Refresh951's Hybrid Ultra Stroker Build
02-28-2014, 01:43 AM
#1186
Rennlist Member
Not sure if you can buy a conical flare (don't know how else to describe it?) that might only be 1" long but just flare out an extra 1/2 to 1" diameter? Then just do a bit of quick cutting and welding...hey presto.
I know that's a simplified way of looking at it but perhaps that's all it would take? Gut instinct says that it would help. Guess you can leave that up your sleeve for next time.
I know that's a simplified way of looking at it but perhaps that's all it would take? Gut instinct says that it would help. Guess you can leave that up your sleeve for next time.
02-28-2014, 02:16 PM
#1188
Rainman
Rennlist Member
Rennlist Member
How much more stroke could be fit into the block with the Mitsu rods if starting with a 3.0 block (and factory-clearanced girdle)...could we get to an even 100mm stroke (with added crank material)?
02-28-2014, 05:27 PM
#1189
Rennlist Member
Thread Starter
Never looked at it from that angle. Pretty sure 96 or 97 may be physically possible with safe clearance. I do not think I would want to go much more than 95 mm stroke due to the low rod ratio. Mine is 1.59 IIRC and ideal is around 1.71.
03-01-2014, 01:10 AM
#1191
Rennlist Member
About 15 years ago I built a 3.4L 944 race engine using the 944 rod journal size, Carrillo rods, and a 4.155" bore (can't recall the stroke, but you can do the math). It had a reasonable rod/stroke ratio, a short compression height, and of course additional rod clearancing. It made a lot of torque, but power was limited by ITB's sized for a smaller engine. Mechanically, it was sound - shifting was never below 8000.
03-01-2014, 10:02 AM
#1192
Race Car
Attached #2 is a comparison of piston position to crank angle for different rod ratios. Extremes are 1.4 and 2.5 (2.5 being an F1 type). Production looks around the 1.6-1.8 range. The difference is quite small. I did this for a guy looking to increase the rod length on a 968, as well, which led to all sorts of issues, including needing to add volume to the dish, which owuld have hurt combustion. Attachment #3 is that 968, rod ratios are 1.80 and 1.71. This is now taking cam events into consideration along with displaced volume. It is FEA of air through the valves (which is not what really happens, as the air will not exceed about 330 m/s early in the stroke). The difference here is minimal. Directionally correct, yes, but when you look at the tradeoffs, is it really worth it? On an all out unlimited budget race car, sure. On a street car, NFW.
03-01-2014, 10:48 AM
#1193
Rainman
Rennlist Member
Rennlist Member
Why don't you want to put more rod clearance in the lower block and girdle? You're talking about maximizing stroke by going to a lot of trouble (adding material to the crank, custom rods, custom pistons); why not do some simple machining to the block for additional rod clearance?
About 15 years ago I built a 3.4L 944 race engine using the 944 rod journal size, Carrillo rods, and a 4.155" bore (can't recall the stroke, but you can do the math). It had a reasonable rod/stroke ratio, a short compression height, and of course additional rod clearancing. It made a lot of torque, but power was limited by ITB's sized for a smaller engine. Mechanically, it was sound - shifting was never below 8000.
About 15 years ago I built a 3.4L 944 race engine using the 944 rod journal size, Carrillo rods, and a 4.155" bore (can't recall the stroke, but you can do the math). It had a reasonable rod/stroke ratio, a short compression height, and of course additional rod clearancing. It made a lot of torque, but power was limited by ITB's sized for a smaller engine. Mechanically, it was sound - shifting was never below 8000.
I'm a long rod fan, but I personally think people get way too hung up on it a times. If you are running a class that limits stroke, then sure, increase the length. If you have already maxed out stroke through whatever other methods, sure, increase rod length. I would NOT let rod ration constrain other things. Generally (i.e. not 944 specific) speaking, you can push the compression height to about 25mm, but that puts the pin into the oil scrapers, and that will increase oil consumption. Attached #1 is a comparison from a stroked Ford 2.3 Turbo I built. You can see how increasing the rod length got into the rings. Factory rod ratio was 1.67. With just the stroke, it would have gone down to 1.53. I recovered some of that and got to 1.61. Obviously this was a short rod engine compared to the Porsche.
Attached #2 is a comparison of piston position to crank angle for different rod ratios. Extremes are 1.4 and 2.5 (2.5 being an F1 type). Production looks around the 1.6-1.8 range. The difference is quite small. I did this for a guy looking to increase the rod length on a 968, as well, which led to all sorts of issues, including needing to add volume to the dish, which owuld have hurt combustion. Attachment #3 is that 968, rod ratios are 1.80 and 1.71. This is now taking cam events into consideration along with displaced volume. It is FEA of air through the valves (which is not what really happens, as the air will not exceed about 330 m/s early in the stroke). The difference here is minimal. Directionally correct, yes, but when you look at the tradeoffs, is it really worth it? On an all out unlimited budget race car, sure. On a street car, NFW.
Attached #2 is a comparison of piston position to crank angle for different rod ratios. Extremes are 1.4 and 2.5 (2.5 being an F1 type). Production looks around the 1.6-1.8 range. The difference is quite small. I did this for a guy looking to increase the rod length on a 968, as well, which led to all sorts of issues, including needing to add volume to the dish, which owuld have hurt combustion. Attachment #3 is that 968, rod ratios are 1.80 and 1.71. This is now taking cam events into consideration along with displaced volume. It is FEA of air through the valves (which is not what really happens, as the air will not exceed about 330 m/s early in the stroke). The difference here is minimal. Directionally correct, yes, but when you look at the tradeoffs, is it really worth it? On an all out unlimited budget race car, sure. On a street car, NFW.
But "the internet" said that the forces of combustion are directed more into the main bearings/block webbing with a longer rod than with a shorter which puts pistons into cylinder walls harder. My concern with going too short on a rod vs stroke would be that we know the 944 cylinders move around a little, so if more strain on the cylinder walls could be taken off by using a longer rod it might help save HG's.
03-01-2014, 12:44 PM
#1194
Rennlist Member
I asked about the 3.0 block because that factory girdle looks to have a large amount of clearance done to accommodate the 3.0 stroke. Shawn's motor with smaller rod ends has ~7mm more stroke than the 3.0 and still fits within the 2.5 block, within 2mm of the girdle. So if adding 5 more mm to stroke, then you'd want at least 5mm more girdle clearance. Not saying the 2.5 girdle can't be modified to work, but starting with a 3.0 engine gives you a better block as well.
03-01-2014, 01:12 PM
#1195
Race Car
In regards to rod ratio...
You could just add deck height :-O (I know of one such engine)
Shorter rods do add a bit of piston speed and higher velocity. They also add a bit of side load on the skirts. In the end it's all relative too the rpm desired. I've built short rod 400 sbc's that outperformed long rod ones when shifted at lower rpms.
personally I don't think that building a much bigger engine is necessary. Building a 3.3 is very easy using this approach (mitsu rod size with base 3.0 parts).
Looking at the 968 heads flow potential, I don't see a need for a larger engine, when you won't even be maxxing out the head on a smaller engine. Also if rataining the 8v you are limited to the head anyway.
You could just add deck height :-O (I know of one such engine)
Shorter rods do add a bit of piston speed and higher velocity. They also add a bit of side load on the skirts. In the end it's all relative too the rpm desired. I've built short rod 400 sbc's that outperformed long rod ones when shifted at lower rpms.
personally I don't think that building a much bigger engine is necessary. Building a 3.3 is very easy using this approach (mitsu rod size with base 3.0 parts).
Looking at the 968 heads flow potential, I don't see a need for a larger engine, when you won't even be maxxing out the head on a smaller engine. Also if rataining the 8v you are limited to the head anyway.
03-01-2014, 01:22 PM
#1196
Rainman
Rennlist Member
Rennlist Member
In regards to rod ratio...
You could just add deck height :-O (I know of one such engine)
Shorter rods do add a bit of piston speed and higher velocity. They also add a bit of side load on the skirts. In the end it's all relative too the rpm desired. I've built short rod 400 sbc's that outperformed long rod ones when shifted at lower rpms.
personally I don't think that building a much bigger engine is necessary. Building a 3.3 is very easy using this approach (mitsu rod size with base 3.0 parts).
Looking at the 968 heads flow potential, I don't see a need for a larger engine, when you won't even be maxxing out the head on a smaller engine. Also if rataining the 8v you are limited to the head anyway.
You could just add deck height :-O (I know of one such engine)
Shorter rods do add a bit of piston speed and higher velocity. They also add a bit of side load on the skirts. In the end it's all relative too the rpm desired. I've built short rod 400 sbc's that outperformed long rod ones when shifted at lower rpms.
personally I don't think that building a much bigger engine is necessary. Building a 3.3 is very easy using this approach (mitsu rod size with base 3.0 parts).
Looking at the 968 heads flow potential, I don't see a need for a larger engine, when you won't even be maxxing out the head on a smaller engine. Also if rataining the 8v you are limited to the head anyway.
By my math, basically maxing out the block at 108mm bore/100mm stroke, comes up within rounding of 3.7L...the Porsche 944 tractor.
03-01-2014, 01:32 PM
#1197
Race Car
One is already in the works using a machined oring groove around all passages.
IMO it is seriously overkill, but should prove to be quite an engine when finished. The nice thing about it is that there are Mitsu 156mm rods that can be used with pistons that still keep the pin out of the ring lands. Even the top of the line mitsu rods are less expensive than most Porsche aftermarket units. Add the race bearings into the mix and you're even further ahead.
IMO it is seriously overkill, but should prove to be quite an engine when finished. The nice thing about it is that there are Mitsu 156mm rods that can be used with pistons that still keep the pin out of the ring lands. Even the top of the line mitsu rods are less expensive than most Porsche aftermarket units. Add the race bearings into the mix and you're even further ahead.
I will take it for a drive and some tuning tomorrow 