LB7: Pistons ?

[TrentNell]

I know we have had many of piston threads , but my main question is wether cutting is nec. or not ? i am going to build my motor this winter and i am not wanting to drop compression other than a grade "c" head gasket . My understanding on cutting pistons is it really helps to remove the ring on top of the piston because the extreme heat and cool down cycles makes that area of the piston weak and want to crack . With my truck i dont see egt's over 1150-1250 at the track and wonder if this still nec. My power goals with the truck are 700 hp . Sad part is i almost have the power i am wanting but i know the truck will not continue to take it . I dont want to put a roll bar in it so 11.49 is my ceeling for power/speed for now anyway . I know keying the cam and crank , rods ,head studs, machining ect are nec . , but dont really want to cut the pistons . If it came down to the fact that the pistons are to week un-cut i would rather run the new Mahle pistons . What are your thoughts ?

 

[othrgrl]

Get them de-lipped at least.
700hp goal on a compound setup made to support ~900???
There is always the 1/8 mile to race in if you don't want a cage.

 

[TrentNell]

Get them de-lipped at least.
700hp goal on a compound setup made to support ~900???
There is always the 1/8 mile to race in if you don't want a cage.

the question was not answered you say do it but why ? i am not saying i wont but want to know more than because i said so , and yes 700 hp out of a compound setup designed to deliver 900 hp , because yes 900 hp till the turbo's over speed but , at 700 hp that compound setup will support that hp amount properly with egt's in check . and have enough air that if 6 mounths from now i decide to go for the gusto the only thing holding me back is myself , i should have stated my question clearer why do we cut pistons and is it primarily due to heat , or is the benifit from lower compression ? what is making the piston stronger ?

 

[malibu795]

the lip tend to get hot faster then the rest of hte piston.. adb weakening the piston... removing said lips mas the piston much more resistant to heat.. and less likely to crack...

mind you if you are boreign it out it will cancle out the delip..

iirc de-liped .040 and C is ~17:1 pending deck protution.

 

[emiller3]

Are you going with a bigger cam?

 

[TrentNell]

the lip tend to get hot faster then the rest of hte piston.. adb weakening the piston... removing said lips mas the piston much more resistant to heat.. and less likely to crack...

mind you if you are boreign it out it will cancle out the delip..

iirc de-liped .040 and C is ~17:1 pending deck protution.

thanks adam , any other opinions on how a stock piston will hold up under low temps , or is egt's not a good indicator of piston temps , i would think it would be ?

Are you going with a bigger cam?

dont know yet depends on funds at the time of the build , i think with the amount of air i can push the only benifit would be spool up and maybe lower d/p ( right now only seing 62 psi with 52 psi boost ) with some more lift on the exhaust valves , we will see .

 

[emiller3]

The size of a cam will determine if you need to cut your pistons.

 

[TrentNell]

The size of a cam will determine if you need to cut your pistons.

unless you are talking about a clearance issue on deck height ? and that would go into the decision process . as stated dont know if i will cut the pistons or not just want to know why i did if i do . and still wondering if the pistons end up any stronger in a cooler invirenment , and what temp levels they can tolerate un-cut ?

 

[slowlmm]

the lip tend to get hot faster then the rest of hte piston.. adb weakening the piston... removing said lips mas the piston much more resistant to heat.. and less likely to crack...

mind you if you are boreign it out it will cancle out the delip..

iirc de-liped .040 and C is ~17:1 pending deck protution.

Hows that? and then y did you have yours delipped ? if it was useless

 

[JoshH]

If you're going to reuse your pistons, I think you can send them to Guy and have him cut/coat them. I think it would be good. In my opinion, it isn't worth the risk to chance having an issue later on. Out of all the sets Guy has sold, I don't think he has had one crack yet. I would say that's a pretty good track record. The machining and coating isn't that expensive; the biggest cost is the price of the pistons themselves. I think removing the lip has something to do with removing a stress riser and eliminating a hot spot in the piston, and it won't drop your C/R by very much. Guy told me it would only drop it by about .5 a point. That would still have you at a higher C/R than a stock LBZ.

 

[othrgrl]

Hows that? and then y did you have yours delipped ? if it was useless

He didn't mean it is useless to have them de-lipped. He meant over bore cancels the affect delipping has on the compression ratio.

Trent you are still going to have high cylinder pressure and temps. For the cost I don't see why you wouldn't have it done. As Josh said the track record on cut and coated LB7 pistons is very good - the track record on stock uncut pistons is not so good.

 

[malibu795]

Hows that? and then y did you have yours delipped ? if it was useless

never said it was useless tommy i said increase bore negate the deliped piston in the c/r area and by check clearances could raise c/r by running a thinner gasket and easily get above 17:1

example
4.055 bore
3.879 stroke
equals 50ci/819cc
for debate reasone compress it into 47cc(non deliped piston) that equals ~17.5:1

a deliped piston increase bowl volume to say 50ccs with nothing else change are CR has droped to 16.38:1 with the same bore stroke combo.
4.095 (.040 over)
3.879
is 51ci/836cc

stock piston is 47cc nets 17.7:1 c/r
deliped piston 50cc is 16.72 c/r

make sense?

 

[slowlmm]

never said it was useless tommy i said increase bore negate the deliped piston in the c/r area and by check clearances could raise c/r by running a thinner gasket and easily get above 17:1

example
4.055 bore
3.879 stroke
equals 50ci/819cc
for debate reasone compress it into 47cc(non deliped piston) that equals ~17.5:1

a deliped piston increase bowl volume to say 50ccs with nothing else change are CR has droped to 16.38:1 with the same bore stroke combo.
4.095 (.040 over)
3.879
is 51ci/836cc

stock piston is 47cc nets 17.7:1 c/r
deliped piston 50cc is 16.72 c/r

make sense?

ok gotcha miss under stood ya i guess

 

[ripmf666]

I would go with deliped and coated, Kinda like in a gasser when running the bottle remove the hot spots any sharp or blunt angles that hold heat

 

[S Phinney]

The advantage to delipping, cutting, and coating would be done for the same reasons as going with the twin turbo kit. The twins allow you to do more and have a much more durable motor at the high hp demands. It is a well spent investment.

 

[TrentNell]

thanks for all the info guys :hug: , sounds like mostly cheap insurance and i agree that it would suck to have the piston crack after the build for that reason i will probably have them cut since if it cracks it is likely to take the block out with it . for argument sake how far hp wise has some one ran un cut pistons and did they last ?

 

[JoshH]

I think Tomac ran the stock engine in his truck in the mid 700 HP range until he bent a rod.

 

[malibu795]

I think Tomac ran the stock engine in his truck in the mid 700 HP range until he bent a rod.

700-750 is the number i get on cast past that lookign into forged form what i have gatheres

NG ran cast to 1000 but runnnign ~13:1 cr

 

[Bako_Dmax]

WOuld delipping and coating LBZ pistons make them stronger or more crack resistant?

 

[LBZrcks]

WOuld delipping and coating LBZ pistons make them stronger or more crack resistant?

Nope.