Rod to Stroke Ratio and Connecting Rod Angle?

Hey Everyone,

I am looking for thoughts on this engine build. I know this is not the optimal way to build an engine but I am trying to build this engine with extra parts that I currently have.

Stock Rod to Stroke ratio is 6.608 divided by 3.9 = 1.694 which many engine builders supposedly strive for 1.700

My 535 rod to stroke ratio is 7.100 divided by 4.5 = 1.577

The 523 that I am looking to build could look something like this... 6.350 divided by 4.400 = 1.443

The reason to build the 523 like this is that I already have a nice piston with a 1.977 pin height. So,m y concern with this combination is certainly the rod to stroke ratio being to low. In addition to the low rod to stroke ratio the rod angle would put greater side loads on the block. This build is only going to be a 10:1 compression ratio and is really a street strip application

What do you think of this situation? Or would I be better off just building a 464 with the 1.7 rod to stroke ratio?

Thanks in advance!

 

Sounds to me like it would be fine for a street strip engine. I am interested to hear what others think... I say build it!

 

There is nothing like big cubic inches for the street or strip. The 523 would be a blast.

A 1.4 rod ratio for a street/strip engine is not a problem as long as the piston is a good forged unit.
There are two local (Spokane) 600 cid street/strip BBC engines with rod ratios in the 1.45 range shifting at 7200 rpm. No problems
The preference is a non silicon 2618 alloy piston. CP, JE, BME ..

The concern is with the Buick 455 cylinder wall thickness and going out of round.
Partially filling the block should help.

I'm thinking do the partial fill, girdle the bottom and go for the 523.

Does the bottom of those piston skirts clear the crank counter weights?

Paul

 

Paul,

Checking for counterweight clearance is good advice! That is something I did not think of. The pistons are JE's and I will check the counterweights!

I have the block bored and honed already. If I put a partial fill in it now do you think it will move the cylinder walls a bunch?

This engine may see 6500 RPM's max.

 

I don't know what your block will do if you do a partial fill now.
It might be a simple matter of just having to re-hone.

We've only dealt with bore and hone after the block fill, so I'm hoping someone else can provide an educated answer.

Paul

 

No John I wouldn't use those pistons. You should sell them to me!!! All kidding aside, considering that this car will see 98% street duty with just an occasional trip down track I don't think it's a real big deal. Using hard block on the street also presents potential peoblems in itself. (cooling) For what this car is being built for, and the method of the build, I would think it would be just fine. BTW I need that Canadian phone number!!

 

I would me more concerned with the top half of the engine, you can get more power there than in the cubic inches.

 

The extra torque from the cubes means a lot though on the street don't need as much spec to get the same power

 

I can't hook my 455's on the street forget about a stroker

 

http://www.highperformancepontiac.com/tech/hppp_1003_gm_performance_parts_lsx_454/viewall.html

1.49 R/S ratio in the 454 LSx above that makes peak HP at 6400. Old conventional wisdom (Smokey Yunick comes to mind) has fell to the wayside due to reasons I don't know, although I suspect lubrication technology and modern head flow have something to do with accepting higher mean piston speeds. Your combo might require a unique cam profile compared to what we are used to. From memory a shorter rod results in the need for more intake port flow (but maybe it was less...)

All of those compromises should still make a VERY fun street engine.

 

Here is a link to an earlier thread;

http://www.v8buick.com/showthread.php?58551

In post #5 there's a chart that shows rod/stroke ratios and rod angles.

Hope this helps!

 

I worry about that much piston speed and the relatively small buick ports. I'd calculate the effective CUI with the cam choice then peak port velocity. It's going to want huge port volumes and a small high lift cam.

http://www.veryuseful.com/mustang/tech/engine/portLimitingVelocity.html

 

Good conversation! After reading through the posts I have come to a few conclusion. If you disagree please feel free to tell me why you disagree!!!

1. Rod to stroke ratio itself makes little difference to an engines performance given the fact that ALL INTERNAL PARTS are equal weight. It may free up a few HP for the prostock guys but for us street/strip racers this is not an issue.

2. Where changing the rod/stroke ratio helps is when you lengthen the connecting rod and thus you can put the wrist pin up in the piston further and reduce piston weight. Lighter pistons equal less rotating weight which equals acceleration.

The links posted by Alan and David were a good read. They lead me to this link... http://www.rehermorrison.com/blog/?p=89

And this from the link.. its a good read so take the time to read the whole article!

"We also wanted to point out some of the common myths and misconceptions about high-performance motors. For example, Ive 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!"

David, the article on port velocity is interesting. Since I will be using an old set of Stg 3 heads this will not be an issue. But, I do have some very reliable information that would indicate a smaller port entrance = BIG HP gains. I started to research this subject after porting a set of Bulldog aluminum heads. I was easily able to get 355 CFM out of the intake port where the port entrance is the size of a stock 455 cylinder head. Unfortunately the exhaust side of the Bdog head stinks!

So, I have decided to go stock crank that will be stroked to 4.00 and a 2.100 rod journal. Use a 6.535 (approximate) inch aluminum rod attached to my existing lightweight JE pistons. This should yield 476 CID. I will try to make the port entrance of the Stg3 head smaller by raising the floor in the intake runner of the cylinder head and the intake. It will be interesting to see how much HP my pump gas (87 octane) engine will make.

Thanks

 

hey john what is the pin dia of your piston and dish volume?

 

Adam,

It is a .927 pin with a 20cc dish. To get the compression ratio correct, I may have to change the rod length a bit and increase the cc's in the dish.

What are you thinking Adam?

Thanks,

 

Sounds like were are building very similar engines . I am also told by others that that is the way to go . Port diameter & raising the port-angle entrance of the intake runner head to manifold .

 

i was thinking a howards 6.5 sbc billet rod, i think they are well under 700 grams and under 800 bucks ,4.05 stroked stock crank,6.5 rod =10.502 deck ,1.6 rs ratio if that doesnt work just sell me those pistons i have a couple of other shelf rod things in mind.

 

Here is another article regarding rod/stroke ratio. This makes total sense to me!

http://www.chevyhiperformance.com/t...0608ch_race_engine_building_tips/viewall.html

Rod Length
"Most people tend to overgeneralize this issue. It would be more accurate to compare different rod-to-stroke ratios, and from a mathematical stand-point, a couple thousandths of an inch of rod length doesn't really change things a lot in an engine. We've conducted tests for GM on NASCAR engines where we varied rod ratio from 1.48- to 1.85:1. In the test, mean piston speeds were in the 4,500-4,800 feet-per-second range, and we took painstaking measures to minimize variables. The result was zero diff-erence in average power and a zero difference in the shape of the horse-power curves. However, I'm not going to say there's absolutely nothing to rod ratio, and there are some pitfalls of going above and below a certain point. At anything below a 1.55:1 ratio, rod angularity is such that it will increase the side loading of the piston, increase piston rock, and increase skirt load. So while you can cave in skirts on a high-end engine and shorten its life, it won't change the actual power it makes. Above 1.80- or 1.85:1, you can run into an induction lag situation where there's so little piston movement at TDC that you have to advance the cam or decrease the cross-sectional area of your induction package to increase velocity. Where people get into trouble is when they get a magical rod ratio in their head and screw up the entire engine design trying to achieve it. The rod ratio is pretty simple. Take whatever stroke you have, then put the wrist pin as high as you can on the piston without getting into the oil ring. What-ever connects the two is your rod length."

 

John

What heads are you using for this build and what is the chamber volume?

Also what is the rod journal size on the 4.40 crank?

Paul

 

Paul,

The cylinder heads will be the T/A Stg3s. I am not sure of the total volume but I would have to guess somewhere near 250 CCs.

The Moldex currently has a 2.200 journal. But I am not going to use this crank. I think I am going to use the stock crank and change the stroke.


Take Care