Questions concerning shaving the head of a KA24E

[JPermen]

Hi. I'm an owner of a (stock) '89 240sx. The other day I was checking out some specs and noticed how low the compression of the sohc is (8.6:1).

My next thought was that I should shave the head to roughly 10.0:1 or so.

I realize I'll have to make a new intake as well as richen the gas/air mixture. What other modifications would I have to make in order for my car to handle the increase in compression? Would ARP bolts be necessary? How much torque/horsepower gain should I expect?

Any instight will be appreciated greatly.

Thanks

[Titan]

First off, not all SOHC's have 8.6:1 compression. As far as I know, some have 9.0:1.

Make a new intake? Do you mean piping from the filter, or a new manifold? Because you certainly won't need a new manifold.

For an all motor, unless your'e going to go nuts, special bolts aren't neccessary. Although, if you have the money, go for it.

Your going to have to run 91 octane minimum most likely.

The increase in power won't be all that much, maybe 5-8% overall. (I think)

Personally, I wouldn't just shave the head for higher compression. I would do a combo of higher compression pistons and a shaved head to acheive what you want.

Plus, if you really want to see an increase in power, I would do a mild port on the head, and mill the cams a bit for longer duration. Just make sure your pistons don't come in contact with your valves. (A.K.A watch your cam lift when milling)

[RedShred]

Honestly, I am not trying to be a search button arsehole... but read as much as you can from this specific forum. Then you will have a better understanding of what your looking for.

Shaving the head alone will not achieve what you want. it would take alot of material to be removed from the head and deck of the block to achieve that compression, also by shortening the distance between the cam and crank that much, will probably make it abit harder to index.

[U12 2NR]

shave the head that much and you're asking for trouble.

[S13Ka24e]

I have hear of a few people saving there heads and they seem to think i worked fine. I talked to a guy once and he told me that if you shave the head 50/1000 of an inch then you increase the compression to 11.1 to 1, so that is an increase of 1 compression point for every 20/1000 so for 10 to 1 that would be 28/1000 of an inch. Now with this said, i hear this from a guy, i have no idea how realiable he is, and don't know if the numbers are reliable. I assume you are having this done at a machine shop, they should be able to do the calculation for this and they will make sure that it is right, just let the shop do the the work.

[DAEDALUS]

If you know the bore and stroke you can calculate the CR.

[4cefed]

It might also be worth it to center bore the cam journals. Almost every head on the planet is slightly warped a bit when removed from the block. when re-assembled they will usually mate back up just fine. When you remove the head and shave it, you may risk a bannana'd cam. I've seen cam shafts break in half because of this. Just check for flatness before you shave it.

[Aries]

here's the deal.To get 10:1 compression by milling the head on a 8.6:1 KA24E you would need to mill 3.7980 from the head. The maximum that can be milled from the head and still allow for valve travel is 1.3240 which would only bring you to 9.113:1 compression.

Fact is you're gonna need to do more than just milling the head. A Full port and polish with larger valves that need less space to travel combined with high compression pistons is truely needed. Or you could get away with doing some custom rods to effectively de-stroke the motor and raise your compression level.

~AriesYour friendly mod

[WideSlide]

About SHAVING YOUR CYL. HEAD, you only SHAVE off .020" to get 10.0:1 CR, not .028". Concerning about ARP bolts, I would suggests them in the LONG RUN, but your stock ones will do for now. If your car is an '89, check your driver door VIN METAL PLATE (not VIN STICKER-'90's has the sticker), 'cuz the '89's are made in '88. If it's made from 7/88, 8/88, 9/88 and/or (I think) 10/88, then your car has a 9.0:1 CR. In able to find out your HP, take it to the DYNO. and get the result in paper. Take one w/ everything stock on it and one w/ all the upgrade, so, you'll know if you made some HP.

[ITA240SX]

Ok, you've awakened the numbers nerd in me (give me a break, I'm an accountant). Check my math:

Single cylinder dsplacement: 597.23ccCombustion chamber volume for 8.6:1 compression: 78.58cc (includes head gasket volume and piston dish volume)Proof - (597.23+78.58)/78.58 = 8.600

Assuming combustion chamber diameter of 89mm at head/block junction, shaving 1mm (.0394") from head reduces combustion chamber volume 6.221cc's, resulting in new compression ratio of 9.25:1.Proof - (597.23+78.58-6.22)/(78.58-6.22) = 9.254

Of course, if the chamber diameter is smaller than 89mm at the head/block junction, the compression ratio increase will be less.

Using the same calculations, if you also throw in a set of .040 over flat-top pistons (9.1:1 compression) to replace the originals, that same 1mm of head shaving will bump the compression up to 9.83:1

Please correct me if I've missed something (like I have to ask )

[Aries]

you're exactly right Sometimes it's good to be a nerd

[JPermen]

So, if the maximum amount able to be shaved is 1.3240 mm, the highest compression atainable is 9.49:1. Almost exactly.

Thanks for your help everybody.

[Open4Cycle]

You shouldn't have to worry about the valves hitting the pistons all the way up to 11:1 ratio. Even with a longer duration cam like the R-4. I don't have exact measurement of clearance but by making the piston deck .060" closer to the valves there was well over .100" running clearance. .032" should be the minimum clearance between the piston top and valve or head. .025" has been achieved but trouble is likely to occure.

[randybunctious]

so can a close to 11:1 cr happen with a shaving of the head, safely?

[ITA240SX]

so can a close to 11:1 cr happen with a shaving of the head, safely?

I don't believe so, but I haven't tried it either, so don't take my word for it. What I will say is if you're thinking of pushing it that close, either you or your machinist needs to carefully check the valve to piston clearance, to find out exactly what you can take off. Also, keep in mind that taking that much off the head is going to throw the cam timing off, so you'll need to get a cam gear with the offset keyway (Nissan Motorsports has them) and re-index the timing. Also keep in miind that this may preclude you ever installing a hotter cam, so if you're planning on going that route you should get the cam first.

One last comment - take my (and everyone elses) free advice for exactly what you paid for it.

Earl

[Open4Cycle]

I didn't only shave my head to acheive 11:1 compresion. I did it by installing pistons with a raised compresion height. Only had to mill the head about .015" with a stock head gasket. The KA24E head looks pretty thick on the gasket surface and I do remember reading that someone milled their KA24E head .050" with no problems. If your looking to increase the ratio by lowering the head then another alternative is getting a thinner head gasket, milling the deck of the block or all of these. Checking valve clearance is an easy procedure. Finall assembly of my engine was with a .046" head gasket. I usually lay the head on without the head gasket and then roll the crank over.Have a beer. If the head doesn't raise up without the head gasket then I know I will have atleast .046" clearance between the pistons and head. Then to check valve to piston clearance I stick clay on the valve heads and then snug the head down and assemble the timing chain set. Drink another beer. Now roll the crank 720 degrees and then carefully remove the head and measure the indentions in your clay. See how simple this is? Now if you have lowered your head by thinner head gasket, milling of the head, or block gasket surface then you are definately throwing your cam out of time. Just as ITA240SX stated! Another good reason to have a degree wheel, dial indicator and piston stop. Hold off on the beer and save up! If your doing a complete rebuild then I would suggest buying some good pistons with a raised compression height.

[DAEDALUS]

Higher pistons/conrods are a good option, sometimes pricey. Useful to know how much you'll be changing the stock cam timing by shaving the head. Don't know the exact cam sprocket diameter, but probably *about* 4". Circumference would be 12.566".Every .010" you remove, then, would throw off your timing by:(.010/12.566)*(360) degrees.=.286 degrees.

This includes any head shaving *and* gasket thinning. So if you remove .050" thickness across the 2, the cam timing would be off (retarded) by 1.43 degrees relative to the crankshaft.

Again, assuming a 4" dia sprocket...whatever it really is will alter the results linearly, up or down, but it'll probably be pretty close.

[randybunctious]

thanks a lot guys, very good information! OPEN4CYCLE, you must be from south carolina including have a beer in the steps! here we do that even when changing oil. my friend got so drunk putting new brakes on that by the time he got to the back brakes, he removed the old pads, put them back on and threw the new pads away, he said they got mixed up!

[Open4Cycle]

No I live in the Heart of Dixie. I hope you and others find this informative. I try to give info that helps others help themselves, because I did learn from dirt track racing that the quickest way to the rear is doing what the other guy said.

[team23rulz]

What type of horsepower gain or we talking if the compression is raised to about 10:1? I bored my cylinder .40 over and also shaved my head and the top of my block. I'm not sure how much I shaved off, it shouldn't be much because I only did it to get the head to sit on the block level again.