Inliners International Forums General Discussion Engines Pistons

I have been collecting parts for a rebuild and pistons are the hardest thing to find for the unique 69 250cid Pontiac OHC L6. EGGE has cast pistons so that is one source but I may want to go with forged pistons. I have looked at Ross but know nothing about having theirs milled down to the spec I need and to have valve reliefs cut.

Any guidance when shopping for pistons?

Many top shelf aftermarket piston companies can accomodate your needs for forged pistons with no problems. Most have hundreds of forgings that can be used for many other applications, just give them a call. Here's a list of prospective manufacturers:

JE
Wiseco
Venolia
Ross
CP Pistons
Diamond

USE 307 FLAT TOP PISTONS, CHEAP AND EASY TO FIND PLUS THERE 9-1 COMP.

Thanks. I really appreciate the list you have posted. It will give me some places to call and get quotes.

The thing about this motor is the wrist pin placement which is unique. The 230cid motors (65-67) can use 283 pistons with no problem (so I've been told) but the 250cid is a weird duck.

My motor has never been apart and as of today it runs great. When I take it apart I want to do it once. As I understand forged pistons might be a little overkill but with today's fuels and an uncertain future I would rather spend the money and time now rather than later.

I keep hearing about the 307 pistons fitting but have yet to see a running motor with them in it. Are they drop in or do they need to be milled for valve reliefs?

I am putting 307 pistons in my 250. they already have valve reliefs but I think they are not in the correct location. No matter to me as I am not needing them as I don't have a high lift cam. I foud a used set of pistons from a marine 307 engine on ebay and they were in pretty good shape. One didn't make it in shipment and another one I ruined trying to get the rods off. So I had 6 good ones left and got the rods off all of them without any damage to the pistons. Original GM so they had a compression height about .020 taller than any new ones you buy so I was happy. Also no chafer around the edge like a lot of new ones have. had the block decked only 20 to give me a little pop out of the pistons. I figure they will come out of block about .005

250 AINT NO WEIRD DUCK THE 292 IS. 307 PISTONS ARE A DIRECT FIT IN A 250 AND GOING PRICE IS 50-75 BUCKS A NEW SET OFF EBAY LAST ONES I BOUGHT

I bought several sets of those new pistons off ebay and returned most of them. Problem with the new ones is they put a huge champher around the edges of them which lowers the compression. the next thing they do is reduce the compression height by about .020 compared to stock. Stock pistons in my 250 were .035 down in the hole. add the gasket of .040 and you get a squish of only .070. with the new pistons they are don another .020 to put you at about .090 squish which is really bad. Ideally you want it to be around .040....distance from piston to head. Improve power gas mileage and reduce detonation with crappy fuel. With a space of .090 you end up with a dog. So my solution was to have the blokc decked .020 and get those original GM pistons with a decent compression height and I will be getting a squish zone of around .038 which is real nice :-) What this does is give lots of turbulence to the mixture so you don't get detotanion with the cheap gas even though the compression will be higher than normal. To get the same thing with the new pistons I would have had to have .040 or more shaved off the block, which was not a good idea. Do a search on "piston squish clearance" to see how it works and how important it is for a good running engine. A tight squish allows high compression and you get more power and gas mileage.

Here is a good article on squish and quench. LOL
Sounds like a sandwich shop.
http://www.chevyhiperformance.com/tech/e...de/viewall.html

Quench, or squish area is typically the flat area on the top of the piston that's almost level with the top of the block deck. It must have a corresponding flat area on the deck surface of the head to qualify as quench.
If you look at a combustion chamber, you will usually see these flat areas, and they will have the volume of the actual combustion chamber between them. When the piston is compressing the mixture, as the piston nears the head, the flat areas on the head and piston come together and force the mixture from those areas to "squish" into the chamber, where the spark plug and burning mixture reside, so you achieve a more complete burn.
The quench area also runs cooler than the rest of the chamber / piston. These lower temperatures are where the "quench" comes from.
When properly designed, the quench areas can have a tremendous effect on the quality of combustion, and allow higher compression ratios, and due to this they are considered "artificial octane" by scientific types.
Bottom line is "properly designed, quench is good".

Since the motor has not been apart it is still running at high compression. From the factory it is 10:5 to 1. I'll bet that it is a little higher from carbon deposits. I use Shell 93 Octane and it does not ping. Everything is stock. I just started using Torco Octane Boost which claims 97 Octane at one mixture (I used this one and (I think) 105 Octane at a full mixture.

As I understand the Fel Pro gaskets that are on the market for the head gasket in this engine are much thicker and will reduce the compression level to 10 to 1. I do have three NOS GM head gaskets for this motor so I can keep it at the factory setting.

Marc

Hell you can't beat that. I see no reason for you to put new pistons in there. most new pistons really suck. However the guy over at 12bolt.com has a nice set of pistons made for the 250 and 292 engines. He has them custom made. http://www.12bolt.com/

Squish- mixture motion caused by "squishing" the mixture out of the tight space between piston and head. Encourages shorter burn time which means more power and less tendency for spark knock.

Quench- mixture cooling "quenching" in close proximity to piston top and head. Spark knock happens in the end gas (last portion of the mixture to burn). Cooling the end gas reduces tendency for spark knock.

Two totally different effects, both good, from a tight space between piston top and head surface.

Fuel/air mixtures start to react (precursor reactions) as temperature and pressure rises but before actually burning. If the reactions proceed too far before burning, the mixture will spontaneously ignite. This is spark knock. Faster burn means less time for precursor reactions, cooler end gas means slower precursor reactions, both reduce tendency for spark knock.

I plan on taking care of the "spark knock" in my engine with the help of water injection to quench the flame and also have put in a Singh groove in the head
http://somender-singh.com/content/view/7/31/
On top of that I will be putting in "plasma Igniton" and HHO to reduce burn time considerably.

Somender Singh has been awarded an US Patent for his discovery of how to gain more engine power and economy by utilizing a small groove cut into the squish area(s) of the typical Internal Combustion Engine (ICE). The Groove(s) allows gasses to expand and compress with a quality that has consistently improved fuel efficiency, lowered operating temperature, improved torque and max power as well as lowering idle speed significantly.

On plasma ignition: I am not sold on the idea. The engine specs would need to be radically different from conventional. Throughout the decades we have fought to prevent detonation, an uncontrolled explosion of the mixture. We wanted to achieve a controlled BURN and expansion of gases. With what plasma claims to achieve, is instant ignition of all mixture at once. From this follows: you cannot have ANY ignition advance before TDC. So any conventional ignition control knowledge is out the window.

This also causes a major change in cam requirements. You have to have a custom cam made to match the plasma timing. By someone who truly understands the plasma theory (if it works, mind you).

And lastly, the "burn rate" is not a burn anymore, but an explosion. You are talking about flame front/ignition traveling at the speed of light, which is much faster than the burn rate of military explosives. Light = 186,000 miles per second, while one of the fastest military high explosives (Pentrite) is a measly 4 miles per second. (I happen to know about this topic, but that's another story). You would see extremely high peak pressures, "intentional detonation" if you will, and you would blow the head off and put holes in the pistons, if you had any kind of conventional compression ratio in the engine. To make it hold together, you would have to reduce the compression ratio very dramatically, if it's even possible, and then you would extract MUCH less energy from the amount of fuel burned.


There is a reason the mixture needs to burn slowly.

For the reasons explained above, I call plasma ignition bullshit until proven otherwise. It either does not work at all as promised, or it would blow your conventional engine into millions of little pieces at the speed of light.

Jan

In the Eagle research books on improving mileage. it explains in minute detail hawe the fuel burn happens in an engine. At 2000 to 3000 RPM the burn rate of gasoline is exceeded by the piston speed After 12 degree after top dead center. So it turns out you have about 3 to 5 miliseconds to burn the fuel to actually give you power. That is not slowly. It takes about 20 miliseconds to burn up all the fuel. All that excess fuel is deliberately put in the engine to put out the fire. With HHO what you are doing is burn as much fuel as you can in that 3 to 5 milisecond window, and yes you have to retard the timing somewhat or have it right at top dead center. then instead of quenching the fire with excess fuel. you quench it with water. So the only modification you really need is the tminig being more retarded than normal. the eagle research books are well worth the read to get a full view on how the combustion works. there are actually two burns. the first one gives power the second burn is all waste. and then the rest is burned up in the catalytic converter. 10% of the fuel used as actually making power 90% of the fuel is total waste used to put out the fire so you don't burn the valves and burned of after the engine. there is enough fuel coming out of an engine to run another entire engine.. All that plasma igtnition is an extra capacitor charged up to around 400 volts that is piggybacked on the regular ignition system. you will have to prove it to yourself becaues nobody is going to do it for you. EVER. So unless you get off your ass and prove it to yourself you will just miss out. the units are not that hard to come by and you can experiment and see for your self what it does. that's the only way you will get proof. nobody is going to hand it to you. Proof is only something you can do yourself.

On raw unburnt fuel coming out of the exhaust: An exhaust gas analyzer says otherwise (emissions test measures hydrocarbons, that's gasoline, that come out of the exhaust pipe... check how little unburnt fuel is allowed).

I know what the plasma ignition is. And I know the theory. And regardless of HOW it's made, the theory is flawed. It may have some benefits, but it cannot do what they promise.If it did, your engine would blow up. Do not believe all the hype you see, try to separate marketing from fact and laws of physics. No more to say about this topic, moving right along.

When you follow the links in the Somender Singh link it leads you back to a repair and mailorder buisiness in Cleveland, but gives a lot of misinformation and is about as useful as snake oil. I still remember the superchargers from JC Whitney that you put under the carbs and the mystery carburetor that gave 100 mpg.

HHO = magic beans

Right I don't go by the hype either I spend a lot of time on forums where guys actually build and try these things on their cars themselves and do extensive experiments on the workbench. Learning by hands-on experiments and comparisons. Its only hype if you haven't done you your homework and not put a wrench to it.

maybe you have a virus or something when I click on the link I go to his own website where it shows how he made the grooves and the results from many people that tried it.

The Singh links seemed to work for me, I got to his grooves pages just fine. It is one of the messiest web pages I've seen in years, absolutely painful to try to read and find the good stuff. Slow to load pictures. (I am on a 15meg line).

I sent an email off to Ross Pistons.

Anyone else I should contact from the list?

I believe the piston offeted by me will work in your application, just fine.

tlowe has good stuff and knows his stuff

I agree I have purchased from Tlowe as well. He has quality parts. he has great side covers.

I have you on the list. The only problem is that I am not sure about boring the block if it doesn't need it. This motor is not going to be raced and I am not looking for more power. I am keeping it as stock as possible and I am not sure boring 40 over is necessary?

I am trying to pick up another block (1bbl) that would be available for power upgrades.

I was able to get the 69 OHC 1bbl motor. When using your Ross pistons what compression ratio should I expect with a 1bbl head? What about a 4bbl head?

I appreciate any guidance.

I do not know the CC of your head.
Can you tell me stock
Deck height
Stroke
CC of head

THE BAD THING ABOUT FORGED PISTONS FOR STREET USE IS YOU HAVE TO RUN MORE SKIRT CLEARENCE. FOR THE STREET A CAST OR HYPERUTECTIC ?/ PISTON IS PREFECT. ALSO UNLESS YOUR GONNA RUN OVER 4K PRM'S ALONG WITH AFTERMARKET RODS/CRANK THE FORGED PISTONS ARE OVER KILL AS ENGINE WILL BREAK AT WEAKEST POINT MOST LIKELY THE RODS. JMO
P/S IF IT WAS ME I'D RIN A SET OF FLAT TOP 307 SBC PISTONS......

Since a 230/250 Pontiac block is completely different than a 230/250 Chevy block, you are assuming they have the same deck height.....better confirm first before you purchase pistons.

For the 250 OHC the block, crank, rods, pistons are all the same. The heads are different between a 1bbl and a 4bbl. The spare OHC I have is a 1bbl BUT I may want to freshen up the pistons in it. Cast pistons are around $350 and I think the forged are $500 so not that much more. I don't think the block needs to be bored (visual check only) but I need to measure it.

I want to run these engines for another 45 years LOL and I don't think the old AL pistons will make it with today's fuels or into the future.

Does this link give you everything you need?

1969 Firebird / AMA Specifications
http://firstgenfirebird.org/firebird/1969/ama/ama_page3.html#general

Nope.
Only the stroke.

I see clearance for the OHC pistons was .003-.005. My Pistons are .004.
They run quite in mine. Far lighter than stock too. That also makes it easier on the rods. If hotroding the OHC, consider using Chevy rods, they are made of better material.

Tom

Deck Height = 9.150
Stroke = 3.53
cc 1bbl head = 80
cc 4bbl head = 65

I hope this is what you are looking for.

Same dimensions as a 250 chevy for the DH and stroke.
80CC head gives 8.64 comp ratio
65CC head= 10.15 comp ratio
That is assuming the block will not be decked and have a .015DH( piston in hole.

Go to this website to calculate
http://www.rbracing-rsr.com/compstaticcalc.html

Assume a .041 gasket, 3.915 bore, 3.53 stroke.

I'd tell you to use the 80CC head, cut .030 from it. Cut the block appr .015 to zero deck it. Compression will be about 9.28.
This is with my 250 Ross pistons with 5.7 rod.