What I am going to do have the block decked a small amount and make up for it in CH of the piston. I made the head gasket .040 to account for the piston sticking .010 out the hole. I'm sorry if I confused y'all with my info.
250 build
- Thread starter 69stang_250
- Start date
The 255 V8's were only used a couple years and not too popular of an engine so all the pistions available are probably based off the originals poor design. There have been severial people that had big trouble with them in their 250 Performance builds its too bad they don't make an updated stronger Hyperectic version, that would probably work well for a 250 six build.69stang_250":1advazyo said:
The 300 rods you needed were 1964-1968 forging #C5TE.
I usually can get cores for under $10 each but my sources have dried up at least for now.
Everyone else wants $25 each for so called reconditioned connecting rods that still would need to be polished, shot peened and resized with ARP bolts.
Not a good deal.
Have you actually CC'd the combustion chambers to know for sure what the volume is?
Not yet. I get the head back next week and will cc it from there. I have some plexi glass and a syringe already and plan on cleaning up the combustion chambers myself. I have been playing around with the stock head and think I have the hang of it. But, once I go pick the head up I will go from there with the rest of the build.
I should have had the head back by now, but It is what it is. I just hope this guy doesn't screw it up.
I should have had the head back by now, but It is what it is. I just hope this guy doesn't screw it up.
Addressing the cam issue again if you don't mind.
It has been my experience that as the .050" duration overlap goes past 15 degrees the idle not only gets very rough, the engine doesn't smooth out until very late killing the very low end of the power band.
The transition into the low end of the power band becomes abrupt like turning on a light switch. Some people like that response.
The 231/231 on a 108 LSA has 15 degrees overlap which is on the border.
The last 300 six cam I had ground was 232/232 on a 112 LSA for an 8 degree overlap so the idle would be somewhat smooth and maybe reasonable gas mileage.
I would recommend the 280 110 LSA cam with 11 degrees overlap since you are going with forged pistons and can afford to let the engine rev.
It has been my experience that as the .050" duration overlap goes past 15 degrees the idle not only gets very rough, the engine doesn't smooth out until very late killing the very low end of the power band.
The transition into the low end of the power band becomes abrupt like turning on a light switch. Some people like that response.
The 231/231 on a 108 LSA has 15 degrees overlap which is on the border.
The last 300 six cam I had ground was 232/232 on a 112 LSA for an 8 degree overlap so the idle would be somewhat smooth and maybe reasonable gas mileage.
I would recommend the 280 110 LSA cam with 11 degrees overlap since you are going with forged pistons and can afford to let the engine rev.
Maybe I am reading the cams wrong.
I know what you mean and I am one of those guys that like that lol
I have a 77 280z and it doesn't "kick in" until 2300rpm. From there that car would get up and go!
So help me understand the difference between the 108 and 110.
I figured the 108 would be similar to the 274 110 cam but with more lift.
I really don't want the engine to be a complete turd off the line, but I can live with a little bit of a delay of power.
I know what you mean and I am one of those guys that like that lol
I have a 77 280z and it doesn't "kick in" until 2300rpm. From there that car would get up and go!
So help me understand the difference between the 108 and 110.
I figured the 108 would be similar to the 274 110 cam but with more lift.
I really don't want the engine to be a complete turd off the line, but I can live with a little bit of a delay of power.
69stang_250":npt296yx said:
The 274 110 and the 280 108 are similar in the intake valve closing point which largely determines the upper end of the power band
but because the 280 cam has 7 more degrees of .050" duration than the 274 and the lobes are 4 degrees tighter to each other, there is a Big difference in valve overlap.
4 degrees overlap for the 274 110 versus 15 degrees for 280 108
If you like the "light switch" response then you are set! LOL
So if I went with the 110, what would be the characteristics of it? I don't want to completely kill the lower end, but ultimately, I do want to be able to run this thing down the strip in the 13s.
Lol I always enjoyed it, but when it comes to it, I enjoy the driving experience as a whole lol
Lol I always enjoyed it, but when it comes to it, I enjoy the driving experience as a whole lol
69stang_250":2zswf9x2 said:
The idle will be a little better with more intake manifold vacuum, the transition into the power band will be smoother
and the power band will be wider because you have improved the low end response with less overlap and increased the upper end by moving the intake valve closing point later.
The ported aluminum head should support the later closing intake valve nicely.
Yes I thin you thought was wrong then.
so the smaller the number, the better performance on the upper end?
so the smaller the number, the better performance on the upper end?
So I double checked my math. When I fake in some numbers to arrive a the piston protruding 0.01" like you were envisioning, I get the exact same values as that online tool for SCR and DCR. I checked into whether or not including the curtain volume around the piston changed things and it only resulted in a change of 0.01 in the SCR using an assumed a wall clearance of 0.0025". I just shoehorned it in to see if it had any effect. I'll consider fulling incorporating it later. Being the nerd I am, it'll probably happen at some point.
Just double check on how you go about getting there. With the 255 piston height of 1.585", and a 58 cc chamber, that would require the block to be milled 0.069" to arrive at those compression ratio numbers. That seems like a lot of milling to me, but I'm no expert on that front. I just like to do math....
Just double check on how you go about getting there. With the 255 piston height of 1.585", and a 58 cc chamber, that would require the block to be milled 0.069" to arrive at those compression ratio numbers. That seems like a lot of milling to me, but I'm no expert on that front. I just like to do math....
So the stock piston has a 1.500 CH and when I measured the deck height, it came out to .117 while the Pistons were still in.
Now take a piston with 1.585 CH and that will drop the deck height to .032. So I'll just have to shave .042 off the deck or get a piston made with that much extra on it.
Now take a piston with 1.585 CH and that will drop the deck height to .032. So I'll just have to shave .042 off the deck or get a piston made with that much extra on it.
BCOWANWHEELS
Well-known member
GONNA take a bunch of engine mods to get in the 13,s car is gonna have to be gutted too to lighten it up.
yes I agree, but I'm gonna get there at some point.
69stang_250":180hp2w4 said:
Generally speaking the higher .050" duration numbers increase upper end performance. The intake closing point is the most dominate factor
Overlap can either hurt or aid performance depending on scavenging from the exhaust system and is application specific in the desired result.
Im sorry,. I meant to say my thought was wrong.
Yes, I understand duration, but I need to read up and learn more on over lap I think.
Yes, I understand duration, but I need to read up and learn more on over lap I think.
I thought the stock piston height was 1.511" and not 1.500". Sounds like the deck has already been milled once as well. Bone stock numbers should leave you with a 0.123" deck height. So it would seem that the engine has been out and decked by 0.006" to true it up. Or there could be deposits on the piston head that are skewing the numbers too? Dunno, just throwing that out there.
But in general:
ht - half stroke - rod - compression ht = deck height
9.469 - (3.910 / 2) - 5.88 - 1.511 = 0.123 <- Stock
9.469 - (3.910 / 2) - 5.88 - 1.585 = 0.049 <- With 255 pistons
This is taking the numbers from the CI website anyway. So that unaccounted for 0.011" in compression height would bring your SCR down to 9.55 from the 9.76 you were expecting. Are you going to notice those 2 tenths? I doubt it.
I've also read in a few places that aftermarket pistons are actually made with a slightly shorter than stock compression height. By how much? I don't remember. Wish I could remember where I read that though. I just know it's something that I'm going to take into consideration when I do ever finally get to working on my block. It won't result in any difference I can feel, but it'll gnaw at me if I don't get it right and find out about it later. But then this is me just being a numbers nerd....
And don't take this as me being negative. I'm just trying to learn as much as I can before I start tearing apart my block. Trust me, I dig what you're doin!
But in general:
ht - half stroke - rod - compression ht = deck height
9.469 - (3.910 / 2) - 5.88 - 1.511 = 0.123 <- Stock
9.469 - (3.910 / 2) - 5.88 - 1.585 = 0.049 <- With 255 pistons
This is taking the numbers from the CI website anyway. So that unaccounted for 0.011" in compression height would bring your SCR down to 9.55 from the 9.76 you were expecting. Are you going to notice those 2 tenths? I doubt it.
I've also read in a few places that aftermarket pistons are actually made with a slightly shorter than stock compression height. By how much? I don't remember. Wish I could remember where I read that though. I just know it's something that I'm going to take into consideration when I do ever finally get to working on my block. It won't result in any difference I can feel, but it'll gnaw at me if I don't get it right and find out about it later. But then this is me just being a numbers nerd....
And don't take this as me being negative. I'm just trying to learn as much as I can before I start tearing apart my block. Trust me, I dig what you're doin!
