OK, no arguements here, just wanted to know why.
If you speak to general race motor builders/degigners they'll tell you a ratio as close to 1.9 would be ideal because it is the ratio where there is the least amount of side loading on the piston skirts. Most VW guys are running something like a 1.6 to 1.7 ratio.
And that 280 deg duration is about the max for a "street" car.
I know a short rod improves torque and therfore enables us to use a larger cam.
What would happen if we all fitted longer rods and slightly smaller cams??
Ideas??
Rod Ratio/Cam choice
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McBernie
- Posts: 62
- Joined: Thu Nov 22, 2001 12:01 am
Rod Ratio/Cam choice
I believe long rods work best with high compression and side loading on the piston only becomes a real problem at high RPM.
Smaller cams have less bleed off at lower RPMs so you could get away with a lower static compression ratio.I think a long rod,smaller cam and lower CR would work OK.By lower I mean under 10 to 1
Smaller cams have less bleed off at lower RPMs so you could get away with a lower static compression ratio.I think a long rod,smaller cam and lower CR would work OK.By lower I mean under 10 to 1
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chad
Rod Ratio/Cam choice
A longer rod is going to reduce the side load on the piston,however, a shorter rod that gives you a ratio closer to 1.6 will increase your piston velocity and increase torque in the lower rpm range. I have heard that 1.6 is as low as you want to go for reliable street use. When using shorter rods like that you also have to make sure you have clearance between the crank/rod and the piston skirt. As for cams, 280 degrees isn't that much duration. Consider the Engle 120 cam with an advertised duration of 294 or the Engle FK-8 with 298. Both of these cams have been used for years on street cars with excellent results, you just have to consider some things like engine size, compression ratio, heads, carbs, etc. Another thing to watch for is if you are looking at an advertised duration or a duration at .050" of lift.
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fugnbug
- Posts: 69
- Joined: Tue Oct 03, 2000 12:01 am
Rod Ratio/Cam choice
The limit for drivability is around 300 degrees or less.
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1964 Bug
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*always under construction*
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1964 Bug
1966 Bug
1967 Bug
1974 Bug
*always under construction*
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danimal
- Posts: 958
- Joined: Wed Jun 13, 2001 12:01 am
Rod Ratio/Cam choice
max, here is some math on rod ratios:
http://www.teknett.com/pwp/drmayf/rods.htm
dan
oceanstreetvideo.com
http://www.teknett.com/pwp/drmayf/rods.htm
dan
oceanstreetvideo.com
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JohnConnolly
- Posts: 3336
- Joined: Thu Apr 13, 2000 12:01 am
Rod Ratio/Cam choice
Interesting link Dan.
6 mos ago I did the math on this (my rod ratio article in progress) to see what happened, and I was quite surprised to find that the difference in piston position from "short" to "long" rods in an AC VW engine was approximately 2 crank degrees! All that fuss over 2 degrees!
I didn't believe it, and had to check about 6 times to see if I made a mistake (no). So, from an ignition timing standpoint, a short and long rod engine differ in ignition timing by 2 degrees at most, but the long rod has more leverage on the crank, since it's pressure is delayed this much.
John
Aircooled.Net Inc.
6 mos ago I did the math on this (my rod ratio article in progress) to see what happened, and I was quite surprised to find that the difference in piston position from "short" to "long" rods in an AC VW engine was approximately 2 crank degrees! All that fuss over 2 degrees!
I didn't believe it, and had to check about 6 times to see if I made a mistake (no). So, from an ignition timing standpoint, a short and long rod engine differ in ignition timing by 2 degrees at most, but the long rod has more leverage on the crank, since it's pressure is delayed this much.
John
Aircooled.Net Inc.
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Shad Laws
- Posts: 1605
- Joined: Fri Aug 24, 2001 12:01 am
Rod Ratio/Cam choice
Hello-
6 mos ago I did the math on this (my rod ratio article in progress) to see what happened, and I was quite surprised to find that the difference in piston position from "short" to "long" rods in an AC VW engine was approximately 2 crank degrees! All that fuss over 2 degrees!
Yup! A few months ago I did a similar thing, and computed distance, velocity, and acceleration of the piston given a constant rotational speed. It did make a difference with the latter two... the problem started out involving piston offset, too (I was curious what it'd do...). But, it got VERY complex VERY fast. If you eliminated it as a variable, it became much simpler. Accelertion had interesting results... I'll have to dig them to remember offhand.
I didn't consider a torque analysis... interesting idea...
The equal area conclusion (5) is because energy is indeed conserved. Piston force times piston distance must equal the area under the torque curve over rotational angle. Since stroke and BMEP never changed, neither would the area.
Take care,
Shad
6 mos ago I did the math on this (my rod ratio article in progress) to see what happened, and I was quite surprised to find that the difference in piston position from "short" to "long" rods in an AC VW engine was approximately 2 crank degrees! All that fuss over 2 degrees!
Yup! A few months ago I did a similar thing, and computed distance, velocity, and acceleration of the piston given a constant rotational speed. It did make a difference with the latter two... the problem started out involving piston offset, too (I was curious what it'd do...). But, it got VERY complex VERY fast. If you eliminated it as a variable, it became much simpler. Accelertion had interesting results... I'll have to dig them to remember offhand.
I didn't consider a torque analysis... interesting idea...
The equal area conclusion (5) is because energy is indeed conserved. Piston force times piston distance must equal the area under the torque curve over rotational angle. Since stroke and BMEP never changed, neither would the area.
Take care,
Shad
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GDRBO
- Posts: 2574
- Joined: Tue Jun 27, 2000 12:01 am
Rod Ratio/Cam choice
Its like 4:30 AM now, so please help me out. What is BMEP?
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Shad Laws
- Posts: 1605
- Joined: Fri Aug 24, 2001 12:01 am
Rod Ratio/Cam choice
Its like 4:30 AM now, so please help me out. What is BMEP?
Brake Mean Effective Pressure. It's actually not the best method of measurement for loads - Indicated Mean Effective Pressure is better, although more difficult to get.
Basically, if you had an engine with 100% mechanical efficiency (no friction, no alternator, etc.), it is the average pressure that each piston would see. This is an average over all 720 degrees of the cycle, including the vacuum of intake, the low pressure of exhaust, the moderately low pressure of compression, and the explosively high pressure of the power stroke.
For a given engine, BMEP is proportional to brake torque... just a different way of showing it. That's why it's so easy to get: get the torque curve from the dyno, divide or multiply by some constants having to do with engine geometry and voila! The result is BMEP, which is a decent benchmark indicator for how well any group of engines with the same type of induction (alcohol vs. gas vs. turbo vs. normally aspirated vs. NOS, etc.) of any displacement or number of cylinders breathes at different RPMs.
Take care,
Shad
[This message has been edited by Shad Laws (edited 12-08-2001).]
Brake Mean Effective Pressure. It's actually not the best method of measurement for loads - Indicated Mean Effective Pressure is better, although more difficult to get.
Basically, if you had an engine with 100% mechanical efficiency (no friction, no alternator, etc.), it is the average pressure that each piston would see. This is an average over all 720 degrees of the cycle, including the vacuum of intake, the low pressure of exhaust, the moderately low pressure of compression, and the explosively high pressure of the power stroke.
For a given engine, BMEP is proportional to brake torque... just a different way of showing it. That's why it's so easy to get: get the torque curve from the dyno, divide or multiply by some constants having to do with engine geometry and voila! The result is BMEP, which is a decent benchmark indicator for how well any group of engines with the same type of induction (alcohol vs. gas vs. turbo vs. normally aspirated vs. NOS, etc.) of any displacement or number of cylinders breathes at different RPMs.
Take care,
Shad
[This message has been edited by Shad Laws (edited 12-08-2001).]
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flying tomato
- Posts: 81
- Joined: Fri Mar 23, 2001 12:01 am
Rod Ratio/Cam choice
Hi Max,
Are you just talking theory or is this an engine for your ruby red car?
cheers
Are you just talking theory or is this an engine for your ruby red car?
cheers
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max, Der Bahnstormerz
- Posts: 195
- Joined: Thu May 10, 2001 12:01 am
Rod Ratio/Cam choice
Mike,
It's just one of the many ideas swirling around in my head. Has anyone over there tried it?? I have a cutomer who is a Forumla 1 engineer/desinger and I just happened to have a 2054cc motor on the counter in the shop and I asked him to have a general look
over it and the first thing he asked me was what rod ratio i was using, I replied 1.66ish and he looked at me a bit odd, and said it was a little on the low side if all I was after was maximum HP.
Anyway, how are you Mike?, didn't get much time to chat to you all last summer. I had Caroline with me, she was getting bored with me talking to "all these guys", ce la ve.
It's just one of the many ideas swirling around in my head. Has anyone over there tried it?? I have a cutomer who is a Forumla 1 engineer/desinger and I just happened to have a 2054cc motor on the counter in the shop and I asked him to have a general look
over it and the first thing he asked me was what rod ratio i was using, I replied 1.66ish and he looked at me a bit odd, and said it was a little on the low side if all I was after was maximum HP.
Anyway, how are you Mike?, didn't get much time to chat to you all last summer. I had Caroline with me, she was getting bored with me talking to "all these guys", ce la ve.