engine and drivetrain Cr Ratio Importance For Hp

[73touring]

OK, here's the background my touring is very good mechanically. Has electronic ignition in the form of an Illumination optical  system and blue coil. A 32/36 Weber. The whole car including the engine and gearbox have only 65k on the clock.

 

It still has its original ball joints and bushes with the exception of the anti-roll bar bushes. The bushes are now a little tired as will be replacing them over the summer but I think that gives you a feel for the car. Consumables have been replaced as required, fuel pump, water pump, head gasket done about 5 years ago. Anyway its very fresh mechanically. 

 

I don't want to mess will a very fresh bottom end. I don't have compression test results for the car but it starts on the first half turn, doesn't smoke, doesn't burn oil noticeably in its 3k service interval. 

 

In the top end there's evidence of some top end work early in its life. marks on the rockers ect but very good. no wear I can determine on the cam or followers and no play on the rockers or shafts.

 

 

 

Bores when I pulled the head to do the head gasket about 5 years ago appeared very good. No determinable ridge from rings ect.

 

Id like to get a bit more get up and go above 3k but naturally don't want to mess too much with a good motor. From researching Compression ratios a bit various sources on the inter-web and and David Vizards book on the A series (Mini) engine. The gains for going from 8.5 to 9.5 seem pedestrian in an engine that already breathes well like a m10.

 

True its a 121 head but it appears to be the later larger valved version being build in May 72 (Stamp on head) If it was a OHV engine like an A series yes there are substantial gains to be made by adding squeeze but from my observations I only see a net increase of 6-7 hp with a piston change to 9.5 so in effect I've no desire to rebuild a perfectly good bottom end for so little gain.

 

Mick McCarthys book states that an-otherwise standard bottom end with a free flowing exhaust, a set of 40s and a 292 cam can achieve 140bhp, is this realistic?? ie if I modified this recipe a bit to a Weber downdraft 38 in place of the twin 40s and put a 292 in the car is 130bhp possible with the correct exaust upgrades. If so I think Id be more than happy, but I have my doubts. Anyone have a Dyno of a similar set-up? Before and after would be nice!. Also what about the 284 cam? is it worth the bother of changing to this. This car is not generally used as city car and spends allot of time on small twisty roads.

 

Any thoughts, Inputs would be appreciated.

 

 

 

 

 

 

 

 

[Cyclone101]

Alpina had the A1, that used a stock bottom end with Weber dcoe 40s that made 115bhp.

There are some gains to be made, but 130bhp sounds optimistic without adding compression.

Dynamic compression ratio means more than static, and adding an aggressive cam can lower it, thus hurting your performance.

I would say, if you don't want to touch the bottom end, then carbs (yes 38s are fine) and a less restrictive exhaust is your best bet. Cams might help up top, but will most likely hurt you low end.

For best results, 9.5 or 10/1 will be the best for a street car. It does make a big difference when used in combination with other mods.

[Cyclone101]

Also remember, that a higher peak bhp does not mean the car is faster. You want the area under the bhp curve to increase for a usable street performance engine.

Do a compression AND leakdown test first before you assume the condition of your engine.

Start with carbs and exhaust, it is bolt on and can be adjusted for cam/compression changes later on.

I prefer to do one mod at a time, that way you can tell the difference each change has made.

Regards

Jacques

[2002dean]

In my opinion if your going to go with a cam, you need to go all in. Proper head work along with bottom end work. Otherwise, like stated above, start with an exhaust system and a carb. Or even just an exhaust, and make sure the carb you have is tuned to its full potential.

Edited May 4, 2014 by 2002dean

[TobyB]

 

Do a compression AND leakdown test first before you assume the condition of your engine

 

This.

 

If you get good, consistent numbers, then sure, breathe on it a little.

 

It's been said millions of times before, but it's worth repeating:  an engine is an air pump system.

As it came from the factory, it was pretty balanced, but had a conservative cam, and mild compression

for what it was.

(note the cam NEVER changed from 1967 thru the emission years!)

 

So you can improve parts (cam, manifolds, etc) and you'll see results.  But then the remaining stock

parts will start to limit you (compression).

 

If you pull the head again, a skim will level the head AND bump you a couple of tenths, with few ill effects...

 

Also, do keep in mind that rings lose their spring over the years, so you may find you don't have very good ring seal...

... or it might be fine.  Thus, the compression and leakdown...

 

t

[73touring]

 Point taken Jacques & Toby, no point wasting money unless you can 100% confirm the state of the bottom end.  I do a compression test next plug change. 

 

Thanks.

[TobyB]

Do leakdown, too- it is a particularly useful way to test ring seal, and the $60 gauges do a good- enough job.

 

hth

 

t

[oldguy]

you will find, in general, pistons will change the power across the board; where cams will rob from the bottom the put up top.  Carbs can fix that somewhat.  DCOE's can be choked to give optimal power to match the new cam power band and develop a synergistic effect.  a downdraft 38 or 40 will never ever  ever be on the same playing field as sidedrafts.  

[kbmb02]

a downdraft 38 or 40 will never ever  ever be on the same playing field as sidedrafts.  

If you've got a sidedraft race car, take a trip out to Southern California and race with VARA (and, be prepared to remove the phrase 'never ever ever' from your mind). -KB

[MKINNA]

If you've got a sidedraft race car, take a trip out to Southern California and race with VARA (and, be prepared to remove the phrase 'never ever ever' from your mind). -KB

So true, they're defiantly giving most of them a run to chase down;)

[jireland2002]

Frist do a compression test.  Piston to valve clearance with 292 cam and stock pistons is pretty close.  If you skim the head you'll need to check the clearance carefully before final assembly.

[73touring]

Thanks Jeff. Any other words for wisdom from the oracle regarding street able power, down-drafts and standard bottom ends?

 

Rob.

[73touring]

Ok so this is a very later follow up. Finally did a compression test results 155,150,145,150. Which is in the Good to Very good range for a standard 02 motor.  Where to now. Dont want to rebuild that in a hurry.

 

Rob,

[Cyclone101]

Well, if you don't want to rebuild, then add some carbs/cams/exhaust and be done with it. There should be some improvement, but there are no guarantees in this business.

Also using old rockers on a new cam can damage the cam lobe, best to resurface the rocker pad or get new ones.