Engine and Transmission Tech

Art, I always appreciate your excellent advice.

I'll check out the prices of the 5.85" and 6.0" Chevy rods and compare that to the cost of working up the rods I have on the shelf. I've got some 283 and 327 small journal rods (they are only slightly different). If they aren't already rebuilt on the big ends, I may opt for some custom rods. Piston selection gets easier with the 5.85" length rods.

The rod ratio is pretty nice at 1.86:1 with the stock rod, though. That's pretty good.

I'd prefer to use the stock rod because it would be less work and less money. But the pin size on those Buick 3.2 pistons is .9398" and the pin size on the stock Olds rods is .875". I don't know much about boring out wrist pin bores, but I was guessing that boring the stock rod pin bores that much might not be a good idea. Would it work?

I just assumed that boring out the Chevy rod pin bores (.927") was not as risky.

I'm not crazy about the dish on that piston, 9.5cc, and I would have preferred a flat-top with notches for the valves, but I just figure I was fortunate that Nic was able to find something that was going to be close enough to work.

The 4.2 crank is costing about $80 including shipping, maybe a little more. It might need to be turned. The pistons are stock-type, so the cost of that would be a wash. So, it's just a matter of modifying a set of stock Chevy 327 rods while they get rebuilt. Not bad.

Edit; with a .030" oversize piston, displacement would be 237"/3.9L.



Edited 1 time(s). Last edit at 12/11/2009 03:14PM by pcmenten.

Ok. good point. I didn't think about the pin dia... OK so let's say you use the 5.85" (+150) SBC rod with the Chevy 2.8L V6 piston. The pin dia in the piston is only .905. Bore that out to .927 and you're good maybe.

The one I'm talking about is the #1478

Gentlemen, If #1478 takes a floating pin, probably better to bush rod to .905" pin size? Reaming out piston pin bores is a "precision" job or the piston becomes an ash-tray. roverman.

Nick/Art,

You might like to read this article by a certain Mr. Iskederian which covers the issues of rod length and cam duration. intersesting, controversial?

[www.iskycams.com]

Kevin.

I think he's correct in that you will not change the output of the engine by changing the rod length alone. Another thing I agree with him about is that an engine is setup the way it is and there are some things that are just going to be the size they are. That's true for us too. I'll get to that. I also agree about the assertion that long rod ratios are meant for low end torque motors. The piston speed at TDC of a short rod ratio is better for that.

I did think it was funny that he started out arguing that rod ratio doesn't make a difference and then countered with a story that it does. LOL! Seriously though, I'd love to sit down and have a conversation with him and some other guys of his generation that were there when all these ideas were being formed: Crower, Edelbrock, Burt Monroe. That would be cool. I'd ask Crower why his LCAs are so wide. I do appreciate Isky's argument that the piston sitting at the bottom of the stroke is better for sustained cylinder filling but this is why it's better for low end torque.

I think he's starting out his article with hyperbole to make a counter point that you are not going to unlock secret power by changing rod ratio alone. What we build in an engine is a system of cooperating components. In our case we are pretty much limited to what Chevy guys would call impossibly small bore size to start with. SO we are limited in our ability to increase valve area. With that in mind short rod ratios are going to hamper what we have to start with. I'm talking about monetary feasibility as well as mechanical and expertise. My point is the rod ratio should work together with the other mechanical functions of an engine to be optimal. The point is, the engine he described is one way to make power. What you need to do is optimize the cam to… Wait a minute. Is he selling me a cam?! LOL.

To use his example as a point that rod ratio is an important cooperating factor. The engine he describes NEEDS more duration to MAKE power on the top end. And he's right changing the rod ratio alone without changing anything else would likely yield no benefits. With our engines, considering the bore limitations as well as the limits of our head castings we have to keep these factors in mind when considering rod length. If we had room for massive intake ports we could get away with shorter ratios as well.

The last thing to consider is engine longevity. Piston side loads and shear are factors to consider when building an engine to last. Just sayin'. In conclusion I agree with him. Rod ratios do matter but are not solely responsible for the creation of power in a 4-stroke engine.



Edited 1 time(s). Last edit at 12/12/2009 09:04AM by NixVegaGT.

Yes, I would one day like to know, half of what "Ed" has long since forgotten. I wish he would have opened that can-o-worms called,"wrist pin off-set". OEM.pistons, uses them almost exclusively. Aftermarket, especially forged,NO! I suspect they don't want the bother and not enough request? As I was taught, decades ago, correct off-set reduces latteral thrust on cylinder walls, reduces piston rock at turn-over and reduces "I" loads and ring flutter potential because rod and piston doesn't completly stop at top,(crank pin past tdc. while piston is not). I defer to my previous post that, not once ,did I elude to greater hp. and torque. I was expousing "some" of the merrits whilst "juggling" for a "combo" of parts to fit a particular build. I suggest a read of all of "Ed's" tech wisdom, especially of that dreaded, .842" flat tappet, maxed-out at .007"lift per deg. vs. rollers. My vote isn't cast as of yet, for oem. hyd. rollers, as I eagerly await for qty. 48 en-route. Time will tell, roverman.

Gang,

The 4.6 has a rod ratio just a hair north of 1.8 - what's the effective ratio with the offset?

Also, does the 4.0 have wrist-pin offset?

Gudgeon pin offset fpr both 4.0 & 4.6 is 0.6mm.

kevin.

To put this in perspective, I believe 305" sbc/oem is 3.175mm.(.125"). Unless I was snoozin in class, I don't think off-set changes the rod ratio. I would like to see the formula an engineer would use to optimize off-set for a given build and how it changes the "I" load at turn-over, roverman.

Mr. Isky didn't mention that silly-long rod ratio,(well over 2/1), that "Smokey" used in those 203"?, sbc's for Indy? Smokey wanted mega-long sqeeze time. Why? Slow burning combustion chambers, slow burning fuel(Methanol) and Turbo's. They ran pretty good for "Detriot wonder metal",cast iron stock blocks, once he deep-sixed the carbs. roverman.

My forged pistons have the stock 0.040" offset.

Jim

Jim, brand? For 340" sbb.? roverman.

Opps! You already said ,Venolia-right? They gave you the off-set option? roverman.

Oh yeah. For what I paid them I could have had them engraved if I wanted. (...for a small additional charge...)

Jim

Jim and clan, while speaking of sticker shock. CP Pistons gets $94.ea., base price for a flat top sbc-305 piston,2618 material. Pin off-set is $6. ea. Pins and clips are extra.Coatings extra. Since it's just a serious put-put, Rover, for the Huffaker, I'm goin with KB 832's in +.03", with pins, inserts/clips at $43. ea. and sort-of move the valve notches. Not my first choice on comp dist.-1.265" but not worth $94. each just because. Nic, your piston?

Art,

"KB 832's in +.03"

Can a Rover be bored that much safely (3.766 + clearance) or are you going to re-sleeve?

Carl,resleeve- Abalutely. Most o' the so-called, "experts", claim .020" os. is "it" on std. Rover-slip-n-slide-sleeves. I got (3) sets of "FSY3780SFM" from Wildcat. Here's how it works, Their made in Japan by Tsubaki,(best noted for good roller chain). Their boxed and distributed By ACL(Aussie),retailed by Wildcat(Wales) and then ,"maybe" sent to "us/Us"(5 mo's later). I'm thinkin they have a"lot" of miles onnum, before they ever get installed? There's "got" to be a better way ! I say, buy US. aluminum and keep your Rover,"pure", except for the made in England part, right Nic'...? roverman of dissent.

I'm definitely looking into alternatives. I'm hoping to have somethings to report in a week or so on that front. I'm still working out the details of using the 350 crank. It could be that by using the "Honda" journal at 1.89" it would help the cam interference issue. I don't have measurements between the rod bolt heads yet but it could afford more room for modification for clearance.

It could be that and a bit of destroking would do the trick to keep away from the cam. I'm not hot for the destroke idea just because if I'm gonna try to put a gigantic crank in here anyway I might as well try to make it as big as possible.

I'm concurrently working on the 4" bore thing too and exploring a potential solution for us there. With that in mind then I was looking at Manley's 1.05" height SBC forged pistons that coupled with the SBC "Honda" journal 6" rod and a slight destroke of 3.82" would yield a zero deck piston. Another option is JE pistons' 1" SBC forged piston with the "Honda" rods and retain the stroke for a piston deck 20 thou down the hole. That feels a bit better to me but the cost is another couple hondo$$$. Might be necessary though.