diff --git a/collects/tests/mzscheme/benchmarks/shootout/nbody.ss b/collects/tests/mzscheme/benchmarks/shootout/nbody.ss
index b15212faaf..f49cd2832e 100644
--- a/collects/tests/mzscheme/benchmarks/shootout/nbody.ss
+++ b/collects/tests/mzscheme/benchmarks/shootout/nbody.ss
@@ -77,56 +77,69 @@ Correct output N = 1000 is
           (set-body-vx! (car system) (/ (- px) +solar-mass+))
           (set-body-vy! (car system) (/ (- py) +solar-mass+))
           (set-body-vz! (car system) (/ (- pz) +solar-mass+)))
-        (loop-i (cdr i)
-                (+ px (* (body-vx (car i)) (body-mass (car i))))
-                (+ py (* (body-vy (car i)) (body-mass (car i))))
-                (+ pz (* (body-vz (car i)) (body-mass (car i))))))))
+        (let ([i1 (car i)])
+          (loop-i (cdr i)
+                  (+ px (* (body-vx i1) (body-mass i1)))
+                  (+ py (* (body-vy i1) (body-mass i1)))
+                  (+ pz (* (body-vz i1) (body-mass i1))))))))
 
 ;; -------------------------------
 (define (energy system)
   (let loop-o ((o system) (e 0.0))
     (if (null? o)
         e
-        (let ([e (+ e (* 0.5 (body-mass (car o))
-                         (+ (* (body-vx (car o)) (body-vx (car o)))
-                            (* (body-vy (car o)) (body-vy (car o)))
-                            (* (body-vz (car o)) (body-vz (car o))))))])
+        (let* ([o1 (car o)]
+               [e (+ e (* (* 0.5 (body-mass o1))
+                         (+ (+ (* (body-vx o1) (body-vx o1))
+                               (* (body-vy o1) (body-vy o1)))
+                            (* (body-vz o1) (body-vz o1)))))])
           
           (let loop-i ((i (cdr o)) (e e))
             (if (null? i)
                 (loop-o (cdr o) e)
-                (let* ((dx (- (body-x (car o)) (body-x (car i))))
-                       (dy (- (body-y (car o)) (body-y (car i))))
-                       (dz (- (body-z (car o)) (body-z (car i))))
-                       (distance (sqrt (+ (* dx dx) (* dy dy) (* dz dz)))))
-                  (let ([e  (- e (/ (* (body-mass (car o)) (body-mass (car i))) distance))])
+                (let* ((i1 (car i))
+                       (dx (- (body-x o1) (body-x i1)))
+                       (dy (- (body-y o1) (body-y i1)))
+                       (dz (- (body-z o1) (body-z i1)))
+                       (distance (sqrt (+ (+ (* dx dx) (* dy dy)) (* dz dz)))))
+                  (let ([e  (- e (/ (* (body-mass o1) (body-mass i1)) distance))])
                     (loop-i (cdr i) e)))))))))
 
 ;; -------------------------------
 (define (advance system dt)
   (let loop-o ((o system))
     (unless (null? o)
-      (let loop-i ((i (cdr o)))
-        (unless (null? i)
-          (let* ((o1 (car o))
-                 (i1 (car i))
-                 (dx (- (body-x o1) (body-x i1)))
-                 (dy (- (body-y o1) (body-y i1)))
-                 (dz (- (body-z o1) (body-z i1)))
-                 (distance (sqrt (+ (* dx dx) (* dy dy) (* dz dz))))
-                 (mag (/ dt (* distance distance distance)))
-                 (dxmag (* dx mag))
-                 (dymag (* dy mag))
-                 (dzmag (* dz mag))
-                 (om (body-mass o1))
-                 (im (body-mass i1)))
-            (set-body-vx! o1 (- (body-vx o1) (* dxmag im)))
-            (set-body-vy! o1 (- (body-vy o1) (* dymag im)))
-            (set-body-vz! o1 (- (body-vz o1) (* dzmag im)))
-            (set-body-vx! i1 (+ (body-vx i1) (* dxmag om)))
-            (set-body-vy! i1 (+ (body-vy i1) (* dymag om)))
-            (set-body-vz! i1 (+ (body-vz i1) (* dzmag om)))
-            (loop-i (cdr i)))))
+      (let* ((o1 (car o))
+             (o1x (body-x o1))
+             (o1y (body-y o1))
+             (o1z (body-z o1))
+             (om (body-mass o1)))
+        (let loop-i ((i (cdr o))
+                     (vx (body-vx o1))
+                     (vy (body-vy o1))
+                     (vz (body-vz o1)))
+          (if (null? i)
+              (begin
+                (set-body-vx! o1 vx)
+                (set-body-vy! o1 vy)
+                (set-body-vz! o1 vz))
+              (let* ((i1 (car i))
+                     (dx (- o1x (body-x i1)))
+                     (dy (- o1y (body-y i1)))
+                     (dz (- o1z (body-z i1)))
+                     (distance (sqrt (+ (+ (* dx dx) (* dy dy)) (* dz dz))))
+                     (mag (/ dt (* (* distance distance) distance)))
+                     (dxmag (* dx mag))
+                     (dymag (* dy mag))
+                     (dzmag (* dz mag))
+                     (im (body-mass i1)))
+                (let ([vx (- vx (* dxmag im))]
+                      [vy (- vy (* dymag im))]
+                      [vz (- vz (* dzmag im))])
+                  (set-body-vx! i1 (+ (body-vx i1) (* dxmag om)))
+                  (set-body-vy! i1 (+ (body-vy i1) (* dymag om)))
+                  (set-body-vz! i1 (+ (body-vz i1) (* dzmag om)))
+                  (loop-i (cdr i) vx vy vz))))))
       (loop-o (cdr o))))
   
   (let loop-o ((o system))