Theoretical maximum BHP
Discussion
Anybody know if it is possible to calculate the maximum theoretical BHP that a normally aspirated engine could produce from a given venturi size?
Reason I ask is that a mate runs short circuit car using a blacktop Zetec engine, the only restriction on modifications to the engine are that the car has to run a 32/36 DGV/A Carb with maximum chokes of 26mm and 27mm, this pretty effectively limits the power and what mods are worth doing to the internals.
We've currently got around 180 flywheel BHP but aren't the quickest car, the guys that go well are obviously the ones that have deep pockets and spend lots of time on the dyno. It would just be nice to know how far off we are power wise.
Reason I ask is that a mate runs short circuit car using a blacktop Zetec engine, the only restriction on modifications to the engine are that the car has to run a 32/36 DGV/A Carb with maximum chokes of 26mm and 27mm, this pretty effectively limits the power and what mods are worth doing to the internals.
We've currently got around 180 flywheel BHP but aren't the quickest car, the guys that go well are obviously the ones that have deep pockets and spend lots of time on the dyno. It would just be nice to know how far off we are power wise.
I reckon you could get 220bhp but you'd have to rev to 8000+ and the last 10% power often costs 1000%. Seen guys spending 3-400 quid and getting 50bhp increases on engines. The next 50 will cost you 3-4000 and the 50 after that is how deep at your pockets.
There's some highly technical BMEP and theoretical max piston and rod speed calcs, but that stuff only really relates to as new build engines and no expense spared. Many 16v engines will keep making power upping the rev limit and adjusting the timing but there's a safe limit on a used road car engine with cast everything. CNC block and head, have a look at Boba Kettler's YouTube channel. 1300+bhp on custom made engines 2.0...
There's some highly technical BMEP and theoretical max piston and rod speed calcs, but that stuff only really relates to as new build engines and no expense spared. Many 16v engines will keep making power upping the rev limit and adjusting the timing but there's a safe limit on a used road car engine with cast everything. CNC block and head, have a look at Boba Kettler's YouTube channel. 1300+bhp on custom made engines 2.0...
mercedeslimos said:
I reckon you could get 220bhp but you'd have to rev to 8000+ and the last 10% power often costs 1000%. Seen guys spending 3-400 quid and getting 50bhp increases on engines. The next 50 will cost you 3-4000 and the 50 after that is how deep at your pockets.
There's some highly technical BMEP and theoretical max piston and rod speed calcs, but that stuff only really relates to as new build engines and no expense spared. Many 16v engines will keep making power upping the rev limit and adjusting the timing but there's a safe limit on a used road car engine with cast everything. CNC block and head, have a look at Boba Kettler's YouTube channel. 1300+bhp on custom made engines 2.0...
I guess you didnt read any of his post then.There's some highly technical BMEP and theoretical max piston and rod speed calcs, but that stuff only really relates to as new build engines and no expense spared. Many 16v engines will keep making power upping the rev limit and adjusting the timing but there's a safe limit on a used road car engine with cast everything. CNC block and head, have a look at Boba Kettler's YouTube channel. 1300+bhp on custom made engines 2.0...
Borla said:
Anybody know if it is possible to calculate the maximum theoretical BHP that a normally aspirated engine could produce from a given venturi size?
I suggest you read this thread.https://www.pistonheads.com/gassing/topic.asp?h=0&...
Rather than try and calculate the max power available and then decide what you might do when you have a theoretical max it may be safe to say your competitors are nearer that max than your engine?
Read the regs then read them again and again until you see what you are allowed to do and what you are not allowed to do. To give you some examples. Grasstrack racing with Mini 1000s. The guy who wrote the regs said needles had to be standard at each end of the needle and gave specific size, no mention of needle dimensions between the maxima so grinding was permissible and legal, result, nearer the power achievable within the regs.
MGCC standard class V8 cam spec allowed for a modified cam without breaking the rules in anyway. Standard MGB racing regs on camshafts vary MGOC to MGCC, both have unrestricted cam duration both have restrictions on valve lift. The Car Club specifies lift on the cam which keeps us to standard lift. The MGOC specifies max lift at the valves. The standard rocker ratio of 1.42ish to1 is incorrect and much lower so usually some judicious work is required on the shape of the pads which lift the valve to get max allowable lift. As the MGOC spec does not restrict cam lift we can fit a cam which takes us to the theoretical 1.42 to 1 and achieve the legal valve lift. Gives around 5 bhp more over the MGCC engine specs. To put it into perspective a standard B makes 62/65 at the wheels and a championship winning MGOC engine makes nearer 120 at the wheels and an MGCC engine around 115 at the wheels. The guy who runs what he thinks of as a standard engine rather than read the rules will get lapped.
Work we have done on downdraught twin choke Webered race cars includes making the throttle butterflies non progressive and fitting an accelerator pump jet arrangement which then feeds both barrels on acceleration. Squeezing the last drop of power out with ignition timing and playing with cam timing if allowed in the regs.
With regard to fixed cam timing, on many occasions we have set cam timing with vernier wheels then owner has had OE wheel/s key way/s welded up and then re-machined to set the timing to the end result of the vernier wheels, legal and very effective.
Read the regs then read them again and again until you see what you are allowed to do and what you are not allowed to do. To give you some examples. Grasstrack racing with Mini 1000s. The guy who wrote the regs said needles had to be standard at each end of the needle and gave specific size, no mention of needle dimensions between the maxima so grinding was permissible and legal, result, nearer the power achievable within the regs.
MGCC standard class V8 cam spec allowed for a modified cam without breaking the rules in anyway. Standard MGB racing regs on camshafts vary MGOC to MGCC, both have unrestricted cam duration both have restrictions on valve lift. The Car Club specifies lift on the cam which keeps us to standard lift. The MGOC specifies max lift at the valves. The standard rocker ratio of 1.42ish to1 is incorrect and much lower so usually some judicious work is required on the shape of the pads which lift the valve to get max allowable lift. As the MGOC spec does not restrict cam lift we can fit a cam which takes us to the theoretical 1.42 to 1 and achieve the legal valve lift. Gives around 5 bhp more over the MGCC engine specs. To put it into perspective a standard B makes 62/65 at the wheels and a championship winning MGOC engine makes nearer 120 at the wheels and an MGCC engine around 115 at the wheels. The guy who runs what he thinks of as a standard engine rather than read the rules will get lapped.
Work we have done on downdraught twin choke Webered race cars includes making the throttle butterflies non progressive and fitting an accelerator pump jet arrangement which then feeds both barrels on acceleration. Squeezing the last drop of power out with ignition timing and playing with cam timing if allowed in the regs.
With regard to fixed cam timing, on many occasions we have set cam timing with vernier wheels then owner has had OE wheel/s key way/s welded up and then re-machined to set the timing to the end result of the vernier wheels, legal and very effective.
Mignon said:
I remember Dave Vizard seeing how much power he could get out of a race Pinto engine using the DGAV. I think it was about 175 bhp but it's a long time ago. The key is plenum volume. The more of that you can fit in the less important the venturi size becomes.
That backs up comments in the thread linked to above, where there is a restriction then gas flow can recover by up to 15% if there is a big enough space after the restriction for it to recover into. This one is worth looking into....Thanks for the other inputs, point was made about looking deeply at the rules, I think you missed what I wrote in the opening post, there are no rules/restrictions on what can be done to the engine, the major restriction on power is the DGV carb, anything else goes (no nitrous/forced induction). The DGV carb has to remain stock, the progressive opening has to stay and no gas-flowing or increasing venturi size. Most people are even using stock Ford camshafts, they give a good spread of power if dialed in well.
Don't get me wrong, we're not a long way off, track length is (at a guess) 400m, race duration is 25 laps and in a dry race we're 1/4 of a lap or so behind the leader. We've won wet races where ultimate power is less important.
Edited by Borla on Wednesday 26th December 22:00
I noted what you said about no restrictions but made the assumption there must be a few? Engine bore and stroke? If not restricted have you got a massive overbore and a stroked crank? If you win in the wet that would point to a softer input of power giving more control over the car , maybe lesser cams than the front runners? Have you set the cam timing on the rollers? Remember skimming the head to up CR retards the cam timing on an overhead cam engine, retarded timing would soften the power input.
Of interest how many times has the car beaten the front runners?
Of interest how many times has the car beaten the front runners?
PeterBurgess said:
I noted what you said about no restrictions but made the assumption there must be a few? Engine bore and stroke? If not restricted have you got a massive overbore and a stroked crank?
Sorry, oversight on my part there, rules don't specifically outlaw a massive overbore, but like a lot of rule books it does not say you can..... Actual and only wording for engine and carb as copied from rule for book is this:ENGINE / CARBURETTOR
Only the Ford x Flow, 2Ltr Pinto or Ford Zetec is permitted to be used.
Only the standard 32/36 DGV/A Carb may be used with a maximum size chokes of 26mm and 27mm. No polishing or re-profiling is allowed. No modifications to the carb body and the interchanging of a carb top from another Weber model is not permitted.
Also, knowing how little metal there is between the piston bores, there isn't the room for a massive over-bore. The rules are there to try and promote reasonably priced racing in this class, the carb really negates the point of doing any radical tuning because it is the power restriction.
PeterBurgess said:
If you win in the wet that would point to a softer input of power giving more control over the car , maybe lesser cams than the front runners? Have you set the cam timing on the rollers?
Yes, cam timing has been optimisedPeterBurgess said:
Remember skimming the head to up CR retards the cam timing on an overhead cam engine, retarded timing would soften the power input.
Yes, head has been skimmed to within an inch of it's life, but as above cam timing has been corrected/optimised.PeterBurgess said:
Of interest how many times has the car beaten the front runners?
Only twice, both times in the wet, car handles very well which helped too.Mignon said:
Very interesting read that, thanks.Thanks for writing down the regs, I found some just before you posted but did not know if they were 'your' class
With the carb being the 'only' restriction my thoughts for a way forward would be to get the engine as large as possible, do Burton do oversize pistons to 2.1? I would look at a stroker kit too Get the cr up as high as you can with whatever fuel you are allowed to run. Bearing in mind the engine will become breathless early you need to stretch it with maybe extra ignition advance at high rpms if you can, the high cr will help stretch the power band beyond the comfort zone of the carb too. Heavy springs in the dizzy can sort high rpm advance, I have seen H and H ignitions do this sort of work very well when customers take in the relevant timing requirements from rolling road testing.
As a bonus for bhp, fill your fuel tank with as cold as you can keep it fuel just before the race, make sure the fuel supply lines cannot pick up heat on the way to the carb as underbonnet cooling will be at a premium with the track speeds on short circuit.
With the carb being the 'only' restriction my thoughts for a way forward would be to get the engine as large as possible, do Burton do oversize pistons to 2.1? I would look at a stroker kit too Get the cr up as high as you can with whatever fuel you are allowed to run. Bearing in mind the engine will become breathless early you need to stretch it with maybe extra ignition advance at high rpms if you can, the high cr will help stretch the power band beyond the comfort zone of the carb too. Heavy springs in the dizzy can sort high rpm advance, I have seen H and H ignitions do this sort of work very well when customers take in the relevant timing requirements from rolling road testing.
As a bonus for bhp, fill your fuel tank with as cold as you can keep it fuel just before the race, make sure the fuel supply lines cannot pick up heat on the way to the carb as underbonnet cooling will be at a premium with the track speeds on short circuit.
So.
Lightweight internals as far as possible, engine size ? maybe irrelevant. Flywheel etc, but actual overall engine weight as well as reciprocating mass.
Camshafts, bound to be torque/power delivery gains to be had within the venturi restriction.
Compression....get it high, and use the best fuel you can, methanol or other ?
Does the engine need to operate over a wide rpm range, or would a narrower power band be fine ?
You mention carb venturi size. But does it actually specify that fuel must be supplied by a carb ? ie leave the carb in place purely for a throttle and restriction, but modify intake for fuel injection for more precise fuel control ?
You also list the rules saying to run a 32/36....does it specifically say only one of them for all 4 cylinders ? lol Could you do 1 per cylinder ?
Do you use a brake servo....the sort that has a large diameter vacuum tube from intake manifold over to the servo, that might "leak" lots of air in under load ? Tuned so fueling would be fine with this of course.
Lightweight internals as far as possible, engine size ? maybe irrelevant. Flywheel etc, but actual overall engine weight as well as reciprocating mass.
Camshafts, bound to be torque/power delivery gains to be had within the venturi restriction.
Compression....get it high, and use the best fuel you can, methanol or other ?
Does the engine need to operate over a wide rpm range, or would a narrower power band be fine ?
You mention carb venturi size. But does it actually specify that fuel must be supplied by a carb ? ie leave the carb in place purely for a throttle and restriction, but modify intake for fuel injection for more precise fuel control ?
You also list the rules saying to run a 32/36....does it specifically say only one of them for all 4 cylinders ? lol Could you do 1 per cylinder ?
Do you use a brake servo....the sort that has a large diameter vacuum tube from intake manifold over to the servo, that might "leak" lots of air in under load ? Tuned so fueling would be fine with this of course.
Edited by stevieturbo on Wednesday 26th December 22:49
stevieturbo said:
So.
Lightweight internals as far as possible, engine size ? maybe irrelevant. Flywheel etc, but actual overall engine weight as well as reciprocating mass.
All our internals are factory at moment, without doubt if budget was available we would use forged pistons and steel rods, probably more for reliability than power. Flywheel is a heavily modified factory item, simply because it was the most cost effective way to go. We used to run a multiplate race clutch (5"?) but again for cost reasons currently run a heavy duty standard type clutch.Lightweight internals as far as possible, engine size ? maybe irrelevant. Flywheel etc, but actual overall engine weight as well as reciprocating mass.
stevieturbo said:
Camshafts, bound to be torque/power delivery gains to be had within the venturi restriction.
Compression....get it high, and use the best fuel you can, methanol or other ?
At the moment we have standard Ford cams, this is the one thing that we will be changing, problem is finding best solution first time. Short circuit cars generally just use first gear just for start then rest of race is in second gear, I've been told mid corner coming back on power is about 3000rpm up to about 7500rpm at end of straight. Compression is about as high as mechanically possible, pistons are at top of deck and head is machined (IIRC) 0.100" which I have been told is safe limit. Rules state engine must use 'roadside' fuel.Compression....get it high, and use the best fuel you can, methanol or other ?
stevieturbo said:
Does the engine need to operate over a wide rpm range, or would a narrower power band be fine ?
Answered above.stevieturbo said:
You mention carb venturi size. But does it actually specify that fuel must be supplied by a carb ? ie leave the carb in place purely for a throttle and restriction, but modify intake for fuel injection for more precise fuel control ?
You also list the rules saying to run a 32/36....does it specifically say only one of them for all 4 cylinders ? lol Could you do 1 per cylinder ?
I like your thinking, I don't think we'd get away with more than one carb, although next time we're out I'll mention it see how it goes down :-)You also list the rules saying to run a 32/36....does it specifically say only one of them for all 4 cylinders ? lol Could you do 1 per cylinder ?
stevieturbo said:
Do you use a brake servo....the sort that has a large diameter vacuum tube from intake manifold over to the servo, that might "leak" lots of air in under load ? Tuned so fueling would be fine with this of course.
Nope, no servo.Edited by stevieturbo on Wednesday 26th December 22:49
Borla said:
Lightweight internals as far as possible, engine size ? maybe irrelevant. Flywheel etc, but actual overall engine weight as well as reciprocating mass.
All our internals are factory at moment, without doubt if budget was available we would use forged pistons and steel rods, probably more for reliability than power. Flywheel is a heavily modified factory item, simply because it was the most cost effective way to go. We used to run a multiplate race clutch (5"?) but again for cost reasons currently run a heavy duty standard type clutch.At the moment we have standard Ford cams, this is the one thing that we will be changing, problem is finding best solution first time. Short circuit cars generally just use first gear just for start then rest of race is in second gear, I've been told mid corner coming back on power is about 3000rpm up to about 7500rpm at end of straight. Compression is about as high as mechanically possible, pistons are at top of deck and head is machined (IIRC) 0.100" which I have been told is safe limit. Rules state engine must use 'roadside' fuel.
1. Make more power from combustion i.e, get more air in and burn it as efficiently as possible.
2. Maximise the power you are creating through combustion.
1. Don't discount improving breathing through head work and camshafts. Reprofiled valve seats and valve angles should be pretty cost effective. Large changes to port area would, I believe, of little help as the chokes would be the greatest restriction, getting better low lift flow would help though. Longer duration faster opening cam profile. Spend time on the dyno!
2. Given you are using only 2nd gear for the entire race I would be looking at the lightest bottom end possible. The effect of lightweight internals on a low gear is much greater than in a high gear. Put the small diameter clutch back in also. Also, once you have optimised the tuning, look at the power curve and set your grearing to get the best from it. You want your accelleration event to be slap bamg in the middle of the power.
It's not really surprising your friend's engine isn't fully competitive. He hasn't done any of the things that would greatly increase its overall performance. Your question about the potential maximum bhp from that venturi size is also pretty much the wrong question to be asking. I've built a bunch of championship winning short track engines over the years but A) I'm retired and B) by the sounds of it your friend couldn't have afforded me anyway.
Do you use a brake servo....the sort that has a large diameter vacuum tube from intake manifold over to the servo, that might "leak" lots of air in under load ? Tuned so fueling would be fine with this of course.
What an excellent 'work around' Stevie, I only just noticed it tucked away at the bottom of your reply, I will salt that concept away for future restrictive carb classes
If I get a chance I will try it and post it.
Sadly, I reckon once noticed by the scrutineers it would be banned!
What an excellent 'work around' Stevie, I only just noticed it tucked away at the bottom of your reply, I will salt that concept away for future restrictive carb classes
If I get a chance I will try it and post it.
Sadly, I reckon once noticed by the scrutineers it would be banned!
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