Which pistons do I buy?

[Stevesr]

I'm doing a 3400 top end swap on a 1991 3.1 and I want to use 3100 pistons. What year/model do I tell the parts man????? Does it matter?

SteveSr

[geoffinbc]

Nope they are all the same for 3100's

[Stevesr]

Ok,one more question. RacerX11 had said in a preivous thread to use a certain Comp camshaft (I don't remember which one) with the 3.1 bottom end and 3100 pistons, will there be a valve to piston clearance issue? I have the 3400 top end for my swap.

SteveSr

[RacerX11]

I have tried the Crane 272 cam, with gen3 heads and 3400 pistons, and I had the valves kissing the pistons when I clayed the engine (with both 1.5 and 1.6 ratio rockers). There are people using this combo with no problems, so I don't know what my problem was. I originally thought I may have had a faulty timing set that indexed the cam wrong, but I tried 1 new and 3 used timing sets, and they all came out the same. I switched back to a milder cam similar to the Crane 262. There are people running the Crane 262 and 272, as well as the Comp 260. Any of these are decent cams, with the 272 being the wildest of the three, although still streetable. This is the one I would go with for your engine.

You might be better off going with the Camaro iron head pistons, to get more compression...this would be a nice combo with the 272.

Marty

[geoffinbc]

How much "kissing" are we talking? I dont remember what the recommendations are but there is only a very small amout of clearance required.

[RacerX11]

They were actually touching, enough to leave a little mark on the piston, but not enough to keep the engine from turning over. Way too close. I think you are supposed to leave at least 0.060" clearance for mechanical lifters, 0.120" for hydraulics. Definitely more than I had.

Marty

[Stevesr]

Here is what you suggested to me a while back Marty ......

SteveSr

Mild combo:

3100 pistons (9.6 CR)
3400-style heads/intake (1996+ 3400, or 2000+ 3100)
Mild performance cam (Crane H260 or equivalent)
62mm TB
Headers

This should get you an easy 200 HP. The late 3100?s (2000+) with the 3400 top end stuff were rated 170-175 HP stock. Throw in the above parts (mainly heads) with a good ECM tune, and 200 should be easy. This combo would be very streetable.

For a more aggressive build, add the following:
3.1 Camaro iron-head pistons (~11.? CR)
More aggressive cam (Crane H272 or equivalent, maybe solid cam)
Ported heads and intake

Two of the biggest bottlenecks in the 3100/3400 engine are the roller cam (lack of aftermarket grinds), and the lack of headers. With your 3.1 bottom end with flat tappet cam, your options are plentiful, and you already plan on custom headers. With the more agressvie build, you should be able to get ~220 HP. This combo would still be very streetable, and would make a nice engine for the 914 chassis.

Marty

[Stevesr]

One other thing.... I don't want to increase the C/R too much,I'm depending on my stock Porsche starter to turn this engine and it was designed for a four cylinder....

SteveSr

[RacerX11]

The more aggressive cam will bleed off compression, giving you a dynamic compression ratio lower than the static numbers, and easier on the starter.

Marty

[Stevesr]

I was thinking of using the mild combo you suggested,I have a 2000 top end from a Grand Am I got from Brad. Do you think the 62 MM throttle body will give that much of a gain? THe bigger opening is only an advantage when the throttle plate is wide open right?
Also will I need custom push rods?

SteveSr

[RacerX11]

With the headers, it will be flowing a lot better, so the 62mm TB won't hurt. I'm not sure exactly how much you will gain. If you port the heads the TB will defineitely help more. You will need custom pushrods with the flat tappet cam and 3400 heads, assuming you are using the 3400 roller rockers.

Marty

[Stevesr]

Marty, the rockers I'm not sure about,I can look at those tonite and get back in the morning. Brad says the heads are ported.
Can I make a adjustable pushrod tool to find the length?And if the rockers are not roller tip?
As far as headers go I'm going to follow Colin's lead and get some bends and start welding.

[RacerX11]

The stock 3400 rockers are roller fulcrum, not full roller. The don't have any adjustment, so the proper length pushrod must be used.

Adjustable pushrods are available, but I remember Brad saying one of the pushrods is an pddball size, and hard to find an adjustable version. I haven't tried this yet, but I had the thought of taking a stock pushrod, cutting it in the middle, and use a die to thread the ends of both halves...then get a standoff nut from the hardware store to connect the pieces...viola..DIY adjustable pushrods. I'm not sure how easy it will be to cut threads into the hardened metal.

Marty

[TurboZ24]

I did that too make an adjustable pushrod too measure the length I needed, but I don't think anyone would recommend actually running a motor with adjustable pushrods (if that's what you meant).

[RacerX11]

I wouldn't run the engine with it either...I was just talking about the tool to find the correct length pushrod to buy.

Marty