I haven't perused my library yet, but are the pistons in a 1971/'72 LT-1 different than a 1970 LT-1? I know the combustion chambers are bigger in the '71 with the different heads, but did they change to a different piston as well to get the lower CR? Somewhere I thought I read that they changed both the heads AND the pistons, but the cam is the same. If the pistons ARE different, is it beneficial/worth the trouble/cost to go with 1970-style pistons to get a little more compression? The rest of the '71 motor is stone stock, with smog.
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Re: C3-1970 - 1971 LT-1 Pistons
NHRA specs call for 1970 LT-1 has a piston with a .109 dome and the 1971 is a flat top piston,no dome -
Re: C3-1970 - 1971 LT-1 Pistons
Recall that a GM management decree required all GM engines to operate on 91 RON unleaded fuel for the 1971 model year. Current 87 PON regular grade fuel has effectively the same detonation resistance as the 91 RON unleaded or yore, so an original '71 LT-1 WILL operate without significant detonation on current unleaded regular. Back in that era, refining technology did not allow the manufacture of high octane unleaded fuel at a competitive cost, but it did arrive in the mid-eighties and is still around, today.
Given the above background, GM increased head chamber volumes and altered piston design as required to allow all engines to operate on low octane unleaded. I can't tell you the physical differences, but the part numbers for '71-72 LT-1 OE and oversize replacement pistons are different than 1970. Maybe Joe Lucia can tell us the differences from examples in The Collection.
The advertised '71-'72 LT-1 CR is 9:1, but as actually built, it is probably closer to half a point lower, which is typical for engines of the era. Most engines were reduced to 8.5 (advertised) and were probably closer to 8:1 as built. The LT-1 can handle more SCR because of the late closing inlet valve, which reduces DCR for a given SCR.
Unfortunately, the combination of low compression and high valve overlap with a late closing inlet valve is the ABSOLUTE WORST engine configuration imaginable and is a formula for poor fuel economy, poor torque bandwidth, and poor top end power relative to the same configuration with "high" compression ratio. The difference between the '70 and '71 LT-1 performance and fuel economy curves is dramatic even though the only EFFECTIVE difference is the CR.
Modern unleaded premium gasolines allow a TRUE SCR of up to 10.5:1 with the LT-1 cam, and this is about what you get with a typical OE machined block, the pre-'71 domed pistons, a 64 cc head that has some chamber overhang relieving (to about 65-66 cc) and a .026" OE thickness gasket, and this can easily be lowered to 10.25 by choosing a readily available thicker head gasket.
My recommendation to all who are restoring '71-up engines is to raise the CR as much as possible to regain fuel economy, torque bandwidth, and power that is there for the taking if you use current premium unleaded fuel, and the increased overall fuel efficiency will offset the ten percent higher cost of premium fuel, but because of the larger head chambers, you will probably still leave some CR on the table.
If you use the 2.4 cc domed '70 LT-1 piston on a block with .025" nominal deck clearance and a .026" thick gasket with the OE 76 cc heads the CR still only comes out to a measly 9.5:1.
I recommend considering custom forged pistons for the '71-'72 LT-1 from vendors like JE, Wisco, or Arias to get the CR up to the range of 10.25-10.50:1 with the OE 76 cc heads. They all use the same forged blanks, likely sourced from Federal Mogul, and a "custom" machined set of pistons really aren't much more expensive than the Speed Pro OE replacements. And since the OE 350 rods are okay to use with just a Magnaflux inspection you don't have to spring for high strength aftermarket rods like 327 owners should do to replace the spindly OE rods on most 327s. Most "custom" pistons are manufactured from off-the-shelf blanks and standard NC program modules to finish machine to specific specfications, including dome volume.
DukeComment
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Re: C3-1970 - 1971 LT-1 Pistons
F/M lists a piston # H617P that gives 10.1 to 10.4 with 76 cc heads. these are a hypereutectic piston which should be fine for any street engine. these should be cheaper than custom pistons.Comment
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Re: C3-1970 - 1971 LT-1 Pistons
You folks confirmed what I was thinking....I know in the real world that the '70's and '71-2's will run pretty close to each other, but I want to get the '70 compression back in my '71, because like what has been said, fuels today are way better than days of old, and my friggin' truck has higher compression than 8:1. I ran 110 racing fuel in my '70 LT-1, and after a little timing adjustment, it would scream and ran wonderful. When I drive this '71, I want it to act in a similar if not better manner, as my '70 had very little torque...it only made power 3000rpm and up. Now, question #2...is it worth it cost-wise, in addition to new TRW pistons, to have the heads ported/polished and the manifold/head ports matched up? I'm a true believer in originality, and I am going to run smog and all, but I want to wring as much out of it as I can without anything being changed externally. But I DO have a budget, and I want as much tire smoke per dollar as I can get. I've never had that done to any of my engines, but if it'll make a big difference, I'll go for it.Comment
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Re: C3-1970 - 1971 LT-1 Pistons
you will have to have the rotating assy balanced because these pistons are heavier than the ones in your engine. duke can run the setup on his EA program and see what the HP and torque difference would be.Comment
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Re: C3-1970 - 1971 LT-1 Pistons
That piston has an 11.8 cc dome that will yield too high compression with an OE .026" head gasket. F-M specs most of their CRs with nominal deck clearance and a .039" gasket.
That would be a good choice for my "Special 300 HP" configuration, but will require a couple of cc taken off the domes or out of the chambers to achieve no more than 10.5 with an OE gasket and nominal 76 cc heads.
For a LT-1 I prefer forged pistons at .004" clearance to maintain the original engine sound. Most hypereutectics including the H617P are designed for tight (.001") clearance like the OE cast pistons.
DukeComment
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Re: C3-1970 - 1971 LT-1 Pistons
i bet if the block is not decked,the heads are not cut,rods reconditioned and a fresh valve job you would be close to 10.5:1. the heads cc are alway larger than specs and the deck is always more than spec.Comment
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Re: C3-1970 - 1971 LT-1 Pistons
Cylinder head massaging - pocket porting/port matching and carefully machined multiangle valve seats is the BEST single "modification" you can make to ANY OE engine, and no one will ever know unless you tell them. The techiniques have been documented in various books like "How to Hot Rod your SB Chevy" and David Vizard's books for over 30 years.
The reason even high compression '70 LT-1s are torque shy is because of the lazy emission oriented spark advance map. Search the archives for a thread I started titled "327 LT-1". This '65 L-76 with massaged heads, LT-1 cam, and all original components (including the aggressive OE spark advance map) and OE or OE equivalent replacement parts except for the connecting rods makes 80 percent of peak torque at 2100, 90 percent at 2500 and useable power to 7200.
With a CR gearbox and 4.11 axle it easily pulls from 1000 revs in fourth gear, and the owner doesn't even to bother downshifting from fourth turning 90 degree corners on city streets, but he buzzes it to 7200 at every opportunity!
You can leave all the emission control equipment intact, but essentially bypass it so you have a 1965 SHP igntion and fuel map and the AIR pump takes little power, and if you gut it, it's basically just an idler pulley.
DukeComment
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Re: C3-1970 - 1971 LT-1 Pistons
I think you're right, but I'm still trying to figure out how much typical rod resizing increases deck clearance.
However, in order to manage CR the user can probably specify how much to take off the rod can cap mating surfaces and use rod resizing as an additional CR management tool.
It's just a matter of good planning.
DukeComment
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Re: C3-1970 - 1971 LT-1 Pistons...wow!
Ok, I feel like I'm in college again!! My block is std. bore, heads have had a std. valve job, stock pistons, crank, everything. the car has 3:73 gears, M21 4 spd. a-la LT-1. Duke, if I install "custom" forged pistons, use a modern .026"(the thin one....) head gasket and balance the whole assembly, in an attempt to get a 10.5:1 CR which is the target I'm shooting for (is it too high, or low, for the LT-1 cam with '71 heads?), how will my engine timing be affected? Will I have to advance the cam a bit, or will it be OK at std. specs? That's also including getting the works port matched. Bear with me, this whole engine dynamics lesson is intriguing me. There's a LOT of variables....I just want to try to get more torque than I had in my '70 without losing the 7000 RPM scream.Comment
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Re: C3-1970 - 1971 LT-1 Pistons...wow!
I consider 10.5 true to be the prudent limit for the LT-1 cam with modern pump premium, so you want to shoot for the range of 10.25-10.5. Retarding the cam will improve the top end power range at the expense of low end torque, so if you want more low end torque you do not want to alter OE cam indexing. With points of max lift at 110/122 the LT-1 cam is already considered "advanced" in aftermarket jive. If you retarded it six degrees to 116/116 the aftermarket would consider it "staight up". Retarding it to this indexing would be a good idea if you want a high revving 3.75" stroke configuration.
To accurately calculate CR you need accurate deck clearance measurements, head cc, piston dome volume, and head gasket diameter and compressed thickness. In order to get a handle on CR, deck clearance should be measured PRIOR to block disassembly. Then you can compute the new CR based on expected changes in deck clearance from any necessary machining operations like rod resizing or main bearing saddle align boring. Then you measure deck clearance again after the block is assembled and select a suitable head gasket. If you didn't measure deck clerance prior to disassembly you're just going to have to wait until the block is assembled, then measure deck clearance and compute the CR with various head gasket thicknesses until you are in the target range. Head gaskets are available from .015" to over .050", so there are a lot of choices. I use the following on line CR calculator:
The key to low end torque is to run as much spark advance as possible at the low end by quickening the centrifugal curve to the limit of detonation, and if you start with the '64-'65 SHP/FI curve (24 deg. @ 2350 with 10-14 initial which yields 34-38 WOT advance at 2350 and up), you are pretty much there. Any single point or TI distributor can be modified to this curve. Convert the ported vacuum advance to full time and use a NAPA VC1810 vacuum advance control.
Yes, there are a lot of variables, and the key is to control them by properly planning and managing your project, which includes careful management of your machinist. Only remove material if it is absolutely necessary and use only OE or OE equivalent parts - no hot rod parts other than minor things like distributor springs to tailor the spark advance curve. This is particularly true of the LT-1. It doesn't have weak rods like 327s, and it has a better induction system. All it needs is the head massaging/multiangle valve seats if you want more top end power without sacrificing low end torque.
There is lots of information in the archives about how to restore Corvette engines for improved performance and fuel economy.
DukeComment
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Stroker crank
Since you are buying new pistons anyway, get a stroker crank too. Nowadays, you can essentially buy a new crank and rods for not much more than the cost of rebuilding your old ones. This is especially practical if you don't know if the originals were abused. In addition, you can sell the old stuff on e-bay and likely come out ahead! By increasing the cubes, you boost the torque of the engine throughout the RPM range.
MarkComment
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Re: Stroker crank
I thought of that....but I want it to still sound and act like an LT-1, as I'd like to have the car judged.....if I make the motor a 383 etc. the judges are gonna know it right away just by the exhaust note. At least in my opinion, you can hear cubic inches more than compression ratios. If I chose that route I'd leave the original engine alone and slap in a ZZ383. Fun to think about, though....Comment
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Re: Stroker crank
The exhaust note will not change if the cam and compression ratio are the same. I have been playing with stroker BB engines for the past 5 years and you can't tell.
MarkComment
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