Was curious if anyone has done much about improving low end horespower. With the GT mapped eproms my car takes off like a race engine coming on cam just below 4k rpm but below that is a little lacking.

I suppose car weight is a factor (probably my weight too) but I'm not sure what I would sacrafice and still have the car visibly stock. I'm fitting the Motorsport X pipe next weekend so that'll free up some HP.

Has anyone installed a GT rear end in a S4 to get the higher gear ratio?

Don't want to be rippin' the tyres off from a standing start but would like a greater push back in the seat when flooring the go pedal at 120kph.

 

Advancing the valve timing will move the start of useable torque down a few hundred rpm per degree advance. It should give you higher torque too (trade for max HP).

 

And.....Bigger throttle body 70-80 mm, Borla exh, cat by pass if you can, advance the ign timing 4 degrees. Make sure their's no vaccum leaks, install an electric engine fan.

 

If yours is an automatic, your problem might be a simple as your 928 starting out in 2nd gear. The 928 automatic is adjustable in both starting out in 1st gear and in shift points and hardness of shifts. Porsche set the 928 automatic primarily for gas mileage and tend to be set very cushy.
If you have a manual, just slide the clutch and you never are at low rpms unless you want to be.

 

A bigger throttle body helping lower-end performance? News to me.

 

It is an auto (see the sig) and yes it starts out in 2nd most of the time but thats not where the problem lies. The auto box shift points are at the wrong road speed between 2nd and 3rd for my style of driving. Accelerating in 2nd is OK as the cars hits 140kph before upshifitng to 3rd but its when its in 3rd is where the problem lies as at 130kph it wont downshift to third and its only revving at 3k............its akin to turbo lag before the magic 4k is reached and off she goes. So other than stripping the tranny and playing with the gear ratios would I get a large difference if I installed the higher ratio rear end...........anyone done it?..............how much work?

 

I thought some adventurous soul once talked about manipulating a vacuum modulator on the transmission that would change the timing of the downshifts...my memory is foggy though, maybe it only changed the quickness of the shift and not the shiftpoint. I know that adjustable modulators are available for US cars, wonder if its the same principle.

 

The modulator, as I understand it, adjusts the strength of the shifts. Thus, they can be hard or soft. Mine shifts with just enough hardness to almost spin the tires and will spin them if I give more throttle or am in a corner. Or I can give moderate throttle and the shift is seamless. I decide how hard I want it to shift and it can shift so hard and quick I doubt I could match it with a stick.

My car also has torque from about 3000 rpm up, and that seems to be a "magic number" with all Porsches I have owned. Below 3000 rpm, the engine is sluggish, and comes on the cam around 3000 with a noticeable surge in power. My original 1600S coupe was this way, my "66 911 with Solex carbs and cams was this way, my 911Es were both this way, my modified Speedster with lots of tweaks and outrageous cams was also this way, my 914/6 with 2.0L engine with E cams was this way, but with Normal cams was NOT this way and had power lower around 2300 rpm. But that is the way 911 Normal engines were. With the 2.7L engine in the 914/6, it was just below 3000 that acceptable power was found. All of them were stick except my 928, and all of them required some clutch feathering to get off a line without bogging or lugging the engine. All of them required use of the shifter to pick a lower gear when accelerating versus cruising if you wanted to stay on the cams. The magic number was 3000 rpm, and if you fell below it you needed Viagra or something like a tow to get back under reasonable acceleration. The 928 is no exception. Once at 3000 the nose picks up and it changes character.

I suggest just flicking the gear lever down into 3 if you want to accelereate or using your kickdown switch. When I get on an on-ramp or in the mountain twisties, I flick the lever down into 3 and the rpm rises to around 2800 and I'm good to go. If I need more, I use the kickdown switch. Then the grin comes across my face.

I must also say that my understanding was the larger the intake throat diameter, the higher the useable torque was realized. Low end torque is enhanced by smaller intake size while larger intake size raises power to a higher rpm with a resulting sluggishness at lower rpm. Other variable come into play though, so this is just a general principle. The factory has already tested the cam-intake valve-throttle body size combinations and options to reach the optimum in driveability while still maintaining acceptable throttle response. Change one variable, and something somewhere else in the power band will change, so it will be a tradeoff if you increase the size of the throttle body; some other characteristic will suffer. Remember that the factory has spent hours and hours of testing the combination of variables to reach those you have in your car. As dramatic evidence of this, watch a Formula One car trying to move forward if it is under 4000 rpm; probably won't move at all, but at 14,000 rpm it starts to scream. Just a little bit too "peaky" for town driving.

 

I tried Garth's "Kickdown Bypass Quickie Trial" and loved the effect so much I could never bring myself to undo it. Now, my shark always seems to be in the "right gear" - 2nd gear starts and low-rev shifts when I'm behind a school bus, 1st gear starts and late shifts when I want to have some fun.

However, I share Malcom's curiousity about a higher geared rear end. What is involved and how much does it cost to change? With the current gearing I often feel like 1/2 of my engine's power is waiting for me to drive above 100 mph, which ain't gonna happen in a metro area like Wash DC.

 

Pappy installed a different torque converter that he said made all the difference. Do a search on them and find the discussion. Was onlt like 500 bucks if you don all of the install yourself. I want to do it to my car. I need more low end to have fun.

 

All that converter does is change the stall speed. Throttle response once in the effective torque curve will be quicker with it. But you still need to be in that effective range. My understanding is that the effective torque range for that converter mod is higher than for stock. I decided against it when I rebuilt my transmission. What I really wanted was a LOWER final drive ratio; I have a 2.2 ratio, but, yes, above 100 mph there is ample power and the car seems designed for optimum operation at and above that speed. I have no trouble pegging the speedometer with at least 800 + rpm left to go. That isn't always possible on the way to the grocery store though, so I thought a 2.72 final drive would be more appropriate for driving through the 'burbs. Mark at Devek set me straight about that desire by informing me that the ratios are all different in the later 2.7 boxes, so I would have been better advised to swap the entire box. I opted to stay with my box and am glad I did. It suit me well, probably because I am not a drag racer, but prefer GT qualities.

 

"My car also has torque from about 3000 rpm up, and that seems to be a "magic number" with all Porsches I have owned."

My 'magic spot' is 2750rpm. From 2750rpm to about 6200rpm the car pulls hard as hell.

 

Ron is on to something with the ratio thing- if you just change the final drive you won't be happy- you need the box aswell. I've looked into it, because I thougt of changing my rear drive. I now have the 2.75 (33/12) a can't floor it in 1.st( at 4k rpm it spins). But when I started took check gear ratio's etc. I found out that 1.st and 2. would not be that much different from what I have now, but 5.th would be "longer", but I don't need that. But I got a great little program from Erkka called cartest, in witch you can test how you car will perform with different gaerratio's....

 

I read with great interest the power you folks have at lower rpm and makes me think I have another issue............I've never been able to spin tyres, even in first but yet the car has good top end power easilly hitting 250kph when wound up.

So where's the low end grunt gone? If its plugged cats or a bad O2 sensor both are being replaced this weekend............bring on the 'X'

 

Advance your ignition timing(via dist on 84 or older, or via chip on 85 and up).

Made a huge difference in low end power on mine.