Offset Grinding 292 Inline 6 Crank Rod Journals

I have a question regarding offset grinding crank journals......

I have one of the 1963 forged steel cranks for my 292 inline 6 build. Also have one of the later model nodular iron cranks with 12 counterweights.

The long stroke, tall deck 292 6's are prone to harmonics issues at engine speeds above 5,500 RPMs. So I have been thinking about offset grinding the crank journals to destroke it a bit. Stock stroke is 4.120" and journals are 2.10", same dia. as 350 V8, only the inline 6 rod has a wider big end (1.030" versus 0.940" for small block V8 rods).

I have heard of these 292 cranks being offset ground for the short deck 230/250 blocks using 5.7" 230/250 inline 6 rods which have 2.00" dia. journals same as a 283 or 327 V8, but as with the 292, wider on the big end.

However, I am interested in offset grinding the throws to fit some used Carillo NASCAR type rods that utilize smaller 1.850" journals. I have seen them with big end widths varying from 0.820" or 0.900", but none as wide as the big end of the stock inline 6 rod.

The reason I am considering offset grinding the rod journals for smaller 1.850" dia. bearings is that I would like to destroke the 4.120" stroke crankshaft to 3.875" and also reduce the counterweight dia. to lighten it. 0.040" overbore along with a 3.875" stroke would be close to 280 cu. in. and I believe the reduction in stroke would go a long ways towards minimizing harmonics issues at higher RPMs.

Is this doable or would you advise against it? Engine will go in my mostly street driven '57 Chevy which will be run at the track from time to time mostly for tuning.

Thanks,

Harry

I'm sure it could be done, at great expense mainly due to the cost of the rods. Then you'd have a 6% power loss with only a miniscule gain in reliability. Maybe even a loss in reliability because you'd be making a noodle of a crankshaft even more flexible.

A much easier way to lose 6% power and gain some reliability would be to cut maximum rpm as well as cruise rpm.

I have to agree with Rick on this one; biggest problem with the inline 6's is the length of the crankshaft, and making it thinner can't help the structural rigidity.

Offset grinding a crankshaft for an SBC does not really change its stiffness enough for it to be a concern. You can cut the journals down from 2.10" to 2" or even 1.8" and offset grind without much if any negative effect. We used to do this for drag race SBCs back in the day to get an engine displacement that matched the car weight to make the minimum cubic inches/weight of the car for NHRA modified and comp classes. This was because the cylinder heads were not as developed as now and a smaller engine would make more power/cubic inch. So the goal was to build the smallest engine/lightest car to meet the class requirements. A narrower goal that what Harry probably has in mind. And maybe not as big a deal for the narrow prospective of drag racing, though to me the lighter car always has an advantage.

The stiffness deal might become more important on a 6, maybe not.

Thing is if you destroke the crank, you lose power and torque. So this is not productive on a street engine with a fixed car weight.

The only 292 I ever drove was in a C60 old state dump truck and the flywheel fell off on the way to the quarry. A buddy of mine hooked us up with a late model low mileage Mr Goodwrench motor complete with all accessory's from a UPS truck. That thing is a torque monster for what it is, I would not worry about spinning it 5500 and take advantage of that low rpm torque of the 4.125 stroke.

First of all...... Thanks to all here for your thoughts on this. Points well taken by all and much appreciated.

Yes, this is a street engine. And yes, I'm an old guy (71). But I still enjoy locking out overdrive and going through the gears every now and again. And I do tend to overthink things at times. Just my nature.

The point of destroking from 4.120" to 3.875" was to minimize the harmonics issues the long stroke, tall deck 292 inline 6 engines are prone to when run at sustained engine speeds of 5,500 RPMs or more. When you spin that 31" long 73 lb. crank at those engine speeds factoring in the heavy rod/piston combo, long stroke and distances between firing pulses, etc...... The forces at play tend to wreak havoc inside these engines. And the longer the stroke, the more it wants to self destruct. So my plan was to destroke a bit, lighten the reciprocating parts substantially and run the best torsional dampner setup I can afford. Figured a square engine (3.875" bore X 3.875" stroke) with a 0.040" overbore = just a fraction less than 280 cu. in. and would allow her to rev a bit.

However, I see now that offset grinding the rod journals from 2.10" dia. down to 1.850" in order to accomplish this creates more issues (oiling and longevity related) than it solves in this instance.

So I will go back to my original plan and build the 292 with 0.040" overbore and I'll lighten both the rotating and reciprocating parts, use the best torsional dampner setup I can afford. And balance the bejavvers out of it, of course. Will use just enough cam to support the engine to self imposed redline of 5,000 RPMs to keep it together. Build it for torque and gear it to run about 2,100 RPMs cruising in high gear overdrive at 70 MPH.

16 years ago, a friend gave me a copy of Dyno2003 to play around with. I don't believe these computer programs are necessarily right on the money, but do feel they can be useful in predicting the comparative results of changes made to a given engine combo. So I used it to compare predicted results for my 292 engine using 2 different cams I designed using Isky solid lifter flat tappet lobes and flow numbers for a Sissel lump ported head with 1.94" / 1.60" valves.

Cam #1 has a nice power band, but needs to spin up to 6,000 RPMs to make full power. Used Isky ZM-89 lobe for both intake and exhaust and tried several LSA, ICA & ECA variations. I found best results with a single pattern cam having 244 degrees duration @ 0.050", ground on 107 deg. LSA with 106 deg. ICA and 108 deg. ECA and 64 deg. overlap. The 194/230/250/292 inline 6's run 1.7 rockers, so valve lift with this lobe worked out to 0.544".

Cam #2 (this is the cam I will run) makes the 292 into a stump puller with max. power between 4,500 and 5,000 RPMs. Used Isky S-50 E4 intake lobe and S-99+10 exhaust lobe, again experimenting with several LSA, ICA & ECA variations. Found best results with 210 deg. intake and 216 deg. exh. duration @ 0.050", ground on 108 deg. LSA with 108 deg. ICA and 108 deg. ECA and 42 deg. overlap. With 1.7 rockers,valve lift is 0.481" for both intake and exhaust.

The interesting thing when comparing these 2 cams (same engine specs except Cam #1 required higher comp. ratio (10:1) whilst Cam #2 required 9.4:1 for correct DCR)......

Cam #1 with 244 deg. duration@ 0.050", 107 deg. LSA and 0.544" valve lift when compared with Cam #2 having 210 deg. / 216 deg. duration @ 0.050" and 0.481" valve lift ...... Made 325 HP @ 5,500 - 6,000 RPMs and 338 ft./lb. TQ @ 4,500 RPMs. Not too shabby for a 292 inline 6 in street tune running through a full length muffled exhaust system. But compared to the milder Cam #2, was down 20 HP and 50 ft./lb. TQ @ 2,000 RPMs and did not make equal HP and TQ to Cam #2 until reaching 4,000 RPMs.

And whilst the milder 210 deg. / 216 deg. cam 'only' made 270 HP @ 4,500 - 5,000 RPMs and 335 ft./lb. TQ @3,500 RPMs (down by 55 HP and 3 ft./lb.TQ at peak)...... If you average all HP and TQ readings from 2,000 RPMs to each cam's respective peak HP and TQ reading...... The milder cam averaged 25 less HP, but 15 more ft./lb. TQ than did the 244 degree cam....... At between 1,000 and 1,500 fewer RPMs.

In fact, the milder cam had the flattest torque curve I have ever seen...... Made a minimum 318 ft./lb. TQ from 2,000 RPMs through 4,500 RPMs with peak TQ of 335 ft./lb. @ 3,500 RPMs. At least according to this old dyno program. So I will run Cam #1 and gear the car accordingly. Interestingly, this cam worked out pretty close to the old Isky E-4 grind from the mid-'50s except with higher valve lift due to the 1.7 rocker ratio (one lobe is actually from the old E-4).

Will let you know how it works out. Should be a torquey little 6- banger. :)

Best regards to all,

Harry

You might get a little more feedback on this site concerning a Stovebolt 6. I'm still trying to get past how a 216ci engine with 85hp at 3200 rpm, was normal back then on my 7200 pound truck.

https://www.stovebolt.com/ubbthreads...gine-shop.html

Harry there is a huge difference between those two cams, and it's reflected in the peak power level.

Thing is, changing the stroke a little bit isn't going to change the potential for vibration at all.

Quote:

---

Originally Posted by 55 Rescue Dog

You might get a little more feedback on this site concerning a Stovebolt 6. I'm still trying to get past how a 216ci engine with 85hp at 3200 rpm, was normal back then on my 7200 pound truck.

https://www.stovebolt.com/ubbthreads...gine-shop.html

---

Thanks, Rescue Dog! I'll check it out! Regarding your question...... Until the early '60s, the Chey inline 6 was pretty much like the engines in most pre-1950 cars. Low compression, small bore, long stroke, designed for low RPMs. The way they got around that (to a degree) was torque multiplication (high numerical rearend gearing and transmission gearing) resulting in a vehicle that might not run very fast on the highway, but one that was able to move quickly enough in town at moderate speeds.

Some vehicles of the period (1930s onward) were equipped with overdrive transmissions or 2-speed rearends and once moving at moderate speeds, engaged these to achieve higher speeds on the highway.

Hope this helps,

Harry

Quote:

---

Originally Posted by Rick_L

Harry there is a huge difference between those two cams, and it's reflected in the peak power level.

Thing is, changing the stroke a little bit isn't going to change the potential for vibration at all.

---

Thanks for your thoughts on this, Rick. Much appreciated. Yes, it was interesting designing the cams. Over several weeks time, I went through dozens of cam lobes from several cam manufacturers looking to put together the right combination of lobe and timing events. The 244 degree cam would work well for the shorter stroke, low deck 230 or 250 inline 6's. These are not affected by the harmonics issue at higher RPMs as badly as are the longer stroke, tall deck 292 engines.

I've been re-reading Leo Santucci's book on Chevy inline 6's and whilst the longer stroke of the 292 engine is a contributing factor in the harmonics issue these engines experience at higher RPM's...... Its only a part of the problem. I was considering destroking the 292 by offset grinding the rod journals in order to strike a balance between increased displacement over that of the 250...... And the harmonics issue. Unfortunately though, that method of destroking only causes other issues with regard to longevity and oiling.

As a custom 4340 billet steel crank with 3.875" stroke is much too expensive for my budget...... I will leave stroke at 4.120" and address the other contributing factors to the harmonics issue as best I can. And limit RPMs, of course. The 210 / 216 degree solid lifter cam should make great power within these self imposed RPM limits.

At these RPM levels, I would normally consider a hydraulic cam for a road car in this state of tune, but I have a 1966 292 block and Isky informed me that 292 engine blocks priour to 1971 did not have the correct oiling provisions for hydraulic lifters. Which seems a bit odd, but they are the cam guys and should know about these things...... So I'll run solids in this engine.

Best regards,

Harry