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Not a stock YB installation but in the never ending quest for more hp the “through stud” design was developed. First was the “long stud” which is similar to our engines from the factory, then came the through stud. The issues the YB encountered are very similar to what we are seeing.
There’s a couple of ways to correct the problems but with very little investment in our platform we are limited in options. Ultimately a stronger stiffer modern crankshaft would be ideal, this would fix the flexing at the source. Second to that you can try and bolt it down and secure it better to the block but without a through stud setup you are limited by block strength and bearing longevity.
This is why I believe the 2.5 crank is the ideal option for an all out approach. The shorter stroke is a benefit of its own in terms of rod ratio and amplitude of flexing motion. But in addition it has more overlap between the webs creating a stiffer crank.
It's a long time since I was into Sierra Cosworth YB tuning, and I believe there may have been expensive custom alloy blocks since then. Rover cars were fitting head bolts that went through to the crank mountings many years ago, and on pretty low powered engines.. I also though the latest design crankshaft dampers addressed most of the flexing issues?
It's a long time since I was into Sierra Cosworth YB tuning, and I believe there may have been expensive custom alloy blocks since then. Rover cars were fitting head bolts that went through to the crank mountings many years ago, and on pretty low powered engines.. I also though the latest design crankshaft dampers addressed most of the flexing issues?
The crankshaft dampener is for torsional dampening through the length of the crankshaft. It’s main purpose is to prevent a damaging harmonic resonance, definitely worth using on any build.
The flexing I’m talking about is a bending along the length of the crankshaft. As the bang gets bigger and the rpm’s go up and because the inner and outer pistons are opposing, the crank will start to whip. The least supported no.1 or no.4 bearings tend to fail first in this scenario.
I am talking about pushing this engine to the limit. Anything less than 600hp and 6700rpm doesn’t need to address these issues
I currently have two engines on the build stand, one is an all out 8v head 2.5 block and crank which is dry sumped. The other is a 968 engine which I’m turbocharging this build will be fairly tame but I’m predicting it will make more power over a broader power band than the all out 2.5. Before spending the many $$$ I have on these engines I did my research and analysed the failures and issues those pushing the limit were facing. There is very little information on power levels north of these figures but many running happily below. Once you can seal the head without blowing a gasket the problem I have seen (from others reporting not personal). Is a loss of oil pressure as both boost and rpm climb, Gustaf reported this on the Revline 968 before finally breaking the block. I know from other platforms that the loss of oil pressure in this manner is usually from increased bearing clearances caused by the crankshaft flexing. I think Gustaf may have mentioned this been the cause as well but can’t confirm of the top of my head. At the end of the day there is very limited actual experience at this power level.
The guys at Nitto performance cover this topic well on JDM engines which led them to develop their own crankshafts to deal with the flex.
Probably also time for a new thread dedicated to an all out build project.
When we were building the engine for the first iteration of the RP968 my engine builder wanted to go with through bolts but I wasn't prepared to take the risk as it is not commonly used, and we never had the time to experiment, but thinking back I do think the idea has some merit. In my experience even at lower hp(450), the oil pressure would droop under boost noticeably, as soon as you lift the throttle it recovers. The block just doesn't seem to have enough rigidity, I really think a billet girdle would help stiffen the whole engine, and shouldn't be crazy expensive.
When we were building the engine for the first iteration of the RP968 my engine builder wanted to go with through bolts but I wasn't prepared to take the risk as it is not commonly used, and we never had the time to experiment, but thinking back I do think the idea has some merit. In my experience even at lower hp(450), the oil pressure would droop under boost noticeably, as soon as you lift the throttle it recovers. The block just doesn't seem to have enough rigidity, I really think a billet girdle would help stiffen the whole engine, and shouldn't be crazy expensive.
On my engine we did a scope in the oil gallery, it has some flaws in the casting.
Oilpump it self, schould have been wider, different pickup system etc etc etc.
I changed from a wild 3.1 build to a 07k.
My fully built 2.5 8v is just going to make 400bhp for two years, before swapping.
I currently have two engines on the build stand, one is an all out 8v head 2.5 block and crank which is dry sumped. The other is a 968 engine which I’m turbocharging this build will be fairly tame but I’m predicting it will make more power over a broader power band than the all out 2.5. Before spending the many $$$ I have on these engines I did my research and analysed the failures and issues those pushing the limit were facing. There is very little information on power levels north of these figures but many running happily below. Once you can seal the head without blowing a gasket the problem I have seen (from others reporting not personal). Is a loss of oil pressure as both boost and rpm climb, Gustaf reported this on the Revline 968 before finally breaking the block. I know from other platforms that the loss of oil pressure in this manner is usually from increased bearing clearances caused by the crankshaft flexing. I think Gustaf may have mentioned this been the cause as well but can’t confirm of the top of my head. At the end of the day there is very limited actual experience at this power level.
The guys at Nitto performance cover this topic well on JDM engines which led them to develop their own crankshafts to deal with the flex.
Probably also time for a new thread dedicated to an all out build project.
I would be interested in your build thread as I have the same two engines waiting for me to get started. In the upper RPM ranges (6500-8000) you could lower boost by 3-4psi while still increasing HP.
When we were building the engine for the first iteration of the RP968 my engine builder wanted to go with through bolts but I wasn't prepared to take the risk as it is not commonly used, and we never had the time to experiment, but thinking back I do think the idea has some merit. In my experience even at lower hp(450), the oil pressure would droop under boost noticeably, as soon as you lift the throttle it recovers. The block just doesn't seem to have enough rigidity, I really think a billet girdle would help stiffen the whole engine, and shouldn't be crazy expensive.
I would like to see a through bolt experiment.
And a welded head experiment.
When we were building the engine for the first iteration of the RP968 my engine builder wanted to go with through bolts but I wasn't prepared to take the risk as it is not commonly used, and we never had the time to experiment, but thinking back I do think the idea has some merit. In my experience even at lower hp(450), the oil pressure would droop under boost noticeably, as soon as you lift the throttle it recovers. The block just doesn't seem to have enough rigidity, I really think a billet girdle would help stiffen the whole engine, and shouldn't be crazy expensive.
Interesting and nice to know my theory is somewhat on the right track. RP968 has been an inspiration and part the reason I chose the 944 platform in the first place. I have thought about upgrading the girdle but ours just looks so solid even when compared to other platforms billet designs. I think the through design will address the issue to a certain point as clamp loading can be increased without any reliance on thread engagement and block strength. Past this I think it comes back to the crankshaft stiffness and rod ratio.
Originally Posted by 944 timbo
I would be interested in your build thread as I have the same two engines waiting for me to get started. In the upper RPM ranges (6500-8000) you could lower boost by 3-4psi while still increasing HP.
My 2.5 is still hydraulic lifter so will be limited to 7200rpm. But you’re right, if your valve train can handle the rpm the hp comes freely. By moving the power band up the rev range the stress on the bottom end and head gasket is lessened.
Originally Posted by 944 timbo
I would like to see a through bolt experiment.
And a welded head experiment.
I do have a build thread, it’s just taking forever. Have you seen my head?
Interesting and nice to know my theory is somewhat on the right track. RP968 has been an inspiration and part the reason I chose the 944 platform in the first place. I have thought about upgrading the girdle but ours just looks so solid even when compared to other platforms billet designs. I think the through design will address the issue to a certain point as clamp loading can be increased without any reliance on thread engagement and block strength. Past this I think it comes back to the crankshaft stiffness and rod ratio.
My 2.5 is still hydraulic lifter so will be limited to 7200rpm. But you’re right, if your valve train can handle the rpm the hp comes freely. By moving the power band up the rev range the stress on the bottom end and head gasket is lessened.
I do have a build thread, it’s just taking forever. Have you seen my head?
I have not seen your head/thread yet, send me a link?. [edit] I think I see it now in the build thread (subscribed); the car with the GTR rear end?
On the flex issue, I did just see that this guy installs dowles around the girdle studs (see here), maybe this solves the problem?
This girdle was dowelled by that company. I agree in regards to the crank whip. The crank in this motor (3.1 8v) was knife edged via LR. I think this may have contributed to the whip motion. We didn't run a harmonic balancer and the motor was good for a season. Unfortunately at the big event at the end of the year (World time attack 2014) the motor self destructed without warning. In that you really only run very minimal laps it was a little strange that it decided to blow that day. The pulley peeled in half. Paul raced back to the shop and machined a new one. We thought we were in the clear but no it just transferred the stress elsewhere. Up until that event the car did well and motor was stout. Shouldn't have run the knife crank and should have run a harmonic balancer. Another expensive lesson.
This girdle was dowelled by that company. I agree in regards to the crank whip. The crank in this motor (3.1 8v) was knife edged via LR. I think this may have contributed to the whip motion. We didn't run a harmonic balancer and the motor was good for a season. Unfortunately at the big event at the end of the year (World time attack 2014) the motor self destructed without warning. In that you really only run very minimal laps it was a little strange that it decided to blow that day. The pulley peeled in half. Paul raced back to the shop and machined a new one. We thought we were in the clear but no it just transferred the stress elsewhere. Up until that event the car did well and motor was stout. Shouldn't have run the knife crank and should have run a harmonic balancer. Another expensive lesson.
Thats a nasty break. Inner rotation parts out of balance? Tiny cracks..
What HP did you make that year?
Thats a nasty break. Inner rotation parts out of balance? Tiny cracks..
What HP did you make that year?
Hard to say what the cause was without any doubt. Suspect crank whip and fugly harmonics. We made 550whp and similar tq at 25psi iirc, but didn’t run it at that boost on the track. More like 16-18 psi.
01-04-2023, 06:28 AM
