Worthwhile Trouble?

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Worthwhile Trouble?

Postby Max2503 » Fri May 19, 2017 9:38 am

Currently I have dropped a 16V bigport with ST ITBs run by an aftermarket ECU into my car. Now before I take it to get mapped, there is a possibility of some 260/252 cams & Smallport Pistons I could get hold of. Is this going to see some gains and worth the trouble?

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Re: Worthwhile Trouble?

Postby aukword1 » Fri May 19, 2017 1:58 pm

You would definitely see some gains with the new cams. Are the smallport pistons any different than the bigport ones? You might as well wait to get it tuned until you figure out what mods you want to do. Tuning isn't cheap.

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Re: Worthwhile Trouble?

Postby jondee86 » Fri May 19, 2017 5:05 pm

Max2503 wrote: Is this going to see some gains and worth the trouble?

To change to smallport pistons you would need to have con rods that take a 20mm
piston pin. Early 3-rib bigport engines have 19mm piston pins so that will be your
first problem. Smallport cranks have a larger bigend journal, so it is not just a
question of installing smallport pistons and rods.

Second issue is the wisdom of installing secondhand pistons (I'm guessing that you
are talking about used pistons and not new pistons) into a used engine. The problem
with fitting used parts is that unless you measure everything very carefully, the result
may not be good, as you can end up well over the factory wear tolerances. You can't
tighten up bore tolerances to suit worn pistons. So when rebuilding an engine the
usual procedure is to bore the block to the first oversize and fit oversize pistons.

Never the less, more compression is always good and the smallport engine has a
nominal compression ratio of 10.3:1 which is a nice step up from the stock bigport.
If you do figure out a way to raise the compression then those cams will give you
a modest improvement across the rev range. You will need to check valve clearances
and re-shim where necessary. If you stay with the factory bigport the engine will run
a little better with those cams, but the result will be barely noticable on the street.

EDIT: Above comments assume that the cams you are considering have near stock
lift (7.56mm) and a bit more duration. These will let your engine rev out better with
a small improvement over stock at high rpm. Cams with similar duration and more
lift (>8.3mm) will deliver a noticeable increase in performance thru the midrange
to top end. Added compression will compound the improvement.

I'd suggest that you think seriously about where you want to finish up with your build,
and decide if you want to try making random parts work, or if you want to get your
car driving with what you've got, and put the money towards a full rebuild with new
parts a bit further down the track. With an aftermarket ECU I'd recommend cams
with at least 272 duration and some decent lift, but again, I don't know if you have
to meet smog requirements.

Maybe some of the above will help you to decide if it is worth it :)

Cheers... jondee86
The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore, all progress
depends on the unreasonable man.

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Re: Worthwhile Trouble?

Postby SgtRauksauff » Mon May 22, 2017 12:49 pm

I've got a similar build:
bastard largeport (7-rib bottom end, largeport head), with first-oversize OEM pistons (prism gsi from a gm dealer, ~200 bucks).
0.8mm TRD head gasket.
home built over-axle 2" exhaust, mandrel-bent, with cat, resonator, and muffler.
I can't remember what I had on the dyno after that build, maybe 108whp/98 lb-ft or something close to that.

Then later, I installed:
TODA 264int/256exh cams.
new valve springs.
random 4-1 header.
then I pulled 133.0hp after 12 dyno runs, the best result was add 4psi of fuel, and advance 4° distributor timing I have adjustable cam gears, but the best power was found by leaving them timed exactly at 0°.

couple years later after beating it on track and autoX events, on a different dyno (Kelly-Moss Porsche shop), it showed 122hp, 105 lb-ft.

bottom line, with only cams and header and FPR after the original build, I increased about 23% in whp. as a straight number 133 is no big deal. But a 23% improvement is HUGE.

So yes, I think you will see some advantage to what you're looking at. I would second Jondee's advice about used pistons in a used block. The factory prism GSi pistons are coated just like the mr2 ones were. at 200 bucks for the set, including rings and pins, I think it was WELL worth it to go OEM.

When I built the engine, though, I was as meticulous as I could be. every intake valve had the same clearance; every exhaust valve had the same clearance; I even plastigauged all of the main journals just to be paranoid. kept everything super-clean, set the ring gaps as identical as possible, with appropriate chamfering as in the TRD bible, etc etc etc... cleaned the chunk flashing out of the intake and exhaust ports, smoothed the intake with 80-grit, and polished the exhaust up with 320-grit. Did not touch the bowls or chamber.