Hot Rods Repro Scot blower can't make real boost

Try removing the air filters and use another gauge, you should be getting around 4psi at 4500rpm


Sent from my iPhone using H.A.M.B.

 

it is the PCV that I have not connected yet...

Will try without the air filter and let you know

 

Just tried during ma lunch break, just tried another boost gauge and no air filter.... no difference

 

The engines are not the same, the one with the 4-71 ran a Max-1 with a lobe center figure of 111* compared to 108* for the L100. There is a bit more overlap with the Literio cam so it might be bleeding off some boost out the exhaust.
Besides the cams do both engines match in other ways? Is porting the same, valve sizes, compression and heads the same? A different intake tract can effect the boost.
The blowers are quite different too, the helix of the GMC gives a smoother flow of air as compared to the “chop” of two lobe rotors in the SCOT. Clearances were mentioned in a previous post, rotor to rotor in the SCOT cannot be changed but you might consider checking rotor to bottom of case. I have no idea what numbers you would be looking for with the H&H blower but if you find clearance excessive you could pull the dowel pins, set the bottom clearance you want and drill/ream for oversized dowels.

 

Thanks for your inputs, It is clear that the cams are different, bore and stroke, porting and blocks are the same.
I will recheck the clearance and cross check with a blower re builder,

thank you

 

H&H you say?..... good luck!!!

 

All this makes interesting reading as we had similar issues. Built a new 276 FH with a mild Max1 cam , ran it in with a pair of 97s then put the new SCOTT blower on with the complete H&H supplied kit . The engine was put together by a buddy who is a very meticulous engine guru. He specialised in flow testing and tuning , the sort of guy that pulls new carbs and distributors all apart to ensure they meet his stringent level .
The car ran well and very strong with out the blower , put the blower on and it seemed to get in the way , #4 in particular seemed to run fluffy. We changed no timing or carb changes initially and it felt aenemic.

In defence of H&H .Max snr at H&H sent me a long email of things to check , which most we had already done . So the blower is still on the altar of the gods of speed looking over my shop.
My engine guru is currently building another big SCOTT flathead for another customer so the outcome will be interesting.
Cheaters Pete ,sent a you a PM.

 

IF the clearance is too large, does anyone think that
Teflon wipers can be added to the rotors, like the NHRA guys do ?

 

What does it hook to?

Bob

 

That PCV valve will bleed off pressure unless it is plugged off.

 

PCVs suck sir from the crankcase. Shouldn't be any pressure there. And the waste should go into the intake above the blower, so no bleed off there.

 

Fordors wrote -
The engines are not the same, the one with the 4-71 ran a Max-1 with a lobe center figure of 111* compared to 108* for the L100.

If the above is true, YES, your boost is going out the tail pipe..! Very simple. Blown engines should NOT have a lot of overlap (108 degrees) if you want the pressure to stay in the cylinders. The overlap (both the exhaust and intake valves open at the same time) is WAY too large (small degree number!) for a blown induction, but very nice for a naturally aspirated (carburetor) combination.
Guaranteed, if you put the same cam in this engine, the boost will go up..! It still will not match your 4-71 blower output though. The 4-71 has three lobe rotors (possibly twisted !?). The (possibly) longer, three lobe rotors will put more pressure, along with less..."beating up" (or less smooth pressure) of the incoming air. This "beating up of the air...also "heats" the air more, which in turn, will, again, lower the overall pressure. How much...don't know, but it will be measurable.

Install the same or similar cam as the other engine has..!

Mike

 

Sometimes it's a matter of how you are measuring boost. Simply revving the engine under light load, or worse, no load will usually show little or no boost. Engine needs to be working under some load, with throttle wide open.
If you have a long uphill straight handy it can be almost as good as a dyno. It all come back to air in versus work done.
Garpo.

 

G'day CheatersPete,

I agree with Mike - methinks your issue may be overlap.

The 1960's were an awesome time in Australia for supercharger development, with a number of local companies producing machines. Among them was Eldred Norman, a seasoned (and eccentric) GP racer who went on to build superchargers. His engineering was incredible. I've spent the last decade researching the Norman supercharger. From Eldred's Supercharge! book, the boost pressure will fall by about 5% of absolute pressure for each 10º by which valve overlap is increased. For example, a motor running 10psi boost with a 10º overlap camshaft will drop to 5psi if the overlap is increased to 50º. Whilst the overlap may be of help at high RPM, it will lead to a low boost, sluggish vehicle at low RPM (and one that tends to bang back through the blower and pop the relief valve).

As a comparison, think through the range of supercharger camshafts available from Weiand (via Lunati) for small block Chevrolets. They have durations that remain long for good exhaust flow, being increased from a typical factory 270º to 290º (especially in the 01006 and 01007 performance cams). More notably, the advertised overlap has been reduced from a typical factory SBC value of 35º to nil.

Food for thought.

Cheers,
Harv

 

It is all about lobe separation angle, as has been mentioned.

My street-screamer Falcon, with a naturally-aspirated 200, runs a custom cam with a 108º LSA.

My 6-71 blown 354 Hemi runs a custom cam with a 115º LSA.

Very different cams, for very different induction. With too narrow an LSA, you are putting boost out the tail pipe.

Swap the cam to one with the widest LSA you can get.

 

Ok, that make sense ! I will look for another cam

Thanks for helping

 

Not sure the cam is the issue , yes agreed long overlap is not ideal for a blower so its easy to point the finger at the cam. We have an L100 cam in our naturally aspirated 32 3W and it does lack a little low down torque but sure is fun to drive. While our 37 has a mild as max1 cam as recommended by the engine builder, we felt the H&H blower lacked the ability to make boost, much the same as cheaterspete though we have no boost numbers as proof. Removed the blower and with zero other tuning changes it ran great , better than with the blower impediment. So still hoping this thread will provide a clue.
cheaterspete is not a newby to hot flatheads so hopefully with his knowledge we can throw some light on a solution.
So many H&H blowers around there must be a solution.

 

Just to let HAMBERS know, I received a few emails, and apparently I'm not the only one that get low boost or NO boost... still waiting for HandH reply on this..

 

Well, they look nice.

 

at least yes... but I m not into show styled hot-rods lol

 

I don't have the numbers, but I remember reading that the L100 cam is available in a "normal grind" and a "blower grind". Which one do you have? I believe that the LSA is the big difference. Just sayn.

 

Street type blowers are set looser then race type, Running for miles the rotors expand as they heat up, too tight and they will touch, and then galling and metal transfer (like walking in mud as a kid and the mud collects on your shoes) Can you achive good boost if you really stomp on the peddle instead of just driving into it? Had a turbo that acted that way--- just passed air up thru the RPMs until I whomped the throttle and started compressing the air and only then the boost started to climb

 

I had an issue with my 6-71 with two 4bbl carbs, on a 427...I was at the races, the car was low on power, and wasn't making much boost at all (about 2 psi, I think, vs. normal 6-8)

The throttle linkage was messed up, and it was only opening the carbs about half way.

I think the problem is that they need to put holes for more carbs on that blower.

 

How about checking the vacuum level between the carbs and blower? If at WOT there is not a significant difference than at idle I would say carbs are not big enough..

 

Get some long feeler gauges, check the rotor clearances, and report back.

Too much clearance means you just have a very expensive turbulence generator.

 

What's all this talk about cams affecting boost? Sure, the cam timing, duration, overlap determine how much of that pressurized air goes through the engine, but how would the cam relate to the blowers actually ability to produce boost? Boost is being measured at the manifold, and should be determined by the blower displacement and rpm, should it not?

Bob

 

The longer both the intake and exhaust valves are both open, the more air/fuel is pushed into the exhaust system.

The smaller the LSA (lobe separation angle) the more the cycle of the intake and exhaust valves overlap. Yes, they are both open at the same time.


The wider the LSA, the smaller the overlap of the intake and exhaust valve cycles, and the sooner the outlet for flow loss is slammed shut.

Boost is produced when there is something blocking the outlet. If that outlet is open, you cannot make meaningful boost pressure. If you have a tire with a hole in it that is the same size as the valve, you will never pressurize it in a meaningful fashion.

Narrow LSA's can work well with naturally aspirated engines, with properly designed exhaust systems, because the scavenging effect produced by the exhaust pulses can help pull the intake charge into the cylinders.

In a supercharged engine, it just lets the boost out.

The bigger this area is, the more flow is lost.

The shorter this area is, the more that flow can be turned into boost.

A blower without a dead-head is just a fan. How much boost does a fan make?

 

Blower to loose or displacement (case) too small ,
I have had serpentine belt slipped , (2 V-belt !!)
I called RCD and had 8 mm pulleys made,
For 144 b&m even Paxton and Vortech when they first started showing up in the early 90s

 

Thanks everyone for your help, much appreciated, Mike from HandH replied and give me a contact to discuss with, I will let you know the outcome

 

Alot of repop parts are alot of hype not horsepower