Forged or Cast pistons ?
Discussion
Both the XE and GJ (ST170) engines were developed by Cosworth ;-) I did a lot of the base engine dev on the ST (feels like a LONG time ago now!) XE was before my time 
it's also worth adding that the reason you can get away with std pistons even in tuned engines is that typically Pmax (peak cylinder pressure) doesn't actually increase linearly with BMEP (higher filled cylinders take longer to burn, so pressure trace gets longer (burns over a longer crank angle) rather than too much taller. This is especially so for "low speed" turbo charged engines, where the ignition angle is required to be retarded, resulting in the mean burn being at a larger cylinder volume. The issue of course is if you get any abnormal combustion (det/Pre-ignition etc) or over-rev conditions, where suddenly the pressure or inertial loading on the piston can quickly exceed the std limits and cause failure.
it's also worth adding that the reason you can get away with std pistons even in tuned engines is that typically Pmax (peak cylinder pressure) doesn't actually increase linearly with BMEP (higher filled cylinders take longer to burn, so pressure trace gets longer (burns over a longer crank angle) rather than too much taller. This is especially so for "low speed" turbo charged engines, where the ignition angle is required to be retarded, resulting in the mean burn being at a larger cylinder volume. The issue of course is if you get any abnormal combustion (det/Pre-ignition etc) or over-rev conditions, where suddenly the pressure or inertial loading on the piston can quickly exceed the std limits and cause failure.
Kokkolanpoika said:
http://www.rosspistons.com/information/automotive_...
My piston setup: Bore is 3.779"/96mm
INSTRUCTIONS ‘B
Bore size 3.476 – 4.499......................................... .004
Huge?
4 thou is not huge.My piston setup: Bore is 3.779"/96mm
INSTRUCTIONS ‘B
Bore size 3.476 – 4.499......................................... .004
Huge?
Which it is, it's a marine engine.
The OEM of this engine specified 7-9 thou clearance using the original Wiseco pistons (They were a custom piston from Wiseco just for this motor).
(Original motor spec listed here FWIW - but not the clearances)
http://www.donzi.net/forums/showthread.php?55352-M...
Block and crank are still the same - most other bits are different.
Going 60 thou over, I used diamond pistons, and they specified 0.0055 as the baseline clearance, adding 0.0005 to 0.001 for closed cooled marine use.
I'd need to check the manual, but I think that the std clearances on a 502 marine big block are still spec'd at 0.005-0.007.
Perhaps it's an urban myth, but it's certainly well documented that a marine BBC needs to run substantially bigger clearances than an "auto" motor.
The OEM of this engine specified 7-9 thou clearance using the original Wiseco pistons (They were a custom piston from Wiseco just for this motor).
(Original motor spec listed here FWIW - but not the clearances)
http://www.donzi.net/forums/showthread.php?55352-M...
Block and crank are still the same - most other bits are different.
Going 60 thou over, I used diamond pistons, and they specified 0.0055 as the baseline clearance, adding 0.0005 to 0.001 for closed cooled marine use.
I'd need to check the manual, but I think that the std clearances on a 502 marine big block are still spec'd at 0.005-0.007.
Perhaps it's an urban myth, but it's certainly well documented that a marine BBC needs to run substantially bigger clearances than an "auto" motor.
stevieturbo said:
Wiseco suggest similar clearances. 6 thou just sounds huge unless it's going to run extreme duties for extended periods.
/\/\/\/\/\ Could that have anything to do with the fact that marie engines tend to spend most of their life at high load and reasonably high engine speeds compeard to an auto engine?
Its not really related but i think one of the US mags (hot rod i think) took a standed iron block 4.8ltr LS engine and with a few mods (stock pistons rods and crank as well as top end) and two 76mm turbos made over 1200Bhp!
Its not really related but i think one of the US mags (hot rod i think) took a standed iron block 4.8ltr LS engine and with a few mods (stock pistons rods and crank as well as top end) and two 76mm turbos made over 1200Bhp!
MattYorke said:
Exactly right.
Think some of this is actually sinking in! Lol I do like max's and puma's posts but they do often need reading more than once! I recently saw a pic of a twin turbo BBC engined boat. The owner posted a pic of the headers glowing cherry red. He said that they are like his for the whole race (between 30 min and 1 hour) so I guess the engines are under a fair bit of thermal load. Edited by MattYorke on Friday 4th November 13:52
chuntington101 said:
Think some of this is actually sinking in! Lol I do like max's and puma's posts but they do often need reading more than once!
Yeah. Max's so you can parse your way through all the spelling mistakes and mine because they contain so much incredibly detailed information. Bazinga! 
Kokkolanpoika said:
http://www.rosspistons.com/information/automotive_...
My piston setup: Bore is 3.779"/96mm
INSTRUCTIONS ‘B
Bore size 3.476 – 4.499......................................... .004
Huge?
Well clearly the spec sheet is bMy piston setup: Bore is 3.779"/96mm
INSTRUCTIONS ‘B
Bore size 3.476 – 4.499......................................... .004
Huge?
ks. If you have a bore size of 4.499" you need 4 thou clearance but if it's 1 thou bigger at 4.500" you need 6 thou????Hogwash. The clearance must by definition be a function of the exact bore size multiplied by a factor. It can't be an increment that leaps up by 2 thou at a given size.
Plus they use 2618 which I bet you don't need.
I'd suggest you could manage very well at 3 thou for most street applications and even less with 4032 which you should have used anyway.
chuntington101 said:
/\/\/\/\/\ Could that have anything to do with the fact that marie engines tend to spend most of their life at high load and reasonably high engine speeds compeard to an auto engine?
Think about it. Every engine ever made has to run clearances that can cope with full power conditions or it will seize. Whether it spends most of its life below that is irrelevant.Every Production car engine made over the last 15 years will have passed a high speed/load durab test (usually 180hrs at peak power, continuously !!). Any issues with piston profiling or bore distortion would have shown up at that point!
(or on the piston scuff test (typically 10-15degC cold start (sometimes subzero depending on manufacturer), 5sec idle, then straight to WOT and peak power rpm, ouch!)
(or on the piston scuff test (typically 10-15degC cold start (sometimes subzero depending on manufacturer), 5sec idle, then straight to WOT and peak power rpm, ouch!)
Max_Torque said:
The homologation engine made 170ps..............
That is all........ ;-)
(early dev engines, before FoMoCo purchasing got involved and told us everything was about 3x too expensive, made an easy 170bhp)
And just to further derail the topic could I tap your knowledge of the ST170 a little more?That is all........ ;-)
(early dev engines, before FoMoCo purchasing got involved and told us everything was about 3x too expensive, made an easy 170bhp)
Does the de-phaser for the variable cam timing get adjusted continuously (i.e. giving a variable amount of advance/retard as RPM changes) or does it simply switch from one extreme to the other at a certain RPM? I'm pretty sure it's the former but would be great if you could confirm this.
So then at what point dose a aftermarket forged piston become required? I know it will depend on the strength / quality of the OEM items, but is there a suggested % increase over stock BHP / Torque?
Also why aren't forged pistons machined the same as stock pistons (mainly this ring to help stop heat getting in the skirt)? Dosen't sound too hard to accomidate....
Chris.
Also why aren't forged pistons machined the same as stock pistons (mainly this ring to help stop heat getting in the skirt)? Dosen't sound too hard to accomidate....
Chris.
They do seem variable took me a while to find one that felt it had all it's pony's alive and kickingWhen the Swedish Rototest institute tested an ST170 on their hub dyno it only made the corrected equivalent of 156 bhp, 138 ft lbs instead of the 170bhp/145 ft lbs claimed. I've seen other low results on a Dastek dyno.
I'm wondering if the hydraulic VCT system is a bit dodgy and doesn't always provide the optimum cam timing at peak power on some engines. I'm not 100% sure how it works internally but it seems to rely on oil pressure to move the cam sprocket and any miscalibration of that could perhaps cause considerable timing errors. I can't think of any other component which would be so variable in quality from the factory.
I'm wondering if the hydraulic VCT system is a bit dodgy and doesn't always provide the optimum cam timing at peak power on some engines. I'm not 100% sure how it works internally but it seems to rely on oil pressure to move the cam sprocket and any miscalibration of that could perhaps cause considerable timing errors. I can't think of any other component which would be so variable in quality from the factory.
Pumaracing said:
When the Swedish Rototest institute tested an ST170 on their hub dyno it only made the corrected equivalent of 156 bhp, 138 ft lbs instead of the 170bhp/145 ft lbs claimed. I've seen other low results on a Dastek dyno.
I'm wondering if the hydraulic VCT system is a bit dodgy and doesn't always provide the optimum cam timing at peak power on some engines. I'm not 100% sure how it works internally but it seems to rely on oil pressure to move the cam sprocket and any miscalibration of that could perhaps cause considerable timing errors. I can't think of any other component which would be so variable in quality from the factory.
Most variable systems have sensors on the camshaft for feedback as to it's position. So any cam timing changes are made in closed loop so they should be meeting their targets. Although can't say if every variable system does thisI'm wondering if the hydraulic VCT system is a bit dodgy and doesn't always provide the optimum cam timing at peak power on some engines. I'm not 100% sure how it works internally but it seems to rely on oil pressure to move the cam sprocket and any miscalibration of that could perhaps cause considerable timing errors. I can't think of any other component which would be so variable in quality from the factory.
stevieturbo said:
Most variable systems have sensors on the camshaft for feedback as to it's position. So any cam timing changes are made in closed loop so they should be meeting their targets. Although can't say if every variable system does this
This is something I have been trying to find out about the ST170, AFAIK the other engines that use the Ford VCT system do have feedback from the cam sensors. An unreliable source insists they work purely in open loop, the ECU applying a certain PWM duty cycle to the solenoid to achieve the required timing, but without feedback I can't see how that could ever be remotely accurate or repeatable over the life of the engine.chuntington101 said:
Also why aren't forged pistons machined the same as stock pistons (mainly this ring to help stop heat getting in the skirt)? Dosen't sound too hard to accomidate....
Chris.
Because it's almost slicing the piston in half and they don't want the top to come off Chris.
The thermal slot in some cast pistons only stops above the pin bosses.Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff