jeejee

iTrader: (3)

When doing aprox 2000-2500 more rpm (8500) then in standard form as it came from the factory,
What would be different on the rod / main clearances ?
should i go bigger or smaller then the suggested manufacturers clearance ?

not specifically an ls engine but more like a 305 sbc (cast iron block and head) based 4 pot with 8500rpm and vortech supercharger ?

suggested manuf. clearance is main :0.023mm-0.064mm (0.0009-0.0025inch)
rods : 0.015mm-0.061mm (0.00059-0.0024inch)

much appreciated

Jeejee

jmalibuss

Who are the bearings made by?

jeejee

iTrader: (3)

i haven't ordered yet, but propably vandervell led,bronze high performance bearings, as they are one of few companies selling bearings for the engine type i'm referring to..

pist0lpete

iTrader: (3)

Why on earth would you waste the time and efford to build a 305? If you are displacement limited then built a 302 much better parts out there. I just can't see the reality of getting the parts together to get a 305 to spin to 8500 reliably on boost especially. However, I hope you can prove me wrong cuz I can admire things that are different.

jeejee

iTrader: (3)

No it's not a 305 v8 but a 4 cyl. with simular architecture..
it's for high speed road racing in a regulated class where you must use the same engineblock as it came from the factory.
(and i allso am building an ls1 in other one too )

and engine building knowledge is limited here with regards to the old 2 valve cast iron dinosaur engines

pist0lpete

iTrader: (3)

Ah its beginning to make more sense now. Well are you limited on bore and stroke? The shortest stroke you can get away with will help. Also, lightweight is gonna be the key especially with the pistons and rods while still maintaining strength. Also the oiling system is gonna be very important. You need a pump that can handle supplying oil at those high rpms.

jmalibuss

I think your best bearing clearance is really going to depend on the journal diameter, journal width and RPM operating range. If you have a lot of surface area I would recommend a bit more clearance. Example, a standard SBC rod journal you usually would run about .002" - .0023" Thats a good starting point. A larger diameter journal and a wider face should see about .0002"-.0003" more clearance. This would allow for some extra oil film and a little better cooling.
The main thing you should look for over and above anything is correct bearing crush. Make sure the housing sizes are correct for the bearing you are using. Make sure they are nice and round, no more than .0002" out of round or taper. The crush will ensure over and above all that you won't spin a bearing. Granted there is adequate oiling.

racer7088

With good parts a good rule of thumb is .001 per inch of shaft diameter plus maybe another .0005-.001 depending on how much you think the housing and shaft deform under high rpm loads. As an example a stock rod probably needs more clearance or a more eccentric bearing to keep from pinching in at the sides under extreme high rpm use than would say a Carrillo rod. The oil is what keeps these surfaces alive especially in continuous high speed and load usage and cooling so a little more clearance is safer than a little less.

The better and more rigid the parts used the less extra clearance you need since they won't move around as much. Also with thinner oils you can run a little less clearance and still have enough volume to keep things happy. I'd still always run at least the .001 per inch though to stay safe. Some people will run aluminum housing bore applications somewhat tighter as they will get larger clearances when hot and be the right clearance at that time. When you do this though you need to have the engine warmed up before you are mean to it though!

Ari G

iTrader: (25)

allso keep your piston speed down if you need the engine to last long

3.4camaro

iTrader: (1)

is there anything that determines piston speed besides stroke and rpm?

Ari G

iTrader: (25)

No,the formula is Stroke/rpm = Pistion speed in fpm

racer7088

RPM will kill a lot of parts where as piston speed really won't on it's own. Piston speed just limits how high your engine will make power as far as engine breathing is concerned usually. The better your engine can breathe the higher the piston speed can go before the power levels off and goes back down.

You can have a low piston speed but very high rpm engine and it will fall apart FAST. You can have a high piston speed low rpm engine that lasts forever.

Piston speed is a misused term most of the time. The forces tearing up parts go up with the square of the rpm but only linearly with stroke. Piston speed doesn't show that! You can have two engines with the same piston speed but the higher rpm one will tear stuff up much faster due to the higher forces on the parts and the increased cycles the parts see as well.

If I destroke and engine by half and then double the rpm I will still have the same piston speed in both examples. The second engine with double the rpm though will have twice the loads and also twice the cycles so the fatigue life starts going away very fast even though both engines have the same "Piston Speed."

Steve Bryant

iTrader: (1)

Erik,
I want to complement you on an excellent explanation of a principle that is often very misunderstood!

Steve

PadreSmith

iTrader: (24)

I've been using a formula for piston speed at a certain rpm. (Stroke x RPM)/6 = piston speed in feet per minute. I dont know if its an appropriate formula or not. I've heard that anything over 4000 fpm is getting into dangerous territory. Does anyone know if this is very accurate or not?

racer7088

No it is not as you can have a lot of piston speed at a lower rpm and it is not dangerous at all.

racer7088

Thanks Steve.

PadreSmith

iTrader: (24)

Is there a better formula out there that takes rpm and piston speed into account that gives you a measure of force on internal components? Or is it just trial and error and experience? Something was mentioned earlier about the square of rpm. This may not even be relevant for those of us using forged and lightweight components but i'm still curious how to determine abuse on the block in relation to rpm.

Forced370GTO

yes there is a better formula... what you are all calculating is average piston speed, true maximum piston speed may deviate by more then 100% of the calculated average piston speed. therefore it is not only possible, but normal to see maximum piston speed at or near double the avergae calculated piston speed. Here's the formula for piston speed vs crank angle:

(stroke/2) x (angular velocity) x (cos theta) x ( 1 + ((sin theta) / sqrt((rod length/(stroke/2)^2) - cos^2theta))

so if you model that on graph paper over 360 degrees of rotation you will find your maximum velocities in the positve and negative vector directions. average velocity is never completely indicative of your engine loads and/or piston accelerations...