New Pistons on stock rods...

[Majik]

I'm trying to build an engine for this old car of mine and I'm trying to figure out the correct redline. What got me going on this was a discussion about piston speeds and G loading on the connecting rods a couple of pages back. Needless what got me thinking is this and I'm trying to make sure that I'm not overlooking anything. The stock piston/pin/ring/retainer weighs in at 737 grams. This new mahle sport race piston I have weighs in at 519 grams. 218 grams weight savings per piston which seems huge to me espically when I start moving the piston about 25m/s. Needless the stock redline on the engine with these heavy pistons was 7250. Bore for this engine is 90mm and stroke is 84mm. When I calculate the g loading on the rod from the previous post it is drastically lower with the lighter piston. What I'm trying to figure out is there any other factors in determining when a rod will fail? My plan is with this superlight piston to run the stock rod at 8500rpm. The only thing done to the stock rod will be rebuilt, race preped, shot peened and installation of ARP rod bolts. Am I barking up the wrong tree? It seems like it should work because of the reduced load on the rod but I might not be considering all factors. Any input would be apreciated. Thanks
Kris

 

[mburgess]

What sort of engine and the intended use???? Reving the engine harder for the sake of it isn't necessary. The limiting factor may well be in the valve train, not only in cam profile but in terms of valve spring stiffness, lift, rocker's and pushrods if any. The existing cam profile may not make it worth running the engine at that high rpm. Anysakes the lighter piston has got to help. One thing to consider is the balance of the bottom end since you have made a significant change.
Regards,
MB

 

[Majik]

THe limiting factor isn't in the valve train as it is capable of many more revs. The valve train consists of titanium valves with double valve springs and titanium retainers with titanium keepers. The cam shaft is a redesigned Schrick Camshaft with 304 degrees of duration and 10.7mm of lift.. The idea that I was going for was a reliable 8500 rpm engine that had super lightweight components in it. The one thing that I hadn't calculated was the connecting rods own force on the crank which would be difficult to calculate as it changes weight throough out it's length. The crank shaft will be dynamically balanced.

 

[bobqzzi]

Given that you've obviously invested quite a bit in this engine, it seems penny wise and pund foolish not to get some racing rods. By the time you get done with reworking and buying bolts for the stock rods, you are only talking several hundred dollars difference for a set of racing rods. What type of engine is it?

 

[Majik]

Well see it's not really that wacky... I'm a machinist so I made my own titanium valves. The engine is a bmw m10 engine from a 1970 BMW 2002. The valve springs have been proven to that RPM with SS valves which are 30% heavier. The Connecting rods are stock bmw m3 rods out of the late 80's bmw m3 engine. The crank shaft is out of the same engine. That engine was designed with a 93mm bore and a 87mm stroke and a 7250 rpm redline. Forged Rods for that engine are $750 anywhere you look. I have stock rods already in my posession. The issue is this. Mathmatically I should be able to run stock rods. The weight different between stock rods in race preped form and race rods is only about 30 grams which isn't a paramount thing. The thing isn't to replace just for the sake of replacing. The thing is to replace if you can see a part has a short comming in it's performance enviroment... the thing is that mathmatically the pull of the piston on the connecting rod at stock 7250 rpm redline is 2511.69 LBS.... with the new lightweight piston at 8500RPM the pull on the connecting rod is 2433.48lbs so honestly I think after a nice race prep on a stock rod it should be well with in spec and capable to run but I didn't know if the extra RPM would have a effect on the rod weight as well. Might just have to spin it up there and see what happens..

 

[mburgess]

Dare say that you have the answer in you last sentence. BMW probably would have used a fair margin of safety in their design in the first place plus the fact that the rods are off an M3 has got to help. Sounds like a good thing. Post on how it goes.
Cheers,
MB

 

[aduetto]

You didn't mention how many miles were on the original engine (and rods). If the rpds have over 70-80k on them, I wouldn't be inclined to use them for a performance application. Sounds like you will have a workout balancing the crank! Cool weight loss!

 

[enginebob]

Calculations are fine. But remember many more vairables involved. Such as inclusions in the material, especially in cast parts. Fatique cycles etc.