ok for us Cali guys got shity 91oct out here....Compression is a means of nice #'s but with retarted timing along with increase of comp.
since most motors liek to responed to more timing....

i was thinking of a lower compression motor say in the 10's with moreadvanced timing vs. a motor with comp in the 12's with with retared timing......

just a thought i was thinking about for a while and couldnt get a str8 answer....

 

would like to know also

 

Just look at Omnis builds.

 

The way I look at it--if you can't run the optimum timing with available fuel because your compression is too high--you are going about getting power the WRONG way. I am not saying alot of timing is better than a little bit of timing. Its whatever your motor likes. But if you have figured out through dynoing or another means that your motor would make more power with more advanced timing (within reason) than what you can currently run due to fuel or compression issues,--then I think you built your motor without being logical. You get to a certain point though, where the motor will make a little more power with the timing really advanced but the cylinder pressure goes through the roof--and the timing is so advanced that its actually trying to push the crank through the bottom of the motor and lift the head. Alot of people throw compression around for bragging rights. Saying "yeah, I run 12.7 on the street"--but what they aren't telling you that the timing is so retarded they lost any advantage and more that the increased compression would have given them. I always say if its a true street motor--that will ONLY see pump gas, and something you would trust in hot weather-to take long trips in--be conservative. I wouldn't go more than 11.0-11.2:1--depending on piston and cc design. Its only when you are trying to eek that last ounce of hp out--that really high comp. will help--and then only if the motor will never see pump gas. Thats how I feel anyway.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by d16dcoe45 &raquo;</TD></TR><TR><TD CLASS="quote">The way I look at it--if you can't run the optimum timing with available fuel because your compression is too high--you are going about getting power the WRONG way. I am not saying alot of timing is better than a little bit of timing. Its whatever your motor likes. But if you have figured out through dynoing or another means that your motor would make more power with more advanced timing (within reason) than what you can currently run due to fuel or compression issues,--then I think you built your motor without being logical. You get to a certain point though, where the motor will make a little more power with the timing really advanced but the cylinder pressure goes through the roof--and the timing is so advanced that its actually trying to push the crank through the bottom of the motor and lift the head. Alot of people throw compression around for bragging rights. Saying "yeah, I run 12.7 on the street"--but what they aren't telling you that the timing is so retarded they lost any advantage and more that the increased compression would have given them. I always say if its a true street motor--that will ONLY see pump gas, and something you would trust in hot weather-to take long trips in--be conservative. I wouldn't go more than 11.0-11.2:1--depending on piston and cc design. Its only when you are trying to eek that last ounce of hp out--that really high comp. will help--and then only if the motor will never see pump gas. Thats how I feel anyway.</TD></TR></TABLE>

Well if you have more compression, and you have to run your timing retarted to drive around on pump gas. You always have the option to run race gas and bump your timing up. It all depends on how much you like to mess around with your car, and what your going after. When building a motor you have to know ahead of time, what numbers your trying to make , and what the motor is for (street, drag, autox, etc)

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Power Rev Racing &raquo;</TD></TR><TR><TD CLASS="quote">

Well if you have more compression, and you have to run your timing retarted to drive around on pump gas. You always have the option to run race gas and bump your timing up. It all depends on how much you like to mess around with your car, and what your going after. When building a motor you have to know ahead of time, what numbers your trying to make , and what the motor is for (street, drag, autox, etc)</TD></TR></TABLE>

this is the route i went...91 on the street with base timing at 12btdc and 100oct at the track with base at 18btdc...i just use my msd timing controller...makes life simple

 

i agree with whaqt you are saying in regards to not getting the benefits of running higher comp set ups on the street.

just getting ideas thinking about a 10 something comp NA build

 

Well from my experience, I run big cams(Crower 404's) adn have PR3 pistons which gave me 10.8:1 static compression/ 10.3:1 Dynamic compression. I had to ran my timing a lil retarded at 14 degrees. The car ran good and still had decent lowend. Well recently i bumped up the compression to 11.5:1(static), which gave me with my cams 10.9:1 dynamic compression. On the retune we still ran 14 degrees timing and retarded both cams 2.5 degrees. The car runs like a champ and has a lot better response and low end. Very driviable on the street now as compared to before.

So IMO, it all depends on your set-up. You have to choose components that are going to work well together and compliment each other.

However I am curious to hear what others say.

 

Here's a question, When motors go out and which valves see it first the exhaust or the intake? Why is the bigger question? Come on ladies you know this

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by allmtrwhore2 &raquo;</TD></TR><TR><TD CLASS="quote"> Here's a question, When motors go out and which valves see it first the exhaust or the intake? Why is the bigger question? Come on ladies you know this

</TD></TR></TABLE>

my set up was the exhaust i believe.....

a major question in regards to this that im asking myself is flame route......thus compression (dome pistons) VS. Timing ( lower comp= flatter pistons)

the way your flame/exhaust is routed out is a big affect on HP right??????

 

This isnt a very easy question to answer, and there is a lot of backlogged information about the way engines work. A lot of people know more than I, so I'll just chime in with my two-cents and see if I can learn something too.

Every engine has a preticular sweet spot for timing due to a huge amount of variable (cams, cam timing, air pressure/density, etc). In every modern engine the fuel mixture is ignited before the piston reaches TDC (whether is be 14deg, 10deg, etc) the engine has a given performance (ie. 'X'). Taking this too the extreme, if we advaned the ignition to 35degrees, the explosive force would be wanting to push the rotating assembly the other way (clockwise to counterclockwise lets say). Taking it to the other extreme, if we retard the timing to 10degrees after TDC the piston would already be traveling down the stroke and our explosive gases would 'miss' the appropriate timing the effectly push the piston down. So where does this put us?

A high compression engine (ie. 15:1) would need such a late (retarded) ignition spark that the fact that the pressurized air/gas mixture is there, the combustion would be so late and hot to make a dependable engine (since the gas would be burning very close to when the exhaust valves open and therfor running a risk of keeping the edges of these valves hot and predetermined to pre-ignition.

A lower compression engine would (as I stated in an extreme situation before) would not make power either. But there are limits to what I have said, and I am sure that there are wholes for my theory of "Keep it logical/within reason".

To a point: you're not going to make more power with low.comp w/lots of timing or highcomp.w w/lots of retard... your going to have to fgiure out where that sweetsport is in your engine's rotation.

Rich

 

There is holes on it, but I want some people to chime in a little more. Yeah What you stated is true in theory, but alot of that info to alot of people who have these races motors are slapping there heads . Lets see where the thread goes.


Remember there are 15 compression small blocks that have flat top pistons. A engine builder just recently did a build in regards to this ( well sort of) He never gave specs of rod length or cam design or even what compression his ( cylinder pressure) block comes out to be. I'd bet you be surprised to find out what it really would be. Someone once told me it's stupid easy making power just Put the 2 together............. just some food for though

 

The exhaust valves hit first because the piston is chasing them up the bore on the exhaust stroke.

I know of engines that run 55 degrees advance at very high RPM at WOT with not so exotic fuel blends. Timing USUALLY equals more power.

 

One more thing, you almost always want peak cylinder pressure at 15 degrees ATDC, give or take 1/2 degrees based on rod length. Knowing the octane rating, you can figure out the flame speed. Knowing the flame speed you can calculate exactly when the best time to ignite the spark would be for maximum power output.

 

SJcivic,
yes i see what you are saying but im not really refering to taking each motor to the max.
im speaking more along the lines of a 10:1-10:5 compression motor with more timing VS. say a 11:8-12:1 comp motor on shitty cali 91 oct gas.....

having the lower comp motor ( flatter pistons) would have more movement of the flame within the cylinder and exiting correct???? also headers and exhaust ports play a factor too


Modified by Snoflake at 11:48 PM 10/9/2005

 

I have found compression of about 11:1 to be optimal for 91 octane fuel. That's the compression I run in my street motor. I can still run about 30 degrees total timing and the motor makes good power without detonating. Less compression and you are giving away power potential. You might be able to get away with a little more, perhaps as much as 11.5:1, but that is about the limit. Then you reach the point of diminishing returns, where you have to start pulling timing and giving back the power that you gain by raising the compression.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by b19coupe &raquo;</TD></TR><TR><TD CLASS="quote">I have found compression of about 11:1 to be optimal for 91 octane fuel. That's the compression I run in my street motor. I can still run about 30 degrees total timing and the motor makes good power without detonating. Less compression and you are giving away power potential. You might be able to get away with a little more, perhaps as much as 11.5:1, but that is about the limit. Then you reach the point of diminishing returns, where you have to start pulling timing and giving back the power that you gain by raising the compression.</TD></TR></TABLE>

i agree with your point Todd,
Thanx Omni for ur inputs

ok lets say a motor thats has a 85mm LsVtec 10:5 comp could good power be extracted from it with timing????
if u look at USDM ITR bottom ends with a good head set up and matched cams over 200whp is found, so with 4mm " in displacement with roughly same comp can 220+ be achived, taking into account a properly done head, extruded/ported Intake manifold etc........

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by SJcivic &raquo;</TD></TR><TR><TD CLASS="quote">To a point: you're not going to make more power with low.comp w/lots of timing or highcomp.w w/lots of retard... your going to have to fgiure out where that sweetsport is in your engine's rotation.
Rich</TD></TR></TABLE>


I agree completely!

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by omniman &raquo;</TD></TR><TR><TD CLASS="quote"> Matching the compression to the octane of gas is very important for optimal power. hence why there are a gazilion types and octanes of race fuel, for all types of different situations.

Im no scientist, of chemical engineer, but i have personally found through expereince that on 91 octane not exceeding the low 11:1 compressions ratio tends to yeild the best balance of performance and longevity. areas that have 93 or 94 can run probably 11.5-11.7 with the same results. total timing tends to like to be around 26-29 depending on the stroke and piston design with it tappering off a bit at or after peak power. the shorter strokes tend to like a little less, the longer ones tend to like a little more. also the rod ratio has some degree of play because it changes the piston dwell time at TDC.

also motors gain compression as they gain miles. carbon builds up and also at high rpms things stretch a little bit. starting out with a few tenths on the lower side of maximum will put you at the maximum after 10k-20k miles. If you can hear knock when you put your car in 5th gear @ 2k and floor it and no matter how much timing you remove it still knocks, its an indication that you have too much compression for the octane that you are using.

lowering the coolant temp with a termostat and larger radiator can help a tiny big to prevent heat soak. also drawing in cold air helps to some degree. </TD></TR></TABLE>

100% Agree, especially with the R/S stuff re TDC dwell and volume change when the piston moves away from TDC.

Also like to add that oil control is also key. The more oil that gets into the chamber the more likely your motor will have detonation.

 

i have been doing a lot of reading on this topic the past year.

i live in cali, i have a b16a2 with ctr pistons and a vey milled head. milled 80 thou but i have a 74 thou gasket to compensate. i ended up having to advance timing to get a sweet spot, smooth idle, smooth/strong accelaration. retarding the timing seemed to make it worse.


strange.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by ( o )( o ) &raquo;</TD></TR><TR><TD CLASS="quote">i have been doing a lot of reading on this topic the past year.

i live in cali, i have a b16a2 with ctr pistons and a vey milled head. milled 80 thou but i have a 74 thou gasket to compensate. i ended up having to advance timing to get a sweet spot, smooth idle, smooth/strong accelaration. retarding the timing seemed to make it worse.


strange.</TD></TR></TABLE>

wowo 80 thou mill!!!

 

ttt for more 2 cents!

 

Does someone have the formula to calculate dwell time?

What degrees points on a 360 degree scale does the piston stay at TDC and BDC? Does it vary with stroke, bore, rod lengths, and weight?

After reading this it got me to thinking and trying to calculate it out for various setups.

And for ignition, the degree that it is set to spark, is that ATDC?

 

It varies with stroke and rod legnth and at a high RPM, probably a little with strecth.

The number set for ignition is BTDC, to acheive maximum cylinder pressure ATDC.

 

Well from taking what I know as far as math goes I calculated for measurements such as Miles Per Hour(MPH) of rotating assemby at a given rpm, Feet Per Second(FPS) of rotating assemby at a given rpm, and Dwell Time(DT) at a given rpm.

From what I found out is Dwell Time(DT), the time the piston stays at TDC during a 360 degree rotation given that the piston only stay at TDC at 0/360 degrees, stays constant no matter the setup. However it does change depending on what RPM the engine is at.

(1/(RPM/60)) / 360 = Dwell Time at TDC or DT

@ 1000: DT = 167 micro-seconds
@ 2000: DT = 83 micro-seconds
@ 3000: DT = 55 micro-seconds
@ 4000: DT = 42 micro-seconds
@ 5000: DT = 33 micro-seconds
@ 6000: DT = 28 micro-seconds
@ 7000: DT = 24 micro-seconds
@ 8000: DT = 22 micro-seconds


Now distance that the rotating assembly travels to achieve these speeds is dependant on stroke. A longer stroke will have to move faster to achieve a given RPM, while a shorter stroke motor doesn't have to work as hard to achieve the same RPM.

To Calculate MPH:

((((Stroke in inches X 2) X RPM) / 12) X 60) / 5280 = Distance traveled by the rotating assmebly in MPH

Comparing the B16A(77.4 mm stroke) against the B18B(89mm stroke) you can clearly see the difference:

B16

@ 1000: 5.8 mph
@ 2000: 11.5 mph
@ 3000: 17.3 mph
@ 4000: 23 mph
@ 5000: 28.8 mph
@ 6000: 34.6 mph
@ 7000: 40.4 mph
@ 8000: 46.2 mph


B18B

@ 1000: 6.6 mph
@ 2000: 13.3 mph
@ 3000: 19.9 mph
@ 4000: 26.5 mph
@ 5000: 33.2 mph
@ 6000: 39.8 mph
@ 7000: 46.4 mph
@ 8000: 53.1 mph

To calculate FPS:

((Stroke in inches X 2) / 12) / 60 = Distance traveled by the rotating assmebly in FPS

Again comparing the B16(77.4mm stroke) against the B18b(89mm stroke).

B16

@ 1000: 8.5 fps
@ 2000: 16.9 fps
@ 3000: 25.4 fps
@ 4000: 33.8 fps
@ 5000: 42.3 fps
@ 6000: 50.8 fps
@ 7000: 59.2 fps
@ 8000: 67.7 fps


B18B

@ 1000: 9.7 fps
@ 2000: 19.5 fps
@ 3000: 29.2 fps
@ 4000: 38.9 fps
@ 5000: 48.7 fps
@ 6000: 58.4 fps
@ 7000: 68.1 fps
@ 8000: 77.8 fps

So no matter whether the engine has a long stroke or a short stroke the dwell time of the piston at TDC of each revolution at a given rpm remains constant. The only thing that changes, dependant of the stroke, is the distance the rotating assembly has to travel to maintain that rpm.