High lift cams bad fro vtec?


GWInquisitor14
04-22-2004, 09:26 PM
Ok so I know this guy and hes building up his honda civic to be a drag car. It isn't his daily driver but he wants it to be a street legal drag race car. He was saying today that Vtec is a horsepower robber. He was saying it is bad to have the vtec because of its high lift at high rpms starve the engine or something like that. i dont understand why he would think that.

GWInquisitor14
04-22-2004, 10:04 PM
Yeah i just re read my post and forgot to add the fact that hes turboing his car and saying that the vtec is especially bad for the turbo. I dont know where i stand on this because the high lift will let u get more boost and fuel into the cylinder but it will also create more overlap at the high rpms. More overlap is bad because u are taking boost and shoving it out the exhaust but it does help the exhaust get out quicker to spool the turbo quicker.

boosted331
04-22-2004, 11:12 PM
That guy is an idiot when it comes to building motors then. The VTEC cams that come stock on all hondas are so incredibly mild that overlap is a non-issue. Even if the VTEC cams did have too much overlap for turbo, he would be an idiot for taking a non-vtec head over a vtec head, solely because of how much better the VTEC head flows, both ported and unported. Sounds like that guy doesn't know squat about building a race car.

YellowITR479
04-23-2004, 07:25 AM
I suggest you dont listen to anymore shit that comes out of his mouth.

ghost94
04-23-2004, 10:27 AM
Well I'm not sure this guy knows nothing , I think the question was not understood. Because I am aware of two full race B16's being built now, one is all motor, and they both have a vtec eliminator on them . The way it was explained to me is the vtec is a hinderance when you are staging and launching above 5000. It doesn't idle like a honda though because it is basically just using the hi cam

GWInquisitor14
04-23-2004, 01:21 PM
check out his cardomain page at www.cardomain.com/id/cavetic

GWInquisitor14
04-23-2004, 01:34 PM
The only bad thing ive read about Vtec, like in actual books, cuz i got one all about honda tuning, is that the guys who make those 800+ hp civics dont use it because its jsut more moving parts that could go wrong under extreme conditions. But neither of us will have a 800+ hp car. If it is a mild cam, then it should be fine for turbo. Crower makes a turbo cam that is high lift w/ lower over lap. That way u have plenty of time for the cylinder to fill w/o blowing boost out the exhaust

boosted331
04-23-2004, 03:48 PM
The only bad thing ive read about Vtec, like in actual books, cuz i got one all about honda tuning, is that the guys who make those 800+ hp civics dont use it because its jsut more moving parts that could go wrong under extreme conditions. But neither of us will have a 800+ hp car. If it is a mild cam, then it should be fine for turbo. Crower makes a turbo cam that is high lift w/ lower over lap. That way u have plenty of time for the cylinder to fill w/o blowing boost out the exhaust

People that eliminate VTEC on turbo and naturally aspirated race cars still use the VTEC head and big cams though, because the vtec head flows much better and bigger cams make more power. You'd be suprised at the cams fast turbo racers use, they are much bigger than you think. If they had tiny amounts of duration and no overlap the motor wouldn't make any power. Heck, HKS sells cams for 2JZ's, RB26's, SR20DET's, etc, with 272 degrees of duration, and those are all turbo motors.

Tranzlogic
04-28-2004, 11:39 AM
look at it this way .. why would anyone build a b16 turbo and then get rid of the Vtec its just stupid and pointless..in that case you would just go with a B18B ! I'm not saying i know everything about every car by any means, but I only talk about what i know and i Learn if i dont!! I think the people you talk cars with need to go to a tech school or just stay home and play with matchbox cars !!

GWInquisitor14
04-29-2004, 01:39 PM
Ya i think ur right, cuz i know this other kid who was saying the other day that geting an adjustable fuel pressure regulator is a dumb thing to get. He said you dont need to increase fuel pressure. Some people just piss me off a lot

casperGSR
04-29-2004, 03:01 PM
just so you know the next time someone starts talking overlap and mentions lift but not duration tell him to blow smoke up someone else's ass... overlap will remain the same if you increase the lift and keep the duration the same which is possible to do... for the most part when you buy an aftermarket cam both the lift and duration will be higher but it's not how far the cam moves the valve open but more importantly (when talking overlap) for how long the cam keeps the valve open... overlap is not a bad thing (with the proper amount), it helps draw more air into the combustion chamber and actually at higher rpms an engine with a lot of overlap will help scavenge the cylinders for better efficiency.

GWInquisitor14
04-29-2004, 03:27 PM
ya but i think hes talking about a turbo motor, where u dont want so much overlap cuz the boost will go out the exhaust and into the turbo. I might have gotten confused w/ lift and duration. I used them interchangably. Lift is the distance the valve opens up and duration is how long it is open, correct?

Tranzlogic
04-29-2004, 03:53 PM
correct

casperGSR
04-29-2004, 04:48 PM
ya but i think hes talking about a turbo motor, where u dont want so much overlap cuz the boost will go out the exhaust and into the turbo. I might have gotten confused w/ lift and duration. I used them interchangably. Lift is the distance the valve opens up and duration is how long it is open, correct?

yes correct... however you have me slightly confused... it seems to me that you are saying that the air, 'boost', will go out the exhaust valve during the overlap and 'into the turbo'... yes the air will go out the exhaust valve but that air will not go back into the turbo... the air that goes out the exhaust valve goes to the downpipe and out the exhaust... as it passes through it spins the turbine wheel which then spins the compressor wheel which draws in more fresh air.

91CRXHF
04-29-2004, 07:39 PM
ok i was looking at cams and boost and what not b4 i ordered myne and this guy who built my engine has a program that he can put in the cam and the turbo as well as the amount of boost. we found out that high performance cams such as a stage 4 spec of Cranes undere high boost actually had a pretty bad reversion problem so we foudn out on high boost (30psi) a smaller cam would be better but hey who is gonna be runnin 30psi first of all and second buying a stage 4 cam...but thats what i found out

GWInquisitor14
04-29-2004, 09:02 PM
How so, and what do u mean by bad reversion problem. See i read its bad to have a lot of overlap for a boosted car cuz it going out the exhaust but also that u dont need as much overlap to push the exhaust out, so u can stay in the power stroke much longer. Also, it pretty much takes some unburned fuel and pushes it out the exhaust path too.

boosted331
04-29-2004, 10:02 PM
How so, and what do u mean by bad reversion problem. See i read its bad to have a lot of overlap for a boosted car cuz it going out the exhaust but also that u dont need as much overlap to push the exhaust out, so u can stay in the power stroke much longer. Also, it pretty much takes some unburned fuel and pushes it out the exhaust path too.

Overlap is not an issue at the vtec xover point of 5000+ RPM's. Yes, if you run some toda vtec killers on your turbo car it's not going to run so great because you don't need THAT much duration with a turbo. Turbos will benefit from more duration, and benefit greatly from increased lift. If you look at some turbo cams for V8's they always favour the intake on the duration side, and the lift is much higher than a comparable duration cam for a N/A motor. Wide lobe seperation too to keep away from overlap but still make great power with lots of duration.

91CRXHF
05-01-2004, 01:15 AM
Reversion, what is it? It is simply the exhaust pulse flowing backwards momentarily during the overlap phase of the camshaft at low cycling rates. During the overlap phase the piston is pushing out the last of the exhaust gases and prior to reaching top dead center (T.D.C) the intake valve and the exhaust valve is still closing. At this point in the engine cycle both valves are in the open position. At high cycling rates the inertia of the incoming intake charge and the outgoing exhaust pulses keep the exhaust flowing in the proper direction. But at low cycling rates, as the piston is pushing out the last of the exhaust gases the intake valve opens and some of the spent exhaust charge is pushed into the intake manifold. As the piston reaches T.D.C. and begins the intake stroke the exhaust valve is still not completely closed. As a result, the piston pulls from the intake and exhaust valves simultaneously causing the exhaust gases to flow in a reverse direction. This is normally not a problem until you add water into the exhaust stream. Reversion can be severe enough to stall the engine, add water to the oil, rust the exhaust seats, etc. This effect only happens at idle speeds, but remember that during shut down the engine encounters the greatest reversion.

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