Jeep Strokers

If Russ is recommending the parts combination I would be confident of his set up choices.

If I were to run an aluminum head, would that help with the quench height issues with the lower ? Because you could run a piston with less dish due to the lower compression of the aluminum?

Because a aluminum head will dissipate heat at a faster rate than a cast-iron head, we generally run a full point of compression more with the aluminum head.

Quench is a term used for the increased combustion efficiency as the piston gets closer to the cylinder head as it approaches TDC

I've been reading that predetonation is a big issue with these 87 octane and 4.6 stroker. Russ, you said that the stroker kit you recommend to me would yield about 9.3 to 9.5 compression, isn't that at the outer limits of 87? Wouldnt going to an aluminum head effectively bring it down to about 8.5:1 given the same combustion chamber volume?

The aluminum head wont change the compression given it has the same cc and shape but it will handle a higher compression before detonation.

Hussler wrote:I've been reading that predetonation is a big issue with these 87 octane and 4.6 stroker. Russ, you said that the stroker kit you recommend to me would yield about 9.3 to 9.5 compression, isn't that at the outer limits of 87? Wouldnt going to an aluminum head effectively bring it down to about 8.5:1 given the same combustion chamber volume?

Jim,
The key to to making torque and running a relatively high static compression ratio for a given octane level is running a tight quench.

Russ Pottenger wrote: Jim,
The key to to making torque and running a relatively high static compression ratio for a given octane level is running a tight quench.

I still don't understand though how you can recommend a higher compression (9.5:1) without the threat of predetonation on 87, unless there's some aspect I'm not understanding

The key is what Russ said in his response. "running a tighter quench".

The combustion chambers of our strokers are quite dated by even modest modern standards.

Generally speaking, tightening up the quench on a given engine allows for a more consistently distributed mixture that burns more uniformly. Which reduces the tendency to detonate.

The combustion process is at (or near) the same temperature regardless of the cylinder head material. The number of BTU`s created during combustion is consistent, look up Boyles Law. Because the transfer of heat is more efficient with the aluminum u can raise the compression level thereby taking advantage of the thermal properties.

Don`t get confused between detonation and pre ignition. They are similar but yet very different.

My 2 cents.....If your in a borderline situation.......And I am not saying Russ`s combo is or is not..........You can buy an awful lot of gasoline with a higher octane that controls "knock" before you will recoup the expense of the aluminum head.

jsawduste wrote:The combustion chambers of our strokers are quite dated by even modest modern standards.

Only 'cause most modern engines have four valves per cylinder instead of two.
The combustion chambers in the 4.0 head are really not that bad for a 2-valve head and were certainly a quantum leap over the 4.2 head.
The quench areas are generously sized and the near kidney shape promotes good swirl for a faster burn, but the non-crossflow design makes it impossible to place the spark plug centrally and the ports are less efficient than they could have been.
Having an aluminum head saves weight and allows a full point more compression without detonation but since the block is cast iron, the head bolts need to be retorqued a few heat cycles after initial installation. An aluminum head is also more prone to warpage and the head gasket is more likely to fail ('cause aluminum & cast iron expand at different rates) if the engine is overheated.

Still don't understand the 9.5:1 compression ratio though, how is that possible on 87?

Cheromaniac wrote:

jsawduste wrote:The combustion chambers of our strokers are quite dated by even modest modern standards.

Only 'cause most modern engines have four valves per cylinder instead of two.
Having an aluminum head saves weight and allows a full point more compression without detonation but since the block is cast iron, the head bolts need to be retorqued a few heat cycles after initial installation. An aluminum head is also more prone to warpage and the head gasket is more likely to fail ('cause aluminum & cast iron expand at different rates) if the engine is overheated.

Those are pretty generic statements Dino.

Think you might find some disagreement with those whom are running typical 2 valve headed engines. As a GM guy I don't think you can argue with the lowly LS series track record. Then there is the plethora of aftermarket aluminum heads for old school v8`s.

Yes the 4.0 is a step above the "older" designs but it is still in a different league to more modern designs.

jsawduste wrote:As a GM guy I don't think you can argue with the lowly LS series track record. Then there is the plethora of aftermarket aluminum heads for old school v8`s.

Yes the 4.0 is a step above the "older" designs but it is still in a different league to more modern designs.

I don't disagree with you there John.

Hussler wrote:Still don't understand the 9.5:1 compression ratio though, how is that possible on 87?

You might want to read this article by David Vizard. He discusses quench about 2/3 of the way down the page under "Combustion Chamber Dynamics". Should help with the how and the why.

http://www.hotrod.com/events/coverage/0 ... r-squeeze/

That was actually really informative, thanks.

I have a line on a 4.2 crank that I can hopefully pick up tomorrow

Is there any sort of guide on using reconditioned rods vs. the scat ones? At what horsepower rating should you get the scat rods? The only articles I can find on bad stock rods is when hydrolocked and they bend. My local guy does a beautiful job reconditioning rods.