|View single post by Esprit2|
|Posted: 11-18-2015 05:47 pm||
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.
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.
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
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:
17734 County Road 9, Lester Prairie, MN 55354
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
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
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.
23245 Agate Lake Rd, Deerwood, MN 56444
Last edited on 11-18-2015 06:33 pm by Esprit2