Tall Block Conversions

[jono]

I have a stripped tall block TC motor sitting in my office and keep thinking I should build it up and put it to use.

Interesting history in that it was built new by Vegantune for Caterham in 1979 (as a dry sumped unit) just before Caterham turned to conventional crossflow (1700) engines for their cars. I bought the engine off the chap who ordered the car from new but sadly it was written off a few years ago. It's a new (at the time) Lotus TC head, ie not the Vegantune belt drive version.

Interestingly it's on std crossflow bores but with pocketed pistons a la lotus but obviously not the 83.5mm lotus bores. I've had the block and pistons measuerd by a good machinist and he declares them to be good so I'm not intending to rebore (but will fit new rings). You can still see some trace of the honing marks, the engine has done just over 50k miles (genuine).

Anyway, everything else being equal if I build the short engine back up and use the same head and cams from my current Plus 2 (the head is nicely ported, the tall block head is completely standard), ie using the 711M bottom end, will the resulting engine be signifcantly better that my current unit?

There's no reason for me to do this other than I am pulling the Lotus engine shortly to fix a load of oil leaks and fit a close ratio 3 rail box. I'm thinking the increased torque might be useful to pull the tall first gear.

Yhe dyno curve for my current engine is attached - already produces good torque but what sort of improvement might I expect with the longer stroke 711M block?

Jon

[bitsobrits]

Jon,

Torque is generally linear, +/-, relative to displacement, i.e. directly proportional. So a 1650cc tall block engine should have about 6% more torque than a standard twin cam all other things being equal. But by putting a standard twin cam head on a tall block with appropriate nominal flat top (valve reliefs only) pistons you will increase compression by a considerable amount (you'd have to calculate it, but is should be around 1/2 a point or more) and that increased compression will add torque. Cam specs and carb jetting also come into play, and there is generally a benefit in the longer rod used with the longer stroke crank.

A Sprint spec engine makes something like 113lbs/ft max. My 1725 crossflow crank standard height twin cam with 10.3/1 compression and special cams/porting made 148lbs/ft on a dyno. So I figure the displacement provided about an 11% increase, with the rest of the 30% increase coming from the other factors.

[ianthomson72]

Hi Jon,
If your tall block is on standard crossflow bores it will be only 1600cc. Very little different from a twin cam at 1558cc.
The longer stroke might alter the torque characteristics but i can't imagine that it would be by much.

Steve, what pistons are you using in your long stroke unit ?

Regards

[BobP]

I haven't got a clue what engine I have in my Elan - I just love its looks; I just drive it (mainly gently) on sunny days; and I just do the basic servicing (no tuning or tweaking). But the guy I bought it from had spent hundreds of hours and thousands of pounds on tuning the engine and suspension. One point he made to me was that at some stage he had fitted a long block engine. He said that the main improvement he found from doing this was that the car was a lot more tractable - much happier to amble along at 30 mph in top gear and then to accelerate away cleanly without changing down. It certainly performs that way for me (but for best acceleration needs to be kept above 3000 rpm).
BobP

[bitsobrits]

My pistons are Honda spec forgings which were custom machined to achieve the required compression height, deck clearance and valve clearance. Do not remember the brand.

[Craven]

General idea, graph from back in 1978.