Choosing gear ratios for a weekend racer/street car

[ManHunter]

I'm currently building a street-legal racing car with the following specs:

Motor: SBC V8 - 640 hp - 6500 rpm redline
Gearbox: Porsche G50/01 5 speed manual
Tires: 18" (26" diameter) stree tires

The car weight 2200 pounds.

I'm wondering if these gear ratios are acceptable:

1st - 2.43
2nd - 1.58
3rd - 1.29
4th - 0.97
5th - 0.76
Final Drive - 3.44

Is 1st gear too tall, given a 2200 pounds car and a 640 hp motor? Any gear too far apart?

 

[ads22]

First does look a bit tall, it's about like 2nd on my VW, but I think a lot will depend on the low RPM tractability of the engine, and exactly how/where the car will be used.

I didn't do the math for you, but ratio spacing looks reasonable by eyeball. You are generally looking for a decreasing % drop gear-gear as you go up through the gears. Sometimes accepting a much larger drop 1 > 2 in order to keep the other drops small, but still being able to easily get the car moving.

Sounds like a fun car!

Al Seim

 

[evelrod]

To start, 1st is okay. With that much power and only 2200 lbs, you could smoke the tires in any gear!!!

Some quick calcs:

Top gear gives 29.547 mph/1000 rpm---top speed 192.058/6500

Engine speeds in top gear:
60mph @ 2031 rpm---100 mph @ 3384 rpm

Top speed in 1st = 60.068 mph
And changes into 2nd @ 4226 rpm dropping 2274 rpm
Top speed in 2nd =92.383 mpn
And changes into 3rd @ 5307 rpm dropping 1193 rpm
Top speed in 3rd = 113.151 mph
And changes into 4th @ 4888 rpm dropping 1612 rpm
Top speed in 4th = 150.479 mph
And changes into 5th @ 5093 rpm dropping 1407 rpm
Top speed in 5th = 192.058 mph

All this discounts tire growth, slippage, etc.

Rod

 

[patprimmer]

2200 # sounds light for a street legal V8 car.

640 HP is a lot from a street-able SBC with 6500 red-line.

The gearing has a big jump from low to second and a small jump from second to third.

If you want to actually drive it on the street as opposed to simply being street legal, it will be very difficult to drive at very low speed (like walking pace) with a 60 mph low gear and a 640 hp small block..

If it is to be street legal, but absolutely no intention to ever be street driven, and if it really is 640 true hp at 6500 or less and it really is only 2200 # then as usual, Rod Is Right



Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ManHunter]

Thanks for the input guys! I'm gonna change 1st gear to 2.83. I should probably adjust 3rd gear too, the change from 2nd to 3rd is a bit too small. Probably 1.25.

The goal is to be able to drive it on the street.

It is a indeed 640 HP carburated, dry sump motor with an accusump to prevent oil starvation under hard cornering/acceleration.

Eric

 

[evelrod]

Just curious why you chose to us an accumulator with the dry sump and to what purpose. I use an electrically discharged Accusump on my wet sump engines but not on my dry sumped ones...Seems a bit redundant, IMO.

I just ran the numbers, but those ratios are a bit on the strange side. Perhaps they were designed for some special engine/car application? A 650/6500 SBC @ 2200 lbs. car certainly does not need a five speed to be affective on the street. Even the new Vette 6 speed I find myself going from 1st to 4th most of the time! (Sure wish I could afford one of my own).

Rod

 

[ManHunter]

About those ratios being a bit on the strange side. That's why I posted on this forum, to get some help adjusting them so they are correct for the car's intented use

So how streetable would 1st gear be with 2.83 ratio?

Eric

 

[Metalguy]

It sorta depends on how your engine runs at real low RPM, and what kind of traffic you'll encounter.

A 9.74 overall 1st gear isn't too bad *if* the engine doens't have a radical cam, etc. My exp. has shown that "street roller cams" make good low RPM power, at least in a BBC.

"When the eagles are silent, the parrots begin to jabber."
Winston Churchill

 

[evelrod]

Most racing gear ratios tend to have a smaller rpm change as you go up through the gears...Your setup seems to be pretty much the oposite. ???

You did not answer the question about the accumulator.

If your engine is a SBC, how much boost are you using to get 650 hp at less than 6500 rpm? (exotic fuel?)

Your going to race on 'street tires'? How many 'milliseconds' do you expect them to last?

I'm having some difficulty in picturing this setup. I've raced a 2700 lb. sportscar with a high horsepower (much less that 650) SBC with BIG slicks and it was a serious handfull if you made even the slightest mistake!!!

Rod

 

[ManHunter]

here's a revised gear ratio charts. It think this one is much better.

1st - 3.00
2nd - 1.79
3rd - 1.29
4th - 0.97
5th - 0.76
Final - 3.44

The motor gets peak power 640hp @ 6500 rpm. The main cause of failure for this particular engine is oil starvation, which should be mitigated by using a dry sump. The accusump is basically a cheap insurance policy against that type of failure, compared to the seriously expensive cost of the motor.

This is a link to the car doing 0-100mph-0, starting in 2nd gear

http://www.pistonheads.tv/clip99

 

[patprimmer]

Those ratios sound like it would be more tractable in slow traffic, but we are still guessing as we have no idea of the full torque curve and off idle characteristics of the engine.

Those ratios sound like a much more tractable street low gear plus an effective 4 speed racing box.

If it is an aftermarket high deck 400 odd cubic inch low compression turboed electronic injected SBC it will pull like a truck at low rpm.

If it is a short stroke SBC normally aspirated with big valves, ports and 290 deg 0.700" lift cam, with twin 4 barrel carbies, you will smoke a clutch any time you try to drive around a crowded shopping mall, or get stuck in stop start traffic on a steep uphill.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[evelrod]

Slight variations...I think I only took pi out to two places on the last post.

1st-48.648 mph
2622 rpm
2nd-81.584 mph
1816 rpm
3rd-113.205 mph
1612 rpm
4th-150.551 mph
1407 rpm
5th-192.151 mph

That looks better. I still think a properly engineered dry sump does not need the added complication of an accumulator with all the check valves and plumbing necessary to make it work in a dry sump application. If, indeed, it is necessary to prevent loss of oil pressure, I think you need to have an engine expert have a look-see. Something is not right. If you are shooting for a light weight like 2200 lbs, you certainly don't need the extra weight!

These are just my thoughts on the subject. I can and, often have been, found out in "left field".

Rod

 

[patprimmer]

In my opinion Rod is right in centre field.

I can't imagine a dry sump being accidently so badly designed that it would have an oil surge problem and therefore require an Accusump.

At 6500 it must be a very bad sump to have a surge problem even with a wet sump.

We run 6000 rpm in centre mount ski boats. The engine is in with the timing cover to the back of the boat, the engine slopes anout 10 or 12 deg, the low end is at the back of the boat, the crank only misses the hull by about 1.5" at the snout end and the ring gear has about 1" clearance on the hull. This greatly restricts the shape and depth of the pan. We only get oil pressure drop off on sudden stops. We get enough windage to present a pressure drop off problem at sustained speeds over 6000 rpm. At that stage we dry sump.

Maybe you have an oil pump that is to big and pumps the sump dry faster than the oil can drain back. Maybe you have to much bearing clearance somewhere, maybe you have a very shallow sump, maybe you have poor design in one or more of the following, baffels, gates, pickup, tray, scraper.

Don't get me wrong. Dry sump is good in low slung sports cars.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ManHunter]

You are right. The accusump is not necessary, and I probably won't install it on the car.

I only got the dyno chart for the second half of the rpm range right now, so I still can't tell how 1st gear is going to be like.

The dyno chart shows from:

RPM TORQUE HP BSFC OIL_TEMP WAT_TEMP
4484 562 480 0.51 145.5 137
4600 562 492 0.50 145.5 136
4700 555 497 0.49 145.5 136
4800 553 506 0.50 145.0 136
4900 557 519 0.48 145.0 136
5000 558 531 0.45 144.5 136
5100 561 545 0.43 144.0 136
5200 561 555 0.41 144.0 136
5300 560 565 0.39 144.0 136
5400 562 578 0.38 143.5 136
5500 559 585 0.38 143.5 136
5600 561 598 0.37 143.5 136
5700 556 603 0.37 143.0 136
5800 551 608 0.37 142.5 136
5900 544 611 0.37 142.5 136
6000 537 613 0.37 142.0 136
6100 532 618 0.37 141.5 136
6200 531 626 0.37 141.0 136
6300 527 633 0.37 141.0 136
6400 525 640 0.37 140.5 136
6500 524 648 0.35 140.0 136
6600 520 654 0.34 140.0 136
6700 518 661 0.32 139.5 136
6800 517 669 0.31 139.5 136
6900 509 669 0.30 139.5 136
7000 503 671 0.29 139.0 136
7100 498 673 0.28 139.0 136
7200 493 675 0.27 139.0 136
7300 485 674 0.26 139.0 136
7400 475 670 0.26 139.0 136
7500 465 664 0.24 139.0 136
7600 450 651 0.25 139.0 136
7700 401 587 0.27 139.0 136

 

[patprimmer]

Is this a simulation or is it measured on a real engine.

If it is a simulation, you have top end power you are wasting with the 6500 red line and I would alter things to get more bottom end with less top end.

If you have the engine already, why not increase the rpm of the red line and go a bit shorter in the final drive.

It does seem to have a nice wide torque band in the figures shown.

I would still like to know what displacement, what induction, what cam duration and actuation, what fuel distribution and metering (carbs, injection etc)

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[evelrod]

I'm with Pat on this one. Is this a 'real' car or just a study project? I have, in my mind's eye, a picture of a Lotus 7 with a gorilla 500 inch SBC out front! Perhaps an old F-5000 with a "Devin" body? For the street??? 2200 lbs is mighty light for any real race car with a 'Chebby' in it, IMO. I've seen suicide T-buckets that were 1500 lbs. but I would not want to try to turn or stop with one of them. I was next to one on the strip when I was a kid and it lifted the left front wheel all the way down...not my cup of tea. Street/race? Tell us more.

Rod

 

[ManHunter]

The dyno numbers I posted are from an actual run of the motor on an engine dyno.

Motor specs: 10.5 compression, CNC ported cyl. heads, Edelbrock 2950 manifold, Comp Cams CS258/264 roller cam, .020"/.022" valve lash, Holley 750 cfm carb, 1 7/8" diam. headers, Jesel rocker shaft, SCP 3 stage dry sump.

The redline will be bumped up to 7300 rpm, maybe 7500.

The car is the Ultima GTR. More on it at:

http://www.ultimasports.co.uk/gtr/downloads.html

 

[evelrod]

Ok. I like it. I think as a full blown race car it would need a LOT of development...the class for this car is pretty heavily over run by OEM and REALLY high dollar teams.
It sure would be fun if I were in my 20's again, even the 500+hp Pantera was a bit much for me on the street in my 40's(terrible brakes). Still, the GTR is a nice looking kit. I think I would opt for a bit more "luxury (LOL)" accomodations with the kit and, perhaps a stock OEM Z06 for the street. Free shipping and insurance to the U.S., too!
Does it come with AC? ;o)

Cheers

Rod

 

[ManHunter]

Indeed, AC is an available option. Heating is an option too

As I mentioned, this will be a weekend racer, not a full blown race car. As for the streets, the Ultima outhandles and outbrakes the Pantera, hopefully I'll be able to tame the beast when street driving. ;-)

Since I'm building the car myself, I can put in all the luxury accomodations I want. But that adds weight.

 

[NormPeterson]

Plan on including a good bit of progression in the throttle mechanism or it will be quite difficult to drive smoothly in traffic. A quick and dirty estimate for acceleration potential in first gear at only 25% of the torque values provided above is approximately 0.5g. A little surprise bump could have you suddenly at a significantly higher torque value. A generally similar project that got magazine attention some years back used something like 8" throttle travel to achieve the same thing.

There is one possible value that I can see for an Accusump in combination with a properly thought out dry sump system, and that is actually for something with significant street duty intention as a short-duration emergency oil supply in the event of dry sump pump drive belt failure. Tied somehow to fuel/ignition shutoff in the event of oil flow out of the Accusump, it should provide protection long enough to take all the load off the engine and let it stop. It's one thing to lose oil pressure at the track, coast or get towed into the pits, roll the thing up onto the trailer, and go home. Quite another to have it happen out in the middle of nowhere, with no idea if the available towing service can properly handle the job. A spare belt and a wrench or two would suddenly be seen as valuable ballast in such instance.


Norm