RyanJ

Ok, so I've been dreaming up combo's since the new SEMA stuff started showing up (ERL decks and World block). I was wondering about rod/stroke and bore/stroke recomendations for builders and the all-knowing in this forum. I know some of you guys are going to be building some big *** motors with these new blocks.


So, I think I read that 1.5+ is what you want for rod/stroke, and 1.2" is a good comp. height for a NA/maybe N2O piston. Thicker for FI.


I was thinking that a 488-499cid motor would be nice with the new world 9.8" deck block (4.5" stroke, 6.4" rod). That gives a 1.15" comp-height and about a 1.42 rod/stroke. Effective and efficient for a street/strip car? Or should it really be around 1.5? A 4.25" stroke w/ 6.5" rods gets me above 1.52, but using the same 4.2" max bore its only 471 vs ~500.

So what's better 500 inches at the ratio's above, or a more efficient 471?

With the ERL stuff (which for a street car IMO would be a big pain with the new size of the block) the deck is heigher so I guess you could run a 1.5 rod/stroke with a 4.5" stroke which I guess is really the optimum.

Also, what size headers would be necessary with 400cfm heads on a 470-500cid motor (f-body platform)? 2" primaries? Larger?

DAPSUPRSLO

iTrader: (14)

I was recommended a 2" primary header for my 368/233 cfm c5-r heads and 434 but could only get kook to make a 1 7/8" to 2" primary header for me. I think 2" would probably be a starting point for something that big. Good luck with that, that should be way nasty.

Big-DEN

On the headers, not sure if 1 7/8" stepped to 2" is really or any worst than 2" straight
header.

We all know exhaust valve is 1.6 or 1.625" and port not much larger, I never thought it made too much since port transition wise to go from a port that is maybe 1.7" jump to a 2" header. We need to keep in mind inside diameter of header tubing, as measurements are external.

I'm sure most cases that 1 7/8" transition to 2" is probably better across the board than 2" straight in case of most huge small blocks.

Ben R

The best Rod/Stroke ratio is 8:1. 8 Rods to 1 Crank.

Old SStroker

Don't drive your engine design with R/S ratio. Start with objectives for what the engne needs to do in the vehicle, and go with the max cubic inches that are economically obtainable. Depending on the block and heads you use and the rpm you expect, bore and stroke will fall in place, as will rod/stroke. Let r/s fall where it may. It's just not that critical.

At the extreme for a 500 cuber, you can go with 4.70 bore and a 3.60 stroke. Using a 9.25 deck height and a 1.2 CH, you could use a 6.25 rod and get a 1.74 R/S. Or you could use a 9.0 deck and use a 6.00 rod for a 1.67. Of course that's the kind of thing that can twist (and make power) well past 9000. That would be a DRCE3 block...not where you are headed, I suspect.

I wouldn't choose a 4.2 x 4.5 BBC unless I was fighting detonation in the extreme and wasn't spinning it. 4.46-4.47 x 4.0 seems a lot better to me. Assuming the heads you choose have sufficient flow, why choose a 471 and give up about 30 cubes? Shoot, go 4.56 x 4.375 and get the 572.

Your engine's potential output will determine the header size. Are you looking at 600 fwhp? 700? more?

My $.02

DAPSUPRSLO

iTrader: (14)

[QUOTE=
Your engine's potential output will determine the header size. Are you looking at 600 fwhp? 700? more?

My $.02[/QUOTE]

How about for 700ish?

NO CATZZ

iTrader: (10)

This is quite a coincident, I have been reading Reher-Morrison Lower Engine Assembly (great book) and just covered a section about rod selection. I don't want to infringe on someones copyright but I will 'paraphrase' the info on rod length.

Conventional racing engine theory states that long rod engines have significantly different characteristics than do short rod engines. The theory is that the rod length affects the position and speed of the piston. Statements are often made that the piston "lingers" near TDC longer on a long rod engine and affects breathing. This turns out to be largely untrue.

Does the piston really "linger" near TDC on a long rod engine? (references a graph the plots piston location for a 6.135" 6.385" and a 6.535" rod on a GM 502ci bigblock...I will just type the info below it)

At 10* ATDC (the time when the most pressure is present in the cylinder on a power stroke), the difference in piston position location between the longest (6.535) and the shortest (6.135) rod engine is only .0004" (four ten-thousandths of an inch). Even at 45* of crank rotation, the difference is only .01". This amounts to only .16 cubic inches per cylinder on a 502 CID engine. Rod length and angularity have very little real impact on engine performance.

The book also gives a tip...Choose your crankshaft and pistons first--they will dictate rod length.

Hope this helps.
Josh

RyanJ

Thanks for the info. I was bouncing around ideas for the new World LS1 block, so bore would be 4.125-4.200" (if it can take that, 4.125" is the standard option) and stroke could be anything up to 4.5". Deck height is 9.8" and I want to keep CH resonable (1.15-1.25"). I guess the real question was would a 1.42 rod/stroke hurt me in the long run vs. a 1.5+ ratio. Its sounds like it wouldnt. 500cid would be nice, but I didn't know if I would be wasting energy and actually get more power from a smaller motor with longer rods. The RM book sounds like its a wash.

Thanks for the info guys!

Old SStroker

Duh! I forgot this was an LS1 forum. Sorry. When I see 500 cubes I think BBC.

What type of LS heads did you have in mind for 500 inches? The expensive extra liter or so (over a 427) won't do you much good unless your heads can flow lots of wind. C5R based heads perhaps?

NO CATZZ

iTrader: (10)

I think you missed the point of the info I posted above. Bottom line, rod length is the least important aspect of choosing your components. The effects of a rod/stroke ratio above or below 1.5 are so insignificant it should not even be a determining factor when selecting engine parts. And if RM is a wash I guess they have just been wasting their time building winning Pro Stock engines. I was just trying to pass on some information...I will be quiet now.

Old SStroker

Some of us are listening, some aren't. "You can lead a horse to water, etc..." Thanks for the RM reference.

Here is a quote from David Reher:

"We also wanted to point out some of the common myths and misconceptions about high-performance motors. For example, I've seen dozens of magazine articles on supposedly "magic" connecting rod ratios. If you believe these stories, you would think that the ratio of the connecting rod length to the crankshaft stroke is vitally important to performance. Well, in my view, the most important thing about a connecting rod is whether or not the bolts are torqued!

If I had to make a list of the ten most important specifications in a racing engine, connecting rod length would rank about fiftieth. Back in the days when Buddy Morrison and I built dozens of small-block Modified motors, we earnestly believed that an engine needed a 1.9:1 rod/stroke ratio. Today every Pro Stock team uses blocks with super-short deck heights, and we couldn't care less about the rod ratio. A short deck height improves the alignment between the intake manifold runners and the cylinder head intake ports, and helps to stabilize the valvetrain. These are much more important considerations than the rod-to-stroke ratio. There's no magic - a rod's function is to connect the piston to the crankshaft. Period."

DAPSUPRSLO

iTrader: (14)

My buddy and I have been having a discussion regarding this statement. Is this in the context that it has not effect on power? Or is this in the context that it has no effect on longevity and wearing a motor? Or is this in the context of both? Obviously if this statement is just regarding a race motor which will be rebuilt very very often then it may not apply to 90% of the people who visit this board. 90% being those with street/strip motors which are required to run along time. Any thoughts.....?

87calais

I always thought the idea of going with a longer rod is because the longer rod is lighter overall then the taller piston... right? long rod short piston is usually lighter then heavy *** piston with a short rod.....

Pro Stock John

iTrader: (34)

"If I had to make a list of the ten most important specifications in a racing engine, connecting rod length would rank about fiftieth. Back in the days when Buddy Morrison and I built dozens of small-block Modified motors, we earnestly believed that an engine needed a 1.9:1 rod/stroke ratio. Today every Pro Stock team uses blocks with super-short deck heights, and we couldn't care less about the rod ratio. A short deck height improves the alignment between the intake manifold runners and the cylinder head intake ports, and helps to stabilize the valvetrain. These are much more important considerations than the rod-to-stroke ratio."

Thanks for posting this, I was going to start a thread about this today.

Rick@Synergy

iTrader: (5)

I know rod ratio is not always important, but it does help with some build characteristics. Take a blower or turbo car. Longer rod is nice because of the wear and side shirt action. With longer rod, you can put less wear on the bearings and make the piston lighter. So I do think there can be some advantages in it. And other times you dont have a choice as to what rod you can use. Sometimes you have to pick what they have. I was able to stuff a 6.3 rod in my turbo motors. The thing I notice on the dyno so far is the sucker doesnt stop making power. The power band is long. Its kinda nice.

Rick

RyanJ

Its cool. There's a thread on the World Products stuff from SEMA in the LS1/LS6 Internal forum I think. World is casting heads that also use the 6-bolts-per-cylinder. They use a LS7 intake side with its valvetrain components and a C5R exhaust side. 400cfm is apparantly attainable (like C5R heads).

1bdbrd

iTrader: (12)

i didnt read all the info post above but rather skimmed it at best so forgive me if this was covered.

but from my understanding, much like Rick, i think rod/stroke ratio is important because it minimizes the wear on the bearings as well as the load on the piston skirts. this enables a car to rev higher safer. now thats not to say have a very good r/s ratio automatically means you can rev it higher. nor does it mean that if you have a bad r/s ratio that you cant rev it high. look at a few engines for example. honda b series engine i believe have a near 1.7 r/s ratio (ill check for sure tomorrow) and some are turning 10k rpms in a street driven setup with a big turbo on top. the 302 SBC from the first gen Z/28 has a perfect 1.75 r/s ratio and was able to turn 7500+ rpms in stock form (i read that someplace). the 377 SBC has somewhere near a 1.7 r/s ratio i believe and is known for its high rpm power making capabilities. and finally the 1jzgte supra engine has a perfect 1.75 ratio. ive seen a couple push over 8k rpms in relatively stock form.

again this is just what ive learned in the past and could be completely off base.

Rick@Synergy

iTrader: (5)

That was the reason I choose the 6.3 rod for my turbo motors. I wasnt too concerned with going to 7K, it was more the fact that the turbo and the wear is what I went after. I want the motor to last longer. Granted I built other similar motors with 6.125 rods, but wanted to try this. And yes, on the dyno I noticed this turbo car has a better HP graph than other turbo cars I have done that has the stock 6.098 rod. Its very flat now. And keeps going. Fun to drive. The cool thing about long rod is it also changes dwell time and lets the heads flow alittle better. For me it was all about new things. Cant really say you dont like something if you dont try it right?

On a side note, I bet I could go to 7K on this motor and it will still make power.

Rick

TransAm

Exactly. That builder doesnt care about r/s simply because his motors are not together long enough to have to worry about piston side loading and bearing wear. RyanJ, a 1.5 r/s would be perfect in your nitrous motor build as far as power curve goes. and also be very forgiving to your bearing and rod bolts while turning 6500+ rpms. building a street motor and a race motor are apples and hamburgers.

TransAm

I bet you mph was also higher.