Spinout007 UberDork
May 19, 2017 7:34 a.m.

For now, just the stock cams. After the challenge I may go with a longer duration intake cam. Miata cams sadly are a 1/4 of a challenge budget. I'm not sure if there is even an aftermarket grind out there for the VVT head engine.

Zomby Woof PowerDork
May 19, 2017 9:48 a.m.

The dwell and phasing of the cams 1-2 degrees will gain you roughly nothing (FYI).

ProDarwin PowerDork
May 19, 2017 10:42 a.m.

Is this a Saturn piston you are working with? Just curious, because they are a .767 wrist pin, and people have gone the other way (BP pistons in a Saturn motor).

Saturns have a lower compression height, so it stands to reason you could use one of them on a longer rod in a Miata motor for better bore/stroke. They won't support more than like 250-300 boosted though.

Spinout007 UberDork
May 19, 2017 1:39 p.m.
ProDarwin wrote: Is this a Saturn piston you are working with? Just curious, because they are a .767 wrist pin, and people have gone the other way (BP pistons in a Saturn motor). Saturns have a lower compression height, so it stands to reason you could use one of them on a longer rod in a Miata motor for better bore/stroke. They won't support more than like 250-300 boosted though.

Actually it's a 2.0 Zetec piston and 4g63 rod.

Spinout007 UberDork
May 19, 2017 1:41 p.m.
Zomby Woof wrote: The dwell and phasing of the cams 1-2 degrees will gain you roughly nothing (FYI).

Thanks for the heads up.

ProDarwin PowerDork
May 19, 2017 6:50 p.m.
Spinout007 wrote:
ProDarwin wrote: Is this a Saturn piston you are working with? Just curious, because they are a .767 wrist pin, and people have gone the other way (BP pistons in a Saturn motor). Saturns have a lower compression height, so it stands to reason you could use one of them on a longer rod in a Miata motor for better bore/stroke. They won't support more than like 250-300 boosted though.

Actually it's a 2.0 Zetec piston and 4g63 rod.

Which ones have a .767 bore? All I am seeing is .787

ProDarwin PowerDork
May 19, 2017 7:38 p.m.

Also, reading the rest of your post... why are you putting so much focus on a longer rod setup when it sounds like the head is by far the biggest restriction? Or is the goal just to increase compression?

Spinout007 UberDork
May 19, 2017 9:26 p.m.
ProDarwin wrote:
Spinout007 wrote:
ProDarwin wrote: Is this a Saturn piston you are working with? Just curious, because they are a .767 wrist pin, and people have gone the other way (BP pistons in a Saturn motor). Saturns have a lower compression height, so it stands to reason you could use one of them on a longer rod in a Miata motor for better bore/stroke. They won't support more than like 250-300 boosted though.

Actually it's a 2.0 Zetec piston and 4g63 rod.

Which ones have a .767 bore? All I am seeing is .787

I think signals are getting crossed here. 4g63 rods have either a 21 or 22mm wrist pin depending on if they're from a 6 or 7 bolt motor. Zetec pistons are 84.8mm bore with a .7874 pin which comes out to 19.99996 pin and a 1.152" compression height. This should give me a 1.7X rod ratio vs the stock 1.5 rod/stroke ratio. Theoretically creating a more freely spinning motor, a small bump in compression, slower moving pistons and a longer dwell at TDC and on the other end. I'm going to be paying just as much if not more attention to the head and concentrating on low lift flow vs peak flow as massive lift without custom cams is not really possible with the way the BP valve train is put together.

ProDarwin PowerDork
May 19, 2017 9:41 p.m.

Yup, wires were crossed in my head. No other rods you can find with a .787 pin that would work on the crank? So this is all in the hopes that a small change in dwell will extract more power?

It will be an interesting experiment to follow. Saturns have this same problem. Actually, the head numbers are not terrible (on par with/maybe slightly better than a 1.8BP). In theory the head should support around 200-220hp depending on lift yet nobody seems to be able to make more than 155whp out of one and even that is very uncommon. BP guys put down better numbers (also uncommon though). The more I read on it, at least in the case of the Saturn, the intake manifold and TB are likely bigger restrictions than the head - is this the case on Miatas as well?

Spinout007 UberDork
May 19, 2017 10:38 p.m.

The head and intake are the cork in the BP system. The long rod engine was an experiment in math. It may turn out no different than a normal bp motor, it may not work at all. I just know that no one seems to have attempted it, or has been demoralized enough by the "just boost it crowd" to even really attempt it, or attempt it and document it. I figure I'll play with it.worst case scenario, it doesn't make a difference, and I'm stuck with a block that's ready for a FM stroker kit.

Zomby Woof PowerDork
May 20, 2017 6:52 a.m.

The camshafts are more of a restriction than the cylinder head itself. There's a lot more potential there than anything you're attempting on paper at the moment.

frenchyd Reader
May 21, 2017 2:26 p.m.

In reply to Spinout007: I did something like that on my Jaguar V12.. The stock crankpin is 2.300 and I had the crankshaft grinder offset grind it to a Honda bearing size the stock car guys use (1.94).. I did have to narrow both the Chevy Rod and the Honda bearing to get everything working but it was simple vertical mill work. I had to go to a custom piston but it was still cheaper than Jaguar pistons were.

iceracer UltimaDork
May 21, 2017 6:30 p.m.

With the same stroke, piston speed will be the same.

How does a longer rod cause freely spinning ?

Just seems like a lot of work for so little gain.

I guess you ae just trying to prove a theory.

Autolex Dork
May 22, 2017 9:33 a.m.
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