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Last Post 27 Oct 2014 05:05 PM by  t walgamuth
New C mod in the INdy region
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Dick Rasmussen
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20 Feb 2011 06:16 PM
t walgamuth wrote:

A couple of things:

1. What is the desired top speed we will need in a C mod?

2. My engine is set up to a Virginia FF club spec's which specify fairly weak valve springs which will allow float at 6500 RPM, so I am planning to use slightly taller gears to work with the rpm range which is possible. Of course I could install stronger valve springs too as far as that goes, but I don't know much about all the other variables that might be taylored to the rpm range it works at.

Thanks very much!

Tom,

Do you have a rev limiter?

You might check with a FF engine builder whether or not using valve float as an autox rev limiter is safe for the engine. In fact, maybe contact the pro builder who did your engine. For me at least, keeping the revs below redline (whatever the rpm) is pretty difficult in an autox.

However, I understand some (if not all) of the fastest CM guys do so it appears I may need to work on "plan B".

Jim's top speed info is consistent with my experience. I did have a local event many years ago that had me on the 6800 rev limit in a 78 mph 4th NUMEROUS times per run.



Jim G
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20 Feb 2011 08:09 PM
Dick Rasmussen wrote:

Do you have a rev limiter?

You might check with a FF engine builder whether or not using valve float as an autox rev limiter is safe for the engine.

However, I understand some (if not all) of the fastest CM guys do so it appears I may need to work on "plan B".




I use a rev limiter and would not ever think of using valve float as a "limiter". And I think the majority of C Mod guys use a rev limiter too. It's one thing to use valve float when you're at 110 mph with a 23/29 gear and the engine acceleration is, well ... "acceleration" isn't really a descriptive term in this situation. But it's completely different when you're using a 13/38 gear or even an 18/xx gear. Protect your investment. It's not expensive.
Dick Rasmussen
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20 Feb 2011 10:07 PM

If you do get a rev limiter there are several choices as discussed in this thread or elsewhere. The Pegasus catalog shows several brands that include "soft touch" electronic limiting. I used a now obsolete Microdynamics at first but since that one was not user adjustable switched to the MSD with the selector switch accessory that covered the target rev range. Summit sold it for less than Pegasus but they don't seem to carry "our rev" range selector switch. Since the MSD is very bulky/large I just bought the current Microdynamics unit which is small and frees up space for a data acquisition box when I get one.

I did try a rev limiting rotor from Ivey last summer but decided that I wanted the electronic "smooth cut" instead.

As usual, lots of choices.

t walgamuth
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20 Feb 2011 10:35 PM

The rev limiting rotor sounds like a good first choice for me. I can see that with our engine rpm range in autross it will be possible to hit the top rpm pretty hard so floating the valves might just float them into the pistons.

Thanks for the thoughts guys!

t walgamuth
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01 Mar 2011 04:19 PM

OK, i think i have figured out what gears I want to buy. I have a 19/32 which I'll use as fourth for now. I won't expect to use it much on course. I am looking for 14/36, 16/34 and 18/33 to buy second hand if possible.

Anybody have some of these to sell?

t walgamuth
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04 Mar 2011 07:37 PM
I looked in the Pegasus catalogue and don't seem to find any autocross gears available. Can someone direct me to a source for the correct gears please?
pru
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04 Mar 2011 08:15 PM
Taylor Race Engineering : Product Categories : Transmissions : MK Series : Gearsets MK
PCalhoun
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04 Mar 2011 10:23 PM

14:36 is too high for first...not matter if you have a 9:31 or 10:31 R&amp;P. As mentioned TRE is the place to buy gears; Pegasus only has a limited selection, -Pru- has the link above to the appropriate TRE catalog section. Optional REM coating also helps w/ gear life if you order from TRE..

Before ordering the ratios you have listed take a serious look at your proposed gears vs those I recommended in an earlier post. I can e-mail you an electronic Excel formatted gear chart to plot them if you supply your e-mail address.

jchiu
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04 Mar 2011 10:32 PM
PCalhoun wrote:

14:36 is too high for first...not matter if you have a 9:31 or 10:31 R&amp;P. As mentioned TRE is the place to buy gears; Pegasus only has a limited selection, -Pru- has the link above to the appropriate TRE catalog section. Optional REM coating also helps w/ gear life if you order from TRE..

Before ordering the ratios you have listed take a serious look at your proposed gears vs those I recommended in an earlier post. I can e-mail you an electronic Excel formatted gear chart to plot them if you supply your e-mail address.

I'm interested in this excel file too please. jimmychiu at comcast dot net

Dick Rasmussen
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05 Mar 2011 07:26 AM

Peter: I would like the gear spreadsheet also.

Tom: For what its worth - That 14:36 with a 10:31 r&amp;p is a 55 mph plus (at 6.8K) gear.

Been there, done that. It can "work" as an interim (budget realities) solution if you have a stock clutch due to the clutch slip needed to launch even on relatively low grip surfaces. Your engine/carb must work well at low revs also with the progressive secondary still functional (no "synchronizing linkage" on the carb). You won't like it since it will be like launching a street car in second gear. After my first Pro Solo which was on high grip concrete I changed to a 41 mph first. It turned out to be a little too short for my taste so I changed to my current 45 mph first.

Of course, my first drives with my car were at a road racing drivers school and subsequent races. First gear was at least a 78 mph gear but we did not do standing starts.

Dick


t walgamuth
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05 Mar 2011 03:23 PM

twalgamuth@comcast.net

In watching Mark launch last year my memory is that he had a lot of wheelspin in first and shifted to second right away and not long after that went to third. A little taller first seems like it might be useful. OTOH I don't know what gears he is running nor what diff.

Jimmy and I were thinking to try to set up the gears so we would not need to shift any more than needed. Fourth would be seldom used but second and third would be the ones to use on course.

Thnaks for the fast feedback guys!

t walgamuth
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05 Mar 2011 03:27 PM


Also we are planning to set up the gears to use the power generated by my engine based on the torque and horspower curves shown by the rebuilder. The engine is set up to have a max rpm of 6500 due to the virginia rules the PO ran under.

I am used to slipping the clutch a bit on takeoff. I suppose changing a clutch might be actually tougher on a FF than a Miata though.;)

Tom

Dick Rasmussen
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05 Mar 2011 07:11 PM

Tom,

Very understandable logic. Lots of tradeoffs and options based on the courses you usually run and personal preference. I did find that with fresh 25B's on a warm day on good asphalt that even my 45 mph 1st needed a lot of revs to gently spin the tires with a racing clutch. This year I will have a lightened flywheel also so it may be harder to launch.

How hard a clutch change is really depends on your car. However if you have a healthy stock clutch it will probably outlast the engine unless you literally "smoke it". My car is "relatively" easy to split for a clutch inspection/change or replacing the clutch slave cylinder seals. I literally support the rear module on a dolly and "roll" it back from the engine.

Dick

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05 Mar 2011 07:21 PM
t walgamuth wrote:

twalgamuth@comcast.net

In watching Mark launch last year my memory is that he had a lot of wheelspin in first and shifted to second right away and not long after that went to third.  A little taller first seems like it might be useful.  OTOH I don't know what gears he is running nor what diff.

Jimmy and I were thinking to try to set up the gears so we would not need to shift any more than needed.  Fourth would be seldom used but second and third would be the ones to use on course.

Thnaks for the fast feedback guys!

 



The part you are missing with the "extra" shift(s) of Marks car compared to a street car is just how much faster a shift can be made. Don't be afraid to shift these cars. Embrace it, it's just another part of making the experience that much better over a street car.

Even with Marks integral 1st gear without the wheelspin off the line the rpms will fall below the powerband and the motor will bog. There were events when I was struggling hitting the 1-2 shift I would come off the line in 2nd. In comparing to Mark using the maxQ he would beat me 3 tenths to sometime 1/2 a second.. That's a ton of time to be behind just coming off the line.
jchiu
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05 Mar 2011 10:48 PM
For those that are still looking for the excel file to calculate the speeds, I made a file. You can get it here:

http://web.ics.purdue.edu/~chiu2/RF...ratios.xls

For reference I have listed ALL of the integral first gear ratios available on TRE.
Edit the RED boxes to change final drive ratio, gear ratios, tire diameter as you want to play around with it.
t walgamuth
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06 Mar 2011 06:55 AM

Thanks all for the great advice!

Jimmy and I will probably get together later today and hash out what to buy.

Tom

PCalhoun
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06 Mar 2011 04:09 PM

For those that requested a copy of the Excel gear chart, which will show the speeds at a given rpm plus plot them on a graph be patient. The chart allows you to change gears, R&amp;P, max RPM, plus tire diameter to compare two different sets of gears. I am in the process of moving this week and they are on my computer which is currently down. I may have another copy here on a memory stick I will check. Nick you have the copy I gave you to share?

As to first gear selection and launch, first you need an integral first no matter the ratio. I you go w/ said 14:36 I guestimate you will lose .3-.5 seconds right off the launch while accelerating.to a shorter geared car. Second a slide-on 1st will not live more than a few events if you launch w/ any sort of authority. On launch you can slip one of two things to save your driveline; clutch or tires. With a race clutch installed tires will generally break loose first to 'cushion' the launch.on gears, CV's, drive &amp; stub axles, not the mention the R&amp;P. Just another reason to use Red Line Shockproof in the 'box too. If you saw Mark spinning the wheels in 2nd it was on asphalt not concrete; at Peru for example the launch RPM can be quite high to get R25s to spin and not bog.

How was the 6500 redline achieved for FRCCA; rpm limiter, rotor, or softer than stock valve springs? With stock valve springs it is not uncommon to go 6800 w/ spikes to 7000 while holding a gear to eliminate a shift. If nothing else you should be adjusting the valves as part of pre-season prep and checking the ignition components anyway. I wouldn't let 6500 stick in your head when choosing gears, use the std 6800, but if you have dyno sheets for the motor that is an important data point too.

PCalhoun
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06 Mar 2011 04:17 PM
Jimmy- Your chart is off as the rear tire diameter is incorrect; use 22.5" as the diameter when computing for a FF1600.
jchiu
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06 Mar 2011 06:20 PM
PCalhoun wrote:
Jimmy- Your chart is off as the rear tire diameter is incorrect; use 22.5" as the diameter when computing for a FF1600.

Yeah I noticed that when Tom and I were going over the ratios today. The 21" is the front tire diameter, 22.6" for the rear, though I guess 22.0" is probably close once you factor in the dynamic rolling radius. The file has been updated to account for the rear tire diameter.

Locked
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06 Mar 2011 07:21 PM
PCalhoun wrote:

For those that requested a copy of the Excel gear chart, which will show the speeds at a given rpm plus plot them on a graph be patient. The chart allows you to change gears, R&P, max RPM, plus tire diameter to compare two different sets of gears.   I am in the process of moving this week and they are on my computer which is currently down.  I may have another copy here on a memory stick I will check.  Nick you have the copy I gave you to share?

As to first gear selection and launch, first you need an integral first no matter the ratio.  I you go w/ said 14:36 I guestimate you will lose .3-.5 seconds right off the launch while accelerating.to a shorter geared car.  Second a slide-on 1st will not live more than a few events if you launch w/ any sort of authority.  On launch you can slip one of two things to save your driveline; clutch or tires.  With a race clutch installed tires will generally break loose first to 'cushion' the launch.on gears, CV's, drive & stub axles, not the mention the R&P. Just another reason to use Red Line Shockproof in the 'box too.  If you saw Mark spinning the wheels in 2nd it was on asphalt not concrete; at Peru for example the launch RPM can be quite high to get R25s to spin and not bog.

How was the 6500 redline achieved for FRCCA; rpm limiter, rotor, or softer than stock valve springs?  With stock valve springs it is not uncommon to go 6800 w/ spikes to 7000 while holding a gear to eliminate a shift.  If nothing else you should be adjusting the valves as part of pre-season prep and checking the ignition components anyway.  I wouldn't let 6500 stick in your head when choosing gears, use the std 6800, but if you have dyno sheets for the motor that is an important data point too.

 



I have the excel file, and have sent it to Tom.

Mark wouldn't have been spinning the tires off the line in 2nd gear...
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