Recessed engine mounting (into the firewall)

[ablesnead]

I only disagree on a quickly reacting car being destabilizing , I belive , balanced , the quicker the better ,remember how 20 laps in , the car had a different feel than the setup you started with. Any movement desired or not is transmitted thru your contact patch , that you try an optimize with your setup . This compromise is evolving to a small degree every lap...a car without a weighted end hanging out , allows this to happen in a more uniform manor....so I found the result of a 50/50 setup both initially quicker and consistently so...I have experienced the quick and relatively violent polar movement off track or in the marbles, but with a normal racing surface and a good tire setup , lateral movement planned or otherwise is mediated by grip or lack of it.

[Leon]

"Quicker reacting" is inherently less stable than "slower reacting". There is no way around it, it's physics. Whether that is good or bad is up to the end user. I wouldn't want a very low PMOI car on a huge oval but I would in an autoX. Road courses are a combo of the two. Weight distribution has nothing to do with this.

 

BTW, your post is confusing, I can barely follow what you're saying.

[ablesnead]

I apologize for the confusion my previous caused you...in person I would talk slower in an effort to make my point. But I think that its interesting that we approach the question of moving back an engine to approach 50/50 weight distribution ..and have different opinions , as to the good or bad , its effect on handling would be . Both of us with years of racing , yet different approaches ...I think it makes for interesting racing ..destabilizing is an interesting word...being that the stable state is a straight line , and road courses have occasional turns

[roger280zx]

I apologize for the confusion my previous caused you...in person I would talk slower in an effort to make my point. But I think that its interesting that we approach the question of moving back an engine to approach 50/50 weight distribution ..and have different opinions , as to the good or bad , its effect on handling would be . Both of us with years of racing , yet different approaches ...I think it makes for interesting racing ..destabilizing is an interesting word...being that the stable state is a straight line , and road courses have occasional turns

 

Not sure if you completely missed it, but the modifications in question go well beyond simple 50/50 weight bias and indeed have a large impact on PMOI. 50/50 is much easier to acheive without mods like this in most cases. In my zx (79) I am gunning hard for weight before the front wheel centerline, beyond that their will be no setting back of the major components, engine and trans. But I do not have a "football feild" between the engine and firewall either.

[Leon]

I apologize for the confusion my previous caused you...in person I would talk slower in an effort to make my point. But I think that its interesting that we approach the question of moving back an engine to approach 50/50 weight distribution ..and have different opinions , as to the good or bad , its effect on handling would be . Both of us with years of racing , yet different approaches ...I think it makes for interesting racing ..destabilizing is an interesting word...being that the stable state is a straight line , and road courses have occasional turns

I don't think you're following what I'm saying. You can be more stable or less stable while turning. A lower PMOI car is inherently less stable, meaning a jerk of the wheel will have a more dramatic effect. What might be good for an F1 driver, may not be for Joe Average. Some stability is good, and the more centered you make the weight, the less stability you have, hence "inherent destabilizing effect".

 

Again, weight distribution and PMOI are 2 separate concepts. Just because you have 50/50 weight distribution does not mean that you have a low PMOI, and a low PMOI does not mean that you have 50/50 weight distribution!

[tube80z]

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.

 

Having been around that car for a long time I can tell you it had nothing to do with the motor placement. It had a lot of other issues. A local racer put the motor even further back in his 510 (Larry Nelson) and it was one of the best handling 510s I've ever had the pleasure of driving. Put another way, the 510 has heaps more PMOI than most sports racers or formula cars and normal humans don't have issues with those.

 

Claude Rouelle shared a number of stories in his seminars about weight distribution and PMOI. Up until the Pirelli tire era most F1 teams arrived and the track and only changed weight distribution according to him. They had sophisticated simulations to determine all the other parameters before they even showed up. Claude shared one story where moving two 7.5 pound blocks was worth three quarters of a second.

 

You can do some simulation based on either data or by guessing by using the free version of Bosch Lapsim. If you have data you can build your own track and create a model of your car. From there you can play with certain parameters and see what happens to laptime. The free version assumes a perfect driver and doesn't really take suspension into account. From what I recall it's predictions matched what John said, which is what you would expect. So if you want to try and optimize you can have lots of fun with that tool. It was fun to build that 1000 HP Z and see what my 0 to 60 time would be.

 

Cary

[proxlamus©]

Can someone explain to me what PMOI is?

[B. Whisky]

Can someone explain to me what PMOI is?

 

Polar Moment of Inertia. In lay terms, the resistance to torsion. In a racecar, referring to the cars resistance to drivers steering inputs. The higher the pmoi, the more the tendency to resist changing direction. The lower the pmoi, the easier it is to change direction, be it from steering input or otherwise, hence the comment about inherent destabilization.

[proxlamus©]

thank you sir! =)

[JustinOlson]

Eventually I'll get back to this. My interest with this has more to do with rear weight bias and traction for roll racing.

 

[weedburner]

Here's a 1st gen RX-7 that i set the engine back 10" in...

 

 

I just cut the entire tunnel / firewall out and trimmed 10" off of the rear, re-installed. 

 

 

18ga cold rolled steel make up the seal panels on the sides up front.

 

 

The sbc distributor just barely wiggles down past the stock windshield.

 

 

The front motor plates are from a 410 sprint car.

 

 

Radiator mounts off of the engine and sits on tabs welded to the front crossmember.

 

 

Still looks pretty stock on the inside.

 

 

The pedal box was spaced rearward with simple 8" aluminum spacers, requiring longer pushrods for the brake/clutch masters.

Seats were re-located rearward too.

 

 

[z1 zonly]

WOW, it's just amazing/creepy that you would bring this thread back.  I'm finally getting ready to do this.  Have the engine (LS1), have the transmission (Tex SR1 4-speed dogbox), have the stripped Z32 chassis, just need a bellhousing before I can start hacking.  On Sunday I was searching around on Google for bellhousings and just happened across your website.  I then spent a significant amount of time there.  You've done a fantastic job on that car and it's truly an inspiration.  Particularly that bellhousing--DROOL. Thanks for responding here--I may have to PM you as I get further into my build.