high compression vs. low compression
- Thread starter mike5
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Ravage70 said:
you dont seem to realize what tuning is and how it works. tuning is a set of compromises. retarding spark, reducing the overlap of the cams, and adding more fuel all REDUCE power. compression, at sea level (for n/a motors) adds 3-4% more power for each full point of added compression. take 2 identical motors, one with 10:1 c/r and one with 11:1, and the 11:1 will make 3-4% more power. on 1 bar of boost (14.7 psi) that gets doubled, to 6-8 %. but the amount of power lost by having to retard the timing several degrees, running a point or two of a/f richer, and changing the cams all combine to lose more power than what is gained by compression- the law of diminishing returns. the compromises required to run any more than 5-6 psi on 11+ c/r make it lose more power by tuning than you gain by the added compression. all you gain by boosting a itr motor to high boost is an extremely unstable and detonation prone motor. thats not opinion, thats fact. go search places like h-t and look at the dyno graphs of itr motors vs comparable gsr's, b16's, even ls motors. they are making NO more power in stock form than any of those (just about all b series motors are good for about 300 whp in stock form), and make similar, if not slightly less power than gsr's with similar setups. there is very little to gain and a lot ot lose by boosting high compression to high boost pressure.
Ravage70 said:
more or less yes.anfrey said:
anfrey said:
its more than ease of tuning, its law of deminishing returns. tuning is the act of losing hp to gain reliability. retarding cam timing, ignition timing, and richening the fuel mixture all make a boosted car safe from detonation, and they all come at a hp penalty. beyond very low boost, having a compression ratio above a 9.x causes you to actually lose power over lower compression because you have to make so many compromises in ignition timing, a/f ratio, and sometimes cam timing to keep the motor from detonating on pump gas. even full race drag cars, which use special race fuel with octane of sometimes 125+ octane, and have access to the best parts and tuning all use a 8.x or 9.x compression ratio because even with race gas, high compression and boost at high levels is too much compromise. for low psi, like a stock motor, its ok to boost an itr, but if you are building a block for boost, putting very high compression pistons in is just plain dumb.
anfrey said:
you are correct, but as i stated earlier compression, by itself at normal atmospheric pressure (a n/a car) adds 3-4% more hp per full point of compression raised (9.0-10.0) that really isnt that much, although it is across the board, from idle to redline. it does spool the turbo a bit faster (more heat) but with a properly sized turbo, the difference is negligable, as is the peak power numbers and the powerband. the gains off boost negate the on boost losses from tuning compromises because most people who turbo their dont care if they make 3% more power at very light throttle crusing or around town driving. or if you have to wait 3% longer for a turbo to spool.
You have said it well "handlebarsfsr" and I couldn't agree with you more.
All of you who are trying to state otherwise are being owned by handlebarsfsr.
I didn't see it said anywhere so I'll break here to you guys here.
Lets say our fuel's maximum detonation threshold is at a c/r of 11.5:1. We'll make two identical engines, one is boosted to attain an actual c/r of 11.5 (Motor A) and one N/A with a c/r of 11.5 @ max VE (Motor B), the boosted engine would actually produce more power (all else being equal). This due to the fact that the boosted engine will take in more fuel and air (the purpose of the FI, right?) therefore when it attains that 11.5 c/r (even though it originally had, say a c/r of 8.5:1) the mixture inside the cylinders are actually much denser. What's this mean? The combustion process would produce higher pressure over the entire process of the power stroke. Whereas in the N/A motor the pressure would drop of gradually as the crankshaft progressed in degrees. Remember that power is proportional to the amount of average pressure over the entire stroke. After about 30 degrees of the crank angle, the N/A motor would begin to drop off in pressure quite a bit. The turbo motor would drop but at a much slower rate than the n/a and that is where turbo power really begins to shine.
Okay, so what if we turbo the 10.6:1 c/r B18C5 to attain a ratio of 11.5 (Motor C). Boost would still have to be kept lower than Motor A (to prevent pre-ignition and/or detonation) and once again, the charge isn't quite as dense as A and once again the pressure after about 30 degrees will drop off quicker than A. Peak pressure will be identical so both engines will take the same amount of strain yet the higher boosted engine produces more power.
If you can boost the hi-comp motor without having to retard timing or add excess fuel (read: power robbing) or you aren't planning to run that much boost, then by god go for it. But if your goal is gobs of boost on octane limited fuel then consider the lower compression for a margin of safety and at times better overall power. It's all compromises, compromises.
I didn't see it said anywhere so I'll break here to you guys here.
Lets say our fuel's maximum detonation threshold is at a c/r of 11.5:1. We'll make two identical engines, one is boosted to attain an actual c/r of 11.5 (Motor A) and one N/A with a c/r of 11.5 @ max VE (Motor B), the boosted engine would actually produce more power (all else being equal). This due to the fact that the boosted engine will take in more fuel and air (the purpose of the FI, right?) therefore when it attains that 11.5 c/r (even though it originally had, say a c/r of 8.5:1) the mixture inside the cylinders are actually much denser. What's this mean? The combustion process would produce higher pressure over the entire process of the power stroke. Whereas in the N/A motor the pressure would drop of gradually as the crankshaft progressed in degrees. Remember that power is proportional to the amount of average pressure over the entire stroke. After about 30 degrees of the crank angle, the N/A motor would begin to drop off in pressure quite a bit. The turbo motor would drop but at a much slower rate than the n/a and that is where turbo power really begins to shine.
Okay, so what if we turbo the 10.6:1 c/r B18C5 to attain a ratio of 11.5 (Motor C). Boost would still have to be kept lower than Motor A (to prevent pre-ignition and/or detonation) and once again, the charge isn't quite as dense as A and once again the pressure after about 30 degrees will drop off quicker than A. Peak pressure will be identical so both engines will take the same amount of strain yet the higher boosted engine produces more power.
If you can boost the hi-comp motor without having to retard timing or add excess fuel (read: power robbing) or you aren't planning to run that much boost, then by god go for it. But if your goal is gobs of boost on octane limited fuel then consider the lower compression for a margin of safety and at times better overall power. It's all compromises, compromises.
Another thing about the B18C1 motor as a better boost candidate is in the head. The GSR's combustion chamber has "real" quench areas. This is an area that remains flat all the way from the head gasket area until the rounded pentroof chambers. On other Honda motors, there is a recessed area, forming a circle like the cylinders. The quench areas "squish" the air fuel mixture in a small area around the spark plug; this keeps the fuel uniformly distributed and helps fight detonation. Of course you could also mill the ITR head and create the quench areas. And for this reason, that's why thick head gaskets used for lowering compression are a weak excuse.
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if im not mistaken, the gsr does not have quench zones. i believe only the d16y8 and the later h22a's had a true quench zone, and that milling a b series or earlier d series head resulted in a pent roof combustion chamber. to get a true quench zone, they need to be welded in and then re ported to eliminate valve shadow. and thanks for the backup, its nice to have someone else who knows what they are talking about.
Here's the GSR head and the ITR head. The GSR is on top.
With all due respect, handlebarsfsr, is there something I'm missing out on here about the quench zones?
With all due respect, handlebarsfsr, is there something I'm missing out on here about the quench zones?
yo anyone who says they its better to boost a low c. motor is mentaly reatrded and or just doesnt have enough practice or skill to fine tune such a motor....(b18c5 i.e)....if u have the time and money... a turbo high comp. motor will blow the door off any low comp. motor...with some advantages ...but with good comes the bad ...
smokedb18c5 said:
another person who doesnt know what tuning is or how its done. please read the thread before you make a dumb comment.
My 1 cents.. I don't know if anyone pointed it out or not but check it: Boosting a lower compression is better because once the air gets inside the combustion chamber, there are NOT alot of ways to lower the heat involved with compressing the air (pv=nrt thinger in chemistry) BUT with a turbo, the air can be compressed, and the heat dissipated through the intercooler. You can also spray NItrous on the intercooler to even further lower it or run a water cooled intercooler. You also have to remember when u have a high compression motor, your not puttin in any extra air/fuel into the motor your just compressing it further. i.e. A GSR with high compression is going to have about the same air/fuel amount as a GSR with low compression(It might even have a little less depending on how much the piston is domed). Even if u tune the engine and retard it to hell and beyond. Your robbing power when u retard the timing by decreasing cyclinder pressure in the motor so in essence you increase cyclinder pressure by high compression, and then release it by retarding the timing, (Kinda retarded eh)) Anyhoot my .01 cents first post. And don't flame because that is immature and all I will do is ignore you.