I was just wondering if someone knows where to find or what the measurements are for:

Optimum Bore to rod ratio

Optimum Bore to stroke ratio

Also in the header formula:

Header pipe length (in inches) = ((850*(360-EVO))/RPM) – 3

Does anyone know what the EVO stands for?

Maybe if there are anymore measurements that are looked at when building an engine they would be appreciated also.

Thanks,

Roman

 

EVO = Exhaust valve opening

 

Thanks.

 

Exhaust valve opening at gross lift .006, not .050 I have the math for it in my Valve event / DCR calculator (but the DCR stuff needs a bit of work).

As for optimal R/S and B/S it depends on what you want to do...

 

exactly what i was going to say, its application specific.

 

Not sure.

What are the dimensions for say......a N/A motor?

And also a full out blown motor?

What are the differences?

Come on, release some of that info.

Thanks,

Roman

 

At least a couple of numbers?

 

Ok, I'll make a few comments since no one else has.

To determine what is optimal, you need to know what your requirements are, and what any limitations are.

Is it RPM?
Is it displacement?
Is it something else?

Ok, I will speak generally first using the LS Series motor as an example.

In looking at the design of the motor, I would choose first to have as much bore as I could get away with without compromising the structural integrity of the motor. The main reason for doing so is simply for better breathing. Look at a flow bench. What happens when you put your heads on a bigger orifice? Flow goes up on the head. You unshround the valves and allow the motor to breath better. So, on the LS motor I would look at bore first.

Obviously then you next look at stroke. If you have no restriction on stroke, then you simply use as much stroke as the motor can take. Now, on the surface that sounds like you put the biggest crank that will fit. Thats not always the case. There are a few items which might hinder you. You have to look at what will fit inside the block. You have to look if you are going to hit something (like the skirt of the piston or the reluctor wheel).

The other issue with stroke is your rods. You have to have a rod long enough to clear the counterweights, but no so long that your piston can't be built. One of the issues with really long stroke motors is that the piston ends up being compromised to get it to fit. The ring pack has to be shifted around, the wrist pins are almost up in the top ring, etc... Its all a balancing act.

The other thing with a lot of stroke is piston speed. As stroke goes up, piston speed, and the forces acting on the rod and piston go WAY up. For instance, in an old school SBC when you go between a stock 350 with a 3.48" stroke, and a 400 crank 3.750" stroke (i.e. you build a 383). The forces on the rods and the rod bolts @ 5000 rpm on that 383 are about the same as a 350 @ 7000 RPM. Can you see that this is a lot to put a stock rod and bolt up against...

Ther eis a lot of argument about rod length, especially when it comes to race motors. In looking at a street car, or even a max effort 346, I would look at the longest rod I can get in there safely. As for why, here it is. I have seen that long rod motors are less sensitive to poor camshaft selection than short rod motors. To me this seems to have a lot to do with the pistons travel up and down the bore, and the dwell at top and bottom, and how that travel is influenced by rod length.

Like I said, I am generalizing. When you get into race motors, (and I mean "real" race motors) this stuff gets a lot more criticial.

For what we do, (if it was me) I'd simply look at the biggest bore first, followed by longest rod and stroke.

Thats for max power with no restrictions.

If you are limited to cubic inches, then you have to look at operating speed which is controlled by intake and cylinder heads, and you have to figure out how tight you want to spin the motor.

Let me give you two examples. I have a B/SM motor in my shop that is limited to ~355 cu inches and a 750 carb. sounds simple until you look at the fact that the motor is a 4.155 bore and a 3.25 stroke and the motor needs to be behind a 5spd, and have a powerband about this wide | . .8xx lift cam and revs to 9200 (shift @ 8800).

We race in some class limited stuff that is intended for ~350 cubic inch motors in a 3200lb car, unported heads, etc... In that case, we are using a 4.030 bore x 3.5625 stroke (363 cubic inches since that is a "sweet" spot in terms of weight vs cubic inches). Motor revs about like an LS1 (makes power to ~6600-6800).

So, to answer your question it depends on your application, and what you are trying to do.

 

VERY good info! Thanks!

 

Wow, thanks. Sorry for the vagueness.

Ok then 4.01 bore iron block spinning to 8500 with a 8:1 compression ratio. With lots of forced induction.

I figured that the 3.65 stroke would probably be the best, or maybe even less?

Thanks for the above response,

Roman

 

This is a question I thought about alot when we did my C5R. In the end, we had to take our best guess. We knew we wanted a bore of 4.155 (the bigger the better so far as it's safe enough.) So the only question was what stroke? I considered...

3.65" (396ci) Now this would be a high RPM screamer! 9000+ RPM shouldn't be a problem at all. Major gearing and a 7000 stall required to make the best use of this engine. This engine would be highly efficient and would make awesome HP per CI. It has an EXCELLENT rod to stroke ratio. However, perhaps it gives up too much CI that it can't make up for with RPM/gearing potential.

4.00" (434ci) We still have the potential for some pretty good RPMs (maybe 8700 or so) at this stroke but we'll have to use 4.something gears instead of 5.something gears if we want to make it across the finish line and a converter around 6400 will work well. That's a bummer but the extra CI should negate that and then some. Rod to stroke is still ok but piston velocity is starting to get high if we can spin the engine in the high 8xxxs.

4.25" (461ci) This engine will feel comfortable only below 8000 RPM with it's short rods and long strokes. Piston velocity gets astronomical REAL quick with this one. Side loading of the piston will be a very notable factor in this engine. It'll have a 5X00 converter and 4.10 gears. What it lacks in RPM and gearing it will try to make up for in shear ground pounding TQ! Afterall, there is no replacement for displacement, right? Weeeell, that's not always the case in drag racing. I think this engine gives up a little too much buzzability for it's air gulping CIs to make up for.

I went with the 4" stroke.

 

Ok, so that helps out.

One of the things you will find in forced induction car (especially a big turbo car) is that RPM helps offset cylinder pressure. In other words in a big boost application, RPM will help you keep from lifting ring lands. This was the article that the guy (I can't remember his name) used when building his twin turbo C4. It made 1200HP. It had a small solid roller (Street Roller) and would rev to around 8000.... But the motor stayed alive... So thre are considerations there too.

 

I did a 388 all bore built around a 4.131" bore wet sleeve aluminum block, a 6.300" (+0.200" longer than stock) rod, and a stock crank. The REALLY cool thing is I was able to use off the shelf pistons (pistons used for a 4.131" bore X 4" stroke 427CI) and off the shelf SBC rods. This made things cheaper while at the same time giving me lots of head flow, CIs, and RPM capacity. I can say this thing revs like nuts. The long rod also helps makes the engine SMOOTH as it keeps the piston from being "snapped" from TDC or BDC but instead "drift" up to speed. I have front and mid plates on my engine (solid mount) and even when durring tunning I had some times the engine fell to 400rpm and "idled" before it died the car DOES NOT VIBRATE (the rotating assembly is smooth as glass.....it's all LOPE ). I snap the throttle and it goes from idle to 4500 instantly (and this is with a 244/253 110LSA cam). It's NICE

 

Thanks J-Rod, So do you think that a 4.01 bore 3.65 stroke with lets say a 6.3" rod (off the cuff) will be ok? I'm actually thinking that maybe a smaller stroke would work even better. Maybe a 3.4 inch stroke 343 c.i.? Or will the rpm gain be lost with the cube loss?

How do we figure out piston velocity and what is a good speed? Thanks Colonel, that's an aspect that I need to look at. That's where the stroke/rod ratio comes in right? I was told that a good S/R is at 1.55 to 1. What do you think?

Roman

 

Formulas for piston speed:
piston speed in fpm = stroke in inches x rpm / 6

rpm = piston speed in fpm x 6 / stroke in inches

 

So with a 3.65" stroke I will end up with 5171 fpm (feet per minutre?)

and with a 3.4" stroke 4817

So what is a good speed?

So these formulas, do you have them memorized or recorded somewhere?

I am impressed.

Thanks,

Roman

 

Roman are you going turbo or SC? That makes a difference.

With the LS1's oiling setup I would look at sticking around or under ~7500rpm max engine speed unless you are going dry sump.

I would look at a C5R block (4.1XX bore) with a 4 inch stroke. (Good piston choice and you can dish the piston a fair bit with that rod/stroke combo) no big reluctor wheel issues or block clearancing problems either.

Shoot for right around 8:1 compression with the pistons ~.005-.008 out of the hole with a BIG reverse dome and put on a large single turbo.

AFR heads with extreme porting and port volumes. (TEA did well with their FI AFR heads. I am not sure what the chamber volume was but it would be nice if they could get out to 80cc's)

Strong turbo cam (preferrably solid)

Are you staying stick or going with a glide or TH400?

You'll need a big *** fuel system, injectors, BS3 tuning and BIG turbo

I think 1600+hp would be possible and VERY cool

 

If you are using an iron 6.0 block then a .0.40 over bore is about it, some have done .060 and .080 but you are really pushing your luck!If you are getting a new crank get a 4.0 stroke no replacement for displacement!

 

Those formulas are pretty common and been around for a while. While the absolute numbers might not hold meaning, its the trends that need payed attention to

 

Would your decision have been any different if you could've turned that 461 to 9,000RPM+ without fear of grenading your motor?

If you don't mind saying, how much do your cylinder heads flow?