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Discussion Starter · #1 ·
I was chatting with my builder/mechanic and we started talking about my eventual turbo setup. I was mentioning things I would want to run with such as cams, valvetrain, pistons etc.

When I mentioned getting forged low comp pistons, he almost blew a gasket.

He's been doing imports for close to 20 years, and all his race cars he says ONLY run 9.5:1 comp on boosted B16s and B18s putting out just over ~360bhp...

Any idea why he's against going low comp pistons with boost? He was telling me its way better goin 9.5:1 on boost.

Unless 9.5:1 is considered low comp.........

(His specialty is also runnin Water/Methanol injection, would that be a reason for high comp on boost?)
 

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I don't understand either...

The way i see it is: 9:5:1 compression yields X amount of boost safely and 8:5:1 yields Y amount of boost safely.

The end result is still the same imo?

How much Cash btw do you have to spend on this turbo set-up?
 

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Discussion Starter · #3 ·
So low comp is safer as it reduces the risk of detonation?

Im at university other than summer not making any money..(Mechanical/Automotive Engineering...woo we get all of Daimlers crashed cars)
Anyways, going to put aside about $5000 Canadian for the build. Year and a half or so.

Get either a kit, or build a custom from a variation of kits etc.
Get all the missing stuff kits dont have, get tuned. Drive around with 6-8psi

Then Eagle Rods to keep it safe

Cams, valvetrain

Then pistons.....dunno low or high yet obviously.
 

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Lower boost is safer for pussies.


Now that we got that out of the way... think driveability. Unless its a drag queen, why would you want low comp pistons?

Here, read this thread. And pay attention to MatT3T4's replies as they are really informative.


I'm gonna leave now and check back in for any dead bodies lying around or bullets flying accross the room.
 

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Discussion Starter · #5 ·
Thanks, that was a great thread to read. So, I could easily and..hopefully smartly, raise my CR to that of a GSR, or just get forged replacements for my own. Those guys on that thread kinda went back and forth lol.

I obviously want the most power with the least amount of boost to prevent detonation. That will come with tuning, but it seems that a good starting point for highest power with least boost is going with higher CRs.

And since he was explaining that even ~10:1 isn't necessarily that high, the 9.5:1 I plan to run will hopefully be the best of many worlds.

One more thing. Stage II cams vs Turbo cams, I see some differences in Lift and Duration...to me they don't seem like huge differences, I hope I'm a little wrong with that assumption. What is the major difference? Do you go less aggressive or more aggressive with boosting? I would think more aggressive since even my Skunk2 I/M really helps more aggressive cams since they can use the extra intake air....and since a turbo puts in even more air, you'd think more aggressive cams...
 

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I didn't read that other thread because reading blows lol


My 2 cents
Lower compression will yeild more boost safely

Higer compression will make more power at the same amount of boost but you are walking a tight rope if its going to be a street car

Compression compared to boost, and this is one that no one ever mentions
efficiency, boost is more efficient.

Its hard to explain so I'll do my best

A N/A engine at WOT will have almost zero manifold vaccuum, if it did it would be 100% efficient (theres a better term, I fogot it). This will never happen on a N/A engine, the closer to zero vaccuum you are at WOT the more efficient your engine is running.
Now when you have boost, zero vaccuum and anything boost is 100% efficency and up.
If I stop by my parents house soon I'll see if I can find my notes from school then I can explain it better, its been 3yrs sence I learned all this shit, I've forgotten most of it lol
 

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capntoll said:
One more thing. Stage II cams vs Turbo cams, I see some differences in Lift and Duration...to me they don't seem like huge differences, I hope I'm a little wrong with that assumption. What is the major difference? Do you go less aggressive or more aggressive with boosting? I would think more aggressive since even my Skunk2 I/M really helps more aggressive cams since they can use the extra intake air....and since a turbo puts in even more air, you'd think more aggressive cams...
To be honest, the cams don't really matter in boost because the air will be forced into the combustion chamber anyways. So regardless of the cams, your still basically getting the same thing.

With NA, you need the vacum from those big cams to take in the air. Small seringe compared to large seringe. The bigger one can suck more volume even though they are the same length.

But if instead of sucking up fluid from the seringe, you are blasting fluid into it, its basically the same thing.

You will need to do some work on the overlap of the cams because they have different opening and closing specs. Either stay with stock cams or get the turbo cams. Although I have never seen a thread where someone actually posts a dyno of a boosted car with turbo cams, its probably the better route to take compared to NA cams.

And yes, VE is very important as already mentioned.These days 10:! is not really hard to do as many people boost their GSR and get great results.

If you are serious, just try and see if you can find some stock CR GSR setup with big boost and see what their results are.

But keep in mind that with low CR and boost, your DD or off boost will suck balls as I mentioned earlier. You will need lots of boost to make the same power as a high CR setup with low boost.

So you will basically have lots of lag and poor off boost driving. Then if and when boost kicks in, you feel a rush of power. Some people may like that, but most people do not.
 

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Your current compression ratio is 9.2:1, which is perfectly boostable. Depending on the turbo, you could run over 8psi daily. However, you can run more boost (safely) with a lower compression ratio. The lower the CR, the more boost you can safely run. Boost is most efficent, so youre better off running more pressure on a lower CR than the other way around.

If I were you, I would do one of two things. 1. Leave the motor stock, boost it, and shoot for ~240whp. 2. Slap on a thicker head gasket, and raise the boost.
 

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Discussion Starter · #12 ·
Yea, sounds like a plan. Although the lower I go for CR, the slower I'll be when I'm off boost. So a bit more lag off the line, but if it saves the engine, then I'm all for it. Stock 9.2 is fine, I don't need to spend so much $$$ to go for 9.5:1. I'll try and find forged replacements for mine.

240whp is more than enough to start me out on mostly stock internals. I already like my setup with I/H/E and I/M. I walk on stock GS-Rs all day. (Although I don't go much past 75mph on the Highway).

Thanks again for all the comments.
 

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The reason for turbo cams, overlap is good for N/A, it uses exhaust scavenging to create more of a vacuum on the intake side therefor you pull more air/fuel into the combustion camber using the vacuum the exhaust exiting has made, to do this the exhaust valve will still be slightly open as the intakes starts to open.
Turbo cams run a minimal amount of overlap, if you have much overlap with boost then you will be foceing air strait into the cylinder and right out the exhaust side, in a sense you are wasting boost that way(that kinda sounds dumb I know) But that is why stock cams are better than N/A aftermarket cams for boost
 

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Sirfallsalot243 said:
Your current compression ratio is 9.2:1, which is perfectly boostable. Depending on the turbo, you could run over 8psi daily. However, you can run more boost (safely) with a lower compression ratio. The lower the CR, the more boost you can safely run. Boost is most efficent, so youre better off running more pressure on a lower CR than the other way around.

If I were you, I would do one of two things. 1. Leave the motor stock, boost it, and shoot for ~240whp. 2. Slap on a thicker head gasket, and raise the boost.

Umm... lower the cr and raise the boost just means that he'll have to use more boost to get the same power.

You people are forgetting that it doesn't matter if you have an 8:1 ratio or a 11:1 ratio you need to tune your car or it's nothing more than a problem waiting to happen.
 

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5gencivic said:
Umm... lower the cr and raise the boost just means that he'll have to use more boost to get the same power.

You people are forgetting that it doesn't matter if you have an 8:1 ratio or a 11:1 ratio you need to tune your car or it's nothing more than a problem waiting to happen.
you are correct. but the higher the CR while boosting the more severe the results when you fuck up and detonate. 9.2:1-9.5:1 are fine boost levels for a daily driven car. you can make 300whp safely and realiably with those numbers. you probably want CR numbers like that anyway for such a small 4cyl. a 1.8 liter with 8.5:1 Cr will suck when out of boost unless you adjust the gear ratios to suit your power band.
i drive an sr20 and it has 8.5:1 from the factory. alot of us who don't plan on running insane amounts of boost actually will raise our CR to about 9.0:1-9.2:1 for better response down low and to make more power with less boost. I'm running 9.1:1 right now actually. so if you are in this for a drag monster then drop your CR because driveablity i probably the least of your goals. if you want a fun car to drive on the street then keep your CR where it is at. keep in mind people run 10:1 Cr on daily driven boosted cars in the 270whp range and they have no problems. it all depends on the tune.
 

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just something for you to thinkabout on driveability side

stock eclipse gsx's came from the factory with 7.8-1 compression on a 2.0
with 11lbs of boost and daily driveing was fine, i have had 6 of them

my current gsx has been finished for a year and the comp is 8.5-1 at 20psi without nos and 11lbs with nos
 

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WOW, 7.8:1 CR, do they even do make shit that low? Ummm no wonder boosted tegs walk all over them. LOL! Just a smarass remark. Disregard, Hehe!

Anyhow, driving VW's and other OEM turbo cars with factory turbo's suck balls really. Unless you hit boost, its just not fun to drive at all. yes the lag tiome is about 1-2 seconds only, but thats enough to stray me away from it.

Keep in mind that factory vehicles have that low CR because they have to take into account more than just a safety margin for boost. Thjey also have to take into consideration of elevation, weather, and climate conditions as well. So that low CR helps keep the car fucntionally properly regardless of where they sell it.

Its similar to the high CR and boost thing where it has to be perfectly tuned.

Even EVO's come with at least 8:1 CR.
 

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RoadRageMotors said:
just something for you to thinkabout on driveability side

stock eclipse gsx's came from the factory with 7.8-1 compression on a 2.0
with 11lbs of boost and daily driveing was fine, i have had 6 of them

my current gsx has been finished for a year and the comp is 8.5-1 at 20psi without nos and 11lbs with nos
stock gsx's are slow as dog piss. it takes 11 psi to make 210 hp and still only run a 15.1 my car in stock form runs mid to low 14's on 7 psi making 205 hp. so yes they have terrible driveability. forget those slow crankwalking balls of garbage. now when modded they can are really nice. but 11 psi and a 15.1 1/4 miles bothers me
 
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