Want to do a long rod 1835

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Ratdaddyvdub
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Want to do a long rod 1835

Post by Ratdaddyvdub » Sat Apr 01, 2017 8:48 pm

Hey guys I'm new here and have a question. I'm wanting to do something a little different. I know every one says stay away from bastardized combos and mini strokers but I like a challenge. So here is my plan. I would like to run a 69mm forged counterweighted crank and thick wall 92 cylinders with a pair of revmaster 40x35 heads.
My question is what is a good rod piston combo.

I like freeway not stoplight driving

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ONEBADBUG
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Re: Want to do a long rod 1835

Post by ONEBADBUG » Sun Apr 02, 2017 1:55 pm

I'm about to build an engine like that. I had some big valve heads cut for 92's, a 69 mm cw crank, and some stroker pistons. So, I'm going to get CB's longest rods, 1.4 rockers, and make a high revving motor. I haven't picked a cam yet.

Ratdaddyvdub
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Re: Want to do a long rod 1835

Post by Ratdaddyvdub » Sun Apr 02, 2017 2:05 pm

How long a rod would you need for the stroker pistons and what would it put your deck height at without shims

Ratdaddyvdub
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Re: Want to do a long rod 1835

Post by Ratdaddyvdub » Sun Apr 02, 2017 2:11 pm

How long a rod would you need for the stroker pistons and what would it put your deck height at without shims

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ONEBADBUG
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Re: Want to do a long rod 1835

Post by ONEBADBUG » Thu Apr 06, 2017 4:19 pm

I didn't really do the math, it appeared that the extra rod length was close to the stroker piston offset.

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Piledriver
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Re: Want to do a long rod 1835

Post by Piledriver » Thu Apr 06, 2017 9:08 pm

B pistons are 35mm pin height
As are around 42.
Stock deck is ~212mm. (zero deck) so you want a +2.75" rod or so.
5.7s would work with some base shims and/or a head gasket.
Rod ratio will be 2.09.
Stock T1 is 1.98, which is mighty high already.
I wouldn't expect more than 3 HP gain at top end, if that.
Will probably just shift the rpm range up by a few dozen...

I can't recall if it was Smokey Yunick or Larry Widmer that said:
"The ideal rod ratio is the one that puts the piston at the right place at TDC"

This sort of swappage makes some sense on t4s, since there are still no off the shelf hipo rods for the 66 cranks, and the deck works out ~stock t4 with t1 rods and t1 b pistons in 2L t4 jugs.
I, for one, regularly embrace our new robot overlords, as I am the guy fixing the robots...

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Marc
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Re: Want to do a long rod 1835

Post by Marc » Fri Apr 07, 2017 2:18 pm

I'm the king of odd-ball combinations, it's always fun to push the limits of the flexibility of the ACVW design. Strongest argument against "trick-of-the-week" engines is that there may be little interchangeability with the next one, which can limit what you can do with the remains if/when it scatters (or you just get bored with it). But IMO this venture isn't worth the effort, it's been done and there's little to gain for your trouble. But if you feel you must, read on.


Pin height difference is 5.6mm ("A"s are 39.6, "B"s are 34mm). That's .220", so to maintain the same nominal deck height you'd need a rod a bit over 5.6" ...assuming the case is decked some in the process of opening up for the larger cylinders, 5.6" should work out pretty close.

That'd give you a rodlength-to-stroke ratio of 2.06:1 (stock being >1.98:1, already one of the highest in any passenger car engine ever made).

I've run 5.7" on 69mm stroke (almost 2.1:1) on circletrack motors that spent most of their time above 5500 RPM and it does make for a "happier" engine at high speed (although the increased piston dwell near TDC gives the piston time to tilt back & forth at least three times at lower speeds - sounds a lot like a diesel at idle). It's eerie when you're rotating the crank by hand and can rock the pulley back & forth by ~15° without the pistons moving.

High rod ratios can make more power at high RPM since the cylinder pressure will be higher due to the volume staying smaller for longer after ignition (which means the octane requirement may be higher); the intake ports can be smaller before the engine runs out of breath at the high end because the slower-descending piston doesn't try to pull as much through them initially. Some engine builders completely discount the effects of changing rod ratios because they "tried it once" and saw no difference in dyno peak numbers, but there's much more involved than they've taken into consideration - changing that one parameter affects almost every other factor of engine design, it's not a simple matter of "all other things being equal" because they're NOT. It has a dramatic effect on low-end manifold vacuum, ideal camshaft timing & LSA. Yunick realized this and as a rule his race motors had the longest possible rod he could squeeze in...but again, that's for roundy-round where once the handling's right you're often running flat-out for extended periods and don't care about low-end tractability.

Unless you're building an engine that will be used mostly at very high RPM I would not advise going over 2:1. The "B" pistons will probably be lighter & shorter than the same maker's "A"s; inertial forces will be lessened so the rods & pins can safely be lighter, and you won't need as much port work, but that's pretty much the only benefits, everything else is a negative for street use.

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Re: Want to do a long rod 1835

Post by farmer » Sat Apr 08, 2017 11:15 am

What Marc said.
If you want a stop light racer you should rather go the other way and use a set of 5,325" rods along with a set of "B" pistons. That will make for a very narrow and sturdy engine which can be built to an incredible throttle reponse with the right combo.
One of our local race engine wizards did a 1600 engine some years back (86 x 69) where he focused a lot on, if not perfect then at least very good cylinder head flow and flow balance in conjunction with the rest of the engine. He kept the stock rods and bought a set of lightweight pistons with about 34 mm pin height, thin rings etc. That engine pulled and still pulls over 190 hp @ 7400 w.o. belt and about 178 in street trim.
Just after break in. https://www.youtube.com/watch?v=VPVqYRrQBA0
Good repeatability. The power is there: https://www.youtube.com/watch?v=9868ihgGVk8
On the quarter in a nice light beetle. https://www.youtube.com/watch?v=v15y0b480GY

I have done a 1680 for a German customer that wanted a small old school engine I "only" made 152 hp @ 6600 rpm. with that one. It drives quite well, but is no match at all to the JPM engine.

T

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4agedub
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Re: Want to do a long rod 1835

Post by 4agedub » Thu Apr 13, 2017 3:00 am

A couple of years ago the racing series which we competed in placed a max engine capacity of 1840cc on our engines. So we came up with this creation..
69mm german welded counter weight crankshaft, de stroked to 65.39mm with chevy big ends. If I remember correctly the rods were 5.8" long and we used 94mm B pistons and barrels. This meant that the engine was still roughly the same width as std.

The camshaft was a CB 2298 combined with a set of CB 044 super pro heads (with tiny combustion chambers, 12:1 compression) and 48mm ida's that we stripped out and made throttle bodies with injectors on the intakes.

The engine made 110kw on the wheels uncorrected at 4300ft above sea level (148.5hp @ 7800rpm). The limiting factor for us was the valve train, we did not have funds for roller rockers and super lightweight push rods. As for torque... there is not much. RPM is your friend with this engine. :)

https://www.youtube.com/watch?v=UUOOYX8pOhA
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WickedWagens
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Re: Want to do a long rod 1835

Post by WickedWagens » Fri Jun 02, 2017 2:51 pm

I'm putting together a long rod 1915cc right now for land speed racing. I have a DPR 69mm crank with Chevy journals. I ordered 5.325" rods to keep the rod ratio in line, but then didn't like that I would have to cut the cylinders down if I ever had to replace one. So now I am going to 5.7" scat H beams with 3/8 bolt rods with AA Forged 94mm B pistons. It's going to be high RPM anyways so we will see how things work.
Piledriver wrote:
Thu Apr 06, 2017 9:08 pm
I can't recall if it was Smokey Yunick or Larry Widmer that said:
"The ideal rod ratio is the one that puts the piston at the right place at TDC"
Bill Miller at BME feels the same. He said use whatever rod gives you the correct deck.
1968 Ghia Land Speed Car
G/CFALT 106.643 MPH 8/2016
G/CGALT 113.131 MPH 9/2016
G/CBGALT 134.606 MPH 8/2017
G/CBFALT 146.715 MPH 8/2017

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WickedWagens
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Re: Want to do a long rod 1835

Post by WickedWagens » Wed Aug 23, 2017 9:20 am

I ended up going with a 5.6" H-beam rod and "B" pistons with a 69mm stroke and ended up with .035" deck height. I added a .010" shim to get .045" deck. Engine runs great.
Last edited by WickedWagens on Fri Aug 25, 2017 4:59 pm, edited 1 time in total.
1968 Ghia Land Speed Car
G/CFALT 106.643 MPH 8/2016
G/CGALT 113.131 MPH 9/2016
G/CBGALT 134.606 MPH 8/2017
G/CBFALT 146.715 MPH 8/2017

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FJCamper
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Re: Want to do a long rod 1835

Post by FJCamper » Fri Aug 25, 2017 2:46 pm


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WickedWagens
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Re: Want to do a long rod 1835

Post by WickedWagens » Fri Aug 25, 2017 5:12 pm

It's all great theory and I'm sure its true, but I doubt that any of us would be able to tell the difference. I have yet to see A-B-A testing on any site when this debate comes up. None of us would know any better due to the fact that you are not going to rebuild your engine with a different rod length just to see if it feels different or if the power band has moved. What is the actual HP difference and how far one way does it move the power band? You would have to do some serious dyno testing to find these answers.

I can see that in certain racing conditions that you can take this theory and use it to your advantage, but you would never know the other side of the plate on how it would run differently with a different length rod and everything else exactly the same. For 98% of all other engines, I don't think it is as big of a deal as people make it to be.
1968 Ghia Land Speed Car
G/CFALT 106.643 MPH 8/2016
G/CGALT 113.131 MPH 9/2016
G/CBGALT 134.606 MPH 8/2017
G/CBFALT 146.715 MPH 8/2017

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