454 pistons--Flat top versus domed

[Scott Sims]

My '70 454 engine is in rebuild and the rebuilder tells me that the flat top pistons are best for today's pump gasoline but the compression won't be as much as factory, he says maybe 10:1. He says that the dome tops, as factory, would produce 11:1 ratio but using today's pump gasoline, even with using octane boosters, would scorch and eventually burn the pistons. Obviously I want what is factory correct but not the potential consequences.
Anyone with experience and knowledge with this? What brand crankshaft and pistons are recommended?

Thanks,
Scott

[Mark #28455]

1970 LS5 had 10:1 compression

The stock LS5 in 1970 had 10:1 compression as rated by the factory (check your console data plate). You can bet the real ratio was a little lower, but not by much - maybe 9.5:1. It runs fine on premium pump gas as long as you also use the stock camshaft grind with the stock ignition timing recommendations.

You need to check which cylinder heads you have as big block stock heads vary from 96 to 120 cc's and were frequently swapped. If you have the original heads for your engine, they will run about 100 cc's. With that head a flat top piston will likely be the best for your application.

What brand pistons? TRW forged or KB-silvolite hypereutectic. What brand crank? Stock, unless you're REALLY racing (as in 7000+ RPM). I have run cast cranks to 6500 RPM and 550 HP with no durability problems whatsoever! Others have pushed them even further.

If you're planning to change to a different piston or camshaft, go to the dynamic compression ratio calculator at www.kb-silvolite.com and see what's going to happen BEFORE you buy the parts. Static compression ratio doesn't mean a whole lot. Your camshaft selection will make or break the combination. Stock cams have a relatively late closing intake valve which bleeds off some of the compression and allows the engine to run on pump gas. Most "performance" cams close the valve sooner, raising the dynamic compression ratio and possibly getting you into trouble with knocking. "Performance" cams also typically run on tighter lobe separation angles which also makes for a rougher idle and may affect your vacuum function - headlights, wiper door, etc. If you don't run open headers, you will likely not see much if any performance gain from the tighter LSA as those cams are meant to run with little exhaust back pressure.

So, my recommendation is if you want an engine that'll be a GREAT DRIVER, just rebuild it with all stock replacement parts for a 1970 LS5 engine (make sure to verify the heads are original). MAKE SURE YOUR MACHINIST DOESN'T DECK THE BLOCK AND REMOVE YOUR NUMBERS!!!!!! (that would knock about 1/3 off the value of your car!).

If you want to play with higher performance stuff which may or may not work, use the dynamic compression calculator before you buy any parts!

Good luck,
Mark

[Duke W.]

Re: 1970 LS5 had 10:1 compression

Excellent advise! I would recommend that your engine builder take all the measurements to compute the actual compression ratio and shoot for a true 9.5:1, which is probably close to how it was originally built. If it didn't ping in its original configuration and the same piston and head gasket thickness is used, it won't ping after the rebuild.

Use all OE or OE equivalent parts from quality suppliers like Federal Mogul. Don't use any "hot rod" parts.

If you want 5-8 percent more top end power and 500 more useable revs at the top end, have the heads/manifolds pocket ported and port matched with a multiangle valve job. This won't affect the smooth idle or stump pulling off idle torque.

The LS5 is a stroked version of the L-36. When you stroke an existing engine configuration, the top end power doesn't change much, if at all, but peak power will occur at about the same mean piston speed, which will occur at lower revs, and torque/power everywhere between off idle and the power peak will be higher due to the larger displacement.

As long as you can live with what is probably not so good fuel economy, the LS5 is a great street/touring engine.

Duke

[Scott Sims]

Re: 1970 LS5 had 10:1 compression

Mark and Duke, thanks for the technical advice and direction. I want to use the stock GM crankshaft per Tony Hurtado's advice. The engine has been bored .30 over. It will be used strictly for pleasure driving. Why anyone would want to abuse a stock engine like that and make it into a pro-street racer is neyond me.
The heads are the 3964290s rebuilt so I'm okay there. I am a fan of keeping a car as original as possible. The engine is not the original engine to the car but a true 1970 Corvette 454 engine with casting and build date numbers so close to the build date of the car it is uncanny including the CGW suffix. I got lucky;it was along block with only 54,000 miles. However, I am not going to have the block decked.
This NCRS board has been a great help.

Scott

[Harold #43147]

LS-5

Scott I have a 70 454 (#'s Correct Block) when I bought the car the motor was already redone with mabey 100 miles on it. I did a tune up and was test driving the car and thought this engine should be putting out a lot more HP. I removed the valve covers and was going to reajust the valves and check the casting #'s and found out they were for a 73 these were the open chamber heads don't know the CC's off hand the pistons were .030 GM Domed pistons. I found a set of correct heads small chamber 290's and rebuiit them. Found out one of the cam lobes was going bad the previous heads had heavy duty race type springs and probably would not let that lifter start rotating on first breakin. Checked the cam timing 6 deg retarded cheap timing chain and gear set I always use Cloyes True roller there about $90.00 but they are dead on all the time. I put a Comp Exstrem Energy Cam in if you want the spec's let me know i also use a thin head gasket .036 to get more compression I figure its about 10.50:1 but the cam grind is designed to in crease low end Cylinder Pressure. The moral of the story is the engine's preformance is a day & night difference. Down side 100 Octane Min or boost does not run as well with boost I run 110 Octane you have to buy it in gas can's they won't let you pump in the car has lead in it off road use only. I am not building a race car I am restoring the car OEM. GM might have used this cam if they had the technolegy they have today car idles great 17 inc Vac.
Good Luck

[Joe L.]

Re: 1970 LS5 had 10:1 compression

Scott-----

Cylinder blocks rarely need to be decked. Many machine shops do it as a matter of "routine", though. Have the block decks checked with a precision straight edge. If ok (which they usually are), you can feel just fine about it. Not only do you avoid losing your numbers, you avoid unnecessary machine work, and you avoid the expense associated with the unnecessary machine work (believe me, they don't deck both block surfaces of a V-8 engine for free).

As far as pistons go, I'd use Keith Black/Silvolite Hypereutectic #KB-202. These pistons will yield a compression ratio in your engine of about 9.25:1. That's just about where you want to be for a big block. You could also use KB-201 which would get you about 10.5:1 with your heads. I don't recommend it for a big block, though. It might work out or it might not. If you end up with pre-ignition on pump gas, then all of the options for correcting it are "burdensome". Also, the KB pistons will be significantly stronger than the conventional cast pistons originally used in your engine.

[Duke W.]

Re: LS-5

The Comp Cams XE series has a high percentage of cam and valvetrain problems relative to OE cams.

There is no "new cam technology"! GM knew the dynamic limits of that BB valvetrain back in the sixties and designed cams that will live.

The aftermarket just pushes dynamics - sometimes over the edge.

The OE cams produce excellent torque bandwidth, and about as much top end power as similar duration aftermarket cams, usually more because aftermarket cams usually have more overlap for the same duration, so they hurt low end torque, while at the same time increasing octane requirement due to the earlier closing inlet valve.

Another thing to consider is that the later the inlet valve closes, the greater the top end power. OE cams generally have close to ideal combination of overlap, inlet valve closing point, and exhaust valve opening point for both medium performance and SHP engie configurations.

Duke