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My rod ratio is 1.58 and so far so good, going into year 5. Did not Pat make a de-stroked high rev motor? IIRC he did. On a street car it is definitely a no-brainer, get as much stroke as you can. My gut tells me it will be no different in a race car, other areas will prove to be the limitations anyway.
In April, it was 4 years. 8 DE's, countless Autocross events, 3 trips to the drag strip, a few 944 Fests, and hundreds (if not thousands) of high boost pulls testing for Josh. So far so good...
My rod ratio is 1.58 and so far so good, going into year 5. Did not Pat make a de-stroked high rev motor? IIRC he did. On a street car it is definitely a no-brainer, get as much stroke as you can. My gut tells me it will be no different in a race car, other areas will prove to be the limitations anyway.
Bah...I wish I could give you the results of this but time and money have thrashed me to within an inch of my existence. Moving very slowly or not at all. Have never fired this up yet.
There is a point past which too much stroke can be cumbersome on a road engine that spends most of its running time in a low load area to comply with speed limits.
There is always the possibility to trim down the ignition profile to massage that excess of torque but that will alter engine response, making it arguably a less pleasant drive at legal speeds than a "shorter" stroke engine making less off-boost torque but with better throttle response.
I suppose I am one of the very few who runs his 3L hard on derestricted Autobahns, and I can say that it's above 5k rpm that engine performance matters, especially with a taller 6th gear. I can't imagine a "long" stroke engine to make peak power durably at sustained high load 6k+ rpm runs. Heat management is probably less of an issue on a race engine that rarely stays too long in the same gear at upper rpm, or on a development car that makes most of its arguably short full load runs on a dyno.
i don't think you can get enough stroke out of a 944 crank to make it "unreasonable" at a 6 or even 7k rpm limit.
another data point, the previous generation BMW M3 V8 has 75mm stroke and does 8300? rpm and then they made a 4.4L out of that engine with 82mm.
then there's the 7000rpm LS7 with a 101.6mm stroke
i think as long as the head and valves can keep breathing (with the right size bore and induction package) the stroke doesn't really matter for revs in the 944's possible rpm range. yes it will make more friction and heat but at 100+ mph there is so much airflow to the radiator that shouldn't be a big issue, right?
I am running 95 mm stroke and I see no ill effects as you describe. Would be pretty difficult to run more than 95 mm stroke on one of these motors. I regularly run my car at 6K+ down the back stretch at Road Atlanta which is about a mile long and I regularly drive my car on the street. I do not see it as an issue. I still say get as much stroke as you can.
Originally Posted by Thom
There is a point past which too much stroke can be cumbersome on a road engine that spends most of its running time in a low load area to comply with speed limits.
There is always the possibility to trim down the ignition profile to massage that excess of torque but that will alter engine response, making it arguably a less pleasant drive at legal speeds than a "shorter" stroke engine making less off-boost torque but with better throttle response.
I suppose I am one of the very few who runs his 3L hard on derestricted Autobahns, and I can say that it's above 5k rpm that engine performance matters, especially with a taller 6th gear. I can't imagine a "long" stroke engine to make peak power durably at sustained high load 6k+ rpm runs. Heat management is probably less of an issue on a race engine that rarely stays too long in the same gear at upper rpm, or on a development car that makes most of its arguably short full load runs on a dyno.
Question for those who have done the Reground stock crank with MITSU ECLIPSE DSM 4G63 rods...
Do you have to use aftermarket rods or can you use the stock MITSU rods is there enough meat in the small end to upsize for the Chevy pistons?
Not planning on running a turbo or anything radical can I use a stock Mitsu turbo con rod?
Thought I would share a few things I have learned as a result of the tear down of my 3.1L 95 mm hybrid stroke motor. About 10,000 miles and 5 years. 13 DE events, 14 Autocross events, drags and more. Pretty hard use. A few long trips.
1) Do not even think about not nitriding the crank. I have made several statements about this in the past, they were all wrong. It must be done to prevent micro-fracture initiation. These micro-fractures then can propagate leading to fatigue failure (ask me how I know). Some great info found here
2) Pay close attention to the fillet radius on the ground crank rod journals. Make sure the fillet radius is as big as possible. This also will help prevent micro-fractures initiation.
3) The larger the stroke, the lower the rod ratio. I saw quite a lot of bore wear with the 95 mm stroke. Several thousands. It should be a consideration especially for a high mileage street car. I think 92 mm is a good place to land on this. Just an opinion.
4) The Eagle rods with NO bushings at the small end of the rod (modified for SBC wrist pin) is not an issue as I saw essentially no wear.
5) On a 2.5L block I would use straight sleeves rather than flanged sleeves. For one, they are WAY cheaper. Two, they anchor at the bottom of the hole rather than the top. Cooler and less pressure at the bottom. If sleeves are installed properly then flanged sleeves work just fine but why pay more and proper installation might not be discovered until something bad happens. Just an opinion based on my limited experience.
If you used the narrower 4G63 rod and not-maximum offset stroke, could you then have your crank grinder build a larger fillet into the journal?
Probably not. You have to consider the rod side clearance.
I like the idea of using the narrower 7 bolt rod bearing in the 6 bolt rod. This will free up a little more room for a larger radius on the crank rod journal. Of course the rod bearing tab will need to be moved slightly so the bearing is centered. There's also a slight tradeoff in durability since the bearing is narrower, so pick your poison.
there was some variant of the Saab I-4 that had PGR, something crazy like 4-5mm clearance on either side of the rod IIRC.
lots of race series (NASCAR?) do that too, easy way to lighten the rod weight by narrowing the big end.
there was some variant of the Saab I-4 that had PGR, something crazy like 4-5mm clearance on either side of the rod IIRC.
lots of race series (NASCAR?) do that too, easy way to lighten the rod weight by narrowing the big end.
That would be more trouble than it's worth. To make PGR work, you'd need pistons with excellent stability and full round skirts to keep the rod squared with the crank, which eliminates modern lightweight piston designs with low comp height and short skirts, and that rules out anything compatible with more than approx 87mm stroke. At that point the risk of crank breakage goes down far enough that you don't need to worry about compromising the rod big end and bearing.
Thanks for reminding me, I checked the pics from my 2.85 build and noticed my pistons don't have full round skirts, even at the relatively tall comp height of 1.46" I also noticed my crank has a good radius, similar to stock. I feel a little more confident in my engine. see my pics in the first post here; https://rennlist.com/forums/944-turb...ig-engine.html
getting further off topic but if you reduced the offset grind (kept it more on center but still cut down to 4G or whatever size) you could just use the "old" rod journal to make new "cheeks" for the side clearance.
Also, you did nitride treat the crank I believe. My build was worst case study, max stroke, almost non-existent fillet radius, and no nitriding. And still I would do it again...
at 95 mm, the fillet radius get pretty tricky. I think my old machinist handled it by making in uniformly small. Be interested in the radius consistency all the way around on your crank.
06-01-2017, 08:53 PM
