How fast are your pistons? I bet you are wondering if red94vette has finally gone crazy after all the programming.

Nope, I just wanted to share some data with you guys since I've been working on a little shortblock proggie. I plan on leaping into horsepower estimations after I get my 1/4 program perfect.

The first graph is the piston speeds and distance from the center of the crank for a 350 LT1 at 6000 RPM.


Ok so what? Well the second graph lets us in on a little secret.


Notice that the piston speeds for a 383 stroker is higher than a 350. This is part of the reason bigger cubic inches make more horsepower.

If the engine were simply an air pump then a 350 could make as much power as a 383, just at a higher RPM. In other words, the same CFM will make the same Horsepower. This is obviously a bad assumption to make.

One reason a 383 makes more overall horsepower than a 350 is because of its higher piston speeds. Your engine draws in air from the partial vacuum it makes in the cylinders. When your pistons are headed down the cylinder faster the partial vacuum is stronger, in other words, higher piston speeds will draw air in faster due to a higher vacuum. This kind of throws out the old "engine is just an airpump theory"

Also, higher piston speeds creates more piston ring drag so you lose some horsepower to friction.

Ok the last table.



This compares max piston speeds, max g-forces on the rods, and % change in g-forces. If you look at the % change you can see going from 5000RPM to about 7000RPM doubles the forces on your rods.

Also a 383 will stress your rods 9.7% more due to higher piston speeds.


:cheers:




[Modified by Red94Vette, 8:21 AM 6/11/2002]

 

That is so :cool: but what about the rod size and piston speed? can you make a graf from 5.7 to 6.0

Jay

 

Rod dimension vs Stroke dimension
With the cube's being the same, the tradeoff is piston speed vs torque.
More stroke, or more rod,... more speed or more torque.
Let's see what the numbers say.

 

Nice information. :cheers:

 

Sure, let me run the numbers 5.7 vs 6.0.

I can almost tell you right now the piston speeds will be close.

correction: they match at 84 degrees. (I put speed there by mistake)



It looks like the 5.7 rod has a higher speed during the first half (not quite half 84 degrees) of the trip down the bore and the 6.0 rod takes over and has more speed down the bottom half.

I will run the bottom to top numbers next.





[Modified by Red94Vette, 10:26 AM 6/11/2002]

 

Ok this is the speeds of the piston starting from the bottom of the cylinder and heading to the top.



So, looking at the two graphs it looks like the shorter rod will have the higher max piston speeds at the same RPM.

This tells me that the 5.7 rod will max out the intake/heads/exhaust system before the 6.0 rod due to having a faster piston (higher port velocity). It's a fact that shorter rods make more torque and these results support that.

The slower piston speeds of the 6.0 rod allows you to turn a few more RPM until you reach the the limit of the intake/heads/exhaust. You make a few more HP with long rods and a little less torque.

This is just looking at piston speeds. I can also calculate piston to rod angle on a 5.7 to 6.0 rod motor.

A longer rod doesn't give you a big performce boost. That's why dyno2000 and some other engine HP software doesn't have the input for rod length. A longer rod will decrease the rod angle and the pistons will not be pushed into the side of the bore.

Anyone care to add to this or correct me if I am wrong?

:boxing


[Modified by Red94Vette, 10:39 AM 6/11/2002]

 

That is what I have allways thought, we have found out and been told that if you want more TQ you go with the 5.7, but if you have bad heads then go to 6.0 because the piston will stay at the top for a longer time so it will give the heads a little more time to fill up the cylinder.
If you look at the small block Pro Stock motors they have a really small rod but they have a lot of TQ.
I friend here that builds dirt track motors put a really long rod in a engine it was so big that he had to put a plate on top of the block so the piston would not push out but he got out run my a small rod motor and it made less TQ then a small rod motor.

Jay


[Modified by MrJay, 10:54 AM 6/11/2002]

 

Wow, thanks for the information!! :cheers: :cheers: :cheers:

 

Thanks for crunching the num's.

Something worth mentioning, the longer stroke = higher piston speeds = higher tendancy for detonation (thermo equations and all)

I've never done the math, but I was wondering how static CR effects cyl pressure. Prior comments have said the two are related exponentially. Wonder if we can confirm this w/ a graph. Care to give it a try?


[Modified by ZylaRace, 12:45 PM 6/11/2002]

 

Yeah I can plot a graph of crank degrees vs cylinder pressure and temperature. I need to get home to my engineering books though to look up the exact equation for the adiabatic compression of a gas.

Just from my memory I think the equation is....

T2/T1 = (P2/P1) ^ (cp-r/cp)

I am not sure on the power factor. I know it's a function of heat capacity and the gas constant R.

I will cunch the numbers at home but there will be some simplifications I must make like (no heat enters or leaves the system during compression). Really the hot cylinder walls will heat up the air slightly but it is a small factor.

Most of the temperature gain will be from squeezing the gas to a small volume. (it's the same equation for boost on a super charger)

:yesnod:

 

Yes interesting stuff, much over my head.

Though it seems most of it has been layed out in books I've read. From what I gather unless you are running a full race motor (aka ProStock) your are best off with the old standbys for a street engine...run as much cubic inches as you can, highest compression, and let rod length fall where it may.

No flame please...very nice to see some of it spelled out:)

 

Well I think the #1 thing you should size rods for is reliability. The stock rod length on the SCB 400 was too short. When a rod is too short the crank rams the piston into the side of the cylinder with a lot of force. This wears out the side of the bore.

Why do you think there's a lot of 383s around? All lot of the 400 blocks with stock rod lengths were ruined. That leaves a lot of spare 400 cranks.


:D

 

Red, curious why you picked the 190s over the 180s. I have a 92 auto with 3:54s and I'm wondering which would be better for me as well as which cam.

 

I have always wondered about the reliability of a 383?

Is a two-bolt 383 SBC really as reliable as its 355 counter part? I have heard a lot of rumors of 383s going south after only 30 or 40K.

I would like to build a 383, but reliability, reliability, reliability!!! I can change out a motor in a weekend, but I don't always have the cash to rebuild it!!! :yesnod:


[Modified by Kieran ***, 2:55 AM 6/12/2002]

 

You are right Red94Vette people sute build for reliability because it is no fun to build a engine that makes a lot of power if it does not last, and you are right too short of a rod would do that.
There are a lot of engines that are build here that are 383 and yes all of them do use the 400 crank but then again they dont use the 400 rod most all of them use the 6 inc rod to get more TQ but if we look at this we see that the 5.7 rod would give you more TQ but the 6 would be better on the walls.

Jay


[Modified by MrJay, 7:56 AM 6/12/2002]

 

however, the reality is that rod length seems to be irrelevant in determining longevity and reliability in street (and most race) engines.

in fact, with high hp, forced induction engines, the shorter rod makes for a more durable engine. (ring locations)

also, the proliferation of 383s is most definitely not due to the high number of defunct 400s, as the 400 crank won't fit in the 350 block. (i realize it could be turned down to fit, but "383" cranks come from other sources.)

the overwhelming reason to get the longest rod you can is to keep the piston weight down.

 

rebuilt engines, regardless of size or parts used, are always extremely suspect. poor machining, and horrid assembly practices are abound, even within the most respected (and expensive) shops.

in other words, you'd be just as likely to see the 30k mile life in a 355 as you would in a 383. not too many people actually make enough power to stress the parts involved, if they were all machined and assembled properly.

 

Ever try to find a 400 SBC shortblock? Hehehe :lol:

Really, this was back before ta lot of aftermarket 400 cranks were around except for the extremely expensive forged pieces. You could find a factory 400 crank but the blocks were trashed.

Now days it's a different story, the 383s today use new cranks.

I was talking about late 70's and 80's about the excessive 400 cranks.


Now about your piston weight, yeah a longer rod will allow a much smaller compression height on the piston, (a lot lighter piston).

I run 6" rods in my 80's 400 smallblock and the pistons are light! But, I can tell you if you run the stock 400 rod (i think it's 5.565") it will wear the side of the cylinders out (I had to bore mine .03 when I got the block out of a chevy pickup)


[Modified by Red94Vette, 8:22 AM 6/12/2002]

 

'70s and '80s? you that old? ;)

 

lol.. I'm not old I just read a lot. I'm 30. :lol: