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Engine build thoughts? (Before I spend alot of money)
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IA-64
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PostPosted: Thu Jan 22, 2015 1:30 pm    Post subject: Engine build thoughts? (Before I spend alot of money) Reply with quote

Looking into doing this to my wrecked SP 1500.

Single relief case checked, line bored, and bored for 90.5/92mm (already done)
SP Heads s/s valves, single heavy duty springs, 1.25 rockers and machined for 90.5/92mm barrels (already done)
74mm c/w crank
90.5 B pistons
5.5 Rods
Scat c25 cam
Full flow kit
30pict 1 carb
SVSA dist.

I know I can get more power with a different carb, and DP heads, but I want the SP heads and that carb.

I want/need to be able to hook heater boxes up to this, as it gets cold here in NB, Canada. I am concerned about the restriction of them, possibly their are some more free flowing units?

Is there another, easier combo I should be looking at. I don't need gobs of power. Just need it to go down the highway well.


Last edited by IA-64 on Thu Jan 22, 2015 3:39 pm; edited 1 time in total
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PostPosted: Thu Jan 22, 2015 2:06 pm    Post subject: Re: Engine build thoughts? (Before I spend alot of money) Reply with quote

IA-64 wrote:
Looking into doing this to my blown SP 1500.

Single relief case checked, line bored, and bored for 90.5/92mm (already done)
SP Heads s/s valves, single heavy duty springs, 1.25 rockers and machined for 90.5/92mm barrels (already done)
74mm c/w crank
90.5 B pistons
5.5 Rods
Scat c25 cam
Full flow kit
30pict 1 carb
SVSA dist.

I know I can get more power with a different carb, and DP heads, but I want the SP heads and that carb.

I want/need to be able to hook heater boxes up to this, as it gets cold here in NB, Canada. I am concerned about the restriction of them, possibly their are some more free flowing units?

Is there another, easier combo I should be looking at. I don't need gobs of power. Just need it to go down the highway well.


The idle will suck with that cam. Stick with a stock cam with single port heads and that carb.
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IA-64
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PostPosted: Thu Jan 22, 2015 2:13 pm    Post subject: Reply with quote

What about the w100 or scat c20? They are both pretty mild.

Or maybe the web cam grind 86
http://vwparts.aircooled.net/Web-Cam-Type-1-Camshaft-Grind-86-1-1-or-1-25-p/00-032.htm
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PostPosted: Thu Jan 22, 2015 2:37 pm    Post subject: Reply with quote

with a blower I would think the idle should be ok. make sure you have a good exhaust & fuel pump. those rods are a little short for that stroke but with a blower probably ok.with out a blower I would go with 76 stroke&those rods.
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IA-64
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PostPosted: Thu Jan 22, 2015 3:38 pm    Post subject: Reply with quote

..... I just saw my top post, so stupid! By 'blown' I meant wrecked motor, not turboed.

I would be ok with using a 76mm crank. I am aware I may have to clearance the case a little, but I want it to go together well and have the heater boxes work.

I want to shim things as little as possible, and have the engine as close to stock size as possible for engine tin etc...
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PostPosted: Thu Jan 22, 2015 3:42 pm    Post subject: Reply with quote

Wouldn't you want to use 5.325" rods with a 74mm crank to keep the engine stock width?
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IA-64
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PostPosted: Thu Jan 22, 2015 5:38 pm    Post subject: Reply with quote

srfndoc wrote:
Wouldn't you want to use 5.325" rods with a 74mm crank to keep the engine stock width?


Is there a math formula for figuring out this? How do you figure out how the parts (crank, rod, a or b pistons) affect things.
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PostPosted: Thu Jan 22, 2015 6:03 pm    Post subject: Reply with quote

There are many variables in engine width, but you will at least have the extra 5mm of the crank, and any difference from the stock rod's 5.392 length. "A" pistons, and "B" pistons vary in difference from manufacture, but you could measure the wristpin height from your stock pistons compaired to your "B" pistons. It's been a while since I measured, but it sticks in my mind Mahles are about .125 difference from "A" to "B". The shop I used to work at used CB's 2280 "cheater" cam, and Crower's 61002 in single ports with good success.
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PostPosted: Thu Jan 22, 2015 10:59 pm    Post subject: Re: Engine build thoughts? (Before I spend alot of money) Reply with quote

normally you go

5.325 or 5.4" rods and A pistons with 74mm stroke
or
5.5" rods and B pistons with 76mm stroke.

As it is, your combo is a mismatch. Either change to A pistons and the shorter rods, or the simpler solution is the 76mm crank instead of the 74mm.

I never had a problem with a C20 or C25 and a stock carb, but did have epic fails with the C35 and the stock carb. :-/
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PostPosted: Thu Jan 22, 2015 11:40 pm    Post subject: Reply with quote

Go the 76 stroke and B pistons, it actually is less work to make it fit.
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PostPosted: Sat Jan 24, 2015 9:37 pm    Post subject: Reply with quote

Thanks for the help. Looks like I'm going to do the 76mm crank. Any real benefit to doing a 78mm crank with the 5.5rods or would I be too wide/mismatched again?
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PostPosted: Sat Jan 24, 2015 9:52 pm    Post subject: Reply with quote

76 or 78 crank with 5.5 rods should be close enough to adjust with cylinder shims.

If you haven't bought anything yet maybe try 74 stroke A pistons and 5.325 rods. It works out abotu the same:close enough to adjust with shims.
SP heads and a tiny carb puts a big cap on power so I don't see a benefit in going larger than 1904cc.
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PostPosted: Sat Jan 24, 2015 11:46 pm    Post subject: Reply with quote

A stock carb is not big enough for a stock engine. I personally would not waste the money building anything larger then a 1776 if you must keep SP heads and a stock carb along with stock heater boxes..
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PostPosted: Sun Jan 25, 2015 12:32 am    Post subject: Reply with quote

I have played around with single port engines a little pretty much stock though, Pict 28, 30, 32mm carbs and W-100 Cam, Counterweighted crank. It should be fine! It will sure run a lot better than it did stock if you can get the right resonance happening. I was getting 32 mpg all around driving average running the Pict 28.
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IA-64
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PostPosted: Sun Jan 25, 2015 5:57 pm    Post subject: Reply with quote

I am buying a new crank, new rods, and a new cam. The heads and case are machined for 90.5/92mm already

Is there any disadvantage to going to the larger 78mm crank and b pistons, other then I won't be using it to it's potential?

I have no problem doing the 74mm crank and A pistons, but if down the road I decide to change the heads and carb to something more performance oriented I'd like the rotating assy. ready.

Is it possible the 74mm crank and A pistons with the SP heads and stock carb would be easier on gas, more lively then the 78mm crank setup?

Feel like I may be overthinking this Rolling Eyes
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PostPosted: Mon Jan 26, 2015 6:56 pm    Post subject: Reply with quote

Well yeah, a 74mm X 90.5 which is 1904cc is going to be a little easier on gas than a 2007cc engine. When Porsche designed the 356 engine they choose a 74mm stroke. I have seen them get 36mpg and also 19mpg. So do the 74 and the 5.325" rods or the 76 and stock rods and B Pistons & Cylinders.
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PostPosted: Mon Jan 26, 2015 7:26 pm    Post subject: Reply with quote

In this case yes, on the highway I think it would actually go slower with 78 stroke, and have reduced mileage also.

I am already assuming you will change the carb and intake manifold later on. Those are too small for a 1600, nevermind a 1900 Shocked
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PostPosted: Mon Jan 26, 2015 7:27 pm    Post subject: Reply with quote

a 78mm crank is going to require more case clearancing than the 76mm. Other than that, no extra work.
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PostPosted: Tue Jan 27, 2015 4:31 am    Post subject: Reply with quote

If you are running the stock carbs, and with single-port heads anyway, I would run a smaller cam with less overlap, more in tune with the single-ports rpm, a CB2280 cheater cam. I'm sure others have even better combos, with web 119 or such on custom LC separation... I think single ports should like split lobes, with an extra attention to intake duration/timing. Keep on looking, I think cam selection is crucial to get the best from this situation. It's kinda like a restrictor engine, no?
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PostPosted: Tue Jan 27, 2015 8:36 am    Post subject: Reply with quote

Hello.
Several things.

1. In this case Dave is right. A stock 30 mm carb will not be nowhere near sufficient to support that kind of displacement, as would the stock sgl port intake manifold. If you modify the carb heavily it can support up to 110ish hp, - in an all out race stage. But not suitable for the street, at all.
- What you should do is buy a CB/Empi centerstion and Empi end castings, so you get more square in the intake. Then use a 34 mm Solex with a 28 mm venturi. That way you can still use an SVDA distributor without getting in trouble with vacum signal.

2. Camshaft. The C25 will work, but as some say, the engine will suffer from a not so steady idle. Sgl port single carbed engines do not like that much cam duration, and they canīt exploit it either. The recommended CB2280 is good. I have used that one several times in sgl port engines with very good results. Keep the 1,25īs on the intake and stock on the exhaust. The exhaust side does not need the lift as it will outflow the intake anyway.

As for the displacement. Well Iīm almost at the verge of saying smaller is better. Reason is that the larger the displacement the lower the hp peak will get. I would keep it at no more than 1904cc, and seriusly considering advocating a 1776 displacement, to get a decent work range out of the engine. Still, this is out from the fact that you will be using the stock intake etc. If you step up and use the 34 mm carb and beforementhioned larger intake manifolds, the 1904 solution would be OK I think.

Stock heater boxes is not a problem on this side of 100 hp.

- I am currently working on improving a 2 liter sgl port Limbach aviation engine to work properly in a street application. And I have had to take a relatively extreme angle to get it to pull peak at 4000 rpm. and still idle like a stock engine.
I have done at least 50 simulations to figure out just how much was needed and how much duration I could and had to have. Based on this, and my experience with smaller displacement sgl port engines I have written the above.

Hope this helps.

T
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