Home 
Home Search search Menu menu Not logged in - Login | Register
> Jensen Healey & Jensen GT Tech > Projects > 19064 - autocross project car for SCCA's FSP class

 Moderated by: Greg Fletcher Page:  First Page Previous Page  1  2  3  Next Page Last Page  
New Topic Reply Printer Friendly
19064 - autocross project car for SCCA's FSP class  Rating:  Rating
AuthorPost
 Posted: 11-18-2015 04:33 pm
  PM Quote Reply
21st Post
Esprit2
Member
 

Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 575
Status: 
Offline
Steve,
I just edited Post #19 to add a cylinder head torque spec. If it wasn't there when you read it, go read it again.

Regards,
Tim Engel

Back To Top PM Quote Reply  

 Posted: 11-18-2015 04:52 pm
  PM Quote Reply
22nd Post
stevegarnjobst
Member


Joined: 10-22-2015
Location: Maplewood, Minnesota USA
Posts: 36
Status: 
Offline
Thanks.

Note - I just heard back from Mike @ Lotusbits. No vernier pullies for the older square tooth belts, but he has a conversion to the newer belts. Not sure that conversion is allowable under the SCCA rules. If it were, is the update worthwhile?

Back To Top PM Quote Reply

 Posted: 11-18-2015 05:47 pm
  PM Quote Reply
23rd Post
Esprit2
Member
 

Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 575
Status: 
Offline
stevegarnjobst wrote:
Were all the stock tappets cast iron?ALL 907s were originally built with chilled cast iron tappets. When Lotus made the change to steel for the mid-late eighties (?) 910, the spec applied retro-actively to all 9XX engines as Service Replacement parts.

stevegarnjobst wrote:
The timing changes are allowed. In fact, I can even add adjustable cam pulleys. I noticed Lotusbits has some vernier units on the shelf - might they be worth having?The adjustable pulleys are a pain to install, since you then have to degree the cams. It's a lot of putzy work, but doable. Given the rules restrictions on mods you can make to the engine (basically none), I don't think the gains on a milquetoast 'stock' engine due to degreeing the cams is worth the effort. Hotrod it, then yes, degree the cams.

On the other hand, whenever the distance between the cam centerlines and the crank centerline changes (composite gasket's crushed thickness or milling the head or block deck), the cam timing changes.

Cut the head, and there's that much extra slack goes into the timing belt. By definition, the cam timing is spec'd at TDC, so the crank can't be allowed to move. So when the tensioner pulls the extra slack out of the long run from the crank to the exhaust cam pulley, the slack must go up over the pulleys, rotating them slightly counter-clockwise, retarding the cam timing. Similar-but-different, the composite head gasket's thicker crushed thickness raises the head, and advances the cam timing. In those cases, the only recourse is to accept the timing change, or install adjustable cam pulleys.

stevegarnjobst wrote:
By the way, do you have any recommendations for head/balance work? I've had good luck with Headwerks in the past, but I have no idea if they have experience with these engines. Is there anyone semi-local you recommend for 907 work?I've not used anyone local for really "porting" the head. But since you can't do that by rule, I have had Aaron's Machine in Scandia port-match several Lotus 9XX engines for LOON members (local Lotus club). He's good, has a reasonable shop rate ($/hr), and has been the LOON's default 9XX machine shop for many years. He's not a 907 expert, but he has done machine work on more of them than any other shop I know of in the Twin Cities.

Aaron is a general automotive machinist (everything except crank grinding) and race engine builder (snowmobiles to outboards to dragsters). He has the fastest boat on the St Croix River, with twin blown big-block V8 race motors in the back (basically AA-Fuel dragster motors).

Aaron's Precision Machine (Lotus Experience)
24014 Olinda Trail N, Scandia, MN 55073-9641
(651) 443-4219

Having said that, I've also been happy with work done by Total Engine in Bloomington, JAX in Hamil (sp?), and Engelke Machine in Lester Prairie. And Cylinder Head Services in Minneapolis (near Top Gear Autoworks) has rebuilt one J-H 907 head that I know of. That was just a straight, by-the-book rebuild. The guy I used to use for Lotus heads has retired... :-(

In the USA, I think West Coast Racing Cylinder Heads in California has the most experience porting 9XX heads.

In the UK, Mike Taylor at Lotusbits does a great job of porting Lotus heads. There's a picture of his "Ported" 9XX head on the Lotusbits website, but ask to see his "Well Ported" head. It has huge ports. So big that the intake manifold can't be port matched to it without the wall thickness getting too thin. So he makes a full billet manifold that 'looks' factory, but with bigger runners. Ask him about ported heads during your visit to Lotusbits.

I have all my crankshaft work done by:
Engelke Machine
17734 County Road 9, Lester Prairie, MN 55354
(320) 395-2982
oengelke@hutchtel.net
They've done a bunch of Lotus cranks.

Crankshaft Supply in Minneapolis can do all crank work as well, and they're closer, but you pay big-city rates for it. Small-town Engelke Machine does quality work at a fraction of the cost.

Other Engelke family businesses (brother/ uncle ??) in LP are:
Lester Prairie Engine (racing engines, primarily circle track & tractor pulling).
Berry Cam Service (regrind, restore, custom).
1948 175th St / PO Box 697, Lester Prairie, MN 55354
(320) 395-2377

Aaron, Engelke, Total, and JAX have all done balancing for me, and did good work. As far as balancing specialists are concerned, I've not used either of the following, I'm just aware of them. Is / was ?? I can't say if either one is still in business.

Minneapolis Electronic Balancing Co
(612) 781-2755
3620 Central Ave NE, Minneapolis, MN 55418
I talked with him about balancing a Lotus engine, once. He was less than enthusiastic about the job, so I moved on.

Superior Balancing
23245 Agate Lake Rd, Deerwood, MN 56444
(218) 546-8417

Regards,
Tim Engel

Last edited on 11-18-2015 06:33 pm by Esprit2

Back To Top PM Quote Reply  

 Posted: 11-18-2015 05:59 pm
  PM Quote Reply
24th Post
Esprit2
Member
 

Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 575
Status: 
Offline
stevegarnjobst wrote:
Not sure that conversion is allowable under the SCCA rules. If it were, is the update worthwhile?Yes. The round-tooth HTD timing belt has much deeper tooth engagement, and is far less likely to jump timing. On an interference engine, that's great mechanical insurance to have.

Also, all trapezoidal timing belts are made with 1960's technology. No new engines have been manufactured using that belt since the introduction of the HTD belts circa 1985-86, so the rubber industry has not invested in upgrading the old trapezoidal belts. They still use old HCR rubber and construction.

The only exception is JAE's Lotus belt. They work with Gates Racing to make trapezoidal 9XX belts in HSN rubber (~1995 technology). It's better than the standard HCR belts, but still not modern day HNBR rubber.

Cheap HTD belts in the Lotus size are still available in old-school HCR; but I strongly recommend that if you're going to make the upgrade to HTD, that you should spend just a few bucks more and use the later 1995-spec HSN HTD belt.

No major rubber company markets a HNBR belt that will fit the Lotus 9XX. However, once again, JAE works with Gates Racing to produce a special run of HNBR HTD belts. They have Gates Racing's signature blue outer layer, so they're easy to spot. It's the best belt you can put on a 9XX. The downside... $165 each. If you're going to use the engine in competitive autocross, consider it mechanical insurance. If you go there, talk with me about how to tension it. The book spec doesn't work for the blue belt.

Having said all that, I autocrossed my Eclat and Esprit S2 with the stock trapezoidal timing belt and never suffered a failure. However, I checked/ re-set the tension religiously, and replaced the belt often. I was always doing 'something' on the engines, so every time the belt had to be slipped off the pulleys for something, I replaced it. My belts didn't get very old.

For trapezoidal belts, the standard service interval is 24,000 miles or 24 months, whichever comes first. Time is just as important as miles. Don't cheat on that.

Regards,
Tim Engel

Last edited on 11-18-2015 06:38 pm by Esprit2

Back To Top PM Quote Reply

 Posted: 11-18-2015 08:57 pm
  PM Quote Reply
25th Post
stevegarnjobst
Member


Joined: 10-22-2015
Location: Maplewood, Minnesota USA
Posts: 36
Status: 
Offline
Thanks again Tim! This thread is quickly becoming a valuable repository of build data.

I'll see what kind of price tag Mike puts on the HTD conversion. It sounds like a "nice to have" upgrade, depending on the cost.

It sounds like I'll have difficulty leaving Lotusbits without at least a few parts, as Mike appears to have a number of items on the shelf that are difficult to find anywhere else.

Back To Top PM Quote Reply  

 Posted: 11-26-2015 10:08 pm
  PM Quote Reply
26th Post
stevegarnjobst
Member


Joined: 10-22-2015
Location: Maplewood, Minnesota USA
Posts: 36
Status: 
Offline
Stopped by Lotusbits for an extended visit this afternoon. Located in a rural area just Southeast of Coventry, the shop is an absolute treasure trove of hard to find parts for the Lotus 907/912 engines. Mike at Lotusbits was extremely helpful. We spent a good deal of time digging through his parts bins and discussing upgrade options for the Lotus engine in my Jensen Healey.

I bought a couple hard to find items while at the shop - an Eclat/Elite airbox and a lightened steel flywheel.

I also talked with Mike about various options for an EFI kit, and examined a couple engines with the Lotusbits EFI conversion installed. VERY nice package, with the cleanest, most secure crank sensor I've seen. It seems about as close to a simple bolt-on conversion as one could hope for. The only significant piece not in the kit is a fuel return line.

-Steve

Back To Top PM Quote Reply

 Posted: 11-27-2015 12:26 am
  PM Quote Reply
27th Post
stevegarnjobst
Member


Joined: 10-22-2015
Location: Maplewood, Minnesota USA
Posts: 36
Status: 
Offline
Esprit2 wrote:
Steve,

The very early 907 Mk I intake ports were 24.5 mm tall. Later, they were increased to 25.5 mm as a running change. At the time of the change, the production of the small port head ended and the large port head became the official standard service replacement. Sorry, but no, I don't know the date or engine number of the port change, but I do believe it was within the life span of the Jensen-Healey's 907 Mk II engine.


Tim (or anyone else who knows 907's really well),

When I was at Lotusbits today, Mike showed me stock Jensen Healey and early Lotus 907 heads side by side. The Lotus heads had substantially larger intake ports (a lot more than the 1mm increase mentioned above). The Lotus and J-H heads had the EXACT same part number cast into the head, and Mike thought some later J-H's may have come with the Lotus head, as supplies of the J-H head ran out. Since it appears Lotus changed the port design without changing the part #, it sounds like the large port heads would be legal for me to run. Can anyone else confirm this? The larger port Lotus heads flow better, which would be useful.

Last edited on 11-27-2015 12:27 am by stevegarnjobst

Back To Top PM Quote Reply  

 Posted: 11-27-2015 08:25 pm
  PM Quote Reply
28th Post
Esprit2
Member
 

Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 575
Status: 
Offline
Steve,
The number cast into a Lotus part is the raw casting number, not the finished part as used on the engine, or that you could buy. Besides the casting number, there was a machined bare head part number, and a fully built-up, ready to install part number.

Lotus didn't fully machine the ports from small-as-cast to various finished sizes. They were used pretty much as cast.

The original AP head's intake ports (AP was the first castings vendor) did go through a series of evolutionary changes (at least three that I know of) before the vendor change to the Zeus head with it's final port config (Zeus is the later castings vendor, circa 1993). The Zeus head is superior, but didn't come along until well after JH was out of production. It would require some fast talking to justify using one on a JH autocrosser.

The early JH Mk I 907 did have the small ports, however, the later Mk II did get larger ports... the ones I believe are 1mm taller. Lotus produced the 907 through 1980, and late 907/ early 912 heads had ports that were larger yet. That may (??) be what Mike Taylor showed to you. The big port head will bolt right on.

I don't know enough about JH to know how far their 907's ports went down that evolutionary path. But the last of the JH engines were pretty much the same spec as the Lotus 907s of the day, with the exception of carbs and distributor advance curves. JH & Lotus Strombergs used different needles and tuning. JH used DHLA 40 Dellortos, while Lotus used the larger 45s.

Do you have pictures you can post of the two heads with small and large ports?

Regards,
Tim Engel

Last edited on 11-27-2015 08:26 pm by Esprit2

Back To Top PM Quote Reply

 Posted: 11-27-2015 09:41 pm
  PM Quote Reply
29th Post
Esprit2
Member
 

Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 575
Status: 
Offline
Steve,

The current head gasket is the Goetze composite gasket (or JAE sells their own aftermarket clone). It requires the use of either Lotus' later upgraded head studs (Lotus Part Number B912E7029Z), or the ARP studs that are available for the 9XX engines, along with a significantly higher torque.

The Lotus spec calls for the use of a Torque Angle Meter instead of a torque wrench. Use the following tightening procedure and notes:

1) Check that the cylinder liner nip is within spec:
... +.001 to +.005 inch (+ .025 to +.130 mm).

2) Use Lotus upgraded cylinder head studs (B907E0224Z) identified by a dimple machined into the top end. Tighten into the block (with oiled threads) to 37 - 41 Nm (27 - 30 lbf-ft).

3) Make certain all mating metal surfaces are absolutely clean and dry (wipe with brake, carb or contact cleaner).

4) Fit the composite head gasket DRY.

5) Put the cylinder head in place before oiling the washers & head stud threads and fitting the nuts. Tighten the nuts using a torque/angle gauge, in a sequence from the center outwards, in the following steps:

i) 20 Nm (15 lbf-ft) initial pre-load via a torque wrench.
ii) +75 Degrees
iii) +40 Degrees
iv) Wait 5+ minutes (longer is better)
v) +20 Degrees

The net result of Lotus’ torque angle spec is a higher final torque/ clamping force than was produced by Lotus’ original 70-75 lb-ft torque wrench spec. Do NOT consider the old spec and new spec as different but interchangeable... they're very different.

I've had mixed results installing the composite gasket dry as instructed. I now prefer to apply a little Wellseal, or Permatex Aviation Form-a-Gasket around the oil passage between the block and head (apply to both sides of the gasket). In my more paranoid moments, I'll also apply a little around the full perimeter of the head & block, both sides of the gasket. That only addresses coolant and oil leaks. At the cylinder liner seal area, the gasket must be installed DRY.

*~*~*
If you use the ARP studs, then ARP specifies the use of a torque wrench, as follows:

6) Apply "ARP Ultra-Torque Fastener Assembly Lubricant" to the washers, bottom of the nuts, and the stud threads (the lube is included in JAE’s ARP kit).

7) Torque the nuts in several smaller steps to a final setting of 110 Lb-Ft. (arbitrary, but something like 50, 75, 100, 110, with the last step being small).

With either procedure, torque the nuts from the center of the head outward, in a criss-cross pattern.

*~*~*
As mentioned in post #19, "IF" you don't replace the original low-spec head studs, then stay with the original 'Lotus' torque spec out of consideration for the lo-spec studs. You won't be getting the full advantage of the composite gasket, but you won't be over-stretching the weaker studs either.

Note that the J-H head torque spec was too high, crushed the old gasket, and caused problems with blown gaskets. Lotus reduced the torque spec to a level that didn't crush the gasket, or over-stretch the studs. If you're going to stay old-school, it would be better to follow the Lotus torque spec instead of the one in the J-H Workshop Manual.

Lotus Cylinder Head Torque Spec:
(Tighten Cold, Oiled Threads & Washers)
70 Lbs-ft - Front & Rear Pairs
75 Lbs-ft - Three Middle Pairs

And again, work up to the final torque in several stages.

For your autocross engine, I strongly recommend that you replace the original head studs with either Lotus' later upgraded head studs (B912E7029Z), or the ARP studs.

Regards,
Tim Engel

Last edited on 11-27-2015 11:51 pm by Esprit2

Back To Top PM Quote Reply  

 Posted: 11-27-2015 10:00 pm
  PM Quote Reply
30th Post
Esprit2
Member
 

Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 575
Status: 
Offline
While I'm on a roll, here's one more thing...

The original steel-asbestos-steel exhaust gasket blows too easily (the ones between the exhaust manifold flange and the cylinder head). The later gasket Lotus used for the 910 Turbo is a 3-laminate stainless steel part that does not blow. I've even re-used them once or twice without experiencing any leaks.

The gasket in Federal 910s was a 2-laminate. But a friend in the UK once sent me some 3-laminate gaskets that were available there, and they were wonderful. I contacted Lotus Cars USA (LCU), but they didn't know what I was talking about and never made them available on this side of the pond. I showed the 3-lam gasket to JAE, and they had them reproduced. The only difference is that the Lotus gaskets are crimped together around the ID, and the JAE version is secured together with eyelets at three ears near the stud holes.

In rare situations, I've had one of the eyelets interfere with the manifold flange (the gaskets are designed for the 910, I'm using them on other engines). In that case, just remove the offending eyelet. Once the gaskets are installed on the engine, the eyelets are redundant anyway.

When I was autocrossing 907s, the steel-asbestos-steel gaskets would blow way too often. After switching to the 3-laminate stainless steel gaskets, there was never another blown exhaust gasket.

The original steel-asbestos-steel gasket "blows" (take that any way you wish).

The Federal 910 2-laminate stainless steel gasket is preferable to the original 907 gasket.

The Euro 910 / JAE 3-laminate stainless steel gasket is pretty much bullet-proof.

Regards,
Tim Engel

Last edited on 04-29-2017 05:37 am by Esprit2

Back To Top PM Quote Reply

 Posted: 11-28-2015 12:19 am
  PM Quote Reply
31st Post
stevegarnjobst
Member


Joined: 10-22-2015
Location: Maplewood, Minnesota USA
Posts: 36
Status: 
Offline
Esprit2 wrote:
...The early JH Mk I 907 did have the small ports, however, the later Mk II did get larger ports... the ones I believe are 1mm taller. Lotus produced the 907 through 1980, and late 907/ early 912 heads had ports that were larger yet. That may (??) be what Mike Taylor showed to you. The big port head will bolt right on.

I don't know enough about JH to know how far their 907's ports went down that evolutionary path. But the last of the JH engines were pretty much the same spec as the Lotus 907s of the day, with the exception of carbs and distributor advance curves. JH & Lotus Strombergs used different needles and tuning. JH used DHLA 40 Dellortos, while Lotus used the larger 45s.

Do you have pictures you can post of the two heads with small and large ports?

Regards,
Tim Engel

I didn't take a photo of the J-H and Lotus heads side by side, but I can probably ask Mike for a shot or two. Mike says the larger port heads were the only ones used on Lotus cars, some of which were produced at the same time as later Mk II J-H's, so it's quite possible some of the Lotus-spec heads were used on the last J-H's. I'm not quite ready to buy a big port head yet, but the possibility is definitely intriguing.

-Steve

Last edited on 11-28-2015 12:20 am by stevegarnjobst

Back To Top PM Quote Reply  

 Posted: 11-28-2015 12:52 am
  PM Quote Reply
32nd Post
Esprit2
Member
 

Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 575
Status: 
Offline
Exhaust Manifold Gasket
(between exhaust manifold flange & cylinder head)

Original thru 1985 Steel-Asbestos-Steel Sandwich:
A907E0004Z = JH 97263 - Martin Robey JH Parts Manual
A907E0004ZC - 907 thru 1985 910LC - Same as above
(Suffix letters are just an internal inventory control code)

Later 1986-96 910HC & 912HC Stainless Steel, 2-Laminate
A910E2235F - HC Carb, HCi Bosch F-Inj & GMP4 F-Inj

A910E__?__F - Stainless Steel, 3-Laminate
From a UK friend's source.
JAE sells their own aftermarket version.

*~*~*
Stud - Cylinder Head to Block:
A907E0224Z = JH 93576 - Stud, M12, Cyl Head/ Block, Orig.
B907E0224Z - Stud, M12, 1993-on, Head to Block, Upgraded
B-prefix stud is a direct fit into early 9XX engines, just stronger.

Regards,
Tim Engel

Last edited on 11-28-2015 12:54 am by Esprit2

Back To Top PM Quote Reply

 Posted: 11-29-2015 03:56 pm
  PM Quote Reply
33rd Post
stevegarnjobst
Member


Joined: 10-22-2015
Location: Maplewood, Minnesota USA
Posts: 36
Status: 
Offline
More helpful info - thanks Tim!

Here are a few more details on the Lotusbits EFI system. It uses Jenvey throttle bodies, typically 45mm for a stockish engine, and 48mm for a 2.2L/2.5L high compression engine. The custom intake won't fit under the J-H hood, so the kit would use a Lotus or J-H Dellorto manifold, depending on the head being used. The kit includes an ECU, wiring loom, coil pack, crank trigger, fuel pump, pressure regulator, distributor plug, and a few other bits. Lotusbits uses a DTA ECU that looks pretty decent. The S40 is the standard unit, which meets my requirements. The more advanced S60 or S80 provide more logging and features like launch control & flat shift.

Mike recommends the Lotus Eclat/Elite airbox, which apparently produces the best power numbers. I picked one up while at Lotusbits, as the airboxes are apparently hard to come by.

I also picked up a lightened steel flywheel, which is a beautiful piece - just the right weight, nicely finished & balanced.

Lotusbits also has a very nice looking header & exhaust system for the J-H. I'm particularly interested in the header, as it's a 4-2-1 design that should produce better mid-range power than the common 4-1 header. It also assembles in a way that's supposed to make installation a bit easier. I'm curious if anyone has first-hand experience with the Lotusbits header on a Mk II J-H? The exhaust system also looks to be a nice piece, although the mufflers use some fairly large cans, so I'm a bit concerned about ground clearance for the front can. Mike claims 95dB with the full system, and about 100dB without the rear can.

-Steve

Last edited on 11-29-2015 03:59 pm by stevegarnjobst

Back To Top PM Quote Reply  

 Posted: 11-29-2015 09:46 pm
  PM Quote Reply
34th Post
NigelK
Member
 

Joined: 07-16-2014
Location:  
Posts: 200
Status: 
Offline
Hi Steve

I've got the Lotusbits big bore exhaust system fitted to my GT and I can't recommend it highly enough. I don't compete in my Jensen, but the exhaust certainly frees up the engine and also quietens it down a lot when I'm pootling around town. I've had no issues with ground clearance with the front silencer (which replaces the front and middle silencers in the stock system). I also second Mike's view re the airbox - with an unrestrictive exhaust system the stock (muffler type) airbox is restrictive, even with a decent K&N filter. Mike found this when dyno testing my GT after upgrading the engine to Lotus spec 10 - replacing the stock airbox with the Excel type increased bhp by over 20%. In terms of originality for autocross purposes, a few of the very last J-Hs (and a majority of the GTs) were fitted with the T75 engine with the Eclat/Elite airbox, so you might be alright.

Hope this helps.

Best wishes,
Nigel

Back To Top PM Quote Reply

 Posted: 12-12-2015 10:52 pm
  PM Quote Reply
35th Post
stevegarnjobst
Member


Joined: 10-22-2015
Location: Maplewood, Minnesota USA
Posts: 36
Status: 
Offline
Spending a little time on interior modifications this weekend.

First, those floor mats were the easiest 11 lb weight reduction ever. Would never have guessed they weigh that much!

Second, I pulled out the stock seats, which weighed in at 28.4 lbs each, with sliders. The Kirkey Pro Street Drag seats I'm using should come in around 20 lbs each, with mounting hardware. For mounting, I'm just using a pair of 15" x 15" aluminum plates mounted to the stock seat mounting points. This will give me a flat, stiff base to mount the Kirkeys exactly where I want them. I have some Corbeau dual locking sliders to provide adjustability.

I also pulled off the Series 2 bumpers, which weighed in at a whopping 111 lbs, versus 29 lbs for my Series 1 bumpers.

Last edited on 12-14-2015 05:15 pm by stevegarnjobst

Back To Top PM Quote Reply  

 Posted: 02-01-2016 07:55 pm
  PM Quote Reply
36th Post
stevegarnjobst
Member


Joined: 10-22-2015
Location: Maplewood, Minnesota USA
Posts: 36
Status: 
Offline
I received a box from Gripper today. Inside was a shiny new limited slip unit for the J-H! I understand all five units produced by Gripper in this batch have been sold. I'm glad my efforts with the fine folks at Gripper helped make this upgrade a reality for a few lucky J-H owners!

-Steve

Attachment: Gripper LSD smallest.jpg (Downloaded 203 times)

Back To Top PM Quote Reply

 Posted: 02-07-2016 07:10 am
  PM Quote Reply
37th Post
Esprit2
Member
 

Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 575
Status: 
Offline
Steve,

Nice looking diff, but... have you ever autocrossed a lightweight car with a limited slip differential?

1) Limited slip under power will tend to make the car go straight, meaning you can't start to feed in the throttle until you have the car straightened out. Is Phil Ethier still autocrossing? Ask him about the LSD a PO had put into the Lotus Europa he used to autocross. He put regular gear oil in it, and just prayed for the day the clutches would wear out. When they did, the car got much better at handling the tight courses.

2) With the near-stock 907's modest torque, I don't think you'd have too much difficulty managing the wheel spin with an open diff. With a hot rod 2.2 or 2.5 it would be a different story.

Regards,
Tim Engel

Back To Top PM Quote Reply  

 Posted: 02-07-2016 01:40 pm
  PM Quote Reply
38th Post
stevegarnjobst
Member


Joined: 10-22-2015
Location: Maplewood, Minnesota USA
Posts: 36
Status: 
Offline
Esprit2 wrote:
Steve,

Nice looking diff, but... have you ever autocrossed a lightweight car with a limited slip differential?

1) Limited slip under power will tend to make the car go straight, meaning you can't start to feed in the throttle until you have the car straightened out. Is Phil Ethier still autocrossing? Ask him about the LSD a PO had put into the Lotus Europa he used to autocross. He put regular gear oil in it, and just prayed for the day the clutches would wear out. When they did, the car got much better at handling the tight courses.

2) With the near-stock 907's modest torque, I don't think you'd have too much difficulty managing the wheel spin with an open diff. With a hot rod 2.2 or 2.5 it would be a different story.

Regards,
Tim Engel


Yup, LOTS of experience with lightweight autocross cars - both with and without limited slip differentials. When set up properly, a car with a limited slip will virtually always be faster than one without a limited slip, assuming the vehicle has sufficient power to spin the tires under acceleration.

The issues you describe are definitely a problem with some limited slip designs, or with limited slip units not properly tuned for autocross use. For autocross, what you want is a differential that behaves like an open diff on corner entry, which minimizes/limits the understeer behavior you describe. However, you want the limited slip to distribute power between the drive wheels when accelerating, such as on corner exit.

With the Gripper unit, diff behavior is adjustable, via ramp angles. The unit permits different diff engagement settings for corner entry/deceleration and corner exit/acceleration. I'll be running the diff with settings appropriate for autocross use.

On a related note - it's fairly common for road racers with older low-powered British cars to run "welded" diffs, which essentially creates a locked differential. Welded diffs display precisely the behavior you describe. It's very difficult to drive a car with a welded diff on an autocross course, as you literally have to break the tires loose and slide the car around every tight corner.

Last edited on 02-07-2016 01:41 pm by stevegarnjobst

Back To Top PM Quote Reply

 Posted: 12-12-2016 03:32 pm
  PM Quote Reply
39th Post
pmebill
Member
 

Joined: 02-08-2012
Location:  
Posts: 1
Status: 
Offline
Greetings Steve! I'm also going to be running my JH (74) in SCCA AutoX FSP class this season. I've been away from the JH game for a good 4 years now, but just picking up my replacement in a week or two. Already has Dellortos, Header, upgraded shocks and sways. Also a roll bar. I'm going to read through your thread in more detail later on today/tomorrow. Perhaps we can compare build/prep notes and meet up at Nationals! :)

~Bill

Back To Top PM Quote Reply  

 Posted: 04-28-2017 02:36 am
  PM Quote Reply
40th Post
stevegarnjobst
Member


Joined: 10-22-2015
Location: Maplewood, Minnesota USA
Posts: 36
Status: 
Offline
It's been quite a while since I updated this thread. Unfortunately, the J-H's season was cut short last year, due to one of those stupid cast iron tappets flying into small pieces at the wrong time. The car has been sitting on the trailer until now, waiting for sufficient funds to finance the repair.

The good news is I just ordered a bunch of parts from LotusBits, including a re-built head, exhaust header, and a complete EFI system. I'm particularly excited about the EFI system, as I've seen the LotusBits kit in person, and it's a very nicely sorted package. I can hardly wait for a modern fuel & ignition system that will deliver reliable performance and not experience fuel starvation on long left-handers.

While the engine is out of the car, I'll install the lightweight flywheel and take a look at the transmission, as the 1-2 shift currently requires much finesse.

The initial round of suspension upgrades and weight reduction are pretty much complete, so I'll be impatiently waiting a couple months for a shipment from the UK.

-Steve

Last edited on 04-28-2017 02:37 am by stevegarnjobst

Back To Top PM Quote Reply

Current time is 12:58 am Page:  First Page Previous Page  1  2  3  Next Page Last Page    
> Jensen Healey & Jensen GT Tech > Projects > 19064 - autocross project car for SCCA's FSP class Top




UltraBB 1.172 Copyright © 2007-2011 Data 1 Systems