My compression ratio makes no sense - help
Racer
Joined: Oct 2000
Posts: 476
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From: Socorro New Mexico
Re: My compression ratio makes no sense - help (steveelsbury)
Steve,
There is a lot of info on how to do this, but here is me: You will need a burette cylinder that holds enough liquid to cc the heads and a flat piece of plexiglass with a small hole in it. Put a couple drops of dish soap in some distlled water and a little food coloring in it (makes it easy to see).
Place the piece of plexiglass (with a little grease around the edge of the combustion chamber for sealing ) over the chamber. Fill the chamber to the edge of the small hole in the plexiglass and read the burette at the miniscus of the remaining fluid.
For the pistons you can do the same for each valve relief if they are flat topped or if dished just do the whole piston top. If not grab a bigh hunk of childrens clay and make a mold of the piston top. Worked for me, I built up a 11.25:1 bigblock for my Camaro using this method (back when we could get high octane at the pump); P.S it is still running and it is 20 years since I did that combo to the tune of ~ 65K miles.
Sorry to read about your experience, 8 grand has to hurt. I have always built my own engines and only farmed out what I was incapable of doing myself. Measure everything, twice or three times, take nothing for granted.
There is a lot of info on how to do this, but here is me: You will need a burette cylinder that holds enough liquid to cc the heads and a flat piece of plexiglass with a small hole in it. Put a couple drops of dish soap in some distlled water and a little food coloring in it (makes it easy to see).
Place the piece of plexiglass (with a little grease around the edge of the combustion chamber for sealing ) over the chamber. Fill the chamber to the edge of the small hole in the plexiglass and read the burette at the miniscus of the remaining fluid.
For the pistons you can do the same for each valve relief if they are flat topped or if dished just do the whole piston top. If not grab a bigh hunk of childrens clay and make a mold of the piston top. Worked for me, I built up a 11.25:1 bigblock for my Camaro using this method (back when we could get high octane at the pump); P.S it is still running and it is 20 years since I did that combo to the tune of ~ 65K miles.
Sorry to read about your experience, 8 grand has to hurt. I have always built my own engines and only farmed out what I was incapable of doing myself. Measure everything, twice or three times, take nothing for granted.
Thread Starter
Instructor
Joined: Mar 2002
Posts: 176
Likes: 0
From: Brentwood Essex
Re: My compression ratio makes no sense - help (Rynda)
Hey Rynda,
That's a fantastic tip and very, very obvious in hindsight (20:20, as always!). I'l get a calibrated burette this weekend and check out the capacities.
Well, 8 grand seemed reasonable for the monster engine I thought I was buying. I assumed the engine builder was trustworthy as he is loosely affiliated to Carroll Supercharging. I guess human nature kicks in when the customer is several thousand miles away, and unlikely to come a-knocking!
Next time (well, this time in fact) I'll build it myself! It would be silly not to have the bore damage at least honed, for which the block must come out. While that's being done, I might as well have the crank ground. I'll use UK engineering shops this time! Before anyone chips in suggesting now would be a good time to go for bigger cubes, I just cannot afford another full set of pistons, rods, and crank. This car has been enough of a money pit as it is!
If I'm doing all that, perhaps it's time to put in a proper blower cam too - how much do those bad boys run? Then again, before I did the damage, she pulled like a train already!
<sigh> Unlikely to see the track for yet another season - I started this project in September 2001!
Thanks to everyone for your help and support.
Steve
That's a fantastic tip and very, very obvious in hindsight (20:20, as always!). I'l get a calibrated burette this weekend and check out the capacities.
Well, 8 grand seemed reasonable for the monster engine I thought I was buying. I assumed the engine builder was trustworthy as he is loosely affiliated to Carroll Supercharging. I guess human nature kicks in when the customer is several thousand miles away, and unlikely to come a-knocking!
Next time (well, this time in fact) I'll build it myself! It would be silly not to have the bore damage at least honed, for which the block must come out. While that's being done, I might as well have the crank ground. I'll use UK engineering shops this time! Before anyone chips in suggesting now would be a good time to go for bigger cubes, I just cannot afford another full set of pistons, rods, and crank. This car has been enough of a money pit as it is!
If I'm doing all that, perhaps it's time to put in a proper blower cam too - how much do those bad boys run? Then again, before I did the damage, she pulled like a train already!
<sigh> Unlikely to see the track for yet another season - I started this project in September 2001!
Thanks to everyone for your help and support.
Steve
Racer
Joined: Oct 2000
Posts: 476
Likes: 0
From: Socorro New Mexico
Re: My compression ratio makes no sense - help (steveelsbury)
Steve,
If you switch to a blower cam, your lower rpm cylinder pressure will probably get a little higher. Generally speaking, I think that a blower cam runs a little milder specs than a stock type cam, but your cam is probably ok. It is a 2000-5500 rpm cam in a normally aspirated engine. An all out street blown motor will have the static compression much lower and drive the blower much faster. Static compression ratios for pupose built blown motors in the range of 7.5-8:1 are typical.
After rereading your posts and looking at the pictures again, since only the lands are cracked and the piston top is undamaged, my vote says that the builder did a lousy job when he slipped the pistons in. If he hung a ring on the edge of the cylinder block and hit it to hard driving it in, he could have cracked the lands and hurt the bearing/crank journal when it slammed down into position. Just a guess on my part.
Once you cc everything, get a set of blower motor head gaskets and lower your static compression ratio a little.
Oh, by the way, you will never hear the detonation that kills your motor. I'd figure out what was causing the false knock counts and hook up and use the knock sensor. Maybe they were not all false counts.
If you switch to a blower cam, your lower rpm cylinder pressure will probably get a little higher. Generally speaking, I think that a blower cam runs a little milder specs than a stock type cam, but your cam is probably ok. It is a 2000-5500 rpm cam in a normally aspirated engine. An all out street blown motor will have the static compression much lower and drive the blower much faster. Static compression ratios for pupose built blown motors in the range of 7.5-8:1 are typical.
After rereading your posts and looking at the pictures again, since only the lands are cracked and the piston top is undamaged, my vote says that the builder did a lousy job when he slipped the pistons in. If he hung a ring on the edge of the cylinder block and hit it to hard driving it in, he could have cracked the lands and hurt the bearing/crank journal when it slammed down into position. Just a guess on my part.
Once you cc everything, get a set of blower motor head gaskets and lower your static compression ratio a little.
Oh, by the way, you will never hear the detonation that kills your motor. I'd figure out what was causing the false knock counts and hook up and use the knock sensor. Maybe they were not all false counts.
Thread Starter
Instructor
Joined: Mar 2002
Posts: 176
Likes: 0
From: Brentwood Essex
Re: My compression ratio makes no sense - help (Rynda)
Hi Rynda
Your last post is an excellent example of the conflicting advice this forum can generate. You're the first person to say my cam is probably OK, and I must admit it's backed up by my experiences with the car, which - like I said - was a bit of a rocket before I broke it!
Interesting point about blower motors, but the V1 I've got generates 12 lbs on paper, and only about 8 in the real world. What on earth kind of blower would you need to generate boost that would need an 8:1 cr?
I'd love to think you're right about the builder, but we'll never know for sure.
Perhaps you're also right that what I thought was false knock generated by the long-tubes was actually real knock! I AM gonna hook a new knock sensor up, and approach WOT very carefully, and only with a pax monitoring the 'puter for me! I'll cc the heads and pistons as soon as I can, then do a *very careful* rebuild. The Beast shall roar again :)
I know *far* more then I did 2 years ago, but still have stacks to learn - and this forum is helping me get there. I suppose it's all part of the fun :lol: I look forward to actually posting some timeslips!
Watch this space....
Steve
Your last post is an excellent example of the conflicting advice this forum can generate. You're the first person to say my cam is probably OK, and I must admit it's backed up by my experiences with the car, which - like I said - was a bit of a rocket before I broke it!
Interesting point about blower motors, but the V1 I've got generates 12 lbs on paper, and only about 8 in the real world. What on earth kind of blower would you need to generate boost that would need an 8:1 cr?
I'd love to think you're right about the builder, but we'll never know for sure.
Perhaps you're also right that what I thought was false knock generated by the long-tubes was actually real knock! I AM gonna hook a new knock sensor up, and approach WOT very carefully, and only with a pax monitoring the 'puter for me! I'll cc the heads and pistons as soon as I can, then do a *very careful* rebuild. The Beast shall roar again :)
I know *far* more then I did 2 years ago, but still have stacks to learn - and this forum is helping me get there. I suppose it's all part of the fun :lol: I look forward to actually posting some timeslips!
Watch this space....
Steve
Racer
Joined: Oct 2000
Posts: 476
Likes: 0
From: Socorro New Mexico
Re: My compression ratio makes no sense - help (steveelsbury)
Steve,
About the 8:1 compression ratio for a supercharged motor. I was refering to a purpose built blown motor. Some recomendations ar to run from 7:1 to 7.5:1! The reason being that although your static compression ratio is low, you can run much more boost without detonation at WOT. The setup will more than compensate for low compression ratios.
If you keep the boost down (as in your case) you can keep the stock type compression ratios and still see good power gains. Just make sure that you are running rich at WOT and retard your spark, helps prevent detonation. What you are doing is science project unto itself, the fun stuff :D
I'll be watching.
About the 8:1 compression ratio for a supercharged motor. I was refering to a purpose built blown motor. Some recomendations ar to run from 7:1 to 7.5:1! The reason being that although your static compression ratio is low, you can run much more boost without detonation at WOT. The setup will more than compensate for low compression ratios.
If you keep the boost down (as in your case) you can keep the stock type compression ratios and still see good power gains. Just make sure that you are running rich at WOT and retard your spark, helps prevent detonation. What you are doing is science project unto itself, the fun stuff :D
I'll be watching.
