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Peak Torque RPM for a 258/4.0 Hybrid

MomoJeep

MomoJeep

Basic User
City
Seattle
State
WA
Hey engine guys!

If a 258 peak torque is around 2100rpm, and a 4.0 is around 2800rpm, then what about:

Hybrid engine: 4.0 Head, Cam and MPI conversion on top of 258 block (bored 0.030)

the 1983 258 gets torquey due to it's crank and rod geometry
the 1995 4.0 gets it from it's better head/cam

...so would the peak torque be somewhere in the middle of 2100 to 2800? Or is it still down low near the 258 range (2000-2200) ?
 
I did have that setup (258 block with 4.0 head, MPI and headers) it was very nice. Unfortunately that block developed a crack between the freeze plugs. When I replaced the engine it was with a standard 4.0 with MPI, headers and slightly hotter cam with roller rockers. This setup is just not as peppy as the previous setup. But the previous setup was worth the effort and time with the changes.
 
gundrvr said:
I did have that setup (258 block with 4.0 head, MPI and headers) it was very nice. Unfortunately that block developed a crack between the freeze plugs. When I replaced the engine it was with a standard 4.0 with MPI, headers and slightly hotter cam with roller rockers. This setup is just not as peppy as the previous setup. But the previous setup was worth the effort and time with the changes.
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That's interesting that the 4.0 engine was less peppy. I've heard different opinions both ways..

What I'm trying to figure out is whether the torque curve for this hybrid engine is more like a 4.0, or more like a 258? (I've been assuming it's low end grunt is due to the 258 block geometry)
 
258 = long stroke = torque

4.0 = shorter stroke = horsepower

Generally speaking. The 4.0 head on the 258 improves airflow helping the 258 make more power at higher RPMs. It also will sacrifice a little power in the lower RPM range.

Long “narrow” intake / head runners = torque

Fat short intake / head runners = horsepower

The rest of the engine needs to be built for the intended RPM range for that generalization to be true. If you build a rev monster and slap long runners on it, nothing will be happy. If you build a torque monster and use short runners, torque will disappear.

Throwing a 258 crank in a 4.0 engine is common practice to get a 4.5 liter stroker motor. Bore it out and you can get 4.7 liters fairly easy.

Remember, horsepower is a function of torque at a specific RPM. (Horsepower = Torque x R.P.M. / 5252). Higher torque at higher RPM = higher horsepower.
 
ag4ever said:
258 = long stroke = torque

4.0 = shorter stroke = horsepower

Generally speaking. The 4.0 head on the 258 improves airflow helping the 258 make more power at higher RPMs. It also will sacrifice a little power in the lower RPM range.

Long “narrow” intake / head runners = torque

Fat short intake / head runners = horsepower

The rest of the engine needs to be built for the intended RPM range for that generalization to be true. If you build a rev monster and slap long runners on it, nothing will be happy. If you build a torque monster and use short runners, torque will disappear.

Throwing a 258 crank in a 4.0 engine is common practice to get a 4.5 liter stroker motor. Bore it out and you can get 4.7 liters fairly easy.

Remember, horsepower is a function of torque at a specific RPM. (Horsepower = Torque x R.P.M. / 5252). Higher torque at higher RPM = higher horsepower.
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Yes, thank you. I get how the 258 uses it's bore/stroke geometry make good torque. The part I don't fully get is how a head/cam (probably mostly the cam) can also increase torque. My understanding is the the 4.0 cam is an improvement on the 258 cam. But in what way? In other words, what does the 4.0 cam/head and MPI do to the stock 258 torque curve?
 
ag4ever said:
There are a lot of factors at play in engine design. Velocity of the air fuel ratio, cylinder scavenging, cylinder pressures, etc…

There are a lot of resources on the interweb that discuss engine design. Here is one that lightly discusses some of the factors at play.

Here is a discussion of the 4.0 head swap on a 258.
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Thanks. When I drive mine, I like cruising at around 2200 at highway speeds. I noticed it can go a lot lower, like down to 1800, but for the health of the engine and optimum efficiency and mileage I want to run at peak torque. Some people go with lowest RPM's for better mileage, but I subscribe to the idea that best mileage is at peak torque (?)

I guess I'll never know exactly where that is without a dyno...!?
 
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