1 point in compression is how many cc off the piston tops?

[dsagers]

I have the chance to put together a 93 4.5 engine.

Wondering if I can cut down the tops of the pistons a bit to decrease my compression?

Anyone know how many cc I need to add in the combustion chamber (cut down top of pistion) to lower the CR by 1 point?

[Olderthanme]

I'd be wary about removing anything from VH pistons. They are already ultra-light pistons with a very small compression height (32mm).I'd reverse-engineer them and get some forged units of similar dimensions.If you are lowering compression then I imagine that you are going to be using forced induction which increases pressures on the piston...and reducing thickness of the pistons while increasing pressure sounds like a road to destruction to me.

I have been looking for someone with a report on the CC's of a VH45 combustion chamber and gasket thickness (guessing around .9mm-1.1mm). If I can't get one online then I'll take a VH head to chem lab this fall and check it myself with a burette.OTM

[sijoko]

I have the chance to put together a 93 4.5 engine.

Wondering if I can cut down the tops of the pistons a bit to decrease my compression?

Anyone know how many cc I need to add in the combustion chamber (cut down top of pistion) to lower the CR by 1 point?

Lately, I've been thinking about how to cost-effectively lower the compression on the VH45. I thought about machining a dish in the pistons but there is the chance of weakening the piston domes.

Anyway, some of you might have heard of Somender Singh and his cylinder head "Grooves". Basically, you machine small grooves in the chamber that helps in better combustion. I've heard that you can expect a 1 cc reduction for each groove, so you could reduce compression slightly. The main benefit would be in detonation resistance, especially important for boost.

Here are some pics of various heads:





What do you think?
Modified by sijoko at 7:57 PM 1/3/2008

[DeXteR]

What do you think?

I think that introduces more "hot spots" that I think would promote detonation... The theory seems to make sense - increasing the volume of the combustion chamber in the head will decrease your compression ratio. I think the grooves need to be a lot smoother (no rough edges) as heat tends to collect at sharp edges. Chamfer the grooves and I'd feel more comfortable with the idea. However, are you (or that guy or whoever) going to be able to remove the exact amount of cc's to keep the volume the same in each combustion chamber? I doubt it.

I think the best way to go (from my experience with inline-4's) would be to find a thicker headgasket. The problem with this solution is that it is going to tweak your timing chain geometries... This would add extra tension to the chain and could pull you slighlty out of time.

You might be better off with some dished forged pistons...

Good luck.

[sijoko]

I think that introduces more "hot spots" that I think would promote detonation... The theory seems to make sense - increasing the volume of the combustion chamber in the head will decrease your compression ratio. I think the grooves need to be a lot smoother (no rough edges) as heat tends to collect at sharp edges. Chamfer the grooves and I'd feel more comfortable with the idea. However, are you (or that guy or whoever) going to be able to remove the exact amount of cc's to keep the volume the same in each combustion chamber? I doubt it.

I think the best way to go (from my experience with inline-4's) would be to find a thicker headgasket. The problem with this solution is that it is going to tweak your timing chain geometries... This would add extra tension to the chain and could pull you slighlty out of time.

You might be better off with some dished forged pistons...

Good luck.

Yes, the grooves have to be smoothed out so that there are no sharp edges. You can measure the chambers using the traditional method since the grooves reach into the combustion chamber.

Here are some more pics:







Someone tried it out on their Volvo:http://forums.turbobricks.com/...66779

"I've tried his grooves on a 530 and with 25 degrees static advance and could not get the engine to ping/knock under any load/speed combination, in my experience anything over 15 degrees produced high load low rev knock on a normal head, the engine set at 10 degrees static advance (normal) will pull happily down to 1000 revs in 5th and thats with a bad exhaust leak, i can't comment for the moment on fuel economy and high rev power because of the exhaust, i've took it up to a hundred and it seemed ok.

I tried this primarily because of the claims of better detonation control before i do my +t, i was sceptical so i tried it on a 530 and kept my my 531 stock, "just in case", i'll be modding my 531 starting this week, i'm glad i tried it on a different head first coz i didn't do a very good job of the first groove, i ended up with a medium cut triangular file which did the job great, talk about low tech,"

[mtcookson]

Yeah, those slits aren't meant to be a way to add chamber volume but a way to control detonation and allow fuel to burn better, if I remember correctly.

There was a website with many people doing that modification but I can't remember what it is off hand. I'll try to search for it.

I've actually been wanting to try it for a while. I may just have to try it out on the VH.

[sijoko]

Check out the pistons from a BBC put together by BES Racing Engines. Looks like a different approach using grooves to promote turbulence.

http://www.popularhotrodding.c....html

[mtcookson]

There was a website with many people doing that modification but I can't remember what it is off hand. I'll try to search for it.

Here it is: http://somender-singh.com

Looks like someone posted a thread in their forum about the VH45. Someone from here?

[sijoko]

Here it is: http://somender-singh.com

Looks like someone posted a thread in their forum about the VH45. Someone from here?

That's me. I wanted to get some input from Mr. Singh. This is the pic that I posted about possible grooves for the VH45.

[ultrapulse]

The reason I thought u had this squish area was to squeeze the mixture towards the spark plug. Surely this is simply an extension of this theory meaning the mixture would direct itself towards the plug. I'd be interested to see if it had a bearing on detonation (if comp ratio was the same), infact at a guess I would have though it would increase the risk with all those edges and hot mixture being forced thru passages at speed. Anyway a nice clean head after this mod would help until the carbon builds up again So prob hard for a valid comparison.

Interesting though, will await feedback.

[mtcookson]

There are actually quite a few that have done that modification and have seen a major improvement in detonation resistance.

I believe the general consensus is that without any other modifications you won't see much of an improvement unless you're already near the limit in which case you may see a noticeable improvement.

Many have noticed fuel economy improvements and interestingly the oil stays much cleaner as well. I definitely want to give it a try and see what happens. I'd probably just settle with one groove on each side but I would be curious to see what they say on that forum. I've seen some people claim that there can be too many grooves. I believe it was a snow mobile engine or similar they put upwards of 5 grooves in and noticed the most improvement with only 3.

[dsagers]

Interesting discussion about the grooves.

But, still hoping to find out how many cc I need to add for 1 point in compression.

Maybe instead of milling down the pistons, I can increase the volume in the heads by taking some material out of the chamber?

[SuperHatch]

Interesting discussion about the grooves.

But, still hoping to find out how many cc I need to add for 1 point in compression.

Maybe instead of milling down the pistons, I can increase the volume in the heads by taking some material out of the chamber?

Approximately 7.5cc's would need to come out of the pistons or combustion chamber.

[DeXteR]

Interesting discussion about the grooves.

But, still hoping to find out how many cc I need to add for 1 point in compression.

This thread was seriously pirated I don't think it was a bad thing though. This has become quite interesting and I'd like to learn more about it.

It sure took a while to get an answer to to the OP.