Ok, me and my friend are having a little arguement, and we've placed a friendly bet on it (loser buys the other cams ) so i need your guys help.
He tryed telling me that a motor with a lower compression ratio lets say for example the integra LS, will be able to run a higher rpm then a motor with higher compression. I think that a motor with a higher compression ratio can rev higher, (example ITR revs alot higher then an LS, and has a higher C/R)

Cliff notes: Friend thinks a motor with lower C/R can rev higher then a motor with a higher C/R, i think hes wrong

 

Well, basically this is just a "bump" for you. Because the biggest thing I've leaned from this page is how little I actaully know about SERIOUS engine building. So, I'll leave it to the more experienced fokes on the page.

But from what I understand.... I think you are both wrong!

Compression Ratios have very little effect on how high an engine can relaible rev.

 

Higher compression ratio doesn't make you rev higher. Lower compression ratio doesn't make you lose rpms. The reason it SEEMS higher compression means higher revs is because more intense camshafts require higher compression. So, neither one of you is right. I say its a draw.

 

your both wrong
the point where your engine makes its max power (redline) is related to the cams and headflow and valvetrain. if you have an extremley powerful set of cams, good strong valvetrain and valve setup. you could rev to 9500 all day and never have a problem.

so if your running an extremley weak set of cams and crappy train/springs you may only make power to 6000 and could damage stuff above that.

remember, more revs dosent mean more power. if you set up a type r motor to rev to 10,000 it would go that high, but wont make power above 8500. unless you upgrade the cams/train setup.

cliffs: compression has NOTHING to do with redline.


[Modified by Kamin, 5:38 PM 1/21/2003]

 

here is what hes trying to tell me. He says if you take a regular motor ( any motor) and drop teh compression, it will be able to rev higher. I just dont see how that is possible.

 

It as ABSOLUTLY NOTHING to do with reving.....

Period.

 

Yea he is wrong.

High compression helps your static CR. When you are running a big cam, some compression is lost from all the valve overlap...raising the CR helps compensate for this.

 

i was thinking along the lines.. the better your rod stroke ratio, the higher you can rev. ITR's have better r/s than LS engines.. that's why they rev higher. as for higher compression.. you make more power when you rev high. i could be wrong though.

 

Oh..My..Gosh...You and your friend are rediculous! Your getting things mixed up..in this case its *Compression Ratio* and *Rod Stroke Ratio* Jeshhh..!
..i have a 9.3:1 CR and rev to 8k everyday! but its mainly up to your Valvetrain that restricts you from reving high..thats why people dont rev high unless they get high performance valvesprings and titanium retainers...

 

i think you and your friends need to buy us cams

 

i understand what you were saying
but your BOTH WRONG

 

i say you both buy the same cams

 

Actually, a higher compression ratio is better for high revs. Increasing the compression ratio brings the oxygen molecules and fuel molecules closer together, increasing their concentration. Since the rate of a chemical reaction increases as the concentration of the reactants increases, the burn occurs more quickly with a higher compression ratio. And when you increase RPMs, you are giving the fuel less time to burn. So anything that decreases the time required for the fuel to burn is better when you are running at high RPMs. Otherwise, the fuel will still be burning during the power stroke and possibly during the exhaust stroke. This causes power output to decrease and can even burn the exhaust valves.

 

I should note that this is one of the reasons why DOHC VTEC motors have higher compression ratios than other Honda motors.

 

Cam profile is what determines the peak power (rpms). For example stock B16 cams will peak out around 7,500 rpms. ITR cams (which have a larger cam profile) will peak a little over 8K, June killer cams will make peak power well after 9,000 rpm. So what I am saying is stock B16 cams can be reved to 10k but it won't make power over 8K. Its basicly pointless. Compression has nothing to do with revs alone but an increase in compression along with larger cam profile and upgraded valve train will lead to higher RPMS.

 

with 10.8-11:1 my R block with GSR head and cams 8K whp/tq at 0,0 and 18 timing is 158/105 with stock GSR I,H,C,E with ITR muffler and stock OBD2 computer/fuel, stock intake manifold

10:1 stock GSR motor stock I,H,E @ 8K whp/tq is 144/97

if there is any math to be done, you would see that this configuration based on several things, one being compression that power is able to be generated all across the rev band including up top with the same cam profile. I would imagine it would be similar to increasing the displacement in a given application, however then you are dealing with R/S ratio and changing the behaviour of the engine's internal geometry. VE in a higher compression engine would seem to raise overall however, I would imagine would shift upward depending on the state of tune the motor is in.

theoretically and in this case somewhat illustrated....

however your MECHANICAL LIMIT has to do with the integrity of the head and block, fasteners, reciprocating parts, valvetrain, fuel delivery, cam profile and maintaining oil pressure.


[Modified by MikeSarr_GSR, 10:41 AM 1/22/2003]

 

a beefed valve train will allow you to rev higher. not compression.

edit: probably doesnt help at all, just saw all the replies. u guys should buy each other cams now.


[Modified by kyle h., 3:26 PM 1/22/2003]

 

I never meant to say that compression increases rev capacity. I just said that since higher compression increases the speed of the combustion event, the engine will operate more efficiently at high RPMs. Ideally, combustion should be complete by the end of the piston dwell time. If combustion occurs during the power or exhaust stroke, energy is wasted. And if a low compression engine is spinning fast, it is very likely that the piston dwell time will not be long enough for combustion to completely occur within it. Therefore, power will be lost. And if combustion is still happening during exhaust, it is possible that the valves could be damaged.

 

get yourself a GSR motor with stock compression, put in a beefed up valvetrain with 260 cams and see how high you can make power. the poweband will be narrow and will fall on its face. without raised compression the engine cannot be as efficient at high rpm. high rpm power is a combination of all these things, not any ONE thing. however if you had to choose one thing to upgrade, compression would be a safe bet with the right fuel tuning. I have seen a GLOBAL increase in power, with less gains up top however with more than there was before with the SAME cam and same (ITR) valvetrain.