Cam duration advice

VBCurtis914 · Post by **VBCurtis914** » Mon May 14, 2001 8:01 pm

I've spent most of the last 2 weeks wandering search engines for T4 advice, finally landing at this forum. I've read most of the posts, and understand that WebCam is the maker of choice for most of you (at least those who follow Jake). I've also seen that having them custom-grind a cam is a common choice for you all, one I'm willing to do as well, if necessary.
My main question: what does a split-duration cam accomplish? I've guessed that split-duration means exhaust duration is longer than intake, but what benefit does that have? If it's an obvious benefit, why isn't a split-duration more common?

VBCurtis914 · Post by **VBCurtis914** » Mon May 14, 2001 8:02 pm

My second question/request for advice: What cam should I use?
I am building up a 1.7 914 motor, for use in my '73 914. I already have IDF44's, but am otherwise starting with only the case and 914 heads (which will get bigger valves/porting/etc). I'll leave off reasons to keep this short, but I wanna go with 96x76, and want a high-revving motor (7K rpm ideally as redline, HP peak around 6K). I'm willing to sacrifice sub-3K power to get that, and a lumpy idle doens't bother me. All my (newbie) cam research has shown duration of 286-294 to be the range to get what I want.
So, a series of questions, all related to acheiving 7K rpm from this motor. Would dropping stroke to 74 help? how esoteric/pricey/lightweight do I have to go on lifters/rods/etc to get 7K? If I do enough to the heads, can a milder cam (286ish) make power that high? I've read somewhere that lift below .480" is easier on the rockers/valves. If I stay in the .460-.480 range, do I have any chance to get what I want?
I'll be happy to talk about "why" for this 7K thing, but want to keep this a readable length.
Thanks ahead 'o time.

Joe Del · Post by **Joe Del** » Tue May 15, 2001 7:54 am

Okay, I'm curious...why is 7,000 RPM so important? My first inclination is to recommend a Type 1 motor if your looking for rev's that high. The T4 already has a very wide power band. It's really strong all the way up to 5,000 RPM. I've been playing around with a desktop dyno just to see where I can expect the performance of my 2270cc (78.4 x 96) motor to compare to other daily drivers on the road today. To make a long story short, I expect my car to fall right between a '91 Acura NSX (14.0 @ 103.5) and a '93 Camaro Z28 (14.6 @ 96.3). I'm also using a set of Porsche 2.0 liter 914 heads (44x36) with a split duration WebCam (.500 lift, 284/300 duration) and 45mm Dellorto carbs.

VBCurtis914 · Post by **VBCurtis914** » Tue May 15, 2001 7:06 pm

Allright, scratch the 7K thing. I feared that would be the answer, so asked it first to get it out of my system.
So, what's the benefit of the split-duration 286/300 cam versus a plain old 286 duration, for a high-power motor like I want to build?
Is 5K roughly where max power is on yours, Del?
How much low-end smoothness/power do you give up moving to a 290 or 294 cam?

VBCurtis914 · Post by **VBCurtis914** » Tue May 15, 2001 7:17 pm

To answer the 7K thing, I grew up on Honda "performance". Though they're not exactly fast, I always admired the engineering Honda did to achieve high engine speeds in their cars. I aimed for 7K simply because I understood and agreed with the tradeoffs Honda makes for HP, and was willing to make the same tradeoffs if similar rules apply to AC motors. Naturally, a 30+ yr-old design doesn't rev as well as modern rice-rockets, nor does it need to for high performance, as 1/4 mile times ya'll achieve indicate. Since it was essentially a whim to aim for 7K, I'll easily drop it-- I already have a T4 base to start from, so I'll do what's best for a hot-street setup with that.

Joe Del · Post by **Joe Del** » Wed May 16, 2001 6:23 am

My 2270cc motor is currently under construction, so I can't back up the desktop dyno figures...yet. However, previous posts confirm that the quarter mile times of the desktop dyno results are VERY accurate.

I'm using the 163/86B cam, cam gear, lifters and dual springs. All these components came as a set. Redline will be around 6,500 RPM. I don't expect much (if any) power increases above 5,000 RPM, so yes, I would consider 5,000 RPM to be the shift point and max of the power band. There are other variables you need to consider like head work, valve size, carb or EFI, exhaust flow restrictions, etc.

It's important to note that my engine is being set up as an "every day" daily driver. I don't plan on overstressing any of the engine components. I'm also expecting 28 to 30 MPG (highway) and well over 100,000 miles. Not bad for a low to mid 14 second quarter mile!

davebuckholts · Post by **davebuckholts** » Wed May 16, 2001 9:54 am

I've ported (mildly..small D-port) and polished my exhaust ports on my 1.8L heads. Should I now take that into consideration when figuring the duration time of the exhaust lobes? I'm just thinking that if the duration is working on a stock head then maybe it would be a tad long for a p & P'd head? Make sense?
Dave

Hot Wheels · Post by **Hot Wheels** » Wed May 16, 2001 3:47 pm

I am Using the same WebCam as Joe but with Quite different expectations. Using the lightweight 39gr. lifters, Aluminum pushrods, mildly ported heads and intakes and Pauter rockers i am building 66X104 to go the a Max of 8500 RPM. Certainaly the Max of the power curve will be below this point but WebCam did not say it would peak out anywhere near 5K. I would expect more like 7-7500 peak HP. Even the ill fated Scat witht the heavy lifters puled some serious R's and pulled high into the band it just didnt survive the experiance. In my case the short stroke and large bore helps the RPM range go higher. I am also fully aware that this is not a daily driver and will not likly see 10K miles in my application. Although durability should be better that the type 1.

I guess what im saying is that RPM's can be had, they are not cheap but if you need them for the application it can be done. In my case with the low gears and Large paddle tires thing work best with a 6K hole shot and 8k shifts. A very torqe oriented motor is a wonderful thing but not for My setup and driving style.

I would also concede that i could probably do the same thing with a larger stroker motor and not nearly so many r's with the proper gearing and tire combos.

Sean

stevestromberg · Post by **stevestromberg** » Wed May 16, 2001 9:58 pm

What are you trying to use this motor for? A 914? If so Are you going to race? Auto-X?
Steve

VBCurtis914 · Post by **VBCurtis914** » Thu May 17, 2001 12:40 am

This motor is going into a 914. The car will see mixed use-- 4-6K miles a year on road, and eventual auto-X or track use, once I think I "know" how to drive it. I am NOT interested in drag racing, and don't imagine I'd ever launch the thing higher than 2500 RPM, so I suppose my use fits pretty darned well into Jake's 2270 motor.
Before finding this forum (where, BTW, I think I've learned more than from the rest of the Web combined-- thank you all), I'd imagined something like Jake's motor, with a little more aggressive head work-- 48x38 valves, the lightweight lifters, dual springs if necessary, etc... Jake's results with 44x36 valves and single springs have me thinking that was a little over-the-top. I already have IDF44's, so no EFI for sure. 96mm pots are a certainty, but I'm still deciding on the stroke-- I'd thought 74 or 76 to help with the revs, but the cc's of 78 probly are worth more than the RPM I'd gain from a shorter stroke.
The cam choice seems the most difficult-- which is why I started my questioning with that topic. I'm sure I'll have plenty more, as time goes on... the split-WebCam seems the right choice, now that I understand a bit more about what's reasonable to expect out of this motor.
Is desktop dyno shareware/downloadable? I'd love to toy with that, see what valve sizes/lift/etc do, since I have NO experience building motors.

Joe Del · Post by **Joe Del** » Thu May 17, 2001 7:17 am

This one's really cool.
http://www.miata.net/misc/software.html

Click on car test to download.
You can play around with bore, stroke, gear ratio's, tire size, etc. Unfortunately, it isn't sofisticated enough to input valve sizes, cam lift and duration, etc.

[This message has been edited by Joe Del (edited 05-17-2001).]