Real Fiero Tech

So on a whim, I bought some reconditioned lz4 rods off eBay from a shop or individual in Michigan.

I was doing some rough calcs based on the fact that the lz9 engines share the same pistons which means the compression height of both is the same.

There are online compression ratio calculators that will calc your compression height of the piston as you input data. Using this website and assuming that the 4 piston, sat down in the bore the same as the 9 piston(.036) I was able to guesstimate that the 4 rod had to be about 5.99 inches long.

So this got my brain thinking that there might be an off the shelf piston readily available based on this rough guestimate and summit racing showed something close

https://diamondracing.net/p-10-compress ... lator.html

So after getting the rods in I did some measurements. My rough measurement came out to 6.030 length which was close to the 5.99 I guessed beforehand. This was based off already know values of .943 small end with bushing installed, and the known machined big end which I think is 2.25? Either way it's the same for both 4 and 9. So that allowed a simple difference measurement and basic math added up the numbers and came to 6.030.

Now here is where it gets good.

Take a 3.9 crank(3.31 stroke vs 2.99 for lz4) add the 4 rods, and obviously your piston will stick way out of the top of the deck, (1.3ish CH stock) unless you lower the compression height.

And the aftermarket comes to the rescue! Off the shelf forged Pistons, with a CH of 1.14 are widely available in stock bore, .005 over, and .015 over in multiple dishes for multiple compression ratios, as well as the stock .943 piston pin bore

1.14 CH piston bring the piston proud of the deck by .007.

Now if my measurement is wrong on the rod, and it's actually shorter, it could be that the piston sits flush, or just short of the deck by maybe .00X

Either way, the 1.14 CH is damn close to perfect.

Now these pistons have three diff dishes. 10 cc, 15 and 29.
10 gives a comp ratio of around 13:1.
15 is about 12:1
29 is close to stock LZ CR of 9.8 for you boost guys.

If getting a stock bore size than a 22 cc is also available which is about a 10.8:1 CR

D.S.S makes these pistons. And they are pretty cheap as you get 8 pistons.
https://www.summitracing.com/search/mak ... 3a1-140-in

"long rod 3900"


Feel free to check my math with the cr calculator and the summit link

This opens up huge options for this engine swap

Good find!
I guess you mean 0.015 over?

The Dark Side of Will wrote: ↑Sat Jan 28, 2023 3:14 pm Good find!
I guess you mean 0.015 over?

Yes. I corrected it.

Drop in forged pistons, factory forged rods, factory forged crank, arp rod bolts available, and it's a bulletproof bottom end with high compression na setups now available without fuss for those not wanting boost.

Know anything about the valve reliefs in these pistons?
A big cam, especially when used with VVT, needs deeper valve reliefs than stock, and in the right place as well.

The Dark Side of Will wrote: ↑Sat Jan 28, 2023 3:36 pm Know anything about the valve reliefs in these pistons?
A big cam, especially when used with VVT, needs deeper valve reliefs than stock, and in the right place as well.

Nobody makes a big cam with vvt for these engines and noone ever will. So that doesn't matter.

Obviously with any cam change you would have to verify PTV clearance. I don't foresee there being any issues however even with an aftermarket non vvt cam

Very cool, are the longer lz4 rods true forged while lz9 are are PM? Or are they both PM?

All the rods are PM. They're good rods.
When GM introduced PM rods for the SBC back in the '90's they said the PM rods were stronger than the prior forged rods, known as "pink" rods

These lz rods in my opinion are similar to gen 4 LS rods with material and construction.

Those factory LS rods have been known to support over 1k HP on the LS v8s. That's 125 HP per rod or greater. So that equals 750 or so HP that the factory rods can handle pending several factors. They might be able to handle more even.

That's a good find.

What do y'all think about long rod/stroke ratios? I know that La Fiera likes them small.

Shaun41178(2) wrote: ↑Sat Jan 28, 2023 4:11 pm Obviously with any cam change you would have to verify PTV clearance. I don't foresee there being any issues however even with an aftermarket non vvt cam

Is there any approach to estimate this besides buying a set of pistons and trying?

Some people have built long rod combos and it didn't pick up any power. But some people swear by it.

Other than doing a full assembly and measuring PTV, I think there are programs out there maybe for free that will run you combo through a simulation to see if there are issues. Running a cam with lower duration will def help preventing contact.

What I've read on SpeedTalk is that the rod ratio itself doesn't allow more power, but can be mismatched to the combination. Darin Morgan of Reher-Morrison says that rod length is, if not completely unimportant, at least far, far less important than getting the ring pack and skirt design right. Of course the engines he builds are anything but port limited.

Most of the arguments I've heard about rod ratio relate to either the strength of draw on carb venturis through a big port with short rods or lengthening the draw period to make better use of an "undersized" port with long rods. Undersized in is quotes because there's an RPM at which any port becomes undersized for the cylinder it's feeding. This loosely aligns with the groups who like long rods and the groups who like short rods.

Longer rods is better for cylinder side loading and frictional loss, which makes them more desirable for running very high cylinder pressures.

If you're going through with this build, I suggest getting Line 2 Line teflon skirt coatings, which will also help with bore & skirt wear and loading at high cylinder pressures.

These long rod 3900 variants make the iron head counterparts look small.

Is there no benefit to the longer dwell period at TDC for high rpm? My understanding was it gives more time to develop cylinder pressure where a shorter rod motor would start outrunning the rate of combustion at a lower RPM.

zok15 wrote: ↑Tue Jan 31, 2023 10:08 am Is there no benefit to the longer dwell period at TDC for high rpm? My understanding was it gives more time to develop cylinder pressure where a shorter rod motor would start outrunning the rate of combustion at a lower RPM.

Yes, you come closer to an idealized Otto cycle with instant heat addition at TDC.

Also, the long-rod piston moves more sinusoidally with crank rotation, so the piston acceleration (and hence vibration) has less higher-frequency harmonics.

Shaun41178(2) wrote: ↑Sat Jan 28, 2023 4:11 pm

Nobody makes a big cam with vvt for these engines and noone ever will. So that doesn't matter.

Obviously with any cam change you would have to verify PTV clearance. I don't foresee there being any issues however even with an aftermarket non vvt cam

I've got a few bad ideas of how to put one together, it's pretty low priority at the moment though.

Is there any performance benefit to the VVT in these motors though? My understanding is it is basically EGR when retarding the cam to make the exhaust valve open for part of the intake stroke.

It helps broaden the overall tq curve

Aftermarket Vvt cams in LS engines show big improvement, but they also do tricks to lock the cam phaser from going too far creating PTV contact.

Most people eliminate their vvt cams in the LS world as they can go with bigger lift and more overall duration for peak power

It's already been proven that a good aftermarket 60 degree cam provides more tq and power throughout the rpm range over the stock vvt cam

zok15 wrote: ↑Thu Feb 02, 2023 12:17 pm Is there any performance benefit to the VVT in these motors though? My understanding is it is basically EGR when retarding the cam to make the exhaust valve open for part of the intake stroke.

yeah, as Shaun said, variable cam timing tends to flatten the peaks, and increase the distance between them.

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The long rod info is cool, FWIW, Engine Masters on Motortrend actually tested the affect of rod length, they built two mostly identical engines, both engines had the same displacement, compression ratio, crankshafts, camshafts, heads, cams, ect. the only difference was the rod length, and the adjustments to the piston to make the different rod work. in their test, the long rod made more low end power, and the short rod made slightly more top end. The longer rod, had a lighter piston, the shorter rod, had a longer piston, they theorized that the lower inertia of the long rod combo lead to the better low end, and the increased stability of the shorter rods piston lead to the increased top end.

here's a screenshot of the two dyno runs, the long rod is ghosted, the short is the bright traces.



I used to be a believer in rod ratio and long rods being important, now I tend to fall into the connect the piston and crank and call it good, as long as the rod isn't crazy oddball sized, everything should be ok. I definitely won't build an engine around the connecting rod length.

this particular scenario is interesting, because there's a piston that works with the existing rod, and no fuss(so far), which opens the door for higher/lower compression ratios, without custom parts. nice find.