|View single post by Esprit2|
|Posted: 04-09-2006 08:45 pm||
|Paul Koehler wrote:
An inquire for those that have been there. If we condiser a stock JH with Z/S carbs and ~140HP as a starting point and fast foward to the same JH at ~200+HP as an ending point, what is the order of progression to get to the end point, if we were to make it into a pay as you go project. Every year an improvement would be done with the caveat that, everything done year to year, would be useful in achieving the end point
IMHO, the best thing that can be done to a 907 is to install the 2.2 liter stroker crank. Even if the engine isn't really hotrodded any further, the increase in low-end torque transforms the engine. W-a-a-y more than you would expect from a silly little 0.2 liter increase in displacement. Go 2.2 and you'll never go back.
Regardless of which stroke you choose, use the cross-drilled version of the crank along along with the plain lower bearing shells (no hole/ no groove). The plain bearings are the "plus" feature, the cross-drilled crank is what makes using them possible. Without the plain bearings, the cross-drilled crank is of marginal incremental value. No harm... but little gain.
Shuffle-pinning the crank (dowel pinning all ten studs) does significantly strengthen the block/MBP assembly. Properly done, it's very worthwhile. However, doing the job properly also involves align-boring the main bearing bore "after" the dowel pins are installed. Go the whole 9 yards, or don't bother at all. Dowel pinning without the align-bore has the potential to do more harm than good.
The semi-hemi combustion chamber is very tolerant of octane/ compression. A 10.9:1 to 11.5:1 compression ratio can be used with the pump gas that's available in most areas of the USA and Canada. 10.9:1 is very comfortable if you don't mind the higher cost of Premium grade fuel. If you live in an area where the available fuel has a low or inconsistant octane rating, then it's probably wise not to increase the CR much above stock. Well, not Federal-stock... 8.4:1 is lame. Something more like 9.5:1 or 10:1.
The stock cast aluminum pistons are a failure waiting to happen. Go forged with the next set. JE Pistons has a forged piston set for the 9XX engines in your choice of compression rations.
There's no need to "upgrade" from carbs to fuel injection just to accommodate higher compression. If you are putting the work into the engine that would support ~11:1 or higher compression, then it makes sense to upgrade to Dellorto DHLA 45 carbs; but IMHO, the cost of a custom fuel injection will not produce a justifiable performance increase. That money could be put to better use paying for other more meaningful upgrades.
Besides, carbs make that lovely noise.
The cylinder head has a lot of un-tapped breathing potential. The easiest head upgrade is to simply install more aggressive cams. The popular 107 cam upgrade is actually milder in terms of duration. Their big advantage over stock is more low-end torque rather than more horsepower. Then to avoid a power loss, the valve lift was increased. In the end, the 107 is actually a pretty milquetoast cam. It's a step up for a near stock street car, but if you are thinking about really hotrodding the engine and doing the stuff discussed here, then the 107 is really too mild to consider.
Stock 907 tappets are are cast iron and not reliable much beyond 7200 rpm. And when they break, things get expensive in a hurry. Current Lotus service replacement tappets (1995 onward) are steel and good for around 9500 rpm in a properly prepped engine. Lotus still set the rev limit at 7500 rpm, but then they were saddled with a warranty program as well.
The three "C's"... compression, carbs, cams. Always keep them in balance. If a significant change is made to the the compression ratio, the full return on the investment will not be realized if the carbs and cams are left stock. Same with any of the other "C's".
Going to a set of DHLA 40's with 34mm or smaller chokes is pretty much a latteral move from the Strombergs. They're easier to tune and feel more responsive, but they don't really make more horsepower than the Strombergs. However, if larger chokes are to be used, especially in a set of DHLA 45's, then the full reward won't be realized unless the compression ratio and cam timing are upgraded as well.
For a hot street engine in an area where emissions testing isn't a limiter, I'd suggest 2.2 liters, 10.9:1 to 11.5:1 compression, DHLA 45's. 36 or 37mm chokes and at least 104 cams. A 2.0 on 104's isn't much fun below 3500-4000 rpm, but a 2.2 is perfectly tractable with 104's. I'm using Dave Smith's DS-2 cams in my 2.2 907 (a step up from 104's) and the engine is comfortably driveable in traffic.
Carbs are only part of the breathing problem. Putting huge carbs on the smallish stock ports doesn't gain much. Next to the stroker crank, the best thing you can do to a hotrod 907 is a generous port job. The original 907 ports were pretty darned small. My 907's head has an aggressive port job and you can roll golf balls down the ports. Well, okay... maybe not. But compared to the OEM ports, it sure looks like it. When I install a new intake manifold gasket, I have to enlarge the openings quite a bit.
In the USA, Westcoast Cylinder Head knows as much about porting 907 heads as anyone and he has a good relationship with the JHPS. But my personal favorite is Dave Smith in the UK. Dave was formerly with Lotus and built all the factory team race engines for the 4-time champion Sunbeam-Talbot-Lotus World Rally cars as well as Chamberlain's LeMans Esprits. He's sorta slow (slower if you push on him), expensive, and it requires big shipping charges round-trip from the USA. But if there's horsepower hiding in a 9XX cylinder head, he knows how to let it out.
Big valves will give high rpm power at the expense of low-end torque. For a hot-street engine, the stock valves might be the better choice. That's a personal call you have to make on your own. For the serious boy-racer, track days or club racing, larger valves are a plus once you get the car launched. The best starting point for the cylinder head would be the current service replacement head from Lotus (1995 onward)... the Zeus head.
The Zeus ports are re-configured in the casting and specially contoured around the valve seat to work with the unique contour on the back side of the larger Esprit S4s intake valves. A different port and runner shape plus more material around the runners allows you to go places with a die-grinder that you just can NOT go with an early 907 head. But that's like the fuel injection... big bucks for a questionable return on a street engine.
If you port the head, then dowel-pin and match the intake manifold to the head and continue the port job all the way to the carb mounting flange.
The engine runs better and makes more power with an enclosed airbox. The open, K&N-style air cleaners make glorious noise, but they breathe hot air from within the engine bay. In a properly tuned system, the total length of the runners, plenum and the cold air trunking to the front of the car all influenced the "tuned length" induction system. I'm Lotus guy and not all that familiar with the J-H's stock induction system (it looks a little huge and convoluted), but I would think the airbox and trunking from a Lotus Elite/ Eclat would easily fit under the J-H's bonnet.
If, rather than pulling the engine and rebuilding it, the plan is to simply bolt some external enhancements onto a 907, then I would start by installing the cams first.
A stock 2.0 907 doesn't have enough torque to peel a grape. Anything that will improve the low-end torque will make the car much more pleasant to drive on the street. A pair of 107 cams will do that... more torque, but not necessarily more horsepower.
The 107/104 cam combination won't specifically help the low-end torque, but it will give a horsepower gain on the top end without hurting the bottom end. Usually, you have to rob Peter to pay Paul, so this is almost like a freebie power gain.
If you're starting with a "Federal" 907, then I'd bump modifying the distributor to the top of the to-do list. The Federal ignition timing totally emasculates the 907. Cuts 'em right off. Simply cranking in more static timing (~14 BTDC) will make a big difference and that only takes minutes. Re-curving the distributor to give about 16 degrees of mechanical advance all in by 2500 rpm would take a little more time and effort, but would put some life back into the engine. (Have a second, stock distributor on hand if annual emissions checks are an issue).
Next, convert to Dellortos. DHLA 40's if you don't plan to stray too far from stock/street. DHLA 45's if you really have any aspirations of reaching that 200 hp level you mentioned (only J-H 907's came with 40's, all Lotus 907's came with 45's... they do work). I'd top them off with a Lotus Elite/Eclat airbox and a cold air duct to the nose. There's a hole in the front bulkhead low and to the right of the radiator that would be about right for an air duct. The Lotus duct is actually tappered (larger at the airbox, smaller at the inlet) and is supposed to make a difference in how well the engine runs compared to a typical straight duct. I haven't dyno tested that myslef, but that's Lotus' story.
As I mentioned earlier, a pair of DHLA 40's with 34mm or smaller chokes won't make much more power on a dyno. But throttle response will be much better so the engine will feel more lively. Plus they are easier to tune and stay tuned longer than Strombergs. If you are thinking "hotrod", then go for DHLA 45's with 36mm chokes as a starting point. 38mm chokes are about the limit of the range for a hot 907 with the right compression and cams, but you won't like them on the street.
The stock exhaust manifold actually flows rather well. However, if you have done the other upgrades, then there's "maybe" 3-5 horsepower max to be gained with a good set of headers. During the bolt-on phase, I'd leave the headers for last.
Beyond those simple bolt on's, you really need to pull the engine and go inside. You "might" see 160 hp with bolt ons. Some of those Shetland Pony kinda horsepower like Lotus' optimistic dyno used to report. But if you really want to pursue more serious horsepower, then you need "build" a proper hotrod motor from the inside out.