Piston choices

jeffnoel · May 18, 2004

I'm looking into a stroker kit for the II, it will probably never see a track I just some want more power. My question is this what type of pistons should I get with the kit forged or hypereutectic (why)? Also how high can I bump the CR and still run pump gas either 87 or 91 octane? Like I said this will be a street motor that likely sees no more than a 2000 miles a year.

78pinto · May 18, 2004

well, if your not running or don't plan to run a power adder (nitrous, blower, turbo) then the hyperutectic would be a cheaper way to go, and they are made a bit bigger (they don't expand with heat as much) and will offer better ring sealing. Most you can go for compression on pump gas is 10.5 to one (really about 11.0:1 but why push it) but that will be premium fuel only.

jeffnoel · May 18, 2004

I was planning on keeping the CR around 9.5 or 10 so would it run on regular or still need premium?

THE COBRAMAN · May 18, 2004

Regular/premium octane numbers vary depending on what part of the country you're in.

Here, regular is 87, premium is 91, 93 (or 93.5 at a few stations)

In parts of Kansas regular is 86, and I've even seen 85 on the turnpike.

Either way, I'd say a TRUE 9.5 or 10.0 to 1 is going to require a premium fuel to avoid possible problems.

jeffnoel · May 18, 2004

Octane ratings here 87reg, 89mid-grade, and 91premium. No 93 available in CA, at least not from a regular gas station.

dmoody · May 18, 2004

jeffnoel, unfortunately your question has no easy answer. CR can be calculated, however pinging or premature detonation depends on a variety of factors. One is the material the head is made out of (aluminum or cast iron). Another factor is the shape of the combustion chamber and the location of the spark plug. The smoothness of the combustion chamber plays a role as well. Unless I were a pro I wouldn't try to go above 9.5 CR and expect to run regular gas in a 302. In my engines I generally shoot for from 9 to 9.3 CR if I want to run regular gas and be sure I won't experience pinging.

d

jeffnoel · May 18, 2004

dmoody said:
In my engines I generally shoot for from 9 to 9.3 CR if I want to run regular gas and be sure I won't experience pinging.

That sounds like good advice, thanks.

Mstng2 · May 18, 2004

On the pistons your best bang for the buck are Keith Black hyperetectic (sp?)
They are lighter than a regular piston and stronger, but not as good as forged of course. If its a street/show car don't waste the money on forged pistons etc.

Blue Thunder · May 18, 2004

I like JE forged, they are EXTREMELY light pistons. (If I recall correctly, my pistons are half the weight of the stockers?) From the loss of piston mass, plus the free-floating wrist-pins, you end up with a motor that has instant throttle response.

COBRAIIW · May 18, 2004

If you change the wieght of the pistons wouldn't the engine have to be re-balanced?

Blue Thunder · May 18, 2004

COBRAIIW said:
If you change the wieght of the pistons wouldn't the engine have to be re-balanced?

So long as the pistons are all identical in weight, I don't think so. But I'm not a balancer, I send it out. I wouldnt put any type of performance engine together without sending the parts to a balancing shop first. (except that time I tossed together a "temporary" grenade motor, and ended up running the hell out of it for a few years and about 35,000 miles. Unbalanced, badger pistons cost me about $8 each from Carolina supply company. Damn that motor held up nicely)

JonDawg · May 18, 2004

Mstng2 said:
On the pistons your best bang for the buck are Keith Black hyperetectic (sp?)
They are lighter than a regular piston and stronger, but not as good as forged of course. If its a street/show car don't waste the money on forged pistons etc.

I have KBs in my 351W. With the shaved heads, I think the CR is 9.73. I dont have any problem with 87 octane, and It runs HOT. But Im 5700 ft above sea level, so I think that plays a part too.

zwhitr · May 19, 2004

JonDawg said:
I have KBs in my 351W. With the shaved heads, I think the CR is 9.73. I dont have any problem with 87 octane, and It runs HOT. But Im 5700 ft above sea level, so I think that plays a part too.

What kind of cylinder pressure are you looking at with 10:1 -- I remember reading something like atmospheric pressure is 14.7 at sea level hence

14.7 x 10 = 147 psi ?????????? flame on

Blue Thunder · May 19, 2004

I've seen a formula for calculating effective cylinder pressure based on compression ratio, but it seems flawed.
So just from personal experience and testing, I've gotten an idea how much cylinder pressure is produced at sea level.

My brother's old ex-police car (a 1969 Ford Custom 429) had something like 11.25:1, and cranked close to 200 psi.
I have 10.9:1 in my motor, and a large solid-lifter camshaft, yet it still cranks over at 185 psi.
When I had 9.5:1 compression, it cranked 155psi, and my old F250 with a factory 360 big block (8.5:1) cranked about 125-130 psi.

How much elevation will affect it, I'm not sure.

dmoody · May 19, 2004

>If you change the wieght of the pistons wouldn't the engine have to be re-balanced?

Yes, if you change or modify any part of the rotating assembly (crank, pistons, rods) then you should definitely have it rebalanced.

>I think the CR is 9.73

At 5700 ft above sea level I wouldn't be surprised if you could run 10:1 or even 11:1 on pump gas. However at sea level I think you would experience pinging unless your engine has been significantly modded. Also remember pinging is most likely to occur at higher rpms due to exhaust scavenging.

At higher elevations, the air loses density. Your CR never changes because the calculation doesn't account for air density.

d

a351Must2 · May 19, 2004

The grind of the cam also affects your effective compression...my 11:1 compression 351W originally had a mild cam with very little overlap and my compression was 225psi. It wouldn't run right on anything short of race gas! When I was living in San Diego, I had Crower grind a custom cam designed for running high compression engines on the street. It knocked my compression down to 175psi and lets me get away with running premium from the pumps with 18 degrees of base timing. It absolutely requires premium though, I accidentally put a few gallons of regular in once and it backfired (intake) and stumbled constantly...there's enough overlap on the cam that regular ignites before the intake valve is shut.

dmoody · May 19, 2004

>The grind of the cam also affects your effective compression...

Typically the way it does this is by limiting the duration and/or lift of your exhaust valve. By doing this you leave more of the previously combusted vapors in the chamber and limit the amount of uncombusted fuel that can enter. This should slightly increase your gas mileage and help your emissions while also enabling you to run lower octane.

I'm not sure how running a lower octane gas would ever allow backfiring through the intake however? Probably something I haven't thought about or am simply overlooking...

d

Blue Thunder · May 19, 2004

Any high RPM cam profile will drop low-speed cylinder pressures, simply because of the increase in valve overlap. (that's why engines with big cams have poor low-speed torque)

Blue Thunder · May 19, 2004

dmoody said:
By doing this you leave more of the previously combusted vapors in the chamber and limit the amount of uncombusted fuel that can enter. This should slightly increase your gas mileage and help your emissions while also enabling you to run lower octane.

Poor exhaust scavenging(an intake charge which is polluted from the previous combustion) drops engine efficiency as well as power. (this is why headers and performance exhaust system add power AND fuel mileage over a stock exhaust setup)
If they ground a camshaft specifically to cause just the exhaust side to lose air flow, the engine's power levels would drop, as well as fuel mileage.

parthos · May 20, 2004

As far as what type and brand to run, I guess it all depends on how much you are willing to spend. Another consideration to think about is what are your future plans, are you going to add other things later, maybe a camshaft that is more agressive, or perhaps a sneeze of nitrous. I'm running DSS forged pistons, I'm glad I went the way I did, because they allowed "growing room" for future plans. Generally you want the lightest, yet strongest slugs that fit your budget.

My .02

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