• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

What compression ratio should I aim for?

OVERDRIVE

OVERDRIVE

Well-known member
Joined
Jul 31, 2017
Location
Brooklyn, Neu York
I'm planning what direction I want to go for building a fresh motor for my '90 244 automatic. I've built similar oem+ motors before so I've got a grasp on the strategy but I haven't built a redblock before and I want to get the CR right the first time. I'll be sourcing an NA squirter motor to start, I will want squish in the 0.030"-0.032" range with a decked block and stock HG, a 530 with cleaned up ports, combustion chambers, and a good valve job, a B cam to start probably advanced to taste. My intention will be to try V or V15 at a later date, those should have a similar dynamic compression ratios hence the choice to go B first. I'm comfortable with tweaking the LH and EZK tables.

My requirements are that it's optimized for 87 octane, I'm looking nice MPG and a powerband window from 2-5k, and I will be keeping EGR. EGR would in theory reduce combustion chamber temps, but also I value not removing emissions equipment.

Just looking for some recommendations on the static CR to aim for, shooting from the hip I feel 10.2:1.
 
I ran a B23 with a 530 head and flat top pistons. It did well with all the cams. A, B, V. No ping. It might have had limited spark advance, since it never felt all that strong. I always felt that you could get close to 11:1 with tighter piston to head clearance, but not much higher, since the chamber is pretty open. I probably had 0.040" clearance.
 
I found that 10-10.5:1 is really the limit with 87 before timing starts getting pulled with LH2.4. If you run premium at that point there’s a slightly noticeable improvement in performance.
That was with a B cam and vx3 and m47 cars.

If you do some chamber work you can probably raise the CR a bit higher. In stock form they leave a lot to be desired. Marking the dish diameter on the head will show you where the squish band is. Everything else overhanging that is just limiting combustion efficiency and making it more knock sensitive.
 
culberro said:
I found that 10-10.5:1 is really the limit with 87 before timing starts getting pulled with LH2.4. If you run premium at that point there’s a slightly noticeable improvement in performance.
That was with a B cam and vx3 and m47 cars.
Click to expand...
Nice, this was exactly what I was looking for
culberro said:
If you do some chamber work you can probably raise the CR a bit higher. In stock form they leave a lot to be desired. Marking the dish diameter on the head will show you where the squish band is. Everything else overhanging that is just limiting combustion efficiency and making it more knock sensitive.
Click to expand...
These are all things I'm keeping in mind, I don't want to do much deshrouding and widen the combustion chamber (just a hunch, no real experience with a redblock remember). I'm tempted to either try a B230A/E piston, or Smorgesborgs's solution of using a flat top 4.8 LS piston and a custom rod, I just want to keep the combustion as much in the chamber as I can if I'm going through the hassle of tightening the squish. Does that all make sense?
 
OVERDRIVE said:
I'm planning what direction I want to go for building a fresh motor for my '90 244 automatic. I've built similar oem+ motors before so I've got a grasp on the strategy but I haven't built a redblock before and I want to get the CR right the first time. I'll be sourcing an NA squirter motor to start, I will want squish in the 0.030"-0.032" range with a decked block and stock HG, a 530 with cleaned up ports, combustion chambers, and a good valve job, a B cam to start probably advanced to taste. My intention will be to try V or V15 at a later date, those should have a similar dynamic compression ratios hence the choice to go B first. I'm comfortable with tweaking the LH and EZK tables.

My requirements are that it's optimized for 87 octane, I'm looking nice MPG and a powerband window from 2-5k, and I will be keeping EGR. EGR would in theory reduce combustion chamber temps, but also I value not removing emissions equipment.

Just looking for some recommendations on the static CR to aim for, shooting from the hip I feel 10.2:1.
Click to expand...
I run 9.8:1 (0.040" mill and a stock head gasket) and a V cam on Shell 87. I suspect that on a stock timing map (-169 EZ116K), it's pretty close to threshold. Last tank of fuel was 170 miles combined commute and 180 miles highway, netted me 28.7 mpg. M47 and 3.31s, though, kid.
 
If you're using a flat top piston, it makes sense to use a compact combustion chamber within reason.
The exhaust valve is heavily shrouded, and opening that up and exposing the spark plug are all good things.
 
I used to run a b230k with a 530 head (10.3:1) and a K-cam. It ran suprisingly well with a b230fb ecu and the 169 ezk with the +3 deg advance timing jumper mod on RON95. The B cam will probably make a bit more dynamic compression however.
 
Swedbrick said:
I used to run a b230k with a 530 head (10.3:1) and a K-cam. It ran suprisingly well with a b230fb ecu and the 169 ezk with the +3 deg advance timing jumper mod on RON95. The B cam will probably make a bit more dynamic compression however.
Click to expand...
I've seen the dyno video for that one, very nice indeed. do you know what your piston to head clearance was?
 
Don't hold back. Start that fresh engine with a V15 in it. It will be worth it. The cam has a wide powerband and good low end torque. But the power doesn't stop till around 6k rpm or more. A great n/a power cam. Plus it also gets good mileage on a stock 951 fuel ecu and a chipped EZK.
 
dl242gt said:
Don't hold back. Start that fresh engine with a V15 in it. It will be worth it. The cam has a wide powerband and good low end torque. But the power doesn't stop till around 6k rpm or more. A great n/a power cam. Plus it also gets good mileage on a stock 951 fuel ecu and a chipped EZK.
Click to expand...
I understand, I have a B cam now in my trunk, and according to the dynamic compression ratio calculator the compression ratio will be near enough the same with a V15. Current exchange rate makes it $420 (nice) before shipping from Sweden, and swapping a cam out isn't too hard. I still have to decide if if I'm going to try to reuse F pistons which is why I started this thread, since that will determine what engine core I buy. This could go from a relatively budget engine to a full custom 0 mile engine really quick.
 
I am not sure how much power you can make with the V15 cam. But it seems like there is a lot of room for more power with it. I'm just using it with minor changes and it is far better than stock. However, with more compression, some headwork, it seems like it would be even better. I would even keep the heavy flywheel because of the good parts of having it heavy. Like good take off torque, and highway smooth cruising.
 
Bumping this thread because I'm going to be taking this head to the machine shop next week. The final spec is V cam adjusted to "straight up", F pistons just a kiss above the deck, maybe .003" I'm going to measure it this afternoon, edit: .002" above the deck just checked, and stock Elring head gasket. The plan had been tight squish but after the headgasket blew in late December I've chosen to opt out of replacing the short block this round, and considering how the old head had .025" of up and down cam wobble at the front and rear bearings, and the block is about .002" out as well, I'm not trusting a .036" Cometic honestly. The replacement head has been previously decked .008" if measuring the height is reliable at all, and the chambers cc out to just under 54cc. Exhaust valves and spark plug have been deshrouded, I want something happy with 87 on the stock LH2.4 initially.

What should I tell the machine shop to take off? .035"? .040? Can I get more greedy with the chamber mods?
20240127_114317-min.jpg
 
Last edited:
You can use the old head gasket to get a sense of how much margin you have. For a mild performance upgrade the work around the exhaust valve looks pretty good.

With the gasket reference you could open it up a bit more by laying over the camber wall to the gasket line. That way you lose minimal CC's to bath tubbing, but gain flow.

As for how much to shave, figure out the compression ratio you are after (10.5 to 1) should be ballpark, and cc out the camber with washer fluid to determine how much to shave.
 
Swedbrick said:
As for how much to shave, figure out the compression ratio you are after (10.5 to 1) should be ballpark, and cc out the camber with washer fluid to determine how much to shave.
Click to expand...
Be careful when using the stock HG as the reference line for opening the chambers, it is a lot larger than the bore and you don’t want any overhang.

Find out what chamber volume you want, put that into the chamber (windshield washer fluid works well as it has low surface tension). Then put a straight edge or flat chuck of metal across the chamber and measure down to the fluid with some calipers using the depth gauge. Subtract the thickness of metal you measured from, and that’s the amount you need to have removed.
 
Last edited:
culberro said:
Be careful when using the stock HG as the reference line for opening the chambers, it is a lot larger than the bore and you don’t want any overhang.

Find out what chamber volume you want, put that into the chamber (windshield fluid works well as it has low surface tension). Then put a straight edge or flat chuck of metal across the chamber and measure down to the fluid with some calipers using the depth gauge. Subtract the thickness of metal you measured from, and that’s the amount you need to have removed.
Click to expand...
That’s a great way to get chamber volume! Never thought of using washer fluid… much easier to see than isopropyl! Although I’ve always had some dye handy, that’s not something everyone has in the shop
 
Nice tip. I always calculate the area and divide by volume to get an amount.
I wouldn’t go too easy on the deck cut, though. Bring those pistons out of their hiding place.
 
You must log in or register to reply here.
Back
Top