For those who have seen my posts and website, you know what is going on. For those who haven't, I blew up the LT4 in my 96 back in December. I'm finally getting around to putting together a new motor.

Currently, I have a used stock shortblock in unknown condition. Right now it is being checked out by a local machine shop. My stock LT4 heads are being ported and fixed by Lloyd Elliot, along with a custom cam that he speced. Projected output on a 350 will be 400 RWHP.

What I can't decide is what to do with the rotating assembly. This engine will never see boost or spray, but what it will see is extended high rpm use. 20 mins sessions where the rpms will between 4000 and 6500 the entire time. The max rpm right now is planned to be 6500. I already have a Canton race oil pan and accusump, as well as an oil cooler.

Will the stock rotating assembly, balanced, with ARP rod and main bolts, stand up to this? If not, what about a quality cast crank and hyper pistons? I would like my compression to end up between 11.0:1 to 11.5:1 so it will run well on 93 with once-milled LT4 heads. I know the "do it once, do it right" motto is quite popular, and it rings true with me, but at some point I have to consider my budget. An all-forged rotating assembly is more than I can afford.

Is using a stock assembly just asking for trouble, or will it being properly balanced and using ARP hardware be enough along with a good oiling system? Are the cheap cast cranks even an improvement over the stock piece?

Currently, I'm not sure if my block will need to be bored or decked, but I should find out soon.

Any suggestions/comments?

 

On a side note, if something happens to the "new" motor you are using as a replacement, I have a shortblock out of my 92 for cheap. Was in good working order when I pulled it but definetly suggest rebuilding it. I am in Houston. Here is a bump for your question.

 

For something being run at 6500 for extended periods I would definitely upgrade.
Even looking into a budget forged piston would be a plus.

 

Stock rotating assembly is suppose to be good for 450Hp ish
The aftermarket (Scat / Eagle) cast steel cranks are supposed to be superior to your nodular iron LT4 crank.
By the time you factor in a possible crank grind on you old crank $170 for a new steel crank may be in your budget.

 

Nothing better happen to this new motor or the car won't be going anywhere for a long, long time.
It's good to know the cheap cast cranks are actually a bit of an improvement. We'll see if the stock LT1 crank that came in my used shortblock is even any good.

 

i think that your rods would be the weakest link in the setup.. if you were spraying or forced induction i would say upgrade.. but i don't think that is going to be a problem with the stock bottom. and you would have to have your rods reconditioned if you put new bolts in cause the new bolts could cause out of roundness of the caps which would bring you up to what some aftermarket rods cost. but if you wanna play you gotta pay..

 

Maybe it's time to step up to a 6.0" rod with new pistons on the stock crank.

 

Turning stock rods and hyper pistons 6500rpm for extended peroids of time is asking for trouble at the 400hp level. There are a few causes.

The stock PM rods are heavy, something like 580grams, and not really all that strong as compared to an aftermarket rod.

Because of the 1.560 compression height even cast pistons are also pretty heavy.

5.7 rods and a 3.48 stroke give you a 1.63 stroke to rod ratio which is actually pretty good. BUT it could easily be better - espc with the extended RPM you intend. At the minimum I would step up to a 6" rod (1.72 stroke to rod ratio). It wouldn't hurt to go to a 6.125 (1.76 stroke to rod). The shorter rods put more load on the crankshaft and the pistons at or near TDC and BDC due to the higher "rod angle", which can exceed the ability of the rod, crank, or piston to stay in one peice at higher RPM. Longer rods reduce this and can allow you to turn more RPM safely.

IMO, You can upgrade the rods and pistons while keeping the stock crank. If you use lighter rods and pistons the stock crank will live a happy life at the HP levels you anticipate as long as it is balanced.

Eagle has a rod that is marketed as their "3D Feather Weight" connecting rods. One of these 6.125 long weighs less than your stock PM 5.7 rod and is far stronger. Many different lenghts and journal sizes are offered.

http://www.cnc-motorsports.com/category.asp?CtgID=8085

Scat makes a "Q-LITE" rod that is a bit stronger than the Eagle and a bit heavier (but still way lighter than stock) also a bit less expensive. Only in Large Journal 6" thou.

http://www.cnc-motorsports.com/produ...412&CtgID=8183

As for pistons. SRP, Probe, KB or the Wiesco "Pro True" (IMO the best of the budget) are budget priced forged pistons and fairly light when coupled with long rods.

http://www.cnc-motorsports.com/category.asp?CtgID=8201

http://www.cnc-motorsports.com/category.asp?CtgID=12725

http://www.cnc-motorsports.com/produ...85&CtgID=12783

Non of the above is going to cut it in NASCAR but they are all good quality pprouducts capable of doing what you want to do and not break the bank. A little $$$ now beats the heck out of blowing up stock motors all the time.
Will

 

Thanks for the info, that is exactly what I was looking for.

 

I agree with Rklessdriver. With the high RPM range, the stock bottom just is not going to cut it in the longevity game. If you are going to do it, do it right is my motto. I would also question the RWHP expectations as well. My thought with your setup would be around 370 -380hp. Just my experience.

 

I respectfully disagree with a few of the posts above. I think you can safely make a stock bottom end work just fine. I personally raced my car for a season with a stock bottom end out of a b-body. At the time the car was making about 400 chp. I eventually spun a rod bearing by overheating the oil because I foolishly added power but no cooling capability.

6500 RPM is not that high. And realistically, while your rev range is higher than most, you're not sustaining 6500 RPM, you're seeing it occasionally before upshifts. You probably spend most of your time between 3000 and 5000 RPM.

My suggestion for a budget car is to disassemble and inspect everything, and depending on condition re-use as much as possible. However, things like rings and bearings are cheap, I'd replace all of them. Use new gaskets and bolts and you should be fine. I'd suggest being a little less aggressive on the compression ratio, maybe keep it stock or go to 11.0:1. Yes, compression makes hp, but it also sacrifices some robustness.

WIth all that said, however, my suggestion is to spend a little now to save yourself the headache later of having to rebuild it in two years. Go lightweight wherever possible if you can afford to...and definately add extra oil cooling!

 

I already have a nice, beefy aluminum radiator with a built in oil cooler. Hopefully it will take care of my cooling 'issues'.

I'll have to see if I can swing the lightweight stuff. Assuming I were to use some of the pistons and rods listed above, would it be safe to spin it higher than 6500? Like say 6800-6900, so I could have a bit of over-rev when I don't want to shift.

 

The PM rods used in LTx engines were advertised as superior to the chevy 'pink' rods when they came out.. back then 'pink' rods were the rods to have.. Out of all the posts on this forum I have rarely read a broken rod post since 1999..

I have heard not so good things about KB pistons.. but i've also read alot of that can be from machine shops not doing the correct machine work for them..

FWIW, I'm running a SCAT stroker crank with stock rods (which have been de-burred and relieved, Keith Black pistons with ARP bolts.. I have run the car on road track HPDE's (i.e. Roebling Road) 3 full days now and haven't had any problems with anything... those motors were pretty dam tough from the factory.. GM did good! Just make sure your oil level is always filled and you are accurate with your tolerances and follow the assembly procedures correctly!!

good luck.. BTW: all that extra HD stuff (fiorged this or that) is usually better if you can afford it.. but the stuff I'm runnin has worked GREAT!

 

Based on several discussions with people smarter than me, the limiting factors are the lifters and the Optispark.

I have my limiter set at about 6800 and hit it once or twice a race and have done so frequently. However, I do have a lighter crank (Eagle) and an aluminum flywheel. But...my pistons and rods are forged so they're actually a little heavier than stock. The one thing I didn't reasearch ahead of time was the lifters, and I got CompCams "stock replacement" LT1 lifters. The stock lifters can't pump up quickly enough to stay oiled. If you got some better ones I'd expect you can go over 7000, I know a few guys here on the forum that do. Comp specifically said not to take the ones I bought above 6500.

You could also go with a solid roller

I've heard stories about the Optispark not being able to handle the high revs, either. I don't know how high is "too high" because I have other limiting factors as stated above. But I can tell you from personal experience that 6700-6800 is no problem, maybe it's over 7k that would force you to an LTCC or Delteq type system. I know 96GS#007 revs that high...

I have lots of video I can send of me holding the car at 6700 halfway down the front straight at Mid-Ohio.

 

Oh, and like 383_blubyu says above, I had a lot of guys try to sell me the pink rods when I was shopping around. The stock PM rods in the b-body block worked just fine.

It's sitting on my garage floor waiting for my 383 to blow up so it can go back into the car right now

 

The pistons listed above were lighter than stock...I think. I'd have to double check. I wonder if the LS7 lifters would work OK; I think they are affordable. I already have an aluminum flywheel and I already converted to an LTCC setup, so I'm good to go there.

I don't think I want to go to a solid roller. I really don't want to be adjusting the valves all the time, as this is a 'fun' car, not a 'race' car.

 

There are a few things I'll address. It's good to have a civil disagreement every once in a while. An honest exchange of ideas and knowlege is good for everyone.

Stock PM rods are safe to 400hp at the crank not the wheels. Maybe 450hp with ARP bolts and spot on machine work. They are way too tight on the brg crush as machined from GM. This is part of the reason stock LT1/LT4's are so tight on the rod and main clearance - far too tight for any serious racing application.

On the rods it all comes down to HP, Rod Weight, Piston Weight, RPM and Rod Angle. When you start stacking that up, the PM rods might live here they might not. I've run them when customers had no money and they did ok but I was careful to not exceed the limits I gave above. IMO they would be marginal in this application even under the best machinest hand. Not to mention for the cost of bolts, having them resized and the little ends floated he is half way to buying a set of the rods I recommended. Which are lighter stronger and longer (if he wants).

As for KB pistons having a bad rep. That is very true of the KB hyperutectic pistons due to "engine builders" not reading the instructions on ring end gaps. The forged pistons are different story completely and have only been on the market for a little over 2yrs... they are very nice for the money but some folks can't get past what's spewed on the internet. If you can't there is always SRP, Pro True, and Probe all for a few more $$$.

Hyd roller lifters. Depends on the spring pressure and the lobes you are running. The LS7 lifter is an OK part... but not really that much better than the stock SBC/LT1/LT4 roller lifters. The internal valving is a little better for higher RPMs but at 6500rpm either will work fine provided you have the correct valve springs. We have modified the stock roller lifters for years in Stock Eliminator... you would be amazed how tough they are. GM also makes a lifter known as the Cadillac lifter. Its better for 7000+rpm than the stock or LS7 lifters by a big margin. Some have liken it to the MOREL lifters... IMO its a good part but not that good. If you have the extra $$$ MOREL makes the best money can buy.

As far "IF you bought the above light weight rods and pistons would they be safe to turn 6800rpm." IMO as long as the balance work was good YES I would have no worries at all. Also FYI the Eagle feather weight rods are rated to 525HP in a endurance application the Scat Q-LITEs are rated to 600hp, so there is plenty of room to grow in the future with them.

Will

 

When revving that high, don't forget torsional vibration problems.

The amplitude of torsional vibration is directly proportional to cylinder torque (it has nothing to do with crankshaft balance, they are not the same).
Each combustion pulse produces a twist and untwist of the crankshaft proportional to the amount of deflection from torque at a frequency determined by the rpm (and firings per revolution).

Therefore, an engine with 300 ft lbs can get by with a damper that will not suffice for an engine with 400 ft lbs. An engine that has a redline of 6K rpm can get by with a damper that is not sufficient for 7K rpm.
That is why the LT4 has a different, dual-mass, damper as opposed to the LT1 single mass damper.

Uncontrolled torsional vibration can cause engine damage (broken timing chains, sheared camshaft bolts, and (on the LT1/LT4) sheared water pump drive teeth on the cam gear). Torsional vibration damage is almost always on the front of the engine.
Since the LT1/LT4 Opti-Spark has one degree of crankshaft resoluton, the torsional vibration transmitted through the timiing chain to the Opti-Spark can cause difficult to diagnose high rpm misses due to Opti-Spark "chatter."

Most people overlook the damper.
You won't feel torsional vibration because it is too high in frequency.
A balance problem is easily felt.

Think of torsional vibration as the "silent engine killer."
When you see posts like these, think "torsional vibration."
http://www.camaroz28.com/forums/showthread.php?t=575451

http://www.camaroz28.com/forums/showthread.php?t=573384

http://forums.corvetteforum.com/show...t=Extreme+duty

http://forums.corvetteforum.com/show...t=Extreme+duty


Tom Piper

 

So would a stock LT4 damper be enough or would an ATI or similar be required?

 

I put a stock LT4 damper on my '92 LT1 with AFR heads and a Crane cam because I figured my torque was about the same as a stock LT4.
I think the LT4 was under-rated at 330 hp, my guess is most LT4s are really about 370 hp.

But, if you are talking over 420 flywheel hp or reving higher than the LT4, I would start thinking after-market dampers.

Just my .02.

Tom Piper