Max CR with 87 octane

[bugaboots]

I am about to have my machinist deck my block but i dont know how much to have him take off. I am trying to figure out what the max Compression ratio is that i can run with 87. This is going to be a daily driver. Here is my current build

4.6L Long rod
KB IC 944 Pistons 21cc Dish .030 over
Stock 4.0 Rods
Comp 68-232-4 cam
New casting 0331 Replacement head
Performance head gasket .043
Bosch 27# injectors

I am not sure what i can safely run for compression and still get away with 87, if i have to run 89 i would be just fine with that as that is what i will most likley do just to be safe. Would love to hear what some others are running and getting away with on pump gas.

[honkysXJ]

That's a good question, I'm not sure anyone has a %100 answer to it theres a lot of variables to the equation/ quench,cr,I hear aluminum heads can handle more compression.
I'm interested in seeing some of the responses because I'm sure there's mixed opinions!
I'm basically doing the the same build as you (different cam ) also decking the block to .05 in the bore for a [email protected]:1@48 quench, I plan on running midgrade at least which I do already, after dumping money in a upgrade like a stroker premium gas is not much more than regular!
I am no expert and can't wait to see what peoples opinions are

[bugaboots]

What cam are you planing on running?

Also dont know if it matters with the 4.0 but I am running a 02 grand cherokee

[honkysXJ]

I'm torn between the mopar 229 and a lunati ill more than likely run the mopar to save some money but other than that Im building the same engine as you cam is officially my last decision to make,machine work is pretty much completed! As far I know it kinda does matter your running fi cause from what I hear carbed moters can typically handle a bigger cam but plenty of people are running the 232 with fi there's piggy backs and tuning you can do if I had enough money I'd probably get a big ass cam for my build but I'll probably get the mopar cause I'm tapped out.
Not sure if you've heard any of the horror stories but some have had problems with comp and wiping out the lobes but it's not isolated to only comp ,one of the things I'm drawn to the mopar for is the wide lobes like the stock cam !
Keep us posted, another notable thing is some might say 28# injectors are to much for a 4.6 ,I've got a few sets of 24s so hit me up if you need to !!! Also that 0331 head will be the week spot in your engine "from what I hear"
Good luck and keep us posted!
So come on guys whats the standard c/r for 87 fuel?

[JPrubo]

The 0331 casting should be fine if it's a new replacement head. The early 0331 castings crack. The stock "TUPY" 0331's and after market replacements should be fine.

[Mad Machinist]

This question is why I registered.

As far as max compression on any motor, it depends. I've ran as high as 11:1 on 87 octane.

There are two different types of compression, static and dynamic. Static is typically what you see listed with the piston on a stock motor, i.e. KB IC944 at 9.1:1. Dynamic compression is what you get after the intake valve closes. Static C.R. looks good on paper, dynamic C.R. is the one to worry about.

Now the cam comes into play. The point the intake valves closes ABDC determines the C.R. Put a big cam in a stock motor and you end up with a dog unless the C.R. is raised to compensate.

For example, my SBC 383 is running 9.8:1 static. But because of the cam I am running (intake closes 41 degrees ABDC) the effective C.R. is 8.3:1. Perfectly safe on 87 octane. And it is pushing 550 horse at 5800 rpm and 550 ft-lbs of torque at 5600. It pulls like a freight train.

Aluminum heads will typically allow one to run almost a full point higher on compression due to the fact they dissipate heat faster than cast heads thus eliminating hot spots in the chamber that cause detonation.

Ideally, the quench distance between the piston and head should be kept between .035 and .045. This helps fight detonation sensitivity due to a more complete burn of the fuel mixture. A lot of power is being left on the table here and I'll see if I can help out here.

David Vizard wrote a great series of books on building motors. The big 3 in the auto industry all consulted with him. The books aren't the cheapest out there, but they are worth the money.

[gdl]

Your elevation will have an effect on octane requirements. At 600 feet in the Dallas/Ft. Worth area I was running 87 octane and every once in awhile I thought maybe I was hearing just the slightest hint of detonation. It was faint and sporadic so I'm not even 100% sure that's what I was hearing. In Denver at 5,300 feet I'm running 85 octane and haven't heard any detonation at all.

My specs:
.030 over KB 944 pistons on stock 4.0 rods, connected to a Scat crank.
Had the head shaved .008 to clean it up, still running stock springs and valves. Very minor port work on the intakes to match to the gasket.
Decked the block .022, leaving the pistons .005 in the hole.
Used a Victor head gasket, quench ended up at .048
Running the Crower 44243 cam installed straight up
Static/Dynamic compression ratio ended up at 9.61 / 7.98

[the grey ghost]

I built nearly the same engine, only its .040 over whith the crower Baja beast cam

[RenoF250]

Your elevation will have an effect on octane requirements. At 600 feet in the Dallas/Ft. Worth area I was running 87 octane and every once in awhile I thought maybe I was hearing just the slightest hint of detonation. It was faint and sporadic so I'm not even 100% sure that's what I was hearing. In Denver at 5,300 feet I'm running 85 octane and haven't heard any detonation at all.

My specs:
.030 over KB 944 pistons on stock 4.0 rods, connected to a Scat crank.
Had the head shaved .008 to clean it up, still running stock springs and valves. Very minor port work on the intakes to match to the gasket.
Decked the block .022, leaving the pistons .005 in the hole.
Used a Victor head gasket, quench ended up at .048
Running the Crower 44243 cam installed straight up
Static/Dynamic compression ratio ended up at 9.61 / 7.98

What did you calculate that from? Using the CR calculator that combo comes up as 10:1 static CR. Regardless no pinging is what I wanted to know. I am planning on something similar but no block or head machining so compression should be lower.

[Cheromaniac]

So come on guys whats the standard c/r for 87 fuel?

There IS no standard CR for 87 octane fuel. There are too many variables involved but the biggest one is the intake valve opening duration of the cam. Make the duration long enough and you could run the engine on 87 octane with an 11:1 CR but it'll have as much low rev torque as a wet sock and a poor idle quality to match.
We usually want our strokers to produce plenty of torque at low rpm and that requires a shorter duration cam. The shorter the cam duration, the higher the cylinder pressure at low rpm and the need to keep the CR lower to prevent detonation.
If you attend to the following details you'll increase your chances of running your engine on 87 octane:

1. Cooling system. Make sure components are up to scratch, especially the radiator. For warmer climates I recommend a 180* t'stat.

2. Combustion chambers & piston dishes. Make sure they're nice & smooth, and feather any rough edges.

3. Quench height. Preferably tight (maximum 0.050").

4. Optimize fuel curve with correct size injectors +/- adjustable fuel pressure regulator or MAP adjuster.

5. Hotter grade plugs are more likely to cause detonation, and cooler plugs are more likely to foul up, so you may need to try more than one grade to find the right balance.

[RenoF250]

I built nearly the same engine, only its .040 over whith the crower Baja beast cam

Can you run 87 octane in it? Crower recommended the baja best to me as well.

Are the IC994s the lowest compression off the shelf pistons?

[SilverXJ]

The IC944/945 is the only off the shelf piston to use with the 4.0L rods. The IC944 being the lower compression piston.

[jsawduste]

The IC944/945 is the only off the shelf piston to use with the 4.0L rods. The IC944 being the lower compression piston.

And is a crappy choice. To bad KB didn't do a better job designing these.

[RenoF250]

The IC944/945 is the only off the shelf piston to use with the 4.0L rods. The IC944 being the lower compression piston.

And is a crappy choice. To bad KB didn't do a better job designing these.

What would you recommend for an low-end torque engine to run on 87 octane?

[beiwulf]

as for pistons there are a lot of choices for custom out there in a reasonable price range for what the KB's are going for.

I recall talking with Clegg Engines and the sales rep (Tieson) said about $4-500 for a set of custom pistons iirc (4-6 week lead time). Clegg deals mainly with reselling parts but the advantage to going with them is they usually have relationships and price breaks with those larger custom companies. Clegg does build engines as well - I don't believe they make any parts at all though. Clegg helped assist with the Scat jeep rods recently. They also use Ross and Diamond also for there custom piston orders. Off Clegg and onto others - CP-Carrillo is charging $93 per slug not including rings and pin/retainer clip. By the time you get done it would be around 130-150 per slug depending on the goodies (and I don't recall if that was including coatings). I enjoyed talking with Dave @ CP-Carrillo east coast office regarding pistons. There is also Dave as Race Tech out in the west coast (personable and willing to educate). All these companies and names are good people to work with from what I have experienced.

Bulltear is another name if have seen and spoke with months ago. Last time I spoke with someone from there they said they had there slugs on backorder. There foreign supplier was backlogged and they said there was not going to be any till something mid year (which is approaching). They get there slugs and machine them down some to there specs <$500 last i saw.


Hope that helps some.

Brian