leakdown and forced induction
05-14-2004, 11:42 AM
#46
Three Wheelin'
Originally posted by rob rossitto
that GTS piston is hardly state of the art anyway - and just because it's forged doesn't mean it's automatically a GREAT boosted motor piston (head dissapation is better w/a step down style), so to go through all that just for something used that weighs 300grams more than a new, properly spec'd slug for boosting is perhaps not so cost effective or optimal in the long run......just my opinion...
bottom line is (IMO), if you are turning your own wrenches, used stuff is fine - if you've got the time...otherwise doing a through job will help increase the odds you won't be doing it again for a bit longer (hopefully)...
that GTS piston is hardly state of the art anyway - and just because it's forged doesn't mean it's automatically a GREAT boosted motor piston (head dissapation is better w/a step down style), so to go through all that just for something used that weighs 300grams more than a new, properly spec'd slug for boosting is perhaps not so cost effective or optimal in the long run......just my opinion...
bottom line is (IMO), if you are turning your own wrenches, used stuff is fine - if you've got the time...otherwise doing a through job will help increase the odds you won't be doing it again for a bit longer (hopefully)...
It really boils down to the questions, how much money are you willing to spend, and how do you plan on running the car?
If money is no consideration, then go all the way and pay dearly; just look at Rob's observations.
If cost is a consideration, then I'll stick to what I said earlier, go with the GTS block and machine the existing pistons for $35-$50 a hole (..providing the pistons and bores are in good shape which is easily checked). You'll also have the engine back in the car and running before the proposed custom crank is even finished. Not only that, it will be comparitively cheap! cheap! cheap!, and you'll be driving and enjoying your car as opposed to waiting around for at least half a year to build the engine.
As for the GTS pistons, I do not think it's an issue that they're not a state-of-the-art design. I have not seen one, but let's suppose they look like an S4's cast piston. The S4 piston is a good design for a boosted engine for several reasons, one of which is the distance from the piston crown to the top of the first compression ring, or in other words, it has a thick top ring land which provides greater thermal mass. Because of this additional mass the piston runs cooler, thus the piston will be stronger.
Now toss in the fact that it's a forged piston whose material is denser than a cast one (..runs cooler) and has a higher tensile strength, and you have a rock-solid foundation to build upon without spending big bucks.
If the application will be strictly a street car with the occassional WOT blast, this combo will more than suffice since that's what I'm running using the factory cast pistons with no compression ratio mods.
If this application is intended solely for a supercharged road-racing car, then I would no doubt consider the 'cost is no object' solution. Even so, I think that I would run the piston mods as above with a Cometic gasket and see what happens.
Keeping the factory RPM redline would allow one to keep the existing rods and crank; in this case, it is the pistons and head gasket that bear the brunt of supercharging, and those items are already accounted for.
05-14-2004, 02:00 PM
#47
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lag has a good point or two (as usual!)... the stock cranks and rods are quite robust, and do not seem to be failing no matter how many miles are on the motors -as long as they don't see high revs or g's on a track...
spec'ing and getting custom parts IS a long drawn out PITA...and decidedly not the short term least expensive option...I'm doing it cause my stock stuff was trashed, and I figured since I'm keeping the car and have done everything else that way - no point in stopping now...it's not necessarily the best option for every situation... when gas is outlawed for hydrogen fuels - I'll still be driving my dinasaour powered shark - even if I have to drill a hole in my back yard for oil (assuming we all aren't under water from global warming 1st)...
if you go custom, be prepared to get up to speed technically - or else!!! not too many of these running around w/custom stuff inside...even less builders...so you'll need to help w/details a bit...rennlist is invaluable for that...
theoretically, the big compression ring and a dish will certianly be much better than a domed piston w/thin rings...forged is better than cast in an HP application...how much better? we just don't really know!!!
a lot of your decision will be based upon subjective opinions...there just aren't enough hard data/trends to base a solid analysis - so there is a bit of wiggle room in all of this...even the cast vs forged issue - those stock pistons are massive and quite strong....we just don't have enough quanititive data to say "it will last 2.1 yrs on average in this situation"...
a sane compromise might be just getting new/larger dia. pistons/nicom, and use the stock bottom end w/fresh bearings, rings, etc....run more boost w/confidence/be up in a jiffy w/minimal expense for a zero time bottom end...
gotta admit - after spending $14K on my motor, my 85Shark will still only fetch about $9K....so from a cost/benefit - custom is dumb....
but, if you are going to keep the shark - long term reliabilty w/all new parts can be justfied (sort of)...
no matter what, make sure YOU are comfortable w/the choices, since you 'll be the one turning the key - and have fun!!
spec'ing and getting custom parts IS a long drawn out PITA...and decidedly not the short term least expensive option...I'm doing it cause my stock stuff was trashed, and I figured since I'm keeping the car and have done everything else that way - no point in stopping now...it's not necessarily the best option for every situation... when gas is outlawed for hydrogen fuels - I'll still be driving my dinasaour powered shark - even if I have to drill a hole in my back yard for oil (assuming we all aren't under water from global warming 1st)...
if you go custom, be prepared to get up to speed technically - or else!!! not too many of these running around w/custom stuff inside...even less builders...so you'll need to help w/details a bit...rennlist is invaluable for that...
theoretically, the big compression ring and a dish will certianly be much better than a domed piston w/thin rings...forged is better than cast in an HP application...how much better? we just don't really know!!!
a lot of your decision will be based upon subjective opinions...there just aren't enough hard data/trends to base a solid analysis - so there is a bit of wiggle room in all of this...even the cast vs forged issue - those stock pistons are massive and quite strong....we just don't have enough quanititive data to say "it will last 2.1 yrs on average in this situation"...
a sane compromise might be just getting new/larger dia. pistons/nicom, and use the stock bottom end w/fresh bearings, rings, etc....run more boost w/confidence/be up in a jiffy w/minimal expense for a zero time bottom end...
gotta admit - after spending $14K on my motor, my 85Shark will still only fetch about $9K....so from a cost/benefit - custom is dumb....
but, if you are going to keep the shark - long term reliabilty w/all new parts can be justfied (sort of)...
no matter what, make sure YOU are comfortable w/the choices, since you 'll be the one turning the key - and have fun!!
05-14-2004, 02:32 PM
#48
Drifting
Thread Starter
cheers you guys, i appreciate your opinions and advice as im finding the decision making at this point very complex - with so many variables to consider not the least of which are time, money and available expert assistance!
