Pumba

Gentlemen,

Those of you who visited the GM Performance display at last Saturday's Woodward Dream Cruise may have noticed the C6R 427 engine display.

Other than the fact that the intake on the display would not fit under a C6R's hood, I found the following factoid interesting:

C6R 427 bore diameter - 4.180 inches

C6R 427 stroke ---------- 3.875 inches

The C6 LS7 uses the following bore and stroke:

LS7 428 bore diameter - 4.125 inches

LS7 428 stroke ---------- 4.000 inches

The reasons is for the difference are obvious:

1) The larger bore unschrouds the exhaust valve for better air flow.

2) The shorter stroke reduces piston speed, rod angle, and side loads on the piston skirt and cylinder wall.

I'll pass on the LS7 bore - stroke combination and build a motor with a 4.185 inch bore.

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AU N EGL

I am sure Katech will build you one just like it for $100,000. Or one of their C5R block motors starting at $28,000

0Randy@DRM

They also have small restrictors, so that changes everything for the engine program.

Randy

RonSS

Better yet go with the 4.180 bore AND the 4 inch crank.

bcseitz

Gosh.....and to think I was satisfied with my cammed LS1 !

Pumba

There is a good reason why the C6R team did not go with a 4.00 inch crank!
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John Shiels

Warhawk block

tlaselva

Darkness

Better yet, 4.185 bore X 4.125 stroke .........and 2 GT40R's

Pumba

Please review the several rod angle drawings for a LSx engine. The C6R team could have any stroke to make up their 427. The 4.00 inch, 4.125 inch, 4.250 inch,and 4.500 inch stroke crankshafts have too great a rod angle for long term reliability.
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Using accepted engineering - racing calculations, the LSx family of engines, with their 9.240 inch deck height, are IDEALLY suited to a maximum stroke of 3.53 inches. Of course, the Warhawk block with its optional deck height would allow longer strokes.
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John Shiels

Wt99C5

Very good information

M_T_0

6.125" rods

Y2Kvert4me

So, long term reliability isn't a factor in production cars with several years of warranty coverage...but is a factor in a racing engine that gets rebuilt at least once a year?

Interesting.

Darkness

I dont think that rod angle is as much of a concern to reliability as piston speed is.
Here's a rule of thumb when it comes to piston speeds:
Mean Piston Speed Result
------------------ ------
Under 3,500 ft/min Good reliability
3,500-4,000 ft/min Stressful, needs good design
Over 4,000 ft/min Very short life

Piston Speed = 0.167 x stroke x rpms

So a 4" crank is going to produce well over 4000 ft/min if reved past 6000rpms, and I'm sure race motors go well beyond that, heck even my 408 has the limiter at 7100rpms!

Just for comparison a HONDA CBR 600 at 13250rpms has a piston speed of 3939ft/min.

0John@SDPC

Might be some other interesting block options in the near future.

MrEracer

In my experience I have NEVER seen any kind of failure that was the result of rod angularity. I have built a lot of short rod 406 cid SBC engines (5.535" rod, 3.75 stroke, 1.48 rod ratio) and never found any evidence of bore wear or bore failure due to the side loads... A properly designed piston uses an off-set wrist pin to eliminate this 'problem'. The durability issue with short rods (low rod ratio's) is a conventional wisdom myth and as usual, 'wrong'... Smoky Yunick was one of the first to recognize the benefits of a 'short rod' in a street driven application. If you want area under the HP/TQ curves, use a short rod...

Pumba: Please add the LS counter weight diameter, reluctor wheel diameter and wrist pin boss diameter to your diagrams... I think you will find that the short rod lengths you show will not clear these items. You may also want to reduce your compression height to 1.00" as that is easily accomplished using a .927" pin in an LS engine. You may come to some different real world conclusions.

I build many 4.100" and 4.125" stroke LS engines and it is nearly impossible to use a rod any shorter than 6.125". A 3.85" crank would not allow anything much shorter...

I just finished a Darton sleeved LS2 454 cid engine with a 4.187" bore and 4.125" stroke using 6.125" rods (1.48 rod ratio and 1.053" compression height). This engine is in a NA street driven C5 and made over 600 rwhp using an LS2/90 intake manifold and ported 317 heads. Can't wait to install LS7 heads and intake manifold on this monster...

I have de-bunked a bunch of conventional wisdom with my engine builds. Here is a short list:

1. Short rods and low rod ratio's result in reliability issues.
BUNK: I have been building these engines for 30 years with ZERO failures.

2. LS engines cannot be bored more than .010" on the stock sleeve.
BUNK: I have built over a dozen LS2 engines bored .030" on the stock sleeve with ZERO failures.

3. LS2 sleeves are too short to use with long stroke cranks.
BUNK: All the engines in 2. above had either 4.100" or 4.125" stroke cranks with ZERO failures. Many are maximum effort road race engines.

4. The LS2 intake manifold is no good.
BUNK: A properly ported and matched LS2 intake manifold is at least as good as a FAST LSX intake manifold and probably better. (ref. the 454 above).

The C6R 427 is highly optimized for the rules and racing requirements... I'm sure the bore/stroke chosen for that application is the best for those conditions... The rest of us driving our big bore, NA strokers on the street are limited by real world budgets. The differences in bore/stroke between a C6R 427 and a LS7 427 would be undetectable on the street... These are my opinions, yours may differ...

Shirl Dickey
SD Racing Enterprises

Pumba

When it is released for production, ask yourself why GM is going away from the 4.00 inch stroke LS7 engine to the 3.622 inch stroke 6.2L Supercharged engine for the next level of Corvette power.
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BLOWNZO6

Very nice post from someone who obviously has hands on real world experience...

Y2Kvert4me

Fuel economy, too high of compression, and the cost to produce the handbuilt engine comes to mind immediately.

Really, was that tough for you to figure out?


Building an engine designed for forced induction from the start is a far more elegant approach than simply taking an existing high compression, big cube n/a engine and slapping a blower on top of it.

I'm sure GM would have LOVED to stay n/a and acheive their desired power goals...but with the EPA and CAFE looking over their shoulder, going FI is about their only logical solution to keep up in the current hp race. The current Z06 is what, less than 1mpg away from a gas guzzler tax?

I'm sure that was more the concern when deciding how to power the next car than the stroke length was.