this week..i've been talking to the owner over the last week and he came up with a piston that will suite my needs..i decided i'm not supercharging it since i don't really care for boost..so with my heads and gasket i'll be around 10.35:1 with these pistons

 

[rz5.0]

Flat tops? Not bad its going to be a different animal once its all together..

 

[whiplash473]

yeah buddy flat tops......yeah it's gonna be different for sure..the only thing the same (internal and bolt on) will be the block

 

[rz5.0]

Striking it?

 

[whiplash473]

yeah rz...331..really looking forward to it