Cam Choice (again i know)

[Deus Ex]

Hey Guys

Just a quick question, i am getting my cam from a N42 reground for my :P90 and i was wondering if anyone had any expierence with a reground cam in a similar set up, ie, im after the duration, timing details.

What i want and what i have

*the cam must be daily drivable (not overly agressive)

*has to idle and start easily

*Triple 45mm Webbers

*Ported and Polished P90

*L28 with Flatops

Any suggestions or details from people with a similar setup would be welcome

[ricksaia]

I have the same engine. I did a 262/272 grind and a 450 duration. It will be out of the machine shop soon. This seems to be thr best grind for every day performance.

[Phred]

:stupid: Sorry to rag on this, but your duration numbers are bogus. Also the 262/272 numbers are useless without reference to int. or exh. and at what lift were these numbers taken? Advertised, or actual timing figures? Different cam grinders will sometimes specify special lift numbers to degree a cam, and then advertise it with theoretical duration numbers. So, whenever you quote cam specs, be sure to get the whole story, or it just adds to the confusion.

The only way to compare cams is to note the lift (usually .050) at which the duration points were taken. The theoretical, or complete duration looks good on paper, but is not usefull. Because of the lash used in a running engine, the duration numbers (the amount of time/distance measured in degrees) will not be representative of the advertised numbers unless the cam is degreed with the lash specified by the cam grinder. Yes, it can be confusing. Just be aware of the facts, and don't throw out numbers without all the specifics to back it up.

Phred

[Deus Ex]

Ok i put my cam in and was recommeneded the following grind

Intake

open 26 Degress Bfore Top Dead Centre

closes 66 Degress Bfore Top Dead Centre

Exhaust

Opens 66 Degrees Bfore Top Dead Centre

Closes 26 Degrees Bfore Top Dead Centre

318 Tho (lift?)

so i guess that makes it 272/272 with 318 lift...does that sound about rigth?

What do you guys think?

[Phred]

Ok class, look closely at the figures Deus posted. All opening and closing events are BTDC. This is imposible. The numbers sound right. So this is the likely corrected cam specs.

Intake: opens @ 26 degrees BTDC (before top dead center)

closes @ 66 " ABDC (after bottom dead center)

add 180 "

total 272 degrees duration

Exhaust: opens @ 66 degrees BBDC (before bottom dead center)

closes @ 26 " ATDC (after top dead center)

add 180 "

total 272 degrees duration

Now to make these figures relavent, we need to know the lash setting, and the point at which the figures were taken. (Usually from .001 to .050 valve or lifter rise) Otherwise you cannot directly compare one cam to another. As each cam may have had readings taken at different points.

As far as your lift figure of .318, This is hopefully actual cam lift. If you figure the rocker ratio of 1.5 (this number will vary, depending on rocker contact point) you come up with .477 valve lift. (you will have to subtract valve lash from this number, so it should be closer to .467 actual valve lift).

There will be a pop quiz tomorrow, so I hope you took notes.

Phred

[Bambikiller240]

Phred, anytime you feel like posting knowledge like this to the Tech Articles section, please consider doing so. Stuff like this can help a lot of people (like me) to make wiser choices and fewer mistakes when considering engine build options.

Thank you for sharing your knowledge!

I hope the test is open book.

[san_maru_zed]

As far as your lift figure of .318, This is hopefully actual cam lift. If you figure the rocker ratio of 1.5 (this number will vary, depending on rocker contact point) you come up with .477 valve lift. (you will have to subtract valve lash from this number, so it should be closer to .467 actual valve lift).

There will be a pop quiz tomorrow, so I hope you took notes.

Phred

I helped Deus Ex put the cam in yesterday, and by the looks of wear pattern on the rocker arm pads (worn towards the lash pad on all of them) the ratio might be slightly higher. It already had bigger lash pads in it so we kept them there. The standard springs are not binding so Im guessing that the valve lift won't be much over .460".

[Deus Ex]

Ok Then,

The cam is in....We thought it was gonna be a mild one, but once we investigated the one i was replacing, we found that it already had a mild cam in it. So we put the new one in. Its quite lumpy now. after letting it warm up and deliberating if we needed to turn the idle up we took it for a test run. I put my foot down which would normally have an effect of pushing me back in my seat. But we got nothing.

We got to a big straight road, and as i kept my foot buried, it gradually reved. And reved. And reved. Im no officinado on L28's, but i havent seen many that rev like this.

So while i was disapointed with the loss in torque, the top end is something wild. So i gave my mechanic a drive, and from the moment he drove, things werent right. It wasnt reving, it stalled, and it wouldnt start again. So i had to run an errand, worried that my car had done its last run. I got the call and the news was something of a bitter sweet mix. It was alive again, but i needed a new air filter. Thats right. My ram filter was melted and sucked into carby number 1. Hows that for a vacum?

But after a thoughrough clean, its all good again. Its got alot more power, it wont drive niceley at constant revs, and it has alot less torque. It seems to be startving for fuel at high revs, since its probably never revved this hard, which is somthing ill have to look into. Silly me to think it was gonna be one of the last mods id do

Well see how she goes

[Gav240z]

You can get an electric fuel pump out of a Nissan Bluebird older model fitted with the L20 I believe. This is good for a carb'd Z.

WHat are you running tripples? or SU's?

Also how high is the compression? (Compression for torque related issues ).

[san_maru_zed]

Im posting for Deus Here,

Hes Got an electric pump, running Triple 45mm webers, 8.75 compression

[Phred]

By any chance did you degree the cam? Your combination of large carbs bigger cam and low compression is a recipe for no bottom end performance. If you have a stock cam sprocket, and installed on #1 hole, you can advance the cam 4 degrees by changing to the #2 hole. Advancing the cam will help pick up the bottom end performance. Conversely, retarding the cam will loose bottom end while picking up the upper end performance. Generally, race cams don't have bottom end power, so its common to advance them a few degrees from straight up. Stock cams have good bottom end, but usually very little top end. For example, formula fords have to run a stock cam. In order to get top end power, formula ford engines are commonly retarded 4 to 5 degrees. Sometimes more.

Phred

[Deus Ex]

We put on Number 3, we did it with the previous cam and lost alittle bottom end (minimal) and gained alittle top end. No i didnt expect it to have much down low, and we just talked to a tuning place in bris and they said i will definetly need new jets and a retune to get the best out of it. DAMN!!!