Engine set-back, how much is worth the hassle?

[fuzzyGN]

I wasn't really sure where to post this but here seemed as good as any. I am working on the chassis for my non-Regal race car. I was looking at the engine placement in the engine bay and noticed 2 inches of clearance between the engine and the firewall. I am going to build a new trans crossmember anyway so I was considering moving the engine back those 2 inches by way of custom engine mounts. I do not have much room to move the engine down in the chassis as the oil pan is already close to even with the front crossmember and the car sits very low. I may be able to squeeze about a 1/2" down but probably not more than that. The car is very light already (about 2300 with driver) but I could save several more pounds by making more efficient motor mount plates and stands on the chassis side. The added benefit of moving the engine back would help load the rear tires on launch etc. So, how much engine setback is required to make it really worthwhile? Opinions and any race car math is appreciated.

 

[Blazer406]

I would say if you can do it..... do it....

I will say that I had heard that moving the battery to the trunk was the same difference in weight transfer as moving the engine back 10"....FWIW.

Every little bit helps. It is a game of inches.... or in our case..... tenths and hundredths..... it will help regardless.

 

[charlief1]

Is this the Vega Erik? I had mine moved back about 1/2" and it helped with handling on the track. I also re-located the battery to the passengers side rear seat to off set my weight in the drivers seat.

 

[fuzzyGN]

Yes, it is for the Vega. I have a fiberglass front bumper, custom aluminum brackets holding it on, battery is in the spare tire well, all unnecessary stuff removed from the car, and a fiberglass hood. I want to keep the original fenders and doors because it is still original paint which is in pretty good condition. The new engine will only weigh about 275 pounds complete minus headers. Already have Areospace front brakes with 5 lug front runners. There is very little left to remove weight wise so maybe MOVE weight is the next best thing.

Charlie, what type of racing were you doing? Road track? Only 1/2" setback was noticeable? That is really amazing. I would think there is enough slop in the stock mounting stuff to move the engine back 1/2" without any effort at all. It would be cool to come up with some nerdy stuff to calculate the advantage of moving the weight around the car. I am sure there is something out there somewhere. Anyone know of any? Cool stuff.

 

[Blazer406]

Yes, it is for the Vega. I have a fiberglass front bumper, custom aluminum brackets holding it on, battery is in the spare tire well, all unnecessary stuff removed from the car, and a fiberglass hood. I want to keep the original fenders and doors because it is still original paint which is in pretty good condition. The new engine will only weigh about 275 pounds complete minus headers. Already have Areospace front brakes with 5 lug front runners. There is very little left to remove weight wise so maybe MOVE weight is the next best thing.

Charlie, what type of racing were you doing? Road track? Only 1/2" setback was noticeable? That is really amazing. I would think there is enough slop in the stock mounting stuff to move the engine back 1/2" without any effort at all. It would be cool to come up with some nerdy stuff to calculate the advantage of moving the weight around the car. I am sure there is something out there somewhere. Anyone know of any? Cool stuff.

If you know where the center of gravity is for the car, that would be a start. Have you weighed it with 4 scales?

 

[fuzzyGN]

I want to do the 4 scale weigh before I tear it down. I am going to cage the car, rewire, etc. but want to know what it weighs now. After the work I will weigh and compare. I am thinking that with smart removal of a few key items and pay attention to the little things I can have a caged car for little to no weight penaltity. That is my hope anyway. You are correct though, the center of gravity is the important number to start with.

 

[charlief1]

It was SCCA street modified class in the 80's Erik. I learned a silly trick when putting the enigne in. I accidently reversed the motor mounts and use a set for the oddfire. The only reason I know it was backed up was because of the colectors on the headers. I blew the head gaskets within a week of getting the engine in and testing so I had to pull the engine out. I didn't pay a lot of attention on the mounts then and when I put it back in the headers were to far foward. That's when I went to the parts store and started looking at the parts and found out I had the mounts on backwards the first time, so I put them on again and everything worked out just right.

 

[fuzzyGN]

Finally got the car on the scales last night. A little tip to everyone who wants to get the weight of their car, don't use the truck stop. I originally weighed the car on a 100,000 pound scale which was in cal but obviously I was using it at the very bottom of the scale. So on the truck scale the car weighed 2380 and since I had removed at least 50 pounds but probably way more. On a 4 corner set calibrated to only 6000 pounds the car weighs 2450. I have to admit I am a little disappointed but it is still pretty light. The amazing thing to me was the corner weights. The front tires were off by only 1 pound! The rear was off by 13 pounds but I forgot to install the nitrous bottle so they would have been almost identical also. Happy accident for sure because I sure didn't try to get them close. Anyway the real information needed was the front to rear weight. Even that was a little unexpected. The front was just over 53% and the rear was just over 46%. Again pretty impressed considering. With the new engine (aluminum heads) and rack steering, it will shave about 60 pounds from the front weight.

So now does anyone have any math or racers wisdom on engine setback? With the current headers I have maybe an inch to work with. Either reworking the existing headers or possibly a different set I could have as much as 2 inches available. I am not afraid to get dirty but I want to make sure it will be worth the effort.

 

[coach]

Yes, it is for the Vega. I have a fiberglass front bumper, custom aluminum brackets holding it on, battery is in the spare tire well, all unnecessary stuff removed from the car, and a fiberglass hood. I want to keep the original fenders and doors because it is still original paint which is in pretty good condition. The new engine will only weigh about 275 pounds complete minus headers. Already have Areospace front brakes with 5 lug front runners. There is very little left to remove weight wise so maybe MOVE weight is the next best thing.

Charlie, what type of racing were you doing? Road track? Only 1/2" setback was noticeable? That is really amazing. I would think there is enough slop in the stock mounting stuff to move the engine back 1/2" without any effort at all. It would be cool to come up with some nerdy stuff to calculate the advantage of moving the weight around the car. I am sure there is something out there somewhere. Anyone know of any? Cool stuff.

Charlie is old!!! and forgets alot. He was racing in the "Pinewood Derby" and that 1/2" is HUGE!!! You should see how big he tells his Mrs. "a foot" is...JK

 

[Blazer406]

Where is the CG of the engine relative to the front tires? What is the wheelbase?

 

[charlief1]

Charlie is old!!! and forgets alot. He was racing in the "Pinewood Derby" and that 1/2" is HUGE!!! You should see how big he tells his Mrs. "a foot" is...JK

Never been Married but I have had a few complaints of bottoming out Donnie.

Where is the CG of the engine relative to the front tires? What is the wheelbase?

Whats the exact weight for each wheel Erik? You might want to go to baseline suspension and use his calculators to figure out what you need. Kevin has some good info on his site as well as the calculators.

 

[coach]

Never been Married but I have had a few complaints of bottoming out Donnie.

Whats the exact weight for each wheel Erik? You might want to go to baseline suspension and use his calculators to figure out what you need. Kevin has some good info on his site as well as the calculators.

Hey Charlie,
Your "Mrs." is anyone that will hold still long enough!!! That was my motto for years.
Never married for me neither.

 

[turbofabricator]

In aviation ....Weight x Arm = Moments. Simple math will tell you what 2" will do for you. (besides get you more dates) Take scale weights and wheel base and detirmine CG (center of Gravity) Measure center of wheel front to cent of wheel back and post the numbers and we can do the math for you. Engine/trans weight will be needed, too. There is even on-line calculators available if you search "Auto Math" a bit.

 

[novaderrik]

moving that weight back 2" on a 2400 pound car makes a hell of a lot more difference than moving it back 2" on a 3500 pound car.

 

[charlief1]

moving that weight back 2" on a 2400 pound car makes a hell of a lot more difference than moving it back 2" on a 3500 pound car.

Well give us some numbers then Derrik. I'd like to see how much moving the engine back !" and dropping it down the same would do in a 3500 pound car as well as a 2400 pound car.

 

[novaderrik]

Well give us some numbers then Derrik. I'd like to see how much moving the engine back !" and dropping it down the same would do in a 3500 pound car as well as a 2400 pound car.

look at the post above mine- the formula is there. there are also a lot of websites dedicated to stuff like this.. i don't know how much the engine/trans comb in question weighs or what the wheelbase or weight distribution is in any particular car, but a little common sense says that moving the same amount of weight back a given amount will make a bigger difference in a lighter car than on a heavier car.

 

[charlief1]

This may or may not help some.

http://robrobinette.com/cg_height_calc.htm
http://www.thedirtforum.com/centergravity.htm

 

[fuzzyGN]

I started a project book and have all the info at home. I will post it up tomorrow and then maybe we can post some real world calcs and examples for future reference. The engine should be around 275 pounds complete (N/A Buick V6 with alum heads) and a TH-350 with brake and 8" converter. The wheelbase is short, like somewhere near 100" but I will measure tonight.

 

[dr_frankenstein]

I know exactly what your thinking. I did the same, moved the motor back 2 inches in my truck. I think were close on wheel base and weight. I cant say weather it really helped, as with the truck bed being so light it might not have improved anything.... but I found that using a straight axle up front with a nose bleed and a set of 72" lift bars made the difference in the 60'

must be that instant roll center change that made the difference.

 

[fuzzyGN]

The wheelbase on the car is 97" and the front weights were 656 and 657 for a total of 1313 on the front or 53.5%. The total weight is 2450. If we assume the existing engine is 325 pounds then we can do some math. Currently the engine is placed such that the drivers side axle centerline is in between spark plug #1 and #3. By moving the engine back 2" it looks like #1 would be at axle centerline thus very little engine weight in front of the front axle.