Effect of lowering compression ratio via larger head CC?
12-01-2004, 11:31 AM
#1
Effect of lowering compression ratio via larger head CC?
Per krismax the VQ35DE found in the 2000-2001 Pathfinder/QX4 shares the same lower intake manifold gasket as the VQ30s. Also, per SAE technical documents, the Pathy heads use low angle intake valves and the engine shares the VQ30s compression ratio of 10:1. So, in theory *IF* the Pathy VQ35 heads "bolt-on" and you can use some kind of cam/gear combo to work with the VQ30 timing chain, we *SHOULD* be able to still use the VQ30 LIM and MEVI/DEK manifold, ie less messy install.
I see the benefits of this combo as the intake port lower angle helps keep a better intake air tumble at low speed like the VQ30s vs. later 10.3:1 VQ35s, intake valves are 1mm larger, the compression ratio would be lowered to I'm GUESSing in the 8.5:1-9:1 range depeding on head cc's with just a simple head swap vs. swapping pistons, superior cooling/antidetonation of the VQ35 heads that allow much more timing due to extended cooling jackets around the long-reach spark plugs/exhaust ports, and finally the lighter valve train/cams with growing aftermarket support.
So, my minimal reading says "bowling" the heads combustion chamber is not the ideal way to lower compression ratio, since you mess with the quench area and flame propagation properties. Now, this typically is a concern to those that use a dremel and open it up, however this is still using what Nissan designed, just for a different piston face/cylinder bore.
Could this be a good alternative to swapping in low CR pistons and gaining the Pathy VQ35 heads/valvetrain to boot withOUT having to use the later VQ35s LIM/intake manifold/accessories/injectors(4th gens)/etc.? What would be the drawbacks?
I see the benefits of this combo as the intake port lower angle helps keep a better intake air tumble at low speed like the VQ30s vs. later 10.3:1 VQ35s, intake valves are 1mm larger, the compression ratio would be lowered to I'm GUESSing in the 8.5:1-9:1 range depeding on head cc's with just a simple head swap vs. swapping pistons, superior cooling/antidetonation of the VQ35 heads that allow much more timing due to extended cooling jackets around the long-reach spark plugs/exhaust ports, and finally the lighter valve train/cams with growing aftermarket support.
So, my minimal reading says "bowling" the heads combustion chamber is not the ideal way to lower compression ratio, since you mess with the quench area and flame propagation properties. Now, this typically is a concern to those that use a dremel and open it up, however this is still using what Nissan designed, just for a different piston face/cylinder bore.
Could this be a good alternative to swapping in low CR pistons and gaining the Pathy VQ35 heads/valvetrain to boot withOUT having to use the later VQ35s LIM/intake manifold/accessories/injectors(4th gens)/etc.? What would be the drawbacks?
12-01-2004, 12:00 PM
#2
Senior Member
Join Date: Sep 2004
Posts: 296
Good thinking actualy.
I'd want to sit down and look at the heads side by side and match up & mock up everything! right down to the push rods and valve lash. Don't leave a stone unturned on that part. If all is good and matches up then give it a wirl! Dropping compression is good for a boosted car especialy a daily driven car. The streighter valve angles will help with flow. Don't learn anything by not trying do we? Go for it if it's not expencive!
Unfortunatly our cars are somewhat limited by traction and transmission from what I'm hearing and learning with my own car. Now that my clutch does not slip the tires do and If I go with slicks in the spring I think I'll bust a tranny! There's a point to every car at which it becomes impracticle or worse undependable with increased power.
I'm all for the power don't get me wrong but there's a limit to practicality & Driveability with the front wheel drive and the parts that are availble to us.
On a similar topic of dremel tool and heads... theres a article in Popular Science that I read about a guy in India (or somewhere like that) He uses files and cutting tools to make triangle like shapes in the cyl head/s combustion chambers in cars and motorcycles. He found that it prevented detonation (almost completely!) and reduced the exhaust temps along with increaseing power&fuel milage. I did a experiment on the kid's PW50 and it seemd to pick it up quite a bit along with keeping the head surface and spark plug clean! I was amazed.. I have yet to try it on a car but It'd be worth looking into!
~Scott
I'd want to sit down and look at the heads side by side and match up & mock up everything! right down to the push rods and valve lash. Don't leave a stone unturned on that part. If all is good and matches up then give it a wirl! Dropping compression is good for a boosted car especialy a daily driven car. The streighter valve angles will help with flow. Don't learn anything by not trying do we? Go for it if it's not expencive!
Unfortunatly our cars are somewhat limited by traction and transmission from what I'm hearing and learning with my own car. Now that my clutch does not slip the tires do and If I go with slicks in the spring I think I'll bust a tranny! There's a point to every car at which it becomes impracticle or worse undependable with increased power.
I'm all for the power don't get me wrong but there's a limit to practicality & Driveability with the front wheel drive and the parts that are availble to us.
On a similar topic of dremel tool and heads... theres a article in Popular Science that I read about a guy in India (or somewhere like that) He uses files and cutting tools to make triangle like shapes in the cyl head/s combustion chambers in cars and motorcycles. He found that it prevented detonation (almost completely!) and reduced the exhaust temps along with increaseing power&fuel milage. I did a experiment on the kid's PW50 and it seemd to pick it up quite a bit along with keeping the head surface and spark plug clean! I was amazed.. I have yet to try it on a car but It'd be worth looking into!
~Scott
12-01-2004, 12:04 PM
#3
i like where you are going with this, but since my max is a dd, i can't really have it down to test.
i wish that i had something to reference, we are messing with i30tmiked's manifolds on friday.
i wish that i had something to reference, we are messing with i30tmiked's manifolds on friday.
12-01-2004, 01:56 PM
#8
Originally Posted by Boosted Maxima
Good thinking actualy.
I'd want to sit down and look at the heads side by side and match up & mock up everything! right down to the push rods and valve lash. Don't leave a stone unturned on that part. If all is good and matches up then give it a wirl! Dropping compression is good for a boosted car especialy a daily driven car. The streighter valve angles will help with flow. Don't learn anything by not trying do we? Go for it if it's not expencive!
Unfortunatly our cars are somewhat limited by traction and transmission from what I'm hearing and learning with my own car. Now that my clutch does not slip the tires do and If I go with slicks in the spring I think I'll bust a tranny! There's a point to every car at which it becomes impracticle or worse undependable with increased power.
I'm all for the power don't get me wrong but there's a limit to practicality & Driveability with the front wheel drive and the parts that are availble to us.
On a similar topic of dremel tool and heads... theres a article in Popular Science that I read about a guy in India (or somewhere like that) He uses files and cutting tools to make triangle like shapes in the cyl head/s combustion chambers in cars and motorcycles. He found that it prevented detonation (almost completely!) and reduced the exhaust temps along with increaseing power&fuel milage. I did a experiment on the kid's PW50 and it seemd to pick it up quite a bit along with keeping the head surface and spark plug clean! I was amazed.. I have yet to try it on a car but It'd be worth looking into!
~Scott
I'd want to sit down and look at the heads side by side and match up & mock up everything! right down to the push rods and valve lash. Don't leave a stone unturned on that part. If all is good and matches up then give it a wirl! Dropping compression is good for a boosted car especialy a daily driven car. The streighter valve angles will help with flow. Don't learn anything by not trying do we? Go for it if it's not expencive!
Unfortunatly our cars are somewhat limited by traction and transmission from what I'm hearing and learning with my own car. Now that my clutch does not slip the tires do and If I go with slicks in the spring I think I'll bust a tranny! There's a point to every car at which it becomes impracticle or worse undependable with increased power.
I'm all for the power don't get me wrong but there's a limit to practicality & Driveability with the front wheel drive and the parts that are availble to us.
On a similar topic of dremel tool and heads... theres a article in Popular Science that I read about a guy in India (or somewhere like that) He uses files and cutting tools to make triangle like shapes in the cyl head/s combustion chambers in cars and motorcycles. He found that it prevented detonation (almost completely!) and reduced the exhaust temps along with increaseing power&fuel milage. I did a experiment on the kid's PW50 and it seemd to pick it up quite a bit along with keeping the head surface and spark plug clean! I was amazed.. I have yet to try it on a car but It'd be worth looking into!
~Scott
12-01-2004, 02:19 PM
#9
Originally Posted by Nealoc187
Hmmm I like where this thread is going even though it's beyond my knowledge scope.
I really like how more and more people on the org are helping each other.
from bernardd's and superdave2 bringing sr and vg knowledge over to icey always asking questions it seems like this forum is trying to bring about new things that can help all of us be faster and more dependable.
Same to be said from the all motor forum as well.
[/group hug]
12-01-2004, 02:27 PM
#11
Originally Posted by mtcookson
i have a **** load of vg heads laying around my house. i could bring them with me when i go to indiana and let you do that to them or try this out myself and see if there are any improvements on our engines. do you have the article about that?
12-03-2004, 08:52 AM
#15
Well read ANOTHER Honduuuh hybrid How-To that did something similar. Damn I wish we had something like they have with these step-by-step, all the parts needed, and model years that you can scavenge from break downs.
Anyways, they took a LS shortblock and put a later VTEC B-series cylinder head with larger intake/exhaust ports and valves. However, the VTEC head had a smaller combustion chamber then the LS's, so it raised the compression.
Dyno improvement was pretty impressive even for a 4cylinder it went from 129hp/124tq to 155hp/142tq with a *HUGE* improvement in area under the curve on the top-end. Peak power was at 6000rpm and shifted to just past 7000rpm.
Yeah, apples vs. oranges, but maybe this could work.
Anyways, they took a LS shortblock and put a later VTEC B-series cylinder head with larger intake/exhaust ports and valves. However, the VTEC head had a smaller combustion chamber then the LS's, so it raised the compression.
Dyno improvement was pretty impressive even for a 4cylinder it went from 129hp/124tq to 155hp/142tq with a *HUGE* improvement in area under the curve on the top-end. Peak power was at 6000rpm and shifted to just past 7000rpm.
Yeah, apples vs. oranges, but maybe this could work.
12-03-2004, 09:19 AM
#17
yeah usually with you can pull about 11.x:1CR easy using all stock stuff...B16 pistons, vtec head and LS block....the motor I'm building right now for my own car will be 11.5:1 and just from simple parts lying around from our old motors. can't wait for my ITBs 
http://www.c-speedracing.com/howto/c...c/compcalc.php
http://www.c-speedracing.com/howto/c...c/compcalc.php
Originally Posted by IceY2K1
Well read ANOTHER Honduuuh hybrid How-To that did something similar. Damn I wish we had something like they have with these step-by-step, all the parts needed, and model years that you can scavenge from break downs.
Anyways, they took a LS shortblock and put a later VTEC B-series cylinder head with larger intake/exhaust ports and valves. However, the VTEC head had a smaller combustion chamber then the LS's, so it raised the compression.
Dyno improvement was pretty impressive even for a 4cylinder it went from 129hp/124tq to 155hp/142tq with a *HUGE* improvement in area under the curve on the top-end. Peak power was at 6000rpm and shifted to just past 7000rpm.
Yeah, apples vs. oranges, but maybe this could work.
Anyways, they took a LS shortblock and put a later VTEC B-series cylinder head with larger intake/exhaust ports and valves. However, the VTEC head had a smaller combustion chamber then the LS's, so it raised the compression.
Dyno improvement was pretty impressive even for a 4cylinder it went from 129hp/124tq to 155hp/142tq with a *HUGE* improvement in area under the curve on the top-end. Peak power was at 6000rpm and shifted to just past 7000rpm.
Yeah, apples vs. oranges, but maybe this could work.
12-03-2004, 12:50 PM
#18
Senior Member
Join Date: Sep 2004
Posts: 296
Mark: I'd love to play with some stock VG heads to see what I can do not only with the combustion chambers but the intake and exhaust runners too. (along with a stock lower intake) If you've got a pair laying around I'll port and mod them for ya for... Um... how about free 
I was going to do this with the parts car heads but since you've got some extras that are off it sure saves me some work!
Thanks man~
Scott
I was going to do this with the parts car heads but since you've got some extras that are off it sure saves me some work!
Thanks man~
Scott
12-03-2004, 08:43 PM
#20
Originally Posted by IceY2K1
Well read ANOTHER Honduuuh hybrid How-To that did something similar. Damn I wish we had something like they have with these step-by-step, all the parts needed, and model years that you can scavenge from break downs.
Anyways, they took a LS shortblock and put a later VTEC B-series cylinder head with larger intake/exhaust ports and valves. However, the VTEC head had a smaller combustion chamber then the LS's, so it raised the compression.
Dyno improvement was pretty impressive even for a 4cylinder it went from 129hp/124tq to 155hp/142tq with a *HUGE* improvement in area under the curve on the top-end. Peak power was at 6000rpm and shifted to just past 7000rpm.
Yeah, apples vs. oranges, but maybe this could work.
Anyways, they took a LS shortblock and put a later VTEC B-series cylinder head with larger intake/exhaust ports and valves. However, the VTEC head had a smaller combustion chamber then the LS's, so it raised the compression.
Dyno improvement was pretty impressive even for a 4cylinder it went from 129hp/124tq to 155hp/142tq with a *HUGE* improvement in area under the curve on the top-end. Peak power was at 6000rpm and shifted to just past 7000rpm.
Yeah, apples vs. oranges, but maybe this could work.
12-04-2004, 08:43 AM
#21
Originally Posted by Boosted Maxima
Mark: I'd love to play with some stock VG heads to see what I can do not only with the combustion chambers but the intake and exhaust runners too. (along with a stock lower intake) If you've got a pair laying around I'll port and mod them for ya for... Um... how about free
I was going to do this with the parts car heads but since you've got some extras that are off it sure saves me some work!
Thanks man~
Scott
I was going to do this with the parts car heads but since you've got some extras that are off it sure saves me some work!
Thanks man~
Scott
I can probably drop by up there and bring you the heads and such and see what can be done with them. The only downside to the trip is I won't be bringing my Maxima.
12-04-2004, 02:40 PM
#22
Good idea... but IMHO you would still need stronger rods to take advantage of the additional power that changing heads would allow you to gain by increasing the boost. Stock 10:1 motor is producing pretty big power as it is under properly tuned boost.
12-04-2004, 07:34 PM
#23
I asked someone in Houston and this is what they told me. I don't know what it means though.
"As long as the head switch doesn't adversely alter the Quench Area of the motor, you should be fine. If the quench area thickens significantly, the motor will become less resistant to detonation and the squish will have less ability to properly create turbulence, outweighing it's simple ability to flow better."
"As long as the head switch doesn't adversely alter the Quench Area of the motor, you should be fine. If the quench area thickens significantly, the motor will become less resistant to detonation and the squish will have less ability to properly create turbulence, outweighing it's simple ability to flow better."
12-17-2004, 03:55 PM
#24
More finds....
2001-2004 Pathys have same heads, so same LIMs.
They use same DEK/350Z injector, so another chance the LIM will bolt-on to at least the heads.
They use flat top pistons vs. later domed style, so flame propagation shouldn't be as bad.
2001-2004 Pathys have same heads, so same LIMs.
They use same DEK/350Z injector, so another chance the LIM will bolt-on to at least the heads.
They use flat top pistons vs. later domed style, so flame propagation shouldn't be as bad.
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