Pelican Parts
Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   Pelican Parts Forums > Porsche Forums > 911 Engine Rebuilding Forum


Reply
 
LinkBack Thread Tools Rate Thread
Author
Thread Post New Thread    Reply
Registered Loser
 
Join Date: May 2001
Location: Worcester, MA
Posts: 2,392
Newbie Question: Can Cams "Shape" The Torque Curve?

I have been wondering...if you were building your motor strictly for street use (or maybe street and infrequent autox) and you wanted to maximize low end torque for acceleration...would it be possible to achieve this effect using only a custom cam grind? Can one lower the peak torque point to 3000-ish rpms? I know this may be sacreligious but I am curious about how cams influence the torque curve.
__________________
Janus Cole
1989 911C4 (Das Drehkraftmonster)
1987 944
1973 911T Targa
Old 01-29-2006, 02:16 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #1 (permalink)
Moderator
 
Bill Verburg's Avatar
 
Join Date: Dec 2000
Posts: 23,181
Garage
to a point yes, that was the way that the old T, E, and S models were differentiated, there were also differences in cr and port flows. But the primary determinant of the character of the torque curve was the cam.

Displacement of course would be another way to modify a torque curve, that was the path used on the '73RS wich had the same cams and heads as the S of that year but had considerably more torque due to the extra 300cc of displacement.
__________________
Bill Verburg
'76 Carrera 3.6RS(nee C3/hotrod), '95 993RS/CS(clone)
| Pelican Home |Rennlist Wheels |Rennlist Brakes |
Old 01-29-2006, 05:56 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #2 (permalink)
Registered User
 
Join Date: Jul 2000
Location: So. Calif.
Posts: 18,301
A simplified answer for more midrange torque would be higher lift and less duration. Higher compression ratio and cubic inches do wonders too.

Sherwood
Old 01-29-2006, 09:36 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #3 (permalink)
Registered User
 
Join Date: May 2003
Posts: 3,232
Yes the cams have a big effect on the torque curve. For best acceleration you need the torque at higher RPMs. Acceleration is a function of HP. Big torque will only get you off the line fast. For example an F1 car with a 3 litre engine probably has less than 250 ftlb of torque. It gets that torque at 12000-18000 RPM. This results in 800 HP and amazing acceleration.

Think of it this way would you get better acceleration at 60mph in second gear or third gear (assuming your engine can rev that high). If you had that F1 motor you could be in second gear at 150 mph!

-Andy
__________________
72 Carrera RS replica, Spec 911 racer
Old 01-29-2006, 10:26 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #4 (permalink)
Registered User
 
Join Date: Jul 2000
Location: So. Calif.
Posts: 18,301
"For best acceleration you need the torque at higher RPMs."

However, you first have to get the engine to high rpms if that's where the torque is, and unless your gearbox has the proper gearing (low gear ratios), it's going to take awhile to get the revs, thus the need for torque at a lower rpm. "Low" is a relative term. For our engines, low speed would be around 2000 - 2500 rpm. An engine with the proper valve timing (among other variables) can help produce more low-end torque for your particular needs.

An F1 engine has a very narrow (and peaky) torque range, thus the need for a 6 or 7-speed gearbox to keep the revs in a certain rpm range for the best acceleration and throttle response. For a street engine, the ideal is a wide and flat torque curve that begins at a relatively low engine speed along with enough breathing to continue creating power at high rpm. "High" is also a relative term. 7,000 - 8,000 would be considered high for our engines. 7K rpm might be off-idle speed for the current 8 cyl. F1 engines that are already into the max. 20+K range. That's high.

Modern engines with variable valve timing and valve lift attempt to optimize horsepower and torque at both ends of the rpm range. Our old-design, fixed valve timing engines must be content with a compromise between bottom and top ends. Like I said, displacement and compression ratio do alot to minimize the compromise.

Sherwood
Old 01-29-2006, 11:00 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #5 (permalink)
Registered User
 
Join Date: Oct 2001
Location: Momence, IL 60954
Posts: 1,854
To further extend Sherwood's simplified answer, properly set up heads are also worth their weight in gold and greatly influence the performance of any given cam. In an ideal world, you would set up the heads and have a custom billet grind made up for the cams. We do this all the time with the Porsche 914. If I'm not mistaken, don't you do this Camgrinder? And yes, compression and displacement help tons- why do you think 2.6+ litre type 4s are so popular, at least after we made Nickies for them. :-)
__________________
Charles Navarro
President, LN Engineering and Bilt Racing Service
http://www.LNengineering.com
Home of Nickies, IMS Retrofit, and IMS Solution

Last edited by cnavarro; 01-30-2006 at 06:12 AM..
Old 01-30-2006, 06:03 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #6 (permalink)
 
Registered User
 
Join Date: Jul 2000
Location: So. Calif.
Posts: 18,301
"...To further extend Sherwood's simplified answer,...."

Simplified answers from a simplfied mind. Glad to assist.

Sherwood
Old 01-30-2006, 08:48 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #7 (permalink)
Registered User
 
camgrinder's Avatar
 
Join Date: Feb 2004
Location: California
Posts: 921
I can make a set of cams to move the powerband down. The trade off is losing high rpm hp. If you want 10 ft lbs of torque at 3000 rpms, it make cost you 15 hp at 6500.
The octane requirement of the engine will go up, due to the increased cylinder pressure.
On an auto-x car this might work well for short, tight corner tracks.
__________________
John Dougherty
Dougherty Racing Cams
Old 01-30-2006, 09:47 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #8 (permalink)
Reply

Thread Tools
Rate This Thread
Rate This Thread:

 


All times are GMT -8. The time now is 04:15 PM.


 
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page
 

DTO Garage Plus vBulletin Plugins by Drive Thru Online, Inc.