85.5x74 (1699cc) revisited

[FJCamper]

Above: The 1300cc / 1500cc Super Porsche engine with roller crank, and Solex single barrels

From the 1300cc Normal in 1951 to the 1600 Super 90 in 1964, Porsche used a 74mm stroke. Piston bore sizes changed, from 74mm (not a typo, 74mm just like the stroke) for the 1300, to 80mm in the 1500's, to 82.5mm for the 1600's.

The only exception from 1951 to 1964 was the 4-cam (Type 547) "Carrera" engine, which in the 1500cc version, had 85mm pistons, and 87.5mm for the 1600, but both used a short 66mm stroke for a 7000 RPM+ redline.

This brings us to the VW engine, and it's use of the 69mm stroke for the 1300/1500/1600 engines. Porsche had opted for the "longer" 74mm stroke (in relation to the bore) for extra torque over a shorter, higher RPM stroke.

The Porsche factory intended for their pushrod engine to live within its 5500 rpm to 6000 rpm range, and sporting intentions considered, a low-stress engine.

We're rebuilding our recently broken 1600 vintage race engine with a 74mm crank. In that this is a racing engine and not a street engine, we have AJ Sims heads with 40x35 valves. The Porsche 912 (which carried on an evolved Super 90 engine) had 40x34 valves for about 102 SAE HP at 6000 RPM.

The point of building an 85.5x74 is to keep within the spirit of the era and under 2-litre vintage class we represent. We're using an Engle 120 cam, 1.25 rockers, and a 1.5-inch 4-into-1 exhaust. The dry-sump case is heavy aluminum alloy, very strong.

Can anyone make an argument against this parts combo, or share any 74mm stroke story's? Cam suggestion?

Remember, engine width is not important nor is the use of use of heater boxes

Thanks,

FJC

[farmer]

Hello.
I can´t say anything bad about going 74 mm stroke. However, unless the W120 is mandatory for vintage purposes I would look in other directions. 10 years ago I used the W120 every now and then. Today, never, unless a customer absolutely wants it. Choice according to the rest of the engine of course.
The 912 pulled 90 hp DIN at 5800 rpm versus the 356 SC´s 95 hp DIN at 5200 rpm (?) Basicly the same engine

You want peak power in the 5500 - 6000 rpm range? How high would you like to pull above the curve, 6500?
Do you know how the cylinderheads flow? Port volume? flange diameter?

Interesting project. I like small efficient engines.

[Piledriver]

2L limit, why build a 1700?
Want Tq? Displacement is good.

92 thickwall by 74 1967cc? With B pistons and long rods should spin just fine and bolt together ~like stock.
If they would ignore a 20cc rounding error go 92x76.

88x82 or 90.5x78 are also common, proven setups with no assembly issues as good "B" compression height pistons exist.
(OK, 90.5x78 is 2007cc, would they let that slide? If not 92x74 with long rods, maybe a C45 or such with 1.4 rockers for historical correctness)

You ARE racing, right ?


Valves won't have shrouding issues at 92mm, but it would depend on if the heads had already had max flycut.
The only reason to stay smaller bore would be if the case has the huge case savers in it.

[FJCamper]

Gentlemen,

Yes, this is road racing, sprint racing actually, and I'm thinking of a 6500 redline.

We have AJ Sims dual spring 40x35 heads. The 85.5mm bore is in case some one protests us. We can remove a head and say hey look --- 85.5!

With the 1.5 inch exhaust and 40x35 valves, I expect the Engle 120 to move the power band up to 3000-6000.

I've built 74 stokers before, but never for high-RPM output.

FJC

[Piledriver]

Gentlemen,

Yes, this is road racing, sprint racing actually, and I'm thinking of a 6500 redline.

We have AJ Sims dual spring 40x35 heads. The 85.5mm bore is in case some one protests us. We can remove a head and say hey look --- 85.5!

With the 1.5 inch exhaust and 40x35 valves, I expect the Engle 120 to move the power band up to 3000-6000.

I've built 74 stokers before, but never for high-RPM output.

FJC

92mm big bores for VW were available from EMPI etc pretty far back...
Is the rule limit 2L or do they actually require the stock bore?
(I keep forgetting, these historic guys make up rules as they go and enforce at whim)

It will be a lot easier the build a 76-78mm stroker with the available B pistons and your choice of rods.
Longer rods may allow the heads to breathe a little higher RPM.
With a 74 you can fit some pretty long rods...
...But boring >85.5 it will actually improve breathing somewhat with those valve sizes, IIRC.

[Marc]

...88x82 or 90.5x78 are also common, proven setups with no assembly issues as good "B" compression height pistons exist...

Source for 88B pistons? Gene Berg lists some with the obsolescent 97/94.4mm cylinders ($410.80). I'm using a set I've had for years in an 88x76 w/5.5" rods in progress; Berg made them from unfinished Cima blanks with a high ring package. These were last used with an 82 stroke and 5.4" rods; a good deal of the skirt and one wristpin boss took significant clearancing with that combo, I wouldn't do that again without 5.4" and/or "Chev" journal rods.

[Piledriver]

Didn't realize the T1 88s had vanished, everyones probably just using the 90.5s.

88mm Pistons for the Honda K24, or 2,3L Ford Duratec from 87.5-92mm can be had in a variety of pin heights and domes, including flattops if you are ordering custom. Pinheight is only 1.18"/1.122 tho.
("Shelf" pistons are going to have valve reliefs in the wrong places)

4G63/64s offer many similar choices in 85.5.

[Marc]

Besides the dome/dish/compression height issues, you also would want them to come with a 22mm wristpin or something close enough that the rods could be modified for it, and that cylinders would be available for. Boring 87s to 87.5 would be sketchy at best; 88 cylinders that are 97mm O.D. top and 94.4mm O.D. bottom are nice & thick (and still would be if bored up to 89 or so) so if you could find an 88.5 or 89 piston, that'd give a second life to a worn set of those jugs. The new "thickwall" 88s from AA are only 90 at the bottom so their lower spigot is already frighteningly thin, those couldn't tolerate more than clearance-honing if necessary.

The ~30mm compression height would be advantageous with a big stroke, that'd allow use of a longer rod without the need for longer barrels/thick spacers.

Honda makes a 13HP air-cooled one-lunger that has an 88mm bore but I can't find specs on the compression height or wristpin. Besides generators and agricultural/industrial applications the GX390 is often used in quads; better rods and cranks are available for racing purposes but the stock pistons apparently don't need upgrading; oversizes come in .25, .50, and .75mm. for ~$40 each. I wonder how they'd hold up in a VW engine

[Piledriver]

The options I listed offer some or all w/22mm pins, but you'd have to be careful with the dish/dome profiles trying to use shelf pistons.

Some Hondas have a very steep chamber/valve angle so the reliefs are not only in the wrong place and too many, but pretty deep.

I only spent a few minutes researching options via Google, so there are probably many more , but we still don't know what the actual rules are.

[FJCamper]

Gentlemen,

The rule books for vintage racing vary a little from organization to organization. This is not just sporting but a sportsmanship event, and that means there is some wiggle room mechanically, but not much in the "spirit" of the event.

We could go out there with a 94x84 and clean house on MG Midgets and 1.6 Alfas.

So, we're left with a 1.6 or 1.7 mouse motor that has enough output to run with the big-$ highly prepped MGB's and 356's, and if we beat them, it's due to driver superiority.

So, will a 69mm "short stoke" wind higher than a 74mm "stroker?" and is the presumed torque advantage of the 74mm coming out of the turns worth any loss of high RPM power the 74 may have?

And is there a better cam for this bore stoke combo than the Engle 120? I'd use a 110 but it runs out of breath near 6000.

FJC

[Marc]

Depends upon the course and where the competition's advantage is. We used to run a 90.5x69 1775 w/42x37.5 valves and 48IDAs w/37mm chokes on methanol; I figure it probably made around 130 honest HP, but in a 1780 lb car which could get it all to the ground at any time and handled very well it was enough to regularly trounce 200-300 HP front-engine RWD cars on a track with short straights. In fact it could easily outrun 600+ HP Late Models on a ¼mi track, but they weighed 3100 and up so that's not a fair comparison - mighty fun to do though.
One track on the circuit was paperclip shaped, with very long straights and short/tight corners, and there we were at a disadvantage - the higher-powered cars could run us down and overtake to get to the corner entry first, and if the car on the inside didn't have its RF wheel even with the one on the outside's door, the latter had the right-of-way and could shut the door...then take up so much track they were impossible to pass on the outside. Conventional tactics weren't working.
The class rules mandated a car weight (wet w/driver) of 1 lb per cc so I had a worn stock-stroke CW crank offset ground to Buick V6 (A.K.A. "Chev") journals at 73.025 and built an 88-bore 1776.6 w/40x36 valves, same cam & carburetion.
That one came out of the corners stronger (the "faster" cars would only light up their tires if they tried to match it) and was therefore still enough ahead at corner entry to take possession of the bottom groove first, and since we were faster in the corners that was all it took to stay ahead. It did quit pulling ~700 RPM sooner than the 90.5x69, but for that one unique situation the increased low-end torque was worth it. I used long rods (5.7" Chev "pinks" so it could still rev happily well past 7000, but the smaller heads had run out of breath by ~6500.

Ever try a Bugpack 4063? Advertised duration is only 2° more than the W-120 but the action's quicker - 15° more at .050", when you graph them out there's a lot more area under the curve. It's not rated for over 1.1 rockers but it makes more power throughout the 4000-7000 RPM range in my experience than either the W-120 or even the W-125 w/1.1s. I've used that cam on everything from 1585 to 1879cc and it's never disappointed.

[4agedub]

FJC

If the rules permit under 2000cc I would got for a 69 x 94 combo. 1914cc. In the past this has been one of the best combos for our road racing cars. The engine is super smooth and still makes pretty good torque.

When our rules changed last year to 1840cc max we went for a slightly different approach, we de stroked a 69mm crankshaft (by using chevy journal rods 5.6" long) to 66mm and used 94mm pistons and liners. Which gave us 1832cc.The camshaft was a CB 2298, CB 044 Super pro heads and stripped out 48mm ida carbs as throttle bodies with EFI, 1 7/8" merged header. This engine made 130hp at the wheels (7800rpm) uncorrected at 4400ft above sea level. The torque as non existent compared to our big engines. Surprisingly the engine ran pretty good on the long flowing circuits and stayed super cool. Biggest problem was valve train failures due to the high rpm (we could not afford pauter roller rockers)

Dyno run
https://www.youtube.com/watch?v=UUOOYX8pOhA

Rocker failure
https://www.youtube.com/watch?v=J3ALId00NxU

Getting back to your under 2000cc limit, I would go for:
69mm x 94mm (b type pistons and liners to make the engine shorter, 0mm deck the barrels and run 1,5mm copper gaskets in the heads) , scat c55 camshaft (will make good torque and peak at around 6500rpm), CB 044 mini wedge port heads, 1 5/8" header, Dual 44mm idf carbs. This is a very similar setup to an engine I built for a customer some time ago.

[farmer]

OK. Played a little with the set up. I can only use flow and port velocity numbers from my own and some of Tims and CB´s heads. But as would have made them they would flow about 160 cfm @ ,600" and with a port volume in the 62-63 cc range, 9,5 CR. With the W120 installed as is, 5,325" rods, 44 IDF´s w. 34 mm venturies, 1½" merge header, (and the 74 mm crank) the engine should peak at about 6300 rpm and 138ish (Din) hp and flatten out for about 400 rpm. So shift point would be in the 6700 - 6800 rpm range. Peak torque will be around 5000 rpm. and in the 120 lbs range, MAYBE 125 lbs.
There is another option (Well there are many, but I am thinking of this) you want to keep the redline at 6500 for whatever reason. Good. Then I would ask you to consider the Web 110 with lightweight lifters, 1,25 rockers, installed on 108 LC When using this one you can use CB 650 outer springs, ti retainers and ACN Aluminum pushrods. Then you will release some power with only having to use one HD sgl spring. That is good for 3-4 hp plus it stresses the valve train much less. You will also pull the entire power range downwards with 200 -250 rpm. But you will KEEP the power if not gain 1 or 2 hp on top.
Food for thought.

T

[Clatter]

Why not keep the stroke at 69 and run the biggest bore you dare?

The mains won't pound out so bad,
And you can un-shroud more...

[farmer]

He stated twice that he wanted to stay with stock bore.

T