So I know this has been gone over a few times, but I need a final word on this............
I cannot, at this time, afford forged pistons for my supercharged 250. I am only going to be running a pretty stock engine with a cam and the supercharger. Factory 5.5lbs of boost (which only comes on above 3/4 throttle).
Which pistons would be better, stock cast, repro cast or hyperutectic?
I am pretty hard on my vehicles. I so desperatly want the inline back into my car that I cannot wait for the funds to buy forged pistons. I will drive the car this way until I have saved enough to pull the engine and build it the way I want it!
Opinions, again. PLEASE.
Cheers!
Factory 5.5lbs of boost (which only comes on above 3/4 throttle).
Don't understand.
What factory?
No supercharger is throttle sensitive. Do you mean an Eaton with a high vacuum bypass?
You do realize that the forged pistons you can't afford will cost twice as much the second time?
Stock cast,if those are your only options.
MBHD
At 6 psi - cast is fine IF you do not rattle the engine. Boost timing retard or methonal/water injection are the 2 options and you better never run out of injection fluid.
After reading Leo's book... he mentioned that for street use and not more than 6psi, factory cast pistons are ok.
panic~ I am using an eaton m90 with the high vacuum by-pass, as you said. All the reading I have done on these leads me to believe that boost will only build once the bypass valve closes... which only happens at around 3/4 throttle and up. As for the 5.5psi boost, everything I read on the 3800 performance forums say that the factory dia. pulley creates this amount. PLEASE, correct me if I am wrong... it has happened in the past (me being wrong)!
I have been in contact with Tlowe about a vacuum canister that will retard timing. I am not 100% clear on the operation of this unit, but I have spoken with another person who is using the same thing on a similar engine. He is very happy with it.
If you're using the stock pulleys the blower speed will be the same, but the boost may not match. Your cam, ports and valves don't match the 3.8, your 250 is slightly larger than the 231" 3800, which may change the boost level, as will how big your manifold plenum is.
Again: that valve is vacuum-sensitive, when it closes doesn't depend on throttle opening or RPM but on falling vacuum, which is affected by gearing, chassis weight, exhaust backpressure - long list. I assume this a draw-through with a carb adapted to the Eaton intake? How big?
As far as the vacuum canisters,those retard timing all over the place. Not one will match the other.
My friend is an expert in this field as he is an expert on Corvairs,which is where they originated from.
Same person that is a distributer & expert in these J&S knock retard set-ups
http://www.jandssafeguard.com/ ,Highly recommended,but pricey I use on on my Syclone & has saved my engine on more than one occasion..
I have stated before,a vacuum canister cannot hear detonation,but the J&S unit does & retards timing accordingly w/out sacrificing too much performance.
I am a firm beliver in modern electronics to help save/protect your engine from detonation.
Two cents thrown.
MBHD
I will be using a carter 600cfm for my carb. I am making a manifold which will be the similar to the clifford one, only with a larger plenum. I will also be making an adaptor for he carb... basically a 90 degree elbow for the carb to sit on top of.
Hank, I will look at the link you posted when I get a few more minutes... I am supposed to be working now.
The J&S is excellent, the only practical vacuum retard method is to try to anticipate how much you need to save the engine, and find or make one with this setting - a lot of work.
What carburetor?
I have two... a carter 6oo or a holley 500 2 barrel. I'd rather use the carter because I find them easier to work with. I konw it is too much carb, but it is what I have to work with for now. Eventually it will be efi or tbi.
Rough prediction:
250" * 90% VE = 430 CFM N/A.
5.5 psi boost @ sea level is 1.374 pressure ratio.
You need 591 CFM to get 1.5" Hg vacuum WOT, any less will increase vacuum a bit.
What are you going to do about the power system?
The vac can I make will pull 10 degrees out with boost. Can't remember right now how much it adds with vacuum.
As a side note, I was able to but off ebay a new set of .060 over forged TRW dished pistons for $41.00. These are not state of the art high tech pistons but I'm going to give them a shot. I'm just saying affordable pistons are out there.
What are you going to do about the power system?
Power system?
Power valve, metering rod.
I didn't realize I needed to change any of it. I figured with it beging a draw through it would work the same as normal... was that a wrong assumption?
You should manifold reference your power valve on a draw through. You can do it yourself with patience and the right tools.
I was just on the turbo forums... there are guys on there that claim to be running upwards of 30psi boost on CAST pistons. Obviously they would have some serious electronics for detonation suppression, but 30psi! Holy crap! There was a youtube link for a guy who made a 1/4 mile pass in his dart(?) making 25psi.
I like to imagine that my lowly 5-10psi would be handled, but who can say for sure.
food for thought... If I am spending the serious money on detonation suppression equipment, to ensure the life of my engine, are the forged pistons REALLY required? I understand that they are stronger and will handle the greater pressures, but if it is the detonation that has to be worried about...
I'll bet they weren't running 30psi on a cast piston in a combustion chamber designed in the 50's though. And I'll also bet they were stretching the truth a bit on the amount of boost they can handle. It's the internet. If for no other reason, forged pistons give you some engineering margin if your tune isn't perfect out of the box or if something goes awry. Multiple people have recommended forged pistons if you want to do it right, once. Take it from me, I've cut every corner known to man and paid for it each and every time. Wait a little while and do it right. For every 25# of boost on cast piston story, there are 19 you don't see about melted pistons the first time they hit the boost without the right tune, right ignition system, or inadequate fuel delivery.
You only have to be wrong once.
If a piston breaks loose from the pin you will lose way more than just a cheap piston. But I'm having the same problem thinking about paying a thousand dollars plus for good rods. I understand where your coming from. Will they hold and is it worth the risk?
as per Blower Drive Srvice compression chart at 6.5 compression then you inject 24# of boost their (chart doesn't go any higher) you end up with an enine that has 17-1 compression at full boost. thats only at 24# their are varibles such as cam timing leeky valvs that figure into this, but this sound like a lot of BS. if your thinking of running a blower get one of bds's catologs it has alot of info on drive raios, boost, compression ratios and a lot more
It's nice that they take the time to put this stuff up there, but even better if it were correct.
Their "math" is simply multiplying the static (mechanical) compression ratio by the pressure ratio, using 14.7 as ambient atmospheric pressure. (24 boost + 14.7 ambient) ÷ 14.7 = 2.63 PR. 2.63 × 6.5:1 = 17.1:1.
Except, that's not what happens in the engine.
Their purpose is to predict the equivalent static CR with the same knock characteristics as a normally aspirated engine, and they simply made this up.
The compression ratio
never changes regardless of boost, the cylinder pressure changes, and the actual math is quite different.
Read my article:
http://victorylibrary.com/mopar/blower-engine5.htm#ECR
If you do not rattle or lean out the engine - at 8 psi your fine with cast pistons/rods.
Either one can spell a fast end to an engine.
I ran 9 PSI on a 10:1 vortec 4200 without any issues BUT I ran efi with timing control so I could retard the timing under boost.
I did rattle it a bit until I got the timing dialed back enough.
I run 22-25 psi range on my Syclone on stock very brittle cast hypereutectic pistons.
I use 91 octane fuel.
I use an Aquamist 1S methanol injection system,soon to go with alky control.
http://www.alkycontrol.com/I also use a J&S safegaurd
http://www.jandssafeguard.com/The Syclone has compression of 8.4:1 .
Guys have run as much as 27-30 psi on occasion w/there stock Sy/Ty engines.
With out using the methanol injection on 91 octane fuel, there would be no way I could run 22-25 psi of boost pressure,+ the J&S is extra insurance to hear & control detonation better than the stock set-up.
As I have always said,you cannot always hear detonation,so why risk your engines life?
Get a good electronic devise that can hear & control your engines detonation.
MBHD
I have a new set of pistons 30 over for a 250 chevy. They are
forged with a dish , good for turbo.They are new TRW.The price is $300.00 for the set.Bob.
Do those pistons have a large chamfer on the top edge,which make the top ring exposed more to detonation?
I have the same set if it is.
MBHD
No.These are not TRW L 2396. These are TRW L 2289 .030 over.They are flat with a .073 deep with 8.5cc cup.They do not have a chamfer.Comp. distance is 1.655 and they use 1/16 x 1/16 x 3/16
rings.These are press fit.Sorry for the delay on answers, but my
computer was damaged last week from a thunder storm.Bob
Ok,
Do you have any pics for people to see what they look like?
Thank you
MBHD