New pistons

speedracer · Post by **speedracer** » Mon Apr 10, 2006 5:17 pm

Is it possible to change your pistons in your driveway?

I was thinking about getting the dohc pistons, which would boost my compression ratio from 9.3:1 to 10.2:1 I believe.

I realize I would need the puller to get the pistons out of the block, and my car has only 36,000 miles, so I don't think it needs to be bored.

Anyone ever do this?

If not, what does it cost in labor to have someone else do it?

Or is the bump in compression ratio not even worth the effort?

quicksilvr · Post by **quicksilvr** » Tue Apr 11, 2006 1:33 am

I would have to say yes, especially if you've done all the mods in your sig yourself. I'm blanking out slightly here, what all do you have to do to be able to drop the pistons out?

Also, which pistons are you wanting to put in? Aftermarket forged ones? There are no stock DOHC pistons that are 10.2:1....1st gen DOHC's were only 9.6:1. Unless that was only in conjunction with the DOHC head, and put in with the SOHC head they come out to 10.2. Is that the deal here?

sllywhtboy · Post by **sllywhtboy** » Tue Apr 11, 2006 1:52 am

dohc pistons + sohc head = 10.2'ish compression.

speedracer · Post by **speedracer** » Tue Apr 11, 2006 6:44 am

I was thinking about the dohc pistons with the anti friction coating on them. I think their like $140 on ebay (kb's I think).

Yeah, because the dohc has a larger combustion chamber, the dohc pistons have a domed top on them, which will raise a sohc's compression to about 10.2:1.

I did everything in my sig except I had help with my head work and struts.

tamadrumr88 · Post by **tamadrumr88** » Tue Apr 11, 2006 7:18 am

from what i hear, you can drop the oil pan, the bedplate (im not 100% on that name), and from there you can take out the connecting rods and slide the pistons out. maybe to make it easier you could put the car in gear and spin the tires (would that cause the engine to spin, or wont it since theres no spark since the ignition is off?) to allow access to diff sides of the crankshaft if they are at a hard to reach place

after a guy explained it in a thread (on the dot org

) it almost seemed TOO easy. i too was looking at just stock dohc pistons

speedracer · Post by **speedracer** » Tue Apr 11, 2006 9:41 am

If done, what kind of power increase is it going to give me? I mean is it worth the trouble?

quicksilvr · Post by **quicksilvr** » Tue Apr 11, 2006 10:35 am

Only a dyno would tell the real power of course, but I would think a whole point in comression boost would be noticed, especially if you use good gasoline. I would say yes, it's worth it, especially if it really is as easy as it looks to be. I've never had the oil pan off my car, but I changed the main rod bearings on my old Nissan Sentra, and that was a piece of cake to do. THis isn't much past that...I say go for it.

OB · Post by OB » Tue Apr 11, 2006 11:31 am

Is it possible for the computer to safely compensate for the raised compression fuel/air wise, or is that not an issue when the compression is raised/changed? also, with the domed pistons is the valve clearance enough? i dont know all that much about this stuff so im interested to know how it works.

rice_eater · Post by **rice_eater** » Tue Apr 11, 2006 12:17 pm

you can do it... instead of dropping the crank (unless you want it "spun" aka balanced) just take the head off... then all you do is undo the two connecting rod bolts and simply push the piston straight up. in the haynes manual htat's how they do it, and that's how i did , but i was swapping on a magnum head at the same time...

Post by **racer12306** » Tue Apr 11, 2006 9:27 pm

^^^ i was thinking that would be a better way to do it. the first way described you would have to drop the crank. when you are putting pistons in be very careful not to nick the crank.

i would think it would be a pain to compress the rings and slide them up from the bottom.

BlackRoseRacing · Post by **BlackRoseRacing** » Wed Apr 12, 2006 7:07 pm

^^^
The way the cylinders are designed on every motor, you can't slide them up from the bottom.
The head WILL have to come off for any pitson or rod change.
On another note, these motors are internally balanced! So my HONEST recomendation for swapping out pistons and rods is to measure the weight of the one your pulling out....lets say its 500grams....
No your replacing it with an identacle setup but it weighs in at 550grams. Take the rod itself to a sander or grinder and smooth out some of the material a little at a time. Try to get it as close to the origional 500gram setup you pulled out as possible. Once you are at that point, assemble everything as the standard procedure states and you should be fine.
I been there done that with KB pistons and forged rods on a 2.2 motor. I weighed each rod and piston that was OE and then weight matched all the aftermarket setup....I got them all within a .005 grams using a collision shops paint scale. That motor ran perfect!
No going off topic, im thinking on rebuilding my old R/T shortblock with SRT4 pistons and rods, aparently the bore is the same, as for stroke the 2.4 has a different crank, and the rods are also shorter. My theory(or planning) is to build a 2.0 sohc w/ 2.4 turbo pistons and rods to make a low compression 2.0 with availability to run wild boost.....
Of course its still a theory right now, I have not done any further research....
Anywho.....

im rambling again.....

unsuper man · Post by **unsuper man** » Thu Apr 13, 2006 12:12 pm

i talked to nemo when srt4s first came out. hes been putting 2.4s into neons since the early 90s. at that time he had a set of stock pistons/rods form an srt sitting there. so i asked if i could swap them into my car, since the bores are the same, he said the rods will not fit onto our crank. thats all he said tho. its been over 2+ years since he told me thaat, so who knows what hell say now, but as far as i know, the rods wont work. maybe get forged 420 rods and use stock srt pistons?

rice_eater · Post by **rice_eater** » Thu Apr 13, 2006 3:33 pm

BlackRoseRacing wrote:^^^
No your replacing it with an identacle setup but it weighs in at 550grams. Take the rod itself to a sander or grinder and smooth out some of the material a little at a time. Try to get it as close to the origional 500gram setup you pulled out as possible. Once you are at that point, assemble everything as the standard procedure states and you should be fine.

very true to a point... what you really want is all the rods and pistons to weigh exactly the same as eachother, withing a few thousands of a gram. you will need a digital scale but that's the general idea, remove material by sanding or in any other way but get them balanced. if you want a race engine its good to have teh crank "spun". it works just like tire balancing, except its for your crank... as long as its balanced you're good.

grindpunk16 · Post by **grindpunk16** » Thu Apr 13, 2006 4:03 pm

unsuper man wrote:i talked to nemo when srt4s first came out. hes been putting 2.4s into neons since the early 90s. at that time he had a set of stock pistons/rods form an srt sitting there. so i asked if i could swap them into my car, since the bores are the same, he said the rods will not fit onto our crank. thats all he said tho. its been over 2+ years since he told me thaat, so who knows what hell say now, but as far as i know, the rods wont work. maybe get forged 420 rods and use stock srt pistons?

pistons wont work either. 2.4 feature different wrist pin locations also to accomidate the increased stroke and crank.

do not drop the bed plate. youll be asking for trouble and you would have to recheck clearances. just pull head and oil pan and pop the pistons out.

BlackRoseRacing · Post by **BlackRoseRacing** » Thu Apr 13, 2006 6:11 pm

rice_eater, I know that much(been there done that...
But he is talking about replacing only one piston and rod. Thats why I tried to give him an example. I did not want him to drop in a lets say 750gram setup in place of a 450gram setup, it'll throw the balance off.
As for weight matching all four thats a different story. The last one I did, I took the weight of the lightest piston and rod assembly and weight matched the other 3 to within a .01 or less of a gram...
Although we might be getting a little bit more technical than what he wants to here to

speedracer · Post by **speedracer** » Thu Apr 13, 2006 6:25 pm

Yeah, so I'm assuming that if I replace all four pistons (brand new) I shouldn't have to worry about weight balancing right?

Thanks for all the replies by the way.

grindpunk16 · Post by **grindpunk16** » Fri Apr 14, 2006 3:07 am

if you just balance the pistons youll bow the crank since each piston is balanced to a specific journal. if i had pics of my crank you would understand how much slop i had in cyls 2&3 respectivally

BlackRoseRacing · Post by **BlackRoseRacing** » Fri Apr 14, 2006 7:13 am

like I said again, if he's replacing one piston&rod, make sure you weight match it to the one your pulling out of the car to try to keep it balanced.

rice_eater · Post by **rice_eater** » Fri Apr 14, 2006 8:41 am

speedracer wrote:Yeah, so I'm assuming that if I replace all four pistons (brand new) I shouldn't have to worry about weight balancing right?

Thanks for all the replies by the way.

no, even new pistons need to be checked since very often even identical new pistons will be off by a few milligrams... it takes so little time, it'd be a shame not to do it considering the rest of hte work involved.

imo, the best thing would be to pull the engine, put it on a stand, and work leisurley on it. it may be a bit more work pulling it and putting it back in, but you have a chance to get the crank out, have it spun and balanced and have properly matched connecting rod bearings. you will be surprised, OEM cranks are not that well balanced at all, and eventhough it may sound like a big project right now, i think you'll be a lot happier in the end

BlackRoseRacing wrote:like I said again, if he's replacing one piston&rod, make sure you weight match it to the one your pulling out of the car to try to keep it balanced.

i always thought he wanted to do all 4

? which one is it??

speedracer · Post by **speedracer** » Fri Apr 14, 2006 1:33 pm

all four.

rice_eater · Post by **rice_eater** » Fri Apr 14, 2006 3:12 pm

looking at your mods, looks like you've already done all the major work (no tb yet) thoroughly, so i dont see you as someone who would cheap out. imo take the time and get the crank balanced, and maybe keep an eye out for someone trying to get rid of a set of eagle rods or similar for cheap. the pistons wont melt, but bent rods wont help

BlackRoseRacing · Post by **BlackRoseRacing** » Fri Apr 14, 2006 4:15 pm

I thought he was looking to do just one...my bad

speedracer · Post by **speedracer** » Fri Apr 14, 2006 4:30 pm

I have a 60 mm lorenzo tb.

The only problem with this is putting my car out of commission for at least a week. This is my daily driver. That's the worst part of doing work on a daily driver. I suppose that's why alot of guys and girls don't mess with internals b/c all they have is the car their working on.

I'm assuming the labor costs on this will be ridiculous! I don't know, I graduate from Georgia State next month and I should have a little more time on my hands.

texasracer · Post by **texasracer** » Fri Apr 14, 2006 5:41 pm

the old hot rodding rule of thumb is 6% more power per point of compression(whole point)

speedracer · Post by **speedracer** » Fri Apr 14, 2006 6:01 pm

So say I'm 145 at the wheels, that mean if I bump it up to 10.3:1 that will give me about 8.5 whp, right? Is changing the pistons worth it?

texasracer · Post by **texasracer** » Fri Apr 14, 2006 11:28 pm

i'd say sure it is - if you're already due for a rebuild. If not...i dunno..bumping the compression up to 10.3 would limit your ability to turbo/nitro it in the future. If you are going straight N/A and you don't mind having to buy premium fuel all the time.

oh and i'd stay away from applying that 6% to WHP - i'd stick with crank horse power. no garantees, for whatever reason, that the 8hp would make it to the wheels -but yes approx 6% per compression point....

on a side note - just got back from the track and my worn out engine has fallen from a 10.5 1/8th time to a 12.1..damnit..but at least i got to see a street driven turbo mustang run a 4.44 at 177mph in the 1/8th(pro mod territory)

2GN.ORG

New pistons

New pistons

heh

heh