Ok, I sure have learned alot from this discussion and I thought of another question on this subject.
From what I've gathered so far it seems the intake window ( runner volume + valve area + intake lobe ramp) needs to be as large as possible for the ci of the engine to allow the most airflow into the engine for the duration that the valve is open right.
So would a very fast opening slow closing intake ramp be optimal? Whats the most effective cam lobe to accomplish this?

 

i sat down and broke out my old geometry tools from highschool (god you should have seen the dust after two years). after thinking about it a LONG time. and drawing out the crank at many different degrees with varying rod length. my thoughts are that it becomes partially an issue of valve timing. it would seem to me that the short rod engine would enjoy more of an earlier opening point like bret suggested above maybe thats another part of why comp advances their shelf cams 4*'s since the average guy isnt running a long rod stroker engine he's running the 5.7 rod. granted the differences are minute when giong from 5.7-6.0 but that doesnt mean they arent there. as for the increased velocity of the piston due to the long rod... i dont want to really believe the long rod piston is moving faster. what i'm thinking now is that they both have the same max velocity for a given stroke. what i see happening is the long rod engine having a much faster piston acceleration curve and maintaining a higher speed longer. while the short rod engine would have shorter dwell time so it actually has more time to move the same distance ie lesser avg. (mean) velocity. as for what 81zmouse said i dont think that the rod angle changes makes the piston actually move any faster i think that the rod changing angles only goes to limit the max piston speed, i'm still thinking they're the same(hope that made sense). something else that occurs to me is why the long rod engine may make more tq. tq is somewhat related to intake port velocity right, well since my theory is that the long rod engine moves at a higher velocity for a longer amount of time than the short rod motor wouldnt that support that the long rod engine makes more tq? on another note if a person has poor heads it may be to their benifit to run the short rod in the case that bad ports can be turbulent and gradually pulling air in over a longer period of time may be conductive to more power with lesser heads.

thats where i quit for now gees my head hurts.

 

81ZMouse,TheNovaMan,

You guys are right. Which is cool, I vaugely remeber something about the max piston speed thing, but it never really took hold, now I guess it will.

So the rules are:

1. Piston speed is first a function of Stroke.

More stroke = higher mean piston speed, and higher peak too

2. Piston speed is a function of rod length.

Shorter Rod = higher peak piston speed

3. Piston acceration is a function of rod length

Longer rod = Slower average acceration rate, which means less g's on the piston/rod/rod throw.

That all sound good?


Now on to the rod length vs RPM vs port size.

Why is this happening? I'm guessing that the duration on the cams used in these engines at these rpm's has alot to do with it. It has to be connected with the valve closing, because valve opening means almost nothing. In a 270 @ .050 duration intake lobe the valve gets open .150-.200 of an inch before air/fuel charge even starts to enter the chamber (which is about 15-12deg BTDC) and then after that all we get is a negative pressure in the intake runner. There is so much less flow entering the cylinder here it's not funny. Now when the valve is closing we have around 6psi of pressure forcing the air/fuel mixture into the cylinder, all along the piston is moving up the bore. If the shorter rod is accerating slower towards TDC again, that means that there is a lower pressure in the cylinder than compared with a longer rod in that part of the stroke (from BDC to ABDC a few degrees) This all works to a point. Once the piston is moving faster due to the shorter rods effect on piston speed then we are then being disadvantaged by it's length. Now if we take this into account on the exhaust lobe we might find soemthing there too.

Now in a low RPM engine, where we use short duration cams. None of this will come into play. In fact a long rod with a longer dwell (which BTW we are talking about before and after TDC and BDC) with the durations that we are looking at in low rpm (1500-7000rpm) will help because it doesn't push upwards on the incoming charge as soon, probably not at all.

This might explain why Smokey liked long rods and Grumpy liked short rods. Smokey worked mostly with lower RPM, lower duration cams on durability circle track engines and Grumpy made the high winding 331cube SBC famous. There is no question that the longer durations on cam lobes are seen in drag racing engines. Get a little more life out of the engine and run a little less timing since you have to use stock parts. Makes sense now why Grumpy and Smokey had the battle they had about this.

"Also, big bore/short stroke is supposed to enhance cylinder filling over the same ci with a small bore/long stroke. But, more stroke means more torque."

I still don't buy long stroke means more TQ. I'll give away to the fact that it's probably true at low RPM, but when you get intake pulse tuning into the equation (starting at around 3500) then the larger bore with less shrouding and more flow will more than make up for it. You can have all the mechanical advantage you want. I'll take less friction and better cylinder filling over a normal street car RPM range any time. Hence my choice of Large Bore Short Stroke, and most likely long rod. The rod length will probably change if the RPM range grows too much more.

Bret

 

Yup, you've got it. As far as stroke goes, I was speaking only of increasing the stroke. Same force x longer lever arm = more torque. But yes, large bore/short stroke is a better combination in most ways compared to small bore/long stroke setups. Chevrolet did testing which confirmed that in the late 60s. They were considering changing the bore and stroke of their big blocks, but they ended up not doing so for some non-engine related reason.

 

Well, I'm going to put some numbers to the madness here. Always helps me to understand better.

First, Bret, you are correct with #1 and #2, but #3 is wrong. Longer rods give lower max accelerations. Max acceleration occurs at TDC. Since F=MA, longer rods put smaller forces on the rod at TDC, with a given piston and pin weight.

Now for the real numbers (I agree the differences are small):

All of these are for a 3.48" stroke rotating at 9,000 rpm:

Rod Max Vel Max Acc
---------------------------------------------
5.7" 8,576 fps 168109 fps^2
6.0" 8,539 fps 166143 fps^2
6.25" 8,513 fps 164649 fps^2


All of these are for a 3.75" stroke rotating at 9,000 rpm:

Rod Max Vel Max Acc
---------------------------------------------
5.7" 9,306 fps 184439 fps^2
6.0" 9,260 fps 182156 fps^2
6.25" 9,227 fps 180421 fps^2


Maybe these numbers explain better than all the conjecture....

Shane

ps. You guys really should e-mail me. I've written an Excel spreadsheet that does all this math for you. No equations to crunch, etc. Just input the variables, hit RUN, and the numbers are given to you. It even allows for offset wrist pins, etc. to see what happens to the velocities and accelerations. It's a big file, and needs a good computer to run, but it works great.

 

Shane,

Changed the rules.

Now how about we get back to the why and how of the engines are making more power with this setup.

CompAirFlow, what kind of gains are we looking at here?

Bret

 

since i'm having "a slow day" can you guys explain to me how the long rod has a slower average acceleration. seems to me that since we all agree the long rod engine has more dwell time at TCD and BDC that means it leaves TDC catches the short rod piston at 90* ATDC, and then beats the short rod to BDC if you wanna tell me that the long rod setup after having left TDC behind the short rod setup isnt accelerating faster i'm going to tell you to draw it out its behind all the way to 90* ATDC where it catches up and passes the short rod setup. after having thought about it ... it actually beats the short rod to BDC to get the longer dwell time once again (it happens on both ends). so what i'm saying is traveling the same distance in less time (crank degrees) the long rod setup not only has to have higher piston speeds, but a steeper acceleration curve as well as higher mean piston speed. it may start out behind but at the end of the stroke the long rod setup is ahead when it gets to BDC. which for what bret does.. tuning the intake to make "faux boost" would seem to work good because you great a lot of pressure in a lesser time amplifying the effect a little. and then you have the dwell time at BDC which is where the effect would become the strongest. after having thought about this a great deal, it seem to me the long rod isnt always the best idea now i think it depends... just another part of a "combo".

so in closing, if anybody cares to explain to me how something can accelerate slower but travel the same distance in less time, feel free (sorry if i misread what you guys said please correct me if so).

 

I'm waiting for the math too.... I'm taking his word on it this time. I still want to see the acceration formula, because my first set of rules had the long rod creating more acceration due to the exact things you talked about.

Bret

 

Bret,

PST cams are normally in the 290-310 range @ .050.

"I think this is interesting because it seems to play with the valve opening point, which is secondary in importance to the Int closing point. Let's look at that area too. The piston is now coming up the bore earlier but at a slower rate"

Totally agree there.

"Why is this happening? I'm guessing that the duration on the cams used in these engines at these rpm's has alot to do with it. It has to be connected with the valve closing, because valve opening means almost nothing. In a 270 @ .050 duration intake lobe the valve gets open .150-.200 of an inch before air/fuel charge even starts to enter the chamber (which is about 15-12deg BTDC)"

I will have to get a degree wheel out and look at the specific numbers, but we also have to take the exhaust exiting during overlap that also creates the pressure differential and starts to get the next incomming charge moving.

"Nope. Slow opening, fast closing on a NA application. The supercharging effect from above is strong at the end of the duration that the intake valve is open, therefore you want the valve to close faster (which means that it is at higher lifts while the supercharging effect takes place) but still have it close at the right time, because the valve is closing while the piston is moving up the bore."

I dont know if I agree or not, I think that you want as fast an opening ramp as you possibly can. My thought on this is to get the lobe open as far as it can be to increase cylinder filling when the piston is reaching higher velocities. I totally agree that the inertial supercharging effect is the strongest at the end of the stroke and the piston is comming back up the bore. I think that the point that the air inertia and piston movement energies are equal, that is where you want to close the intake valve and I will spend the rest of my life trying to find that point on every specific motor. This is why the intake closing point is the most important to me, In fact I would almost say it is more important than the other three valve events put together. I have seen intake closing points up to 80-90* ABDC on pro stock stuff! The thing with a rapid closing rate to me sounds good, but, It also lends itself to valve bounce at an earlier rpm.


"This might explain why Smokey liked long rods and Grumpy liked short rods. Smokey worked mostly with lower RPM, lower duration cams on durability circle track engines and Grumpy made the high winding 331cube SBC famous. There is no question that the longer durations on cam lobes are seen in drag racing engines. Get a little more life out of the engine and run a little less timing since you have to use stock parts. Makes sense now why Grumpy and Smokey had the battle they had about this."

My sentiments exactly.


"CompAirFlow, what kind of gains are we looking at here?"

Ok, well that is a hard thing to say as every cobination of parts is different, But I can say I have seen quite a bit of improvement. There are many other things that are nice side affects from doing this,

The shorter Deck height you have, the cylinder heads are closer together and improves line of sight and a better entry angle, in my opinion, this may be just as important as the rod length.

Ok, Now due to all of these other posts, it looks to me like the consencus is that a shorter rod not only leaves TDC sooner, but also faster, am I correct in this statement???

My rule of thumb is

1.8-2.0 R/S ratio- Extremely restricted
1.7-1.75 - Restricted intake up to 5500-6000
1.6-1.7 - Decent heads and intake to 7000
1.5-1.6 -Excelent heads and intake to 9000+
1.45-1.5 -Pro Stock type levels. 11,000+

Man, there are just so many things that corrolate(sp) and that run through my head, that If I typed them all out we would end up with a never ending thread and my posts would be pages in themselves! So I am trying to stay on topic...

NosFed

Ok, on a restrictor plate motor the reason for not using a long stroke, small bore is due to the fact that they still turn about 7000-7500 and the piston speed on a larger stroke motor is going to hurt power production. This is why they use such a short stroke and a long rod, as this slows actual piston speed more. As far as the bore, they are running sb2's and still have about a 2.170 int valve in the thing, now, they also need to have cubic inches in the thing and that is why the large bore. You are right that the long stroke stuff on ebay is old and outdated, They tried that and dyno'd it and found out that the piston speed was more beneficial due to all of these things that we have been discussing in this thread.

"Also on the subject of restricted motors, it seems that a long stroke engine would tend to make it's power at a relatively lower RPM range, which appears to be the setup for a limited cfm induction system, since a high rpm engine would appear to need more induction"

Correct.

"My intuition tells me that a short rod, long stroke, short deck, small bore, large csa engine would lend itself well to restricted induction..."

Not exactly. You want a longer rod where you have a restricted induction system. Now the difference in my eyes between a cup car and a street car is that in a cup car they are turning a high almost constant rpm, So, I would think that in a street car, the stroke length isnt as important as the cubic inches you can gain.

Also, the short deck thing, We are talking about pro stock type of technology and also the unlimited budgets of these classes. For most people (unless you are running in a class that requires stock type restrictive components as part of the rules), you would not be trying to work on all of these things without trying to get the restriction out of the intake system first. Of course, IMHO, Cylinder heads and intake system is the most important part of the motor and all other aspects of engine design is based off of the intake system characteristics.

"Lets say you were building a 360" or less engine that was limited to the restrictor plate setup. If'n you don't mind, what bore/stroke/csa/rod length/bench curves would you do, and what would you expect the tq/hp curves to look like?"

Really, I need more info, What heads, what block, what intake, camshaft type, rpm range, etc...get the info and I would be glad to help.

Sorry, I dont know how to use the quote function yet so...

 

Ok, on a restrictor plate motor the reason for not using a long stroke, small bore is due to the fact that they still turn about 7000-7500 and the piston speed on a larger stroke motor is going to hurt power production. This is why they use such a short stroke and a long rod, as this slows actual piston speed more.

Yep, that's the key it's all a reduction in friction now. 1.771" Rod journals? The mains are most likely 2.30 or smaller, it's crazy the things they do just for friction reduction.

As far as the closing rate on the cams go, that was the last thing I heard on WCS restricted engines. They just slam the door shut on the air/fuel charge. It's also going to help gain a little lift in th same amount of duration ebcause the intake ramp/flank is that of a larger duration cam. Asymetrical lobes.

The short deck thing also is helpfull because the tuning length of the intake runners is so much shorter in that given deck height that it makes the runners almost a perfect length for high rpm engines. Hell i'll take all the deck height I can get if I'm building a low RPM truck V engine. The FE Fords are one of those type of engines, that setup leads to stupid long intake runners. The entry angle is a big part of it too, but a short runner gives you a better shot at entry angle then a long runner ever will.

"Lets say you were building a 360" or less engine that was limited to the restrictor plate setup. If'n you don't mind, what bore/stroke/csa/rod length/bench curves would you do, and what would you expect the tq/hp curves to look like?"

Throw in the budget, application and the rules. That would put you in the right spot.

Bret

 

Yep, that's the key it's all a reduction in friction now. 1.771" Rod journals? The mains are most likely 2.30 or smaller, it's crazy the things they do just for friction reduction.

Yeah, that sounds about right,

How does this sound, They run about a 30 lb crank, rod side clearance of about .200!!!-yeah thats right, .200 I can explain that later. The total bobweight is around 1425, single compression ring, etc...etc...

As far as the closing rate on the cams go, that was the last thing I heard on WCS restricted engines. They just slam the door shut on the air/fuel charge. It's also going to help gain a little lift in th same amount of duration ebcause the intake ramp/flank is that of a larger duration cam. Asymetrical lobes

Yeah, that sounds possible. plus, with the extreme lightweight components and rpm they turn with the spring pressures they also run it makes more sense. Can you believe they run about 450-475 open pressure on a flat tappet??!! You should see the cam tunnel thing they build and the break in procedure, its insane!!

The short deck thing also is helpfull because the tuning length of the intake runners is so much shorter in that given deck height that it makes the runners almost a perfect length for high rpm engines. Hell i'll take all the deck height I can get if I'm building a low RPM truck V engine. The FE Fords are one of those type of engines, that setup leads to stupid long intake runners. The entry angle is a big part of it too, but a short runner gives you a better shot at entry angle then a long runner ever will.

Couldnt have said it better myself.

 

Bret, I know there are two topics going on here, and I hope I'm not "muddying" the water, but to answer your and WS6's questions, I'll continue with my topic......

The first thing you have to do is remember what acceleration is. Acceleration is the change in velocity with respect to time. The next thing you must understand is MAX ACCELERATION occurs at TDC, and it differs from AVERAGE ACCELERATION. Ironically enough, what you are saying is correct. The problem is you are comparing accelerations at two different points in the rotation. Yes, the longer rod has lower acceleration values at TDC, where MAX acceleration occurs, and therefore has a lower MAX acceleration. And, yes, the longer rod has higher acceleration values during part of the downward stroke, actually between 46 degrees ATDC and 48 degrees BBDC.

As for AVERAGE accleration values, they are the same between TDC and BDC. It only makes sense..... at the same rpm, the piston MUST travel from TDC to BDC in the same amount of time, so the average of these numbers must be equal.

Again, if you would like to see this for yourself, including charts of the velocities and accelerations for different rod lengths plotted against each other, e-mail me and I will send you the spreadsheet I wrote. It really helps to see the curves, they tell a lot about what is really going on.

Shane

 

Shane--check your Private Messages. I still think that the long rod has a higher max acceleration (near TDC/BDC), but I'd like to compare our equations. All this talk of averages seems pointless to me since it doesn't take the rod length into consideration.

 

I still think that the longer rod has a high acceleration. Throw up those equations boys and see who's right.

Bret

 

I am having a diffcult time with the concept of "average acceleration". In terms of stress on parts, peak acceleration is what is important. Clearly, the short rod has higher peak acceleration. So in that sense, long rods win though the magnistude of the difference is so small as to probably be irrelevant, as Shane's numbers indicate.

Now, how do the differences in piston acceleration and velocity relate to producing power in an NA application? There is no simple answer. If there were no restrictions except displacement and number of cylinders, and you could pick any B/S ratio along with any rod/stroke ratio, the number of possibilities are staggering. If you limit the discussion to sticking with a given bore and stroke, and just vary rod ratio, here's how I see it. Piston velocities across top and bottom center are slower with higher rod/stroke ratios, and faster in the mid-part of the stroke. This would favor a cam with shorter duration, narrower LSA and as high lift as possible. With a lower rod/stroke ratio, the velocity of the piston is higher near TDC and BDC. This should decrease the sensitivity of the combo to changes in valve opening and closing points. But I'll bet that if the cam were truly optimized, within the realm of practical R/S ratios, the power would be so close to identical as to make no difference.

And how does this relate to runner volume? Not at all as far as I can tell.

This spreadsheet may also help visualize piston kinematics http://e30m3performance.com/tech_art...kinematics.xls

Rich Krause