Compression in an engine

Free Ranj Egg · Post by **Free Ranj Egg** » Tue Apr 26, 2016 4:51 pm

Question time Luders

Ludes generally run high compression in comparison to other cars, im assuming this is due to the power being made further up the rev range or due to the vtec.

So when people get forced induction and the compression on an engine is lowered, does vtec still work properly?

Ranj

Post by **Merlin** » Tue Apr 26, 2016 4:58 pm

VTEC is not dependent on compression ratio. It is oil temp & pressure and speed related.

OEM VTEC engines have compression ratio's from 10:1 up to 11:1 while the non VTEC engines are lower, as little as 8.8:1.

You don't have to put lower comp pistons into a boosted engine but it means that you can have more boost if you are running a lower compression ratio.

Post by **NafemanNathan** » Tue Apr 26, 2016 5:25 pm

Merlin wrote:VTEC is not dependent on compression ratio. It is oil temp & pressure and speed related.

Speed as in rev speed though.

njord · Post by **njord** » Wed Apr 27, 2016 12:20 am

Vtec is speed related,

Some jap cars depending where the speedo converter is placed you loose vtec in 1st gear

njord · Post by **njord** » Wed Apr 27, 2016 12:35 am

Ideally you dont want vtec in a forced induction engine,

as you want the exhaust to be closed when the inlet valve opens as otherwise the pressured air will just go into the exhaust.

I belive the new turbo civic uses vtec in reverse you get overlap lower down as this helps to spool up the turbo, similar to anti lag, compressed air going through the exhaust turbine spins it up

Post by **NafemanNathan** » Wed Apr 27, 2016 8:41 am

njord wrote:Vtec is speed related,

Some jap cars depending where the speedo converter is placed you loose vtec in 1st gear

Sorry yeah, it's at least 30mph or something isn't it? Sorry I was getting ahead of myself and just thinking about the engagement being set to a rev speed.

Post by **Merlin** » Wed Apr 27, 2016 9:49 am

In my haste I forgot about RPM

From memory the car needs to be over 18mph (roughly 30 kph) before VTEC will engage. 18mph seems to be the magic number for Ludes, it is the speed where the electronic 4WS changes its behavior.

Free Ranj Egg · Post by **Free Ranj Egg** » Wed Apr 27, 2016 9:55 am

Interestingly yes speed does have something to do with the vtec engaging.

Before I had my 5g years ago I had an import 4g which was an automatic. When I brought the car the speedo wasn't working and forced the car into the limp mode. As soon as the kmh to mph converter went on I had vtec again.

It was an auto but god I miss that car. It was the best cruiser I ever had, plus IMO the 4g was way better than the 5g.

The reason why I asked the initial question was that again IMO you buy a vtec to have a high revving high pitch screamer of a car. So wouldn't any type of F/I nullify the purpose of having Vtec in the first place? What Njord said in theory makes sense, but then again im no mechanic lol

njord · Post by **njord** » Thu Apr 28, 2016 12:02 am

The vtec isnt as important when having a FI lump. As your blowing the air fuel mix into the cylinder rather than sucking it into the cylinder with NA

Yes it helps having higher lift and more duration which vtec gives

The pistons are the only real issue as you cannot run very high boost, I think 7-8 psi is the max you can really go before running into problems

Post by **RattyMcClelland** » Thu Apr 28, 2016 10:16 am

I say yes to boost and vtec. It's where the magic happens. Especially a roots supercharged vtec. Behaves just like a standard h22 but with dollops more torque and power and a similar power curve.