Recessed engine mounting (into the firewall)

[z1 zonly]

It's been a long term goal of mine to mount an engine way back into the firewall--literally into the cabin. For example:

 

 

 

I'm confident that I could actually accomplish this now, but I recognize that it creates a whole slew of jobs that need to be done simultaneously--moving the driver position back just as far, rebuilding the floor, brake lines, etc. etc. It's far more than cutting out the firewall and mounting the engine back a foot or two.

 

I'm wondering if the ends justify the means. Searches have yielded little on the actual results of doing this. It was rumored that the S14 pictured above ran slower lap times than other similarly prepped (but normal engine position) 240SXs, but there was no evidence of that.

 

The only reliable account I've seen was from the builder of the above Supra. He said that on the track the car had great turn-in, but was unpredictable on corner exit, and essentially said the car did not feel as balanced as he had expected. He said he would be attacking the suspension geometry, but never reported back.

 

So does anyone have any evidence that a SUBSTANTIAL (12+in.) engine setback would substantially positively impact performance driving conditions? I'm wondering if front engine cars simply don't work any other way...

[johnc]

Moving an engine back in a front engined sedan is done for two reasons:

 

1. Increase weight on the rear (drive) wheels.

2. Decrease chassis yaw inertia (MOI).

 

Number 1 helps with putting power down and must be matched with wider rear tires and changes to spring, anti-roll bar, and shock damping rates. It also reduces forward weight transfer and helps with braking by keeping more weight on the rear wheels.

 

Number 2 makes transitions (any time the car changes directions) quicker and generally makes the car more responsive.

 

Both of the changes above, if done correctly, will reduce lap times and make the car more responsive to driver and track input. They also make the car more difficult to drive at the limit and over the limit behavior can be sudden and extreme if the driver is not skilled. This difficulty increases over the course of race as the tires lose grip.

 

I'm not surprised at all to hear that such radically modified Supras and S14s were actually slower initially. That's to be expected until the car gets sorted and the drivers get better. But there's a huge Catch 22 in getting these cars sorted: Until the drivers get better the car cannot be driven consistently at the limit, lap after lap, to provide data to sort the car. The driver's start to distrust the car and drive slower. Any chassis/suspension changes made using the data from these slow lap times is useless and just works to make the car even slower. The crew chief starts losing his mind, the driver is scared, frustrated, and just wants to go home, and the car owner wonders why he bothers to read anything on the Internet.

[Leon]

Perfectly summed up.

[z1 zonly]

So, the closer to 50/50 F/R weight bias, the less predictability and more skill it takes to drive at the limit? Gotta say, that upsets my intuition. I guess that's why I'm a bean counter and not an engineer .

 

Perhaps I'll leave it woefully front biased until I become good friends with a setup guru. Sigh.

[johnc]

No, I was not talking about weight distribution. I was talking about yaw inertia. Think of a dumbbell. It's has a 50/50 weight distribution but the weight is at the extreme ends of the barbell. It has a high yaw inertia. But moving weight to the center of the car you decrease the yaw inertia and make the car easier to spin.

[z1 zonly]

So is there an 'ideal' target for MOI weight positioning, or is it a moving target, or solely a function of setup?

[zero]

I'd think of it like this. Pretty much anyone can balance a broom on a finger with the head of the broom in the air. The effective moment is very high so the reaction of the broom to perturbation is relatively slow. This means you don't have to correct very quickly to keep it balanced

 

Remove the broom head and try balancing just the broom shaft and it gets more difficult. This is because you've reduced the moment and now these angular changes happen much more quickly. You need a much faster hand to keep the broom upright.

 

That's sort of the issue here. Reduce the moment and the car will want to rotate more quickly, which is good if you're a driver good/experienced enough to catch it, but if not it can make the car feel nervous or twitchy at the limit. Obviously there are many many more factors in the case of a car, but I think that's a decent way to picture the effect of MOI alone.

[rnye]

 

I'm not surprised at all to hear that such radically modified Supras and S14s were actually slower initially. That's to be expected until the car gets sorted and the drivers get better. But there's a huge Catch 22 in getting these cars sorted: Until the drivers get better the car cannot be driven consistently at the limit, lap after lap, to provide data to sort the car. The driver's start to distrust the car and drive slower. Any chassis/suspension changes made using the data from these slow lap times is useless and just works to make the car even slower. The crew chief starts losing his mind, the driver is scared, frustrated, and just wants to go home, and the car owner wonders why he bothers to read anything on the Internet.

 

Epic.

[johnc]

So is there an 'ideal' target for MOI weight positioning, or is it a moving target, or solely a function of setup?

 

No, yes, maybe.

 

Are you building the car to:

 

1. Compete on a race track?

2. Compete at a car show?

3. Impress your friends?

4. Improve your fabrication/engineering skills?

5. Do everything above?

[z1 zonly]

Racetrack. My local track is tight, but Road Atlanta and VIR aren't far away either. VH45DE Z32, by the way.

Edited June 19, 2012 by z1 zonly

[ablesnead]

I'm new to Charlotte, NC where is our local track ? the tight one ?

[ablesnead]

I am enjoying this thread , and would like to add from my experience as a race driver in 50/50 cars and others..( I have no engineering expertise )...when a car breaks loose with a 50/50 weight distribution , ( with a reasonable race setup) , both ends react in a balanced manner...easy to control, and use , for a quick time. With oversteer or understeer , one end reacts quicker , and can be more tuner intensive to set up, I belive the well balanced car easier to drive as tire wear occurs .. the reaction speed of the driver is equally vital to both setups and I belive moot to the discussion .

[z1 zonly]

Ablesnead--Carolina Motorsports Park (CMP) in Kershaw, SC. I'll let the others conjecture about your second post. Thanks for your input!

[johnc]

Racetrack. My local track is tight, but Road Atlanta and VIR aren't far away either. VH45DE Z32, by the way.

 

OK, what race sanctioning body are you competing with? What class will your V8 powered 300ZX be put in? What are the category and class rules?

 

If you're in a completely open class (most likely) then what is the competition like in that class? What are the class lap time records at RA and VIR? How do those times compare with the lap records for SCCA GT1/GT2?

 

What you want to determine is how much engineering you have to put into your Z32 to win. If the class is weak its pretty silly to spend tens of thousands of dollars and two years of work to dominate 3 guys who can't beat their grandma in a Camry.

[z1 zonly]

Just a trackday car, no competition. Building is the funnest part for me, so time doesn't matter so much. I just want to build an extreme car and have a blast personally to reap the accomplishment. But I certainly won't be having a blast if the car doesn't improve markedly after such an intensive effort. Hence the thread.

[johnc]

If an engineering/fabrication project is your main goal, then something like this will be a lot of fun to create. But, as I mentioned above, the testing and setup once the car hits the track will determine how much (if any) of lap time improvement you see. That part of the effort is far more important then the basic engineering and fabrication and is generally more difficult to do. Look at Ferrari in Formula 1. They showed up this year with a shit car and, over 7 races and NO real testing, have made it a front runner (almost).

[z1 zonly]

That's what they've been doing for the past several years; key word being "almost," heh! Go McLaren!

 

Anyway, same conclusion, wait until I befriend a setup genius. Thanks for all the replies!

[JMortensen]

You might talk to "Uncle Joe" Benson. He has a LS1 Z32. I believe he said it was something like 5 seconds faster than the 6 cyl when he first put it together, and that was a year or two ago. Yes, that is the Uncle Joe from classic rock radio. I don't believe he moved the firewall back that far, but I do seem to recall the firewall did have to be altered.

 

Another anecdote is Dennis and Peggy Hale's aluminum V8 (215???) 510. They did move it way back for lower PMOI, did just what John described, put it together, found it undriveable and gave up on it. I think that a car like that could be sorted but it takes a lot of time and effort. They have quite a few racecars from what I understand, so they just put it back in the garage and took out a different one.

[z1 zonly]

I've seen the Uncle Joe's car floating around the Internet and saw they had LS1 swapped it. Although I'm not positive, I think firewall mods are necessary on LS1 swaps in Z32s just as a cost-of-doing-business for getting the engine in the car. My VH has a football field between the engine and firewall, meanwhile (which is the impetus for wanting to move it back). The transmission is in the stock position.

 

That's a really interesting tidbit on the 510. Gah!

[Leon]

I am enjoying this thread , and would like to add from my experience as a race driver in 50/50 cars and others..( I have no engineering expertise )...when a car breaks loose with a 50/50 weight distribution , ( with a reasonable race setup) , both ends react in a balanced manner...easy to control, and use , for a quick time. With oversteer or understeer , one end reacts quicker , and can be more tuner intensive to set up, I belive the well balanced car easier to drive as tire wear occurs .. the reaction speed of the driver is equally vital to both setups and I belive moot to the discussion .

FWIW, a 60/40 car can be made to oversteer and a 40/60 understeer. It's all about setup. A properly setup racecar will be neutral or near-neutral steer, no matter what the weight distribution is. PMOI is a different subject, and very much affects how the quickly the car reacts and thus, how it feels. Putting the weight closer to the CG will make it react quicker to inputs which is inherently destabilizing.