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@aaron, Warren has them at his shop. I will measure and post. These have been worked on to make lighter.
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All engines suffer from harmonic issues. Cranks break when the harmonic frequency gets close or at the cranks natural frequency.
Don't get confused with dynamic balance and harmonic balance. Torque pulses twist the crank back and forth, this is the harmonic. Dynamic balance is when you consider only the weights of the components without any torque pulses. Smaller journal shafts will have less overlap, hence less stiffness which doesn't help. All these engines should be fitted with a damper. We have proven multiple times engines run better and make more power when fitted. The cam timing stays where you set it as the vibrations are far less and they do not travel up the chains and play havoc with the camshafts. I have been told that a known Porsche engine builder saw 55 extra HP after fitting one of our dampers. This was never proven and seen very high, but I'm sure if the cam timing was not controlled at high engine speeds you could lose 55 HP. To fit one of our dampers to an early engine, the front main bearing has to be machined to move the front seal back so the damper has the correct amount of overlap onto the crankshaft. |
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Was the space cam modified/changed on MFI build? |
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With pin. |
Hi Jay, My mistake on the weight. That was for the 3.8RSR.
Not as obese as the 3.8 but still calorically challenged. I have an 85.7 Mahle ST piston in house. It weighs 477 grams piston and pin. no rings or clips. The 90mm should be similar weight to the ST piston/pin. What is does show is a valve change from 46mm to 49mm caused a 60 to 70 gram increase in weight. |
No, the 3.8 rsr pistons weigh 470. These are the 12.5 compression pistons with the large valve reliefs that can fit a 53mm intake.
Actually, it seems that most of the Mahle Motorsport pistons are under 500 grams. It’s the stock street versions that weigh 650 plus. |
As I ponder using this engine in a new street build I'm working on I have a few questions/concerns.
As I mentioned, I'm thinking of a 66mm crank to get the C.R. down to something that we can run on pump gas and I also think a short stroke 2.6 might be kind of cool. I'm a little worried that the 42.5 intake and 39mm exhaust ports simply won't work on a street build even a rev happy short stroke engine. It was suggested to me that I might be able to choke the ports down by using phenolic spacers between the heads and the manifolds. Any of you experts have any thoughts on this? http://forums.pelicanparts.com/uploa...1534027996.jpg |
Just use a smaller cam. There’s nothing wrong with large ports IMO.
Never in my life, I’d admit, that I’ve ever heard someone being worried about too big of ports. This is a 1st. Me, I’d celebrate! ;) |
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Velocity is every bit as important as flow, if not MORE so. Big ports without velocity kills everything but high-RPM power and that makes a "dog" of an engine that won't accelerate. Everything must be designed and configured to work together: intake, heads, cams, CR, rod ratio, and the exhaust system. |
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If you don't want to go another set of heads then you may need to run a smaller than normal cam to get the midrange up and let the big ports improve the top end. Like a Mod Solex grind. (compression would need to be adjusted appropriately to the cam). Applying a choke only increases velocity @ the choke. The big ports will still be slow. |
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This is what I was concerned with is well. |
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This is why I am overly concerned about trying to turn this race engine with the large ports into something I can use on a car that will get driven on the street. I like the idea of a race car for the street but it has to be something that is driveable on the street! I'm very tempted to give it a try but don't really know what I will end up with if I do. |
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So Steve in your opinion could the heads that I'm referencing be at all something I could run on the street? ...71 MFI heads 42.5 intake and approx 39/40 exhaust depending on where you measure. I'm not sure if I go MFI, EFI or carbs yet.
- I need to come up with something for my 68 SWB sports purpose build and I already own this engine. |
The secret (if one can call it that) to getting something like this streetable would be to go some sort of fuel injection, EFI or MFI, but EFI would be easier. DC60 or thereabouts cam with high twin plug compression further helps. The lack of port velocity would make a vacuum signal to carburetors non existent at lower rpm.
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But, I remember a lot of 930 guys going from 930 heads to 3.2 heads and not noticing a drop in low end torque. But yeah, 42.5 mm ports are pretty darn big on a small motor. Guess the OP just needs to swap with me since I'll put them to good use on a turbo'd 3.4.... :D:D:D |
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I would use something with 36mm-38mm I & 35-36mm E ports with cam of choice, decent CR, & twin-ignition. |
Can’t for a fairly modest amount a small sleeve be installed in these heads to choke it down to the 37-38mm mark? And make the heads usable?
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