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Hi everyone.

Very soon I will be starting the rebuild of my 1943 gpw engine and wondered if any of you have had any success with mild tuning of this unit ?

What I am planning is this...

GPW block, I may use a spare CJ block but for now the plan is to use the proper one.

Hurricane crankshaft, polished and balanced with the flywheel.

Hurricane rods, shot peened and balanced unless anyone knows of any better rods that are available ?

+.030 pistons, std jeep ones, balanced set, I'm not too keen on the split skirt design, any suggestions on what else I can use ?

Gear driven CJ camshaft, I will clock up the hurricane camshaft to compare it to the CJ one in regard to lift, duration, lobe separation, unless anyone has the spec for a tried and tested profile ?

Cyl head, for now I only have a std GPW head, I would like an alloy supersonic head or similar if anyone has one, or some details of combustion chamber shape and volume ?

The valve seats will be hardened and the guides are new iron ones.

Not sure about valves, I may stick with the std sizes as I have not had chance to work out the lift to diameter ratio yet.

Best valves to use ?

Has anyone experimented with squish and compression ratios ?

That's the basics for now,

I may also post this on the g503 to see what they all think.

Cheers for reading.

Skelly..

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Hi everyone.

Very soon I will be starting the rebuild of my 1943 gpw engine and wondered if any of you have had any success with mild tuning of this unit ?

What I am planning is this...

GPW block, I may use a spare CJ block but for now the plan is to use the proper one.

Hurricane crankshaft, polished and balanced with the flywheel.

Hurricane rods, shot peened and balanced unless anyone knows of any better rods that are available ?

+.030 pistons, std jeep ones, balanced set, I'm not too keen on the split skirt design, any suggestions on what else I can use ?

Gear driven CJ camshaft, I will clock up the hurricane camshaft to compare it to the CJ one in regard to lift, duration, lobe separation, unless anyone has the spec for a tried and tested profile ?

Cyl head, for now I only have a std GPW head, I would like an alloy supersonic head or similar if anyone has one, or some details of combustion chamber shape and volume ?

The valve seats will be hardened and the guides are new iron ones.

Not sure about valves, I may stick with the std sizes as I have not had chance to work out the lift to diameter ratio yet.

Best valves to use ?

Has anyone experimented with squish and compression ratios ?

That's the basics for now,

I may also post this on the g503 to see what they all think.

Cheers for reading.

Skelly..

 

I rebuilt my Jeep engine on similar lines to the above in 1996 and put it on a dyno to measure the power.

 

Basic truths.. side valve engines don't respond to tuning anywhere near like OHV designs.

Because side valve engines are 20% less efficient than OHV they pump 20% more heat into the block and head that needs cooling somehow.

The std carb with 90 degree bend into it strangles the engine. Fancy valves etc will do diddly squat.with a std intake system..

Skimming the head to increase horsepower, reduces gas flow from the valves into the cylinder. so no power gain.

Any power gain you do get will raise engine temps alarmingly you will need an uprated radiator at least.

raised block temps will lead to cracking between the valve seats...been there done that...

Any playing with combustion chamber shape risks cracking the head in the centre of the combustion chamber to the water jacket as it is very thin there..i had 2 heads fail like that..

 

Having said that engine once sorted is still in my Jeep, and has covered 14,000 miles. It clocked 74.7 mph ( 4270 rpm ) with the screen down through timing lights in 1996 as part of an article I wrote for windscreen magazine.

It wont look very original, but putting 2 x 1 1/2" SU carbs on the engine and nothing else will get it going a treat, that what a lot of Yanks did in the late 40,s early 50,s in USA desert racing..( they did have some fancy US alloy heads as well )

good luck anyway...

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Interesting conclusions Don, one question occurs, did you keep the original cam profile and if so at high revs was valve bounce an issue ?. I'm impressed with the 74.7mph top speed with the screen down, I think I would have to wear my best brown corduroy driving pants if I was behind the wheel :-D. There are additional issues with increasing the overall road speed of course, not least the handling characteristics which at even at design speeds can be unpredictable on some road surfaces. Still as an engineering exercise interesting.

 

Pete

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Interesting conclusions Don, one question occurs, did you keep the original cam profile and if so at high revs was valve bounce an issue ?. I'm impressed with the 74.7mph top speed with the screen down, I think I would have to wear my best brown corduroy driving pants if I was behind the wheel :-D. There are additional issues with increasing the overall road speed of course, not least the handling characteristics which at even at design speeds can be unpredictable on some road surfaces. Still as an engineering exercise interesting.

 

Pete

 

with regard to the valves , everybody goes for big valves on tuned engines, you can massively increase the efficiency by cutting the valve seat back ,if you leave a narrow seat and machine the edge off , you are increasing the valve size buy the amount that is removed , flow the porting and your gaining power for free

i`m pure yorkshire so love things for free

jonny loot

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Jonny,s point are very valid,, opening the ports out at the valve throat is an option, but you still need at least 1/8" seat to facilitate valve cooling as it shuts.

The one thing Jeep needs with a 3 speed gearbox is plenty of torque ,,,which of course its famous for. Having a Hotter cam with more overlap will loose low speed torque to get more top end power. That's a simple fact of life for all engines...unless you increase valve lift a little at the same time....BUT as I said yesterday...the Carb and std inlet system is the restriction to power..unless you change that you are wasting your time. Remember its a 2.2 litre Engine with a piddling little carb on it, which is the rev limiter effectively to stop the engine overrevving ( note other US Vehicles had a specific device to limit revs fitted between the carb and manifold )

 

cut a hole in the bonnet, put a 6" long ram pipe on the carb and with cold air going in you will find 5 bhp ..hey presto...( take out the vapouriser gizmo between the manifold and carb. )

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Hi yes, porting is no problem as long as you remember to stick to a few rules and ratios.

Cutting the valve seat back in a side valve engine is not so good for flow though.

Don 74.7 mph ? blimey thats impressive, was that with std tyre sizes ? overdrive ?

I dont have a rev counter on my jeep so not sure what revs it is pulling at say 55 mph.

I think just ballance everything nicely and a gear driven cam sounds like the most sensible option.

carbs and head can be changed anytime as time/cash allows .

I'l post results as I get them.

Cheers.

Skel.

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For my money, I would start with everything external to the block and head, such as intake, exhaust and cooling. Only when everything was flowing in and out of the engine with no awkward corners and restrictions would I start considering the engine internals as serious objects for tuning.

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