how many 7 liter strokers are out there????
02-13-2018, 06:43 AM
#123
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My changes don't come from induction related issues but of concern over piston speeds in the larger stroke engines.
Drag racers rarely worry about piston speeds....they spent only a few tenths of seconds at rpms that would tear a road racing engine apart. Road racing is very different.
The engineers at Chevrolet found that with a 3.750 stroke engine, 7,600 rpms was the absolute limit, even with super light pistons and titanium rods ( with engines used in Trans Am racing.)
Mark Anderson's 928 race engine used an extremely "beefy" connecting rod (heavy) with relatively heavy pistons for years. He reved this engine 8,000+ for years. The only side effects were cracked blocks, pounded bearings, and rapid cylinder wear.
When we switch to a lighter rod, engine failure (rod failure) occurred very quickly. Analysis of the remaining pieces of the failed rod (not all of the pieces remained) at Carrillo found no defects or problems (with the pieces we supplied.) They said that the rod failed from excessive loading (too many rpms) and asked for any recorded data to see what rpms the rod had been subjected to. We had no such data.
Final engineering findings resulted in no changes to the design of these rods, but simply a suggestion to reduce the piston speeds (or use a lighter piston) and "gather" some recorded rpm data.
"Research" resulted in finding Chevy's information about piston speeds with a 3.750" crankshaft.
Drag racers rarely worry about piston speeds....they spent only a few tenths of seconds at rpms that would tear a road racing engine apart. Road racing is very different.
The engineers at Chevrolet found that with a 3.750 stroke engine, 7,600 rpms was the absolute limit, even with super light pistons and titanium rods ( with engines used in Trans Am racing.)
Mark Anderson's 928 race engine used an extremely "beefy" connecting rod (heavy) with relatively heavy pistons for years. He reved this engine 8,000+ for years. The only side effects were cracked blocks, pounded bearings, and rapid cylinder wear.
When we switch to a lighter rod, engine failure (rod failure) occurred very quickly. Analysis of the remaining pieces of the failed rod (not all of the pieces remained) at Carrillo found no defects or problems (with the pieces we supplied.) They said that the rod failed from excessive loading (too many rpms) and asked for any recorded data to see what rpms the rod had been subjected to. We had no such data.
Final engineering findings resulted in no changes to the design of these rods, but simply a suggestion to reduce the piston speeds (or use a lighter piston) and "gather" some recorded rpm data.
"Research" resulted in finding Chevy's information about piston speeds with a 3.750" crankshaft.
Actually it is not much difference between a 6.5L and a 7.0L engine. The 7.0L is about 7.5% larger and should produce the same amount of extra torque everything else being the same. 7600 rpm limit for 3.750" stroke make sense, it is a mean piston speed of about 24 m/s. Over here in Europe 25 m/s is considered as an upper safe limit. In my case I will never race the engine, it is just a toy, a boy dream I like to come true. I expect and hope the engine will stay together for some occasional high rpm bursts in the lower gear above the 25 m/s limit. If it breaks, that´s it. I can always put the stock engine back or drive any of the other two cars when considering how to build a new high performance engine. By the way the oldest car will get a 6.5L two-valve engine with four Dellorto Twin-choke carbs (see my avatar picture).
Åke
Åke
02-13-2018, 10:10 AM
#124
Nordschleife Master
My opinions:
The big issue with rpms is piston weight. As piston weights have come down, maximum rpms and rules of thumb about piston speed limits have increased. It's the piston inertia due to mass and the acceleration forces that it creates that mainly limits the rpms. A heavy piston requires a heavy connecting rod with a large cross-sectional area. The two important determinants of the rod strength under tension loads are material and cross-sectional area. Tension is what matters for the normally aspirated motors, ability to hold up under compression also matters for turbo engines that can make real power. If the pistons are heavy, and then connecting rods must be heavy, the bearing loads are high. This will then require long bearings, a lot of bearing area, and high viscosity oil. If the pistons are heavy and there's rod bearing damage due to excessive loads, it's logical that replacing the heavy connecting rod with a light one (holding material constant) will increase likelihood of rod failure and catastrophic engine damage. The right way to go about it is to replace the pistons with lighter ones. For example, running an ancient 968 piston design isn't the way to go for a high rpm engine, when you can easily save more than 25% of the piston mass with a more modern design. The problem with our engines is of course that then the whole engine needs to be rebuilt with Nikasil coated cylinders or with some other solution like liners. Building a normally-aspirated engine from scratch without a budget constraint (money or time), though, I'd go with modern piston design for sure.
Even taking heavy pistons as given, a long-stroke engine will make more average power and will be faster than a short-stroke engine as long as there's no displacement limit (i.e., the bore is the same). This is because the power is proportional to rpm times the stroke, while for long-rod engine the destructive forces on the rotating assembly are proportional to the rpm squared times the stroke (the first term in the piston acceleration equation). Therefore, holding piston acceleration constant, you'll make more power by increasing stroke and reducing the rpm. You can go to absurd extremes where this may not hold, but for car engines it does.
The big issue with rpms is piston weight. As piston weights have come down, maximum rpms and rules of thumb about piston speed limits have increased. It's the piston inertia due to mass and the acceleration forces that it creates that mainly limits the rpms. A heavy piston requires a heavy connecting rod with a large cross-sectional area. The two important determinants of the rod strength under tension loads are material and cross-sectional area. Tension is what matters for the normally aspirated motors, ability to hold up under compression also matters for turbo engines that can make real power. If the pistons are heavy, and then connecting rods must be heavy, the bearing loads are high. This will then require long bearings, a lot of bearing area, and high viscosity oil. If the pistons are heavy and there's rod bearing damage due to excessive loads, it's logical that replacing the heavy connecting rod with a light one (holding material constant) will increase likelihood of rod failure and catastrophic engine damage. The right way to go about it is to replace the pistons with lighter ones. For example, running an ancient 968 piston design isn't the way to go for a high rpm engine, when you can easily save more than 25% of the piston mass with a more modern design. The problem with our engines is of course that then the whole engine needs to be rebuilt with Nikasil coated cylinders or with some other solution like liners. Building a normally-aspirated engine from scratch without a budget constraint (money or time), though, I'd go with modern piston design for sure.
Even taking heavy pistons as given, a long-stroke engine will make more average power and will be faster than a short-stroke engine as long as there's no displacement limit (i.e., the bore is the same). This is because the power is proportional to rpm times the stroke, while for long-rod engine the destructive forces on the rotating assembly are proportional to the rpm squared times the stroke (the first term in the piston acceleration equation). Therefore, holding piston acceleration constant, you'll make more power by increasing stroke and reducing the rpm. You can go to absurd extremes where this may not hold, but for car engines it does.
02-13-2018, 10:17 AM
#125
Developer
Although the 7.0L version currently performs under the 6.5, I know we haven't been giving it enough air. I feel we have shown the kind of advances in the intake manifold and cams that it is going to benefit from. The story of the 7.0L motor is not over - later this year I should have another that we can feed correctly and realize its true potential.
02-13-2018, 11:40 AM
#126
Pro
Although the 7.0L version currently performs under the 6.5, I know we haven't been giving it enough air. I feel we have shown the kind of advances in the intake manifold and cams that it is going to benefit from. The story of the 7.0L motor is not over - later this year I should have another that we can feed correctly and realize its true potential.
I also think a liner engine has its own advantages if the technique can be tested and standardised. No more alloy coating problems, new piston designs, possible standard parts.
For every cloud there is a silver lining, it’s about seeing past the cloud
02-13-2018, 02:13 PM
#127
Fun. That's it. This is for fun. If you get fun from a stock engine boosted to within an inch of its life - great. If you get fun from building a 75k motor with shiney bits - great. Just remember you only get one go - so if what you are doing with your hobby isn't fun, don't do it.
02-13-2018, 02:58 PM
#128
Rennlist Member
Fun. That's it. This is for fun. If you get fun from a stock engine boosted to within an inch of its life - great. If you get fun from building a 75k motor with shiney bits - great. Just remember you only get one go - so if what you are doing with your hobby isn't fun, don't do it.
02-13-2018, 03:45 PM
#129
Rainman
Rennlist Member
Rennlist Member
Fun. That's it. This is for fun. If you get fun from a stock engine boosted to within an inch of its life - great. If you get fun from building a 75k motor with shiney bits - great. Just remember you only get one go - so if what you are doing with your hobby isn't fun, don't do it.
still makes 5.4L but tiny combustion chamber (can you fit 4v head over 95mm bore?) makes for easier tuning control and holding the head on easier.
02-14-2018, 04:38 AM
#130
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Join Date: Feb 2011
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Although the 7.0L version currently performs under the 6.5, I know we haven't been giving it enough air. I feel we have shown the kind of advances in the intake manifold and cams that it is going to benefit from. The story of the 7.0L motor is not over - later this year I should have another that we can feed correctly and realize its true potential.
Åke
02-14-2018, 06:14 AM
#131
Rennlist Member
Boosting the 4.5 was the original plan with my 81 until I decided to build a 5.0. Also, S3 heads are a direct swap to the 4.5 engine, S4 heads would give super high compression in excess of 12:1 IIRC.
02-15-2018, 03:07 PM
#132
Again, I think 7 liters is going in the wrong direction. Why is so much displacement needed to achieve more power? Although I own a 6.57L stroker that made 602hp with the custom manifold shown on my thread, it begs the question as to why the motor has to be so big to achieve that power level. As a counterpoint, I just placed an order for the Huracan Performante - 5.2L V10 normally aspirated, 640hp. Many people think this is underrated since the Performante smashed the Nurb lap time of the 918 Spyder, a much more powerful car by like 5 seconds.
I know, the new motor has DFI and a bunch of other doodads, but at the end of the day 640HP is being achieved by a motor that is smaller than that of the 928 GTS even. To me, that is a way more exciting challenge at this point - to build a super high horsepower 5.0 or 5.4.........
I know, the new motor has DFI and a bunch of other doodads, but at the end of the day 640HP is being achieved by a motor that is smaller than that of the 928 GTS even. To me, that is a way more exciting challenge at this point - to build a super high horsepower 5.0 or 5.4.........
02-15-2018, 03:27 PM
#133
A stock setup from memory is 760 grams for the S4 piston and about 890 for the rod and bearings? For a total of 1650 grams.
Well that weight difference is greater than I thought, 600 grams x 8 = 4,800 grams and then the stroke goes to around 82.5 mm from offset grinding. I don’t know how much weight I will be able to trim off due to costs. This particular engine has a cost factor involved. The rev limit will be 8,000 just because there is no point to going higher. The camshaft I suspect will make its max power in the 7250 to 7500 range. It has 251 degree at 0.050”. It’s a two valve engine. We are currently designing a CNC intake manifold which uses a LS throttle body.
02-15-2018, 04:00 PM
#134
Drifting
251 degrees at 0.050" is a big cam!
02-15-2018, 04:06 PM
#135
Mmmm, the lift is 0.542” nett, (solid tappet engine) the exhaust is 240 degrees with 0.495” nett . The exhaust side flows very well hence smaller duration, the engine capacity will be around 350 CI. As such I think my guess where the power range will be is pretty accurate, we’ll see.